Symposium CG
Progress in Nano-laminated Ternary Carbides and Nitrides (MAX Phases) and Derivatives Thereof (MXenes)


Session CG-1 - Transport and Electronic Properties, Ab Initio Calculations and Structural Characterization of MAX and MXene Phases

CG-1:IL01  Anisotropy of MAX Phase's Transport Properties
S. DUBOIS, W. YU, V. MAUCHAMP, V. GAUTHIER-BRUNET, T. CABIOC'H, Institut PPRIME, CNRS/Université de Poitiers/ENSMA, UPR 3346, Bât. SP2MI, Futuroscope-Chasseneuil Cedex, France; L. GENCE, L. PIRAUX, Institute of Condensed Matter and Nanosciences, Université catholique de Louvain, BSMA/FHyN, Louvain-la-Neuve, Belgium

MAX phases electronic properties and especially the anisotropy resulting from the nanolaminated structure is not yet fully understood. Although intensively studied, it remains a complex issue since the majority of transport experiments were performed on polycrystalline samples thereby averaging the basal plane and c-axis transport properties. To circumvent this point, several alternative approaches have been investigated such as the probe of single grains plasmon excitations anisotropy using Electron Energy-Loss Spectroscopy (EELS), the comparison between data obtained on (000l) oriented thin films and bulk polycrystalline samples, or the comparison between thin films with different grain populations.
An overview of MAX phase transport properties will be discussed in a first step. In a second step, emphasis will be placed on the results obtained at PPRIME Institute on the anisotropic transport properties of Ti2AlC and Ti3SiC2.
The anisotropy of Ti2AlC transport properties are understood from the compared study of both a highly-oriented (000l) Ti2AlC thin film and a Ti2AlC polycrystalline sample.
The basal plane (BP) and c-axis resistivities were measured on highly-oriented (000l) and (11-20) Ti3SiC2 thin films. The latter is 4 times higher than the BP resistivity at RT.

CG-1:IL02  A Genomic Approach to Properties of MAX Phase Compounds 
WAI-YIM CHING, S. ARYAL, R. SAKIDJA, University of Missouri, Kansas City, MO, USA; M.W. BARSOUM, Drexel University, Philadelphia, PA, USA

Using a genomic approach, we have established a large database on the properties of 792 MAX (Mn+1AXn) phases (M = Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo; A = Al, Ga, In, Tl, Si, Ge, Sn, Pb; X = C, N and n = 1-4). Based on the calculated elastic and mechanical properties of these crystals, their mechanical and thermodynamic stability is rapidly screened resulting in a refined database of 665 crystals. All the experimentally reported phases are shown to be stable but there are many other equally stable phases that have not been tried in laboratories. Concurrently, the electronic structure and bonding in these MAX phases are calculated using density functional theory. Detailed analysis of the correlations between the electronic structure (charge transfer, bond order, density of states at Fermi level, etc.) and the mechanical properties (elastic coefficients Cij, bulk modulus K, shear modulus G, Young's modulus E, Poisson's ratio η and Pugh ratio G/K, etc.) provide the fundamental understanding of the key elements that lead to the most desirable properties of MAX compounds for practical applications. This highly accurate large database on MAX phases is used to test the efficacy of several data mining algorithm and machine learning tools with calculated properties as descriptors.

CG-1:IL03  Atomic Vibration and Anisotropic Transport in MAX phases
G. HUG, L. ANDREA, ONERA-CNRS, Chatillon, France; L. CHAPUT, IJL Université de Nancy, France; A. Togo, Kyoto University, Japan

Due to their crystallographic structure with high c/a aspect ratio, MAX phases have been predicted to possess highly anisotropic intrinsic properties. As an example, it has been shown that the thermoelectric tensor in Ti3SiC2 exhibits components of opposed signs in the basal plane or along the c axis leading to a compensated null value in macroscopic measurements. However, experimental determination of e. g. transport properties are impeded by the lack of macroscopic single crystals. Similarly, theoretical ab initio determination of transport properties requires solving of a Boltzmann transport equation of a charge density or phonon density for electric or thermal conductivity, respectively. Besides its difficulty to solve the Boltzmann equation contains one important ingredient, the lifetime, which is not straightforwardly given by the standard DFT. The lifetime term accounts for the damping of excited states of elections or phonons via electron-electron, electron-phonon and phonon-phonon interactions. Recent calculations of the later term allow to recover a realistic energy- and band-dependent lifetime, which can be used to solve the Boltzmann equation and to make new analysis and prediction of orientation-dependent transport properties like thermal conductivity.

CG-1:IL04  Magnetic MAX Phases Based on Mn from First Principles and Thin Film Synthesis
A.S. INGASON, J. ROSEN, Thin Film Physics, Department of Physics, Chemistry and Biology (IFM), Linkoping University, Linkoping, Sweden

First-principles calculations have been used to study trends in phase stability for potential magnetic MAX phases based on Cr2GeC, Cr2AlC, and Cr2GaC, with Mn substituting Cr over a range 0 - 100 %. The theoretical results predict stable MAX phase alloys, as well as a new phase, Mn2GaC, introducing an entirely new M element to the MAX phase family. The theory also show the possibility to use atomic configurations as well as composition to tune the magnetic behavior of these phases. This has been explored experimentally by thin film deposition through arc as well as sputtering methods. Predicted new MAX phases have been successfully synthesized, and the magnetic properties studied, in accordance with observations from theory. Intriguing results include ferromagnetic behavior above room temperature, non-collinear magnetic ordering, and sharp transitions between magnetic states accompanying significant volume changes.

