Mn+1AXn Phases Research Articles

Synthesis and elastic and mechanical properties of Cr2GeC

Herein we report on the synthesis and characterization of Cr2GeC, a member of the so-called Mn+1AXn (MAX) phase family of layered machinable carbides and nitrides. Polycrystalline samples were synthesized by hot pressing pure Cr, Ge, and C powders at 1350 °C at ∼45 MPa for 6 h. No peaks other than those associated with Cr2GeC and Cr2O3, in the form of eskolaite, were observed in the x-ray diffraction spectra. The samples were readily machinable and fully dense. The steady-state Vickers hardness was 2.5 ± 0.1 GPa. The Young’s moduli measured in compression and by ultrasound were 200 ± 10 and 245 ± 3 GPa, respectively; the shear modulus and Poisson’s ratio deduced from the ultrasound results were 80 GPa and 0.29, respectively. The ultimate compressive strength for a ∼20 μm grain size sample was 770 ± 30 MPa. Samples compressively loaded from 300 to ∼570 MPa exhibited nonlinear, fully reversible, reproducible, closed hysteretic loops that dissipated ∼20% of the mechanical energy, a characteristic of the MAX phases, in particular, and kinking nonlinear elastic solids, in general. The energy dissipated is presumably due to the formation and annihilation of incipient kink bands. The critical resolved shear stress of the basal plane dislocations—estimated from our microscale model—is ∼22 MPa. The incipient kink band and reversible dislocation densities, at the maximum stress of 568 MPa, are estimated to be 1.2 × 10−2 μm−3 and 1.0 × 1010 cm−2, respectively.

Synthesis and Mechanical Properties of Fully Dense Ti2SC

Because the layered machinable ternary carbide, Ti2SC, has a significantly shorter c‐lattice parameter—as compared with most of the 50+ other so‐called Mn+1AXn (MAX) phase family (M=early transition metal; A=A group element and X=C or N and n=1–3) to which it belongs—it was postulated that its mechanical properties would be significantly different than the other MAX phases. In this work, fine‐grained (FG) and coarse‐grained (CG) polycrystalline fully dense Ti2SC samples were fabricated. Hot pressing Ti2SC powders, at 1500°C under a stress of ∼45 MPa for 5 h resulted in FG (2–4 μm) samples which and upon further annealing for 20 h at 1600°C resulted in CG (10–20 μm) ones. No peaks other than those associated with Ti2SC and an impurity anatase phase, with a volume fraction of ∼6 vol% were observed in the XRD patterns and micrographs. The average Vickers hardness in the 2–300 N range is 8±2 GPa with the FG samples being slightly harder. This hardness is the highest of any of the MAX phases characterized to date. Also in contrast to all MAX phases, cracks extended from the corners of Vickers indents in the FG samples. From these cracks the fracture toughness was estimated and found to increase more or less linearly with load from ≈4 to 6 MPa·m1/2 as the Vickers force was increased from 50 to 300 N, respectively. The room temperature compressive stress of the FG samples was 1.4±0.2 GPa; the failure mode was brittle. Its Young's modulus—also one of the highest for a M2AX phase measured to date—was 316±2 GPa. There was no evidence for incipient kink band formation during simple compression. The latter is attributed, in part, to the fine grain size of the hot‐pressed material.

Microstructure and nonbasal-plane growth of epitaxial Ti2AlN thin films

Thin films of the Mn+1AXn (MAX) phase (M: early transition metal; A:A-group element; X: C and/or N; n=1–3) Ti2AlN were epitaxially grown onto single-crystal MgO(111) and MgO(100) substrates by dc reactive magnetron cosputtering from Ti and Al targets in an Ar∕N2 gas mixture at a temperature of 690 °C. To promote the nucleation of the MAX phase, a fcc (Ti0.63Al0.37)N seed layer was deposited before changing to Ti2AlN growth parameters. The nucleation processes have been studied by real-time in situ specular x-ray reflectivity. Independent of substrate orientation, the seed layer shows no roughening until its final thickness of approximately 100 Å, indicating pseudomorphic layer-by-layer growth. The MAX phase shows heteroepitaxial layer-by-layer growth on MgO(111), with increased surface roughening up to approximately 200 Å, whereas on MgO(100) the growth mode changes to Volmer-Weber-type already after three monolayers. X-ray scattering in Bragg-Brentano geometry of the final, approximately 1000 Å thick, Ti2AlN film reveals lattice parameters of c=13.463Å and a=2.976Å on the MgO(111) substrate and c=13.740Å and a=2.224Å on the MgO(100) substrate. From pole figure measurements the orientational relationship between film and substrate lattice was determined to be MgO{111}⟨110⟩∕∕Ti2AlN{101¯2}⟨1¯21¯0⟩, regardless of the substrate orientation. This tilted, nonbasal-plane growth leads to a threefold grain orientation of Ti2AlN along the MgO⟨110⟩ directions and a polycrystalline morphology confirmed by cross-sectional transmission electron microscopy. The growth can be assumed to take place in a lateral step-flow mode, i.e., emerging low surface free-energy (0001) planes, on which arriving atoms can diffuse until finding a step where they are bound to A facets. This growth process is irrespective of orientational relationship between substrate and film. However, in the present low-temperature case the partitioning of arriving Al and Ti atoms during nucleation is suppressed, which as a result of interfacial adaptation between substrate and film induces standing a-type planes during growth.

Compressibility of Nb2AsC to 41 GPa

Room temperature synchrotron x-ray diffraction experiments were performed on the layered hexagonal ternary carbide Nb2AsC in a diamond anvil cell to a pressure of 41 GPa. The structure is stable with no phase transitions observed in this pressure range. The bulk modulus is 224±2GPa, with a pressure derivative of 4; the former is the highest bulk modulus for a Mn+1AXn (MAX) phase, including Ti3SiC2, measured to date. Nb2AsC is also the first MAX phase for which the compressibility along the a direction is higher than along the c direction; in other words, the c∕a ratio increases strongly with applied pressure.

Growth of Ti3SiC2 thin films by elemental target magnetron sputtering

Epitaxial Ti3SiC2(0001) thin films have been deposited by dc magnetron sputtering from three elemental targets of Ti, C, and Si onto MgO(111) and Al2O3(0001) substrates at temperatures of 800–900°C. This process allows composition control to synthesize Mn+1AXn (MAX) phases (M: early transition metal; A: A-group element; X: C and∕or N; n=1–3) including Ti4SiC3. Depositions on MgO(100) substrates yielding the Ti–Si–C MAX phases with (101¯5), as the preferred orientation. Samples grown at different substrate temperatures, studied by means of transmission electron microscopy and x-ray diffraction investigations, revealed the constraints of Ti3SiC2 nucleation due to kinetic limitations at substrate temperatures below 700°C. Instead, there is a competitive TiCx growth with Si segregation to form twin boundaries or Si substitutional incorporation in TiCx. Physical properties of the as-deposited single-crystal Ti3SiC2 films were determined. A low resistivity of 25μΩcm was measured. The Young’s modulus, ascertained by nanoindentation, yielded a value of 343–370GPa. For the mechanical deformation response of the material, probing with cube corner and Berkovich indenters showed an initial high hardness of almost 30GPa. With increased maximum indentation loads, the hardness was observed to decrease toward bulk values as the characteristic kink formation sets in with dislocation ordering and delamination at basal planes.