A Study on Amorphous Formation of Alloys Solidified Under High Pressure

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On Several combinations of volume-expanding and -shrinking elements on solidification, their high-pressure phase diagrams were estimated and three alloy systems where amorphous formation could be expected, i.e., Bi-Sb alloys (homogeneous solid solution-type), Bi-In alloys (intermediate compound formation-type) and Al-Ge alloys (eutectic-type), were subjected to the high-pressure experiment. In the X-ray diffraction pattern of Bi-10-30Sb alloys, a broad peak was detected at 2rheta=30-40deg.and the peak was thereafter disappeared and, instead, the normal pressure phase was observed. In the microstructure, a fine-grained structure was observed and considered to appear as a result of phase transformation from a high-pressure phase to the normal-pressure one. In Bi-20at%In alloy solidified under high-pressure, no distinct change was observed in the X-ray diffraction pattern. However, an increase in the amount of Bi/BiIn eutectic was found, suggesting that the eutectic composition was shifted to Bi-rich side, and the high-pressure phase diagram of Bi-In system was estimated. In the X-ray diffraction pattern of Al-40at%Ge alloy, solidified at about 20kbar, the peaks of metastable phases, gamma_1 (Al_2Ge) and gamma_2 (AlGe), were detected and these metastable phases still existed after 2 months. The gamma_1 and gamma_2 phases were appeared as a laminar eutectic in the BSE comp. image and the average Ge concentration of the gamma_1/gamma_2 eutectic was found to be higher by 8at%Ge than that of normal eutectic, alphaAl/dia.cub.Ge. The high-pressure phase diagram of Al-Ge system was estimated and the critical pressure for the generation of the gamma_1/gamma_2 eutectic was estimated to be 10-20kbar.