CG-1:IL05  MAX Phases and MXene Valence Electron Excitations with Nanometre Scale Resolution
V. MAUCHAMP, D. MAGNE, T. CABIOC'H, Institut Pprime, University of Poitiers-CNRS-ENSMA, Poitiers, France; M. BUGNET, G.A. BOTTON, CCEM, Mc Master University, Hamilton, Canada; M.W. BARSOUM, Drexel University, Philadelphia, USA

Probing the intrinsic response (i.e. at the single grain level) of MAX phases and MXenes is a key point in the understanding of the structure-properties relationship in these materials. In this context, Electron Energy-Loss Spectroscopy (EELS) in the Transmission Electron Microscope (TEM) is a powerful tool to investigate the electronic structure at the nanometre scale with controlled crystallographic orientation.
Combining EELS with Density Functional Theory calculations, we focus on the characterization of the collective oscillations of valence electrons (so-called plasmons) in Al based MAX phases and related MXenes. We analyze the complex screening mechanisms in MAX phases and we show that their dielectric properties can be understood by describing the MAX as atomic scale superlattices. The relevance of this picture to describe other physical properties (chemical bonding, behaviour under ion irradiation) will be discussed. Based on these results, bulk and surface modes of Ti3C2Tx MXenes will be analysed with particular emphasis on their dependence on the number of MXene sheets.

CG-1:IL06  Itinerant-electron Magnetism of Cr-based MAX Phases
Z. LIU, T. WAKI, Y. TABATA, H. NAKAMURA, Department of Materials Science and Engineering, Kyoto University, Kyoto, Japan

Aiming to find magnetic states in the MAX phases, we measured systematically magnetic properties of Cr-based M2AX compounds synthesized via a solid-state eraction. It was found that all known carbides, Cr2AlC, Cr2GaC, and Cr2GeC, are nonmagnetic down to the lowest temperature, although Cr2GeC is exchange-enhanced and located in the vicinity of a ferromagnetic quantum critical point [1]. On the other hand, a magnetic phase transition, most probably to the spin-density-wave
(SDW) state, has been found in Cr2GaN at 170 K [2]. The origin of the SDW transition is interpreted in terms of possible Fermi-surface nesting
in the two-dimensional-like electronic structure. We also found that the ferromagnetic band polarization is induced immediately by Mn doping to Cr2GeC [1]. The Curie temperature and the spontaneous moment increase almost proportionally to the Mn concentration.  The strong concentration dependence of p_eff/p_s, where p_eff is the effective moment in the paramagnetic state and p_s is the spontaneous moment in the ferromagnetic state, indicates that the ferromagnetism appearing in the Mn-doped Cr2GeC can be classified as a typical itinerant-electron ferromagnetism in a wide range of the degree of electron localization.
[1] Z. Liu et al., Phys. Rev. B 89, 054435 (2014).
[2] Z. Liu et al., Phys. Rev. B 88, 134401 (2013).

CG-1:L07  Temperature Dependent Phase Stability of Tin+1AlCn MAX Phases from First-principles Calculations
A. THORE, M. DAHLQVIST, B. ALLING, J. ROSÉN, Linköping University, Linköping, Sweden

Methods based on first-principles calculations have proven effective for predicting the existence of new materials, including MAX phases. However, the vast majority of the predictions are based on 0 K calculations, which means that until now, little has been known about the effects of temperature on the phase stability of existing phases, causing a considerable uncertainty for stability predictions of new hypothetical phases.
In this work we combine first-principles calculations with an optimization procedure to calculate the phase stability as a function of temperature for Ti2AlC, Ti3AlC2 and Ti4AlC3 MAX phases with respect to their most competing phases in the Ti-Al-C phase diagram, in a temperature interval from 0 to 2000 K. To model nonzero temperatures, we include effects from the electronic and vibrational free energies to the Gibbs free energy, using approximations including or excluding thermal expansion. We show that, in our material case, the results of neither approximation differ significantly from the calculated 0 K formation energies, and thus provide further evidence for the hypothesis that MAX phase stability is primarily governed by the 0 K energy terms.

Session CG-2 -  Room Temperature Mechanical Properties of the MAX Phases

CG-2:IL01  Neutron Diffraction Evidence for Incipient Kink Bands in Highly Textured Ti2AlC
E.N. CASPI, O. YEHESKEL, Nuclear Research Centre - Negev, Beer-Sheva, Israel; M. SHAMMA, S. AMINI, A. ZHOU, V. PRESSER, M.W. BARSOUM, Drexel University, Philadelphia, PA, USA; B. CLAUSEN, S.C. VOGEL, D.W. BROWN, LANL, Los Alamos, NM, USA

The MAX phases are a large family (> 60 members) of ternary early transition metal carbides, carbonitrides, and nitrides with a layered hexagonal structure and a Mn+1AXn, chemistry, where: "M" is an early transition metal, "A" is an A-group element, "X" is carbon or nitrogen. Upon cyclic loading, these phases trace fully and spontaneously reversible, rate-independent, closed hysteresis loops. The microscopic mechanical mechanism driving this unusual non-linear elastic behavior is still under debate, with the formation of fully reversible Incipient Kink Bending (IKB) of the hexagonal basal planes as one of the intriguing models. In an attempt to better understand this unique mechanical behavior, in-situ stress-strain neutron diffraction measurements of the Ti2AlC MAX-phase compound were performed on SMARTS at the Los Alamos National Laboratory. Detailed diffraction analysis revealed an unexpected and complex loading direction dependent behavior of the diffraction planes with the applied load. This behavior is accompanied with the never seen before expansion and contraction of the width of selected diffraction peaks as a function of loading and unloading cycles. These observations can be explained by the combination of mechanical slip and formation of IKBs, with the cyclic peak broadening and contraction as a strong evidence for the latter.

