High-temperature elastic constants of gold single-crystals

[摘要] Non-linear deviations of Young;;s modulus (modulus defects) have been observed in many polycrystalline metals at elevated temperatures ($>$0.7 $Tsb m$). As the quasi-harmonic theory of interatomic crystalline forces predicts a linear temperature dependence of elastic constants up to $Tsb m$, the observed modulus defect is thought to result from grain boundary effects. Comparison of polycrystalline modulus defects with single-crystal elastic constants at elevated temperatures should indicate if grain boundaries are involved in the modulus defect phenomenon. Young;;s modulus measurements were made from 300 $leq T leq$ 1275 K on high purity polycrystalline gold wires using the thin-line ultrasonic pulse-echo method. The adiabatic elastic constants $csbsp{m ij}{m S}$ ($csb{11}$, $csb{12}$, $csb{44}$) were measured on two, high purity, gold single-crystals: (100) and (110) orientations. A modulus defect was observed for each of the $csbsp{m ij}{m S}$ at elevated temperatures. The $csbsp{m ij}{m S}$ were averaged according to Hill;;s arithmatic and geometric methods and the results compared to the measured polycrystalline Young;;s modulus. The modulus defect of the polycrystalline specimen was essentially identical to the averaged $csbsp{m ij}{m S}$ at elevated temperatures, indicating that the large modulus defect of gold is not due to grain boundary effects. A modified quasi-harmonic model is proposed to account for the non-linear temperature dependence of the gold $csbsp{m ij}{m S}$ at elevated temperatures. The temperature dependence of the strictly harmonic terms due to thermal expansion are approximated using a Born-Mayer interatomic potential function and the temperature dependence of the atomic vibrational frequencies are included in the model by use of Gruneisen;;s approximation. The model suggested provides satisfactory agreement with measured $csbsp{m ij}{m S}$ at elevated temperatures.

[发布机构] Rice University

[效力级别] Materials science