Tailoring of the thermal expansion of Cr_2(Al_x,Ge_(1-x))C phases

摘要

We report thermal expansion coefficients of the end members and solid-solution compounds in the Cr_2(Al_x,Ge_(1-x))C system. All samples studied were essentially phase-pure Cr_2(Al_x,Ge_(1-x))C except the Cr_2GeC sample, which contained a substantial fraction of Cr_5Ge_3C_x. X-ray diffraction performed in the 25-800 °C temperature range shows that the in-plane thermal expansion remains essentially constant at about 14 ± 1 × 10~(-6) K~(-1) irrespective of Al content. The thermal expansion of the c axis decreases monotonically from 17 ± 1 × 10~(-6) K~(-1) for Cr_2GeC to ~12 ± 1 × 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 Cr_5Ge_3C_x 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 A and 4.88 A, respectively. We also measured the thermal expansion coefficients of the Cr_5Ge_3C_x phase. They are found to be 16.3 × 10~(-6) K~(-1) and 28.4 × 10~(-6) K~(-1) along the a and c axes, respectively. Thus, the thermal expansion coefficients of Cr_5Ge_3C_x are highly anisotropic and considerably larger than those of the Cr_2(Al_x,Ge_(1-x))C phases.