CG-2:IL02  Pressure-enforced Plasticity in MAX Phases: from Single Grains to Polycrystals
A. GUITTON, A. JOULAIN, L. THILLY, C. TROMAS, Pprime Institute, CNRS - University of Poitiers - ISAE-ENSMA, France; S. VAN PETEGEM, H. VAN SWYGENHOVEN, Paul Scherrer Institute, Villigen, Switzerland

It is commonly believed that plastic deformation mechanisms of MAX phases consist in basal dislocation glide, thus forming pile-ups and walls. The latter can form local disorientation areas (kink bands). Nevertheless, the elementary mechanisms and the exact role of microstructural defects are not fully understood yet. We present here a multi-scale experimental study of deformation mechanisms of the Ti2AlN MAX phase.
At the macroscopic scale, in-situ compression tests at room temperature coupled with neutron diffraction were performed. They brought new insight into the deformation behaviour of the different grain families in the polycrystalline Ti2AlN. At the mesoscopic scale, deformed surface microstructures were observed by SEM and AFM. These observations associated with nanoindentation tests showed that grain shape and orientation relative to the stress direction control formation of intra- and inter- granular strains and plasticity localization.
Finally, at the microscopic scale, a detailed dislocation study of samples deformed under confining pressure revealed the presence of dislocation configurations never observed before in MAX phases, such as dislocation reactions, dislocation dipoles. In the light of these new results, mechanical properties of MAX phases are discussed.

CG-2:IL03  Microstructure Design of MAX Phases with High Strength and Toughness
CHUNFENG HU1,2, DONG QU2, K. SATO1, M. ESTILI1, S. GRASSO3, H. YOSHIDA1, K. MORITA1, T. NISHIMURA1, T. SUZUKI1, B. KIM1, Y. SAKKA1, 1National Institute for Materials Science, Japan; 2Ningbo Institute of Material Technology and Engineering, CAS, China; 3Queen Mary University of London, UK

Owing to the special nanolayered microstructure, MAX phases possess excellent machinability, physical and mechanical properties. Generally, the flexural strength and fracture toughness of MAX phases are 200-600 MPa and 6-9 MPa·m1/2, respectively. In order to further enhance the intrinsic properties, the microstructure design was adopted to modify the relationship between property and microstructure. Surely the textured MAX phases could be successfully fabricated by strong magnetic field alignment (SMFA) followed by spark plasma sintering. It was proved that the tailored Nb4AlC3 ceramic showed high flexural strength above 1200 MPa and high fracture toughness above 18 MPa·m1/2. Also, Ti3SiC2 ceramic could be formed as nanolaminar structure ranging from nano-scale to milli-scale. The textured Ti3AlC2 ceramic and its composites would be systemically discussed. The investigation achievements of these works contributing to further understandings of advanced MAX phases.

CG-2:L04  Mechanical Properties of Ti3AlC2 and Ti3AlC2/TiC Composites
T.A. PRIKHNA1, STAROSTINA A.V.1, BASYUK T.V.1, DUB S.N.1, OSADCHIY A.A.1, LOSHAK M.G.1, CABIOC'H T.2, CHARTIER P.2, SVERDUN V.B.1, KARPETS M.V.1,3, DEVIN L.N.1, 1Institute for Superhard Materials of the National Academy of Sciences of Ukraine, Kiev, Ukraine; 2Universite de Poitiers, CNRS/ Laboratoire PHYMAT, UMR 6630 CNRS-Universite de Poitiers SP2MI, Chasseneuil Futuroscope Cedex, France; 3Institute for Problems of Materials Science of the National Academy of Sciences of Ukraine, Kiev, Ukraine

Ti3AlC2/TiC composites, with a content of TiC varying from 0 to 100 mass %, were produced by pressureless annealing and by hot pressing (30 MPa or 2GPa) of TiC, TiH2, Al and C mixtures.
Mechanical testing by nanoindentation showed that nanohardness, HB, and Young modulus, E, increased with the TiC content from 2.0±0.4 to 23.6±1.2 GPa and from 137±21 to 447±11 GPa, respectively. Wide hysteresis loops were observed during nanoindentation tests for samples with low TiC contents. This high loss of elastic deformation energy during cyclic deformation evidences the high damping ability of the MAX phase. This was confirmed by a resonance curves method which allowed the determination of the logarithmic decrement of damping, δ, increasing values of δ (up to 1,42±0,14, 1.7 times higher than gray cast, i.e. very high) being obtained when the porosity and the TiC contents decrease. This is attributed to a better connectivity between Ti3AlC2 grains and between Al2O3 inclusions with the MAX phase too.
A more extensive study of mechanical properties of a 99.3 % dense Ti3AlC2 (91 mass.%) was performed indicating that the bending and compressive strength were 500 MPa and 700 MPa respectively. Values of KIC=10.2 MPa.m-0.5, HRA=70 GPa, HV=4,6 GPa were also obtained.

