Tailoring of the thermal expansion of Cr2(Alx,Ge1-x)C phases

We report thermal expansion coefficients of the end members and solid-solution compounds in the Cr-2(Al-x,Ge1-x)C system. All samples studied were essentially phase-pure Cr2AlxGe1-xC except the Cr2GeC sample, which contained a substantial fraction of Cr5Ge3Cx. X-ray diffraction performed in the 25-800 degrees C temperature range shows that the in-plane thermal expansion remains essentially constant at about 14 +/- 1 x 10(-6) K-1 irrespective of Al content. The thermal expansion of the c axis decreases monotonically from 17 +/- 1 x 10(-6) K-1 for Cr2GeC to similar to 12 +/- 1 x 10(-6) K-1 with increasing Al content. At around the Cr-2(Al-0.75,Ge-0.25)C composition, the thermal expansion coefficients along the two directions are equal; a useful property to minimize thermal residual stresses. This study thus demonstrates that a solid-solution approach is a route for tuning a physical property like the thermal expansion. For completeness, we also include a structure description of the Cr5Ge3Cx phase, which has been reported before but is not well documented. Its space group is P6(3)/mcm and its a and c lattice parameters are 7.14 angstrom and 4.88 angstrom, respectively. We also measured the thermal expansion coefficients of the Cr5Ge3Cx phase. They are found to be 16.3 x 10(-6) K-1 and 28.4 x 10(-6) K-1 along the a and c axes, respectively. Thus, the thermal expansion coefficients of Cr5Ge3Cx are highly anisotropic and considerably larger than those of the Cr-2(Al-x,Ge1-x)C phases. (C) 2012 Elsevier Ltd. All rights reserved.