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The shock-compression method enables dense solidification at relatively low temperatures, and fabrication of bulk material is possible without thermal alteration of the nonequilibrium phases. A metastable phase of an equiatomic Al-Ti-V-Cr-Si high-entropy alloy was prepared by mechanical alloying, and the obtained alloy was solidified by shock-compression to retain the metastable phase of AlTiVCrSi. The bulk form composed of a metastable body-centered cubic (BCC) solid solution was attained after shock solidification at a pressure of 11 GPa. However, when shock stresses of 21 GPa and 31 GPa were applied, the intermetallic compounds (Ti, V)5Si3and Al8(V, Cr)5were formed in addition to the BCC phase. The formation of these intermetallic compounds during shock compression was consistent with thermodynamical predictions and empirical parameters for phase prediction. The bulk AlTiVCrSi samples shock-solidified at 11 GPa and 21 GPa exhibited Vickers hardness values greater than 1000 HV, whereas that for the sample shock-solidified at 31 GPa was ~ 400 HV. The decrease in hardness was attributed mainly to the formation of a large number of cracks.

Molybdenum(VI) oxide (MoO3) is a promising semiconductor material that can be used in several functional applications. Understanding the structural response of MoO3under high-pressure and high-temperature conditions is important for designing a material used in device applications. An X-ray free electron laser (XFEL) enables the structural response under extreme conditions to be probed at the nanosecond timescale. We describe laser-driven shock compression experiments on MoO3using an XFEL to directly observe the structural evolution of MoO3. When a laser-driven shock wave arrives at a pressure of 61 GPa, MoO3melts immediately and remains in the molten state for a few nanoseconds. Rapid recrystallization to the α-MoO3phase and a high-pressure phase MoO3-II is also observed on nanosecond timescales during pressure release. Our results provide insights into the kinetic and phase transition under shock compression and represent the advancement toward the understanding structural response of MoO3under high-pressure, high-temperature conditions, which has not been studied.