Session CG-3 - High Temperature Mechanical, Oxidation and Thermal Properties of the MAX Phases

CG-3:IL01  Critical Review of Creep and Oxidation Resistance of the MAX phases
M.W. BARSOUM1, D. TALLMAN1, B. ANASORI1, M. RADOVIC2, 1Department of Materials Science and Engineering, Drexel University, Philadelphia, PA, USA; 2Department of Mechanical Engineering, Texas A&M University, College Station, TX, USA

One of the more promising applications for the MAX phases is in high temperature structural applications. Before the latter can be realized, however, their resistance to creep and oxidation need to be well understood. Of all the MAX phases, the most resistant to oxidation in air in the 900-1400 °C temperature range are Ti2AlC, Ti3AlC2 and Cr2AlC. A literature review, however, shows that while many claim the oxidation kinetics to be parabolic, others claim them to be cubic. Whether the kinetics are parabolic or better is of vital practical importance. By carefully re-plotting the results of others and carrying out one oxidation run for ≈ 3000 h at 1200 °C on a Ti2AlC sample, we conclude that the oxidation kinetics are better described by cubic kinetics and that even that conclusion is an approximation. The creep of Ti3SiC2 and Ti2AlC samples, with varying grain sizes in the 1000-1200 °C temperature range in both tension and compression, suggest that dislocation and grain boundary sliding are the dominant creep mechanisms. The high plastic anisotropy results in large internal stresses during creep. A comparison of the creep properties with other high temperature structural materials suggests that the MAX phases can be used at higher temperatures but lower stresses.

CG-3:IL02  Critical Review of the Oxidation of Cr2AlC
DONG BOK LEE, School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon, South Korea

The successful deployment of Cr2AlC requires the understanding of the high-temperature oxidation characteristics of Cr2AlC such as the oxidation kinetics, the mechanism, and the oxide scales formed. Hence, the high-temperature isothermal and cyclic oxidation behavior of Cr2AlC is described. Cr2AlC oxidizes according to the following equation during the isothermal oxidation between 900 and 1300 °C in air.
Cr2AlC + O2 → (the Al2O3 oxide layer) + (the Cr7C3 sublayer) + (CO or CO2 gas)
The oxide scale consists primarily of the Al2O3 barrier layer that forms by the inward diffusion of oxygen. The consumption of Al to form the Al2O3 leads to the enrichment of Cr immediately below the Al2O3 layer, resulting in the formation of the Cr7C3 sublayer. At the same time, carbon escapes from Cr2AlC as CO or CO2 gas into the air. During the cyclic oxidation between 900 and 1100 °C in air, Cr2AlC similarly oxidizes according to the equation; Cr2AlC + O2 → (the Al2O3 oxide layer) + (the Cr7C3 sublayer) + (CO or CO2 gas). However, during the cyclic oxidation at 1200 and 1300 °C, Cr2AlC oxidizes according to the equation, Cr2AlC + O2 → (the Al2O3/Cr2O3/Al2O3 triple oxide layers) + (the Cr7C3 sublayer) + (CO or CO2 gas), because the thermo-cyclings facalitate the formation of the intermediate Cr2O3-rich layer between the outer Al2O3-rich layer and the inner Al2O3-rich layer.
Acknowledgement. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Korean Ministry of Education, Science and Technology (Project No. 2010-0023002).

CG-3:IL03  Decomposition Kinetics of Max Phases In Extreme Environments - A Critical Review
I.M. LOW, Department of Imaging & Applied Physics, Curtin University of Technology, Perth, WA, Australia; W.K. Pang, The Bragg Institute ANSTO, Kirrawee DC, NSW, Australia

A critical review of the susceptibility of MAX phases to thermal decomposition in extreme environments is presented. At elevated temperatures and in inert environments, MAX phases tend to become unstable and decompose to binary carbide (e.g. TiCx) or binary nitride (e.g. TiNx), primarily through the sublimation of A-elements such as Al or Si, which results in a porous surface layer of MXx being formed. Positive activation energies have been determined for decomposed MAX phases with coarse pores but a negative activation energy when the pore size was less than 1.0 µm. The kinetics of isothermal phase decomposition at 1550 °C have been modelled using a modified Avrami equation. An Avrami exponent (n) of < 1.0 was determined, indicative of the highly restricted diffusion of Al or Si between the channels of M6X octahedra. The role of pore microstructures on the decomposition kinetics is discussed. The implications of this work are enormous and the insights for tailor-design of MAX phases with controlled thermal stability and intercalated MXenes for energy storage are addressed.

CG-3:L04  Oxidation and Crack Healing Behavior of Ti2Al(1-x)SnxC/Al2O3 Composites
GUO-PING BEI, B.J. PEDIMONTE, M. PEZOLDT, T. FEY, P. GREIL, Ceramic and Glass Group, Department of Materials Science, University of Erlangen-Nürnberg, Erlangen, Germany

Ti2Al(1-x)SnxC/Al2O3 (x=0.5) composites with different Ti2Al(1-x)SnxC volume content have been prepared by pressureless sintering at 1350°C for 4h under Ar atmosphere. After sintering, the Ti2Al(1-x)SnxC distributed homogenously in the Al2O3 matrix with a density of 91-95% that of the theoretical density. The oxidation behavior of the Ti2Al(1-x)SnxC/Al2O3 composites have been investigated by TG-DTA meaurement at the 1200°C for 12h. XRD and SEM coupled with EDS were used to determine the mechanism. It has been found that the oxidation of Ti2Al(1-x)SnxC controls the oxidation behavior of the composites. The thickness of the oxidation zone was determined as function of the oxidation time at 700°C and 900°C. Afterwards, Vickers indentation was used to generate the cracks in the composites. The healing process were performed at 700°C and 900°C for 0.5-96h. It has been found that the with higher Ti2Al(1-x)SnxC repair filler and temperature, rapid recovery of mechanical properties was obtained in the composites. The healing mechanisms were determined by XRD and SEM as well as EDS map analysis. The interfaces between different oxidation products were investigated by TEM.

