Shahram Amini

Thermal expansion of select Mn+1AXn (M=earlytransitionmetal, A=Agroupelement, X=C or N) phases measured by high temperature x-ray diffraction and dilatometry

Herein we report on a systematic investigation of the thermal expansion of select Mn+1AXn phases. The bulk dilatometric thermal expansion coefficient αdil was measured in the 25–1200 °C temperature range and the thermal expansion of more than 15 of these phases was studied by x-ray diffraction in the 25–800 °C temperature range. The coefficient of thermal expansion for the a axis αa ranged between (2.9±0.1)×10−6 °C−1 (Nb2AsC) and (12.9±0.1)×10−6 °C−1 (Cr2GeC) while the coefficient for the c axis (αc) ranged between (6.4±0.2)×10−6 °C−1 (Ta2AlC) and (17.6±0.2)×10−6 °C−1 (Cr2GeC). Weak anisotropy in the thermal expansion was seen in most phases, with the largest value of αc/αa belonging to Nb2AsC. The Gruneisen parameters along the a and c directions were calculated from ab initio values for the elastic compliances and were relatively isotropic. A good correlation was found between the thermal expansion anisotropy and the elastic constant c13 and we conclude that the anisotropy in thermal expansion is relate...

Synthesis and elastic and mechanical properties of Cr2GeC

Herein we report on the synthesis and characterization of Cr 2 GeC, a member of the so-called M n +1 AX n (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 Cr 2 GeC and Cr 2 O 3 , 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 × 10 10 cm −2 , respectively.

Nanocrystalline Mg-Matrix Composites with Ultrahigh Damping Properties

Recently, we reported on the processing of 50 vol.% Ti2AlC-nanocrystalline magnesium, nc-Mg, matrix composites using a pressureless melt infiltration method. Herein we report on composites with up to 80 vol.% Mg. These composites are readily machinable, relatively stiff, strong and light, and exhibit ultrahigh damping. Increasing the nc-Mg volume fraction leads to lighter composites with higher damping characteristics at lower stresses (~30% of the mechanical energy is dissipated at 250 MPa). In some cases the Mg nanograms are also extraordinarily thermally stable which renders these composites good candidates for applications at temperatures higher than ambient. Due to the simple inexpensive melt infiltration technique used to fabricate these novel nanocomposites, it is possible to produce samples as large as ones made via normal powder metallurgy methods.

On the Stability of Mg Nanograins to Coarsening after Repeated Melting

Herein we report on the extraordinary thermal stability of approximately 35 nm Mg-nanograins that constitute the matrix of a Ti(2)AlC-Mg composite that has previously been shown to have excellent mechanical properties. The microstructure is so stable that heating the composite three times to 700 degrees C, which is 50 degrees C over the melting point of Mg, not only resulted in the repeated melting of the Mg, but surprisingly and within the resolution of our differential scanning calorimeter, did not lead to any coarsening. The reduction in the Mg melting point due to the nanograins was approximately 50 degrees C. X-ray diffraction and neutron spectroscopy results suggest that thin, amorphous, and/or poorly crystallized rutile, anatase, and/or magnesia layers separate the Mg nanograins and prevent them from coarsening. Clearly that layer is thin enough, and thus mechanically robust enough, to survive the melting and solidification stresses encountered during cycling. Annealing in hydrogen at 250 degrees C for 20 h, also did not seem to alter the grain size significantly.

Isothermal Oxidation of Ti2SC in Air

The oxidation behavior of fully dense Ti 2 SC was studied thermogravimetrically in air in the 500-800°C temperature range. The oxidation product was a single-layer of rutile in all cases. At 800°C, the oxide layer was not protective and the oxidation kinetics were rapid. At 600 and 700°C, and up to ∼50 h, the kinetics were parabolic before they became linear. It was only at 500°C that the weight gain reached a plateau after a 50 h initial parabolic regime. Mass spectrometry of the gases evolved during oxidation confirmed that both CO 2 and SO 2 are oxidation products. The overall oxidation reaction is thus Ti 2 SC + 4O 2 ― 2TiO 2 + SO 2 + CO 2 . On the basis of this and previous work, we conclude that oxidation occurs by the outward diffusion of titanium, sulfur, and carbon, the latter two either as atoms or in the form of CO 2 and SO 2 and, most probably, the inward diffusion of oxygen. Mesopores and microcracks were found in all rutile layers formed except those formed at 500°C. The presence of these defects is believed to have led to significantly higher oxidation rates as compared to other rutile-forming ternary carbides, such as Ti 3 SiC 2 .