Martensite-like ultrasonic signatures in Pd40Ni10Cu30P20 bulk metallic glass under repeated thermo-acoustic cycling

Martensite-like ultrasonic signatures are observed in the highly stable Pd40Ni10Cu30P20 bulk metallic glass during repeated thermo-acoustic cycling under extremely low strain amplitudes (∼10−8) at cryogenic temperatures. Successive thermal cycles under high-frequency elastic perturbation give rise to hysteretic step-like anomalies and satellite-like features in ultrasonic velocity and attenuation, which progressively strengthen over repeated thermo-acoustic cycles. X-ray diffraction measurements confirm the initially amorphous state of the sample and reveal no detectable crystallization after the complete ultrasonic cycling protocol. The observed cumulative and history-dependent ultrasonic response is consistent with metastable cooperative local structural evolution associated with nanocluster-like correlated regions within the amorphous matrix. These results demonstrate the exceptional sensitivity of high-frequency ultrasonic response to subtle local structural evolution, even in Pd-based metallic glasses with high thermodynamic stability.