CG-3:IL05  Current Understanding of Tribology of MAX Phases and Their Composites during Dry Sliding
S. GUPTA, Advanced Materials Research Group, Dept. of Mechanical Engineering, University of North Dakota, Grand Forks, ND, USA

MAX phases are thermodynamically stable nanolaminates displaying unusual and sometimes unique properties. These phases are so-called because they possess a Mn + 1AXn chemistry, where n is 1, 2, or 3, M is an early transition metal element, A is an A-group element and X is C or N. They are highly damage tolerant, thermal shock resistant, readily machinable, and with Vickers hardness values of 2-8 GPa, are anomalously soft for transition metal carbides and nitrides. Some of them display a ductile-brittle transition at temperatures > 1000 ◦C, while retaining decent mechanical properties at these elevated temperatures. Moreover, their layered nature suggests they may have excellent promise as solid lubricant materials. Recently, first generation MAX Phase based composites shafts were successfully tested against Ni-based superalloy at 50,000 rpm from RT till 550 ◦C during thermal cycling in a foil bearing rig. This study further demonstrates the potential of MAX Phases and their composites in different tribological applications.
The main objective of this talk is to present recent progress and scientific understanding about the tribological behavior of MAX Phases and their composites during dry sliding.

CG-3:IL06  High Temperature Oxidation, Thermal Shock and Crack Healing Behaviors of MAX Phases
SHIBO LI, Center of Materials Science and Engineering, School of Mechanical and Electronic Control Engineering, Beijing Jiaotong University, Beijing, China

MAX phase materials, including the layered ternary carbides and nitrides, have attracted much attention because of their unusual combinations of attractive properties up to high temperatures, such as high strength, high oxidation resistance, ductility and nonsusceptibility to thermal shock. These properties make MAX materials attractive for high temperature applications such as gas turbines, heat exchangers, solid oxide fuel cells and nuclear reactors.
In addition, upon application in high temperature environments in which materials are exposed to thermal cycles, mechanical loading and oxidative environments, MAX materials are highly expected to possess autonomous crack healing ability. The crack healing capability could significantly extend their service life, and improve their structural integrity and reliability.
For practical applications, it is necessary to fundamentally understand their high temperature properties. Here, the high temperature oxidation resistance, abnormal thermal shock, and crack healing for MAX phase materials are presented, together with a discussion of their future for high temperature applications.

CG-3:L07  Study of the Thermal Stability in Air of Ti2Al(C1-xNx) Solid Solutions
T.A. PRIKHNA1, D. LITZKENDORF2, T. CABIOCH3, T.V. BASYUK1, A.V. STAROSTINA1, P. CHARTIER3, D.V. TURKEVICH1, M.V. KARPETS1, 4, V.V. KOVYLAEV1, 1Institute for Superhard Materials of the National Academy of Sciences of Ukraine, Kiev, Ukraine; 2Institut für Photonische Technologien, Jena, Germany; 3Universite de Poitiers, CNRS/ Laboratoire PHYMAT, UMR 6630 CNRS Universite de Poitiers SP2MI, Chasseneuil Futuroscope Cedex, France; 4Institute for Problems in Material Science of the National Academy of Sciences of Ukraine, Kiev, Ukraine

MAX phases of 211 structural type to which belong solid solutions Ti2Al(C1-xNx)y are considered to be promising functional materials for high-temperature applications. Porous samples of Ti2Al(C1-xNx)y and Ti3AlC2 (for comparison) were synthesized under Ar 0.1 MPa pressure and then densified in thermobaric conditions at 2 GPa, 1400 °C, for 1h. The Differential Thermal Analysis (DTA) and thermogravimetry (TG) study in air of these samples reveal that Ti3AlC2 is more stable than Ti2AlC, these carbides being more stable than Ti2Al(C1-xNx) solid solutions. Densification changed the kinetics of oxidation and improved oxidation resistance. When heated in air up to 1300 °C, porous samples of Ti3AlC2 and Ti2Al(C1-xNx) have a mass increase of 3,5 wt% and 8.5 -9.5 wt%, respectively, and only of 1 wt% and 1.5-5.8 wt%, respectively, after thermobaric treatment. X-Ray diffraction showed that oxidized surface layer contained TiO, TiO2 and Al2O3 oxides. While the N content was increased from x=0 to x=0.75, the oxidation resistance decreased and a hump (770-830°C) was observed on the DTA curve, its temperature decreasing with the nitrogen content. This effect was not observed for Ti2AlC and Ti3AlC2.

Session CG-4 - Synthesis and Fabrication of MAX and MXene Phases and Composites

CG-4:IL01  Synthesis of the MAX Phases by Pulse Discharge Sintering, a Review
ZHENGMING SUN, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan

This talk introduces the pulse discharge sintering (PDS) process, also referred as Spark Plasma Sintering (SPS), applied for the sintering and/or synthesis of a few representative MAX phases, such as Ti3SiC2, Ti3AlC2, Ti2AlC, Ti2AlN, Cr2AlC, Cr2GaC, Ti2GaC, as well as the MAX-based solid solutions and some composites. The effects of processing parameters on the synthesis reactions, densification, microstructure, as well as the properties will be presented.
Ref1. Z.M. Sun, Int. Mater. Rev., 56 (2011) 143-166.
Ref2. Z.M. Sun, H. Hashimoto, W.B. Tian, Y. Zou, Int. J. App. Cer. Tech., 7 (2010) 704-718.

CG-4:IL02  MAX Phase Single Crystal Synthesis               

Single crystalline platelets of nanolaminated MAX phases can be produced by high temperature solution growth, with areas now in the range of a few cm2. We detail the conditions which make possible the synthesis of such crystals, and the technical challenges which still have to be overcome in order to produce very thick samples. We discuss the cases of Ti3SiC2, Cr2AlC and Ti3SnC2. We show that such crystals can be cleaved or delaminated parallel to the basal plane, and might thus be put to good use for producing large areas of MX-ene layers, a novel class of 2D compounds.

CG-4:IL03  MAX Phases Thin Film Synthesis by Thermal Annealing Techniques              
T. CABIOC'H, M. JAOUEN, D. MAGNE, M. ALKAZAZ, M. BUGNET, V. MAUCHAMP, Département de Physique et Mécanique des Matériaux, Institut P', University of Poitiers-CNRS-ENSMA, Chasseneuil-Futuroscope, France

In the field of MAX phase thin film synthesis, high temperature direct deposition by PVD techniques has been mostly used. It allows for growing high quality thin films but its industrial scale transfer is difficult to achieve(use of high temperature, very narrow set of experimental parameters,..). Therefore, new approaches based on thermal annealing of thin films deposited at room temperature were recently developed.
In this contribution, three different ways of obtaining MAX phase thin films by thermal annealing are described. The most simple method consists in synthesizing, at room temperature, amorphous or nanocrystallized thin films with the appropriate stoichiomety and then to anneal the samples. In a second approach, multilayers (MX/MA or M/AX or MX/A)are deposited at room temperature and annealed to allow for interdiffusion processes between the layers that lead to the MAX phase formation. For the last method, interdiffusion processes between the substrate and a thin film deposited at room temperature control the MAX phase thin film formation during a thermal annealing.
Several examples of MAX phase thin films synthesized by these three indirect approaches will be discussed.

CG-4:IL04  Structure Evolution during Low Temperature Growth of MAX Phase Thin Films
J.M. SCHNEIDER, Materials Chemistry, RWTH Aachen University, Aachen, Germany

V-Al-C and Cr-Al-C and thin films were deposited by magnetron sputtering. The formation temperatures for V2AlC and Cr2AlC during sputter deposition are compared to the amorphous - crystalline transition temperatures in these material systems. The transition temperatures are determined by DSC and XRD. Based on the significantly lower synthesis temperature for Cr2AlC and V2AlC during vapor phase condensation compared to the bulk diffusion mediated amorphous - crystalline transition temperatures surface diffusion is identified as the atomic scale mechanism enabling the low temperature synthesis of MAX phase thin films. This notion is consistent with the phase formation data obtained utilizing HPPMS where the formation of nano-crystalline V2AlC MAX phase is observed in a (V,Al)2Cx matrix. An ion energy flux of >5.7 times of the conventional DC magnetron sputtering flux was identified to be prerequisite for V2AlC MAX phase formation. The data underline the potential of HPPMS for the low temperature synthesis of the MAX phase V2AlC. It is reasonable to assume that these findings are also relevant for other MAX phases.

CG-4:IL05  Cold Spraying of MAX Phases
F. GAERTNER, H. GUTZMANN, T. KLASSEN, Faculty of Mechanical Engineering, Helmut Schmidt University, Hamburg, Germany; D. HOECHE, C. BLAWERT, Department of Corrosion and Magnesium Surface Technology, Helmholtz-Zentrum Geesthacht GmbH, Geesthacht, Germany; B. ANASORI, M. W. BARSOUM, Department of Materials Science and Engineering, Drexel University, Philadelphia, USA

Cold spraying was applied to deposit Ti2AlC on different substrate materials. The study of single impacts by scanning electron microscopy indicates that bonding of the first layer is mainly attributed to the deformation and shear instabilities occurring at substrate sites. Nevertheless, as compared to the feedstock particles, the splats appear flattened by the impact. This deformation seems to be attributed not only to local, internal shear but also to internal fracture.
By applying up to five passes under optimized spray parameters, Ti2AlC-coatings with thicknesses of about 110-155 microns were achieved. XRD analysis of the coating proved that the crystallographic structure of the feedstock was retained during cold spraying. The coating microstructures show rather low porosity of about <2%, but several cracks between spray layers. Successful build-up of more than one layer can probably be attributed to local deformation of the highly anisotropic Ti2AlC-phase.
Results concerning the oxidation behavior reveal that mixed titanium/aluminum oxides form at the surface and in the vicinity of internal coating cracks. The occurring lack of protective alumina layer formation can be explained by fast inter diffusion and changes in coating composition.

CG-4:IL06  On the MAX-phase Matrix Composites Processed using Spark Plasma Sintering
M. RADOVIC, Department of Mechanical Engineering Materials Science and Engineering Program Texas A&M University, College Station, TX, USA

The unusual, and sometimes unique, combination of the MAX phases' properties recently fueled exploratory research activities on developing new types of functional and structural MAX phase matrix composites. In this talk emphasis will be given to two groups of MAX phase composites processed using Spark Plasma Sintering (SPS): (a) MAX phases - metal composites and, (b) ceramic fiber-reinforced MAX phase composites. Recently, several different types of MAX phase metal composites were processed with different metallic phases such as Mg, Al, and shape memory alloys (such as NiTi), etc. using either co-sintering or melt infiltration methods. The attractiveness of those composites for different structural applications comes, among others, from their exceptional mechanical damping capabilities that significantly exceed the damping capabilities of pure MAX phases - that are already high - or most metallic constituents. Most recently, we processed MAX phases composites reinforced with Al2O3 and SiC fibers using combination of slip casting and SPSing. The preliminary results suggest that further enhancement of the fracture and creep resistance of MAX phases can be achieved by their reinforcement with ceramic fibers.

CG-4:L08  Towards Understanding the Formation Mechanism of MAX-phase - In Situ TEM Studies on the Crystallization of V2AlC Thin Film
JIE ZHANG1*, M. BORNHOEFFT2, M. BABEN1, L. SHANG1, J. MAYER2, J.M. SCHNEIDER1, 1Materials Chemistry, RWTH Aachen University, Aachen, Germany; 2Central Facility for Electron Microscopy, RWTH Aachen University, Aachen, Germany; *current address: High-performance Ceramic Division, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China

The formation mechanism of MAX phase material was revealed by in situ heating transmission electron microscopy experiment on amorphous V50Al25C25 thin films. The crystal structure evolution was studied by a real-time monitoring on the signals on the TEM diffraction mode. Upon heating, the amorphous V50Al25C25 sample initially crystallized at 577oC. By two more intermittent heating and cooling procedures, the crystallization was completely achieved at 650oC. The microstructure evolution was investigated by recording the BF images via heating. Randomly distributed nuclei appeared in the amorphous matrix at 516oC, and then normal grain growth occurred. Afterwards, grain-growth stagnation was observed. Therefore, it was speculated that the crystallization of V2AlC was achieved by local rearrangement of amorphous V50Al25C25 phase with the nucleation-dominated mechanism.

CG-4:IL10  A Novel MAX Phase-derived Composite having Unexpectedly Excellent Wear Resistance and Anomalous Flexural Strength
H. ZHANG, XIAOHUI WANG, Z.J. LI, M.Y. LIU, Y.C. ZHOU, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China

A novel twinning plateletes strengthened TiC-Ni2AlTi composite was designed and fabricated by in situ reactive hot pressing of blended powders of Ti3AlC2 and Ni. Upon the de-intercalation of Al from Ti3AlC2 during the heating, TiC twinning platelets with width in submicrometer size were introduced in the composite. The obtained composite exhibits excellent wear resistance, which is comparable to the commercial Co- cemented WC. Unexpectedly, the flexural strength of the composite increases with temperature rising from 500 to 800 degree C, reaching a maximum of 936 MPa at 800 degree C, benefiting the application as cutting tools.

CG-4:IL13  In-situ Fabrication of Ti3SiC2-strengthened Composite by SPS
LIANJUN WANG, Donghua University, Shanghai, China

Ti3SiC2 has good mechanical properties, electrical and thermal conductivities damage tolerant, readily machinable, and resistant to thermal shock, so it is a good candidate as reinforcement to improve the toughness and strength However, fabricating high-purity Ti3SiC2 without TiC impurity is still difficult.  A novel method to in-situ fabricate Ti3SiC-strengthed composites via SPS was provided in this work. Our work mainly included synthesis of Ti5Si3/TiC/Ti3SiC2、TiSi2/SiC /Ti3SiC2、SiC /Ti3SiC2 nanocomposites via SPS. At the time, we also investigated microstructure and mechanical properties of sintered samples.

Session CG-5 -  Functional Properties and Applications of the MAX and MXene Phases

CG-5:IL01  MXenes: 2D Hosts for Ions in Electrochemical Energy Storage Systems
M. NAGUIB, M. LUKATSKAYA, O. MASHTALIR, Y. GOGOTSI, M. BARSOUM, Department of Materials Science & Engineering, and A.J. Drexel Nanotechnology Institute, Drexel University, Philadelphia, PA, USA

Recently we developed a new family of two-dimensional (2D) early transition metal carbides and carbonitrides, that we labeled MXenes. MXenes are produced by selective etching of the A-group layers from MAX phases. The latter is a large family of ternary layered metal carbides and/or nitrides. The etching process is carried out in an aqueous hydrofluoric acid. Thus, the as synthesized MXenes surface is terminated by O, OH and/or F. To date the following MXenes have been produced: Ti3C2, Ti2C, V2C, Nb2C, Ta4C3, TiNbC, (V0.5Cr0.5)3C2, and Ti3CN. Unlike conventional transition metal carbides, MXenes were found to be promising electrode materials in lithium-ions batteries (LIBs). MXenes showed an excellent ability to handle cycling rates that are considerably faster than commercial graphite anodes can handle in LIBs (up to 40C). MXenes can also be used in electrochemical capacitors. At >300 F/cm3, the volumetric capacitance of MXenes was superior to that of activated carbon – the material of choice at this time. Herein we report on the latest progress in synthesis and use of MXenes as hosts for ions in electrochemical energy storage systems.

CG-5:IL02  MAX Phases for Nuclear Applications               
E.N. HOFFMAN, B.L. GARCIA-DIAZ, R.L. SINDELAR, Savannah River National Laboratory, Aiken, SC, USA; D.J. TALLMAN, M.W. BARSOUM, Drexel University, Philadelphia, PA, USA

Some of the MAX phases have a unique set of properties such as excellent heat conduction, oxidation resistance, and high temperature strength that make them attractive candidates for nuclear applications. One such application is providing corrosion protection to the nuclear fuel during a Loss-of-Cooling Accident (LOCA) similar to the one that occurred at Fukushima.  In-core use of MAX materials requires research on the irradiation effects on coatings and understanding how coatings and components constructed from MAX phases will perform chemically, thermally, and mechanically.  Characterizations of the oxidation resistance, thermal diffusivity, wear, and tensile strength of MAX phase coatings and materials have been evaluated for non-irradiated and irradiated materials.  Results demonstrate that the MAX phases give good thermal performance when combined with existing in-core materials as coatings.  Irradiation of MAX materials can develop passive layers on components, but basic thermal conduction properties are retained for many relevant MAX compositions.  Cold sprayed MAX phase coatings showed good adhesion to cladding surfaces and superior resistance to oxidation.  Rendering these coatings as an oxidation barrier for the cladding, however, remains a processing challenge.

CG-5:IL03  Ion Irradiation of MAX Phases and Implications for Use in Nuclear Reactors               
D.P. RILEY, S.C. MIDDLEBURGH, G.R. LUMPKIN, S. MORICCA, Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW, Australia

Operational performance of MAX Phases has attracted considerable attention due to their unique combination of physical properties. Widely ascribed to the layered crystallographic structure inherent to these materials, their unique combination of metallic and ceramic properties manifests macroscopically as material ductility, high thermal/electrical conductivity, machinability, high temperature stability, resistance to oxidation and low surface friction. As many of these attributes potentially suit applications within high radiation environments, recent interest in assessing the radiation tolerance of these materials has markedly increased.
High energy ion beam irradiation (used as a simulant to neutron bombardment) serves as a rapid and relatively inexpensive method for evaluating the tolerance of materials to radiation damage. Potential applications within nuclear reactors may therefore be assessed through the investigation of ion beam induced defects, which may be broadly differentiated by those damage mechanisms that spontaneously recover (short-lived defects) verses those that are retained (long-lived defects). Overall, material degradation and its effects on performance will be a combination of all defect mechanisms and hence will require a thorough review before any potential nuclear application can be realised. An overview of recent theoretical and experimental results will be provided.

CG-5:IL04  Ion Irradiation of MAX Phase Thin Films: Influence of the Nanolaminated Structure and the Chemical Composition
M. BUGNET, Department of Materials Science and Engineering, CCEM-McMaster University, Hamilton, ON, Canada; V. MAUCHAMP, T. CABIOC'H, F. MORTREUIL, M. JAOUEN, Institut Pprime, CNRS-Université de Poitiers-ENSMA, Poitiers, France; E. OLIVIERO, CSNSM, CNRS-IN2P3-Université Paris-Sud, Orsay, France; P. EKLUND, Thin Film Physics Division, Linköping University, Linköping, Sweden

Thanks to their exceptional mechanical and thermal properties, and their strong damage tolerance at high temperature and refractoriness, MAX phases appear as potential candidates for applications as structural materials in Generation IV nuclear reactors [1]. The work presented here aims to shed light on the behavior of these materials under extreme environment such as ion irradiation.
Selected Ti-based and Cr-based MAX phase epitaxial thin films were exposed to low energy (150-340 keV) Ar2+ and Xe2+ beams. The microstructural modifications are investigated by XRD and TEM, and probed locally by spectroscopy techniques. The interpretation of spectral features by first principles calculations in Ti3AlC2 evidences that Ti6C octahedra layers are very resistant to irradiation damage and on the contrary, Al layers are strongly disordered [2]. Although Ti-based materials are still crystalline after irradiation at high fluence, Cr-based compounds rapidly amorphize [3]. We suggest that the behavior of these materials in irradiative environment can be tuned by the chemical composition and the stacking sequence at the nanoscale.
[1] J.-C. Nappé, J. Nuclear Materials 385, 304 (2009)
[2] M. Bugnet et al., Acta Materialia 61, 7348 (2013)
[3] M. Bugnet et al., J. Nuclear Materials 441, 133 (2013)

CG-5:L05  Effect of Neutron Irradiation on Select MAX Phases
D.J. TALLMAN1, E. HOFFMAN2, E.N. CASPI1*, B. GARCIA-DIAZ2, G. KOHSE3, R.L. SINDELAR2, M.W. BARSOUM1, 1Department of Materials Science and Engineering, Drexel University, Philadelphia, PA, USA; 2Savannah River National Lab, Savannah River Site, Aiken, SC, USA; 3MIT Nuclear Reactor Laboratory, Massachusetts's Institute of Technology, Cambridge, MA, USA; *On sabbatical leave from the Nuclear Research Centre - Negev, Israel 

Gen IV nuclear reactor designs require materials that can withstand long term operation in extreme environments of elevated temperatures, corrosive media, and fast neutron fluences (E>1MeV) with up to 100 displacements per atom (dpa). Full understanding of irradiation response is paramount to long-term, reliable service. The Mn+1AXn phases have recently shown potential for use in such extreme environments because of their unique combination of high fracture toughness values and thermal conductivities, machinability, oxidation resistance, and ion irradiation damage tolerance. Herein we report on the effect of neutron irradiation of .5 and 1 dpa at 70°C and 700 °C on Ti3AlC2, Ti2AlC, Ti3SiC2, and Ti2AlN. Evidence for irradiation induced dislocation loops and their effect on electrical resistivity is shown, revealing a healing effect at higher irradiation temperatures. X-ray diffraction refinement of the resultant microstructures is provided. Based on the totality of our results, it is reasonable to assume that the MAX phases, especially Ti2AlC, are promising materials for high temperature nuclear applications.

Cimtec 2014

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