Diffusionless (martensitic) phase transition of Fe90Ni10 alloy up to 6 GPa

<title>Abstract</title> <p> The phase relations and transformation kinetics of Fe-Ni alloys are essential for constraining the stability and dynamics of metallic cores in terrestrial planets and satellites. However, phase relations below 10 GPa remain poorly understood, with previous studies often overlooking the role of sluggish diffusion kinetics at relatively low temperatures. In this study, we conducted in-situ synchrotron X-ray diffraction experiments on an Fe ₉₀ Ni ₁₀ alloy using a Kawai-type multi-anvil apparatus at pressures up to 6.2 GPa and temperatures up to 1473 K. Our time-resolved observations reveal that during cooling, the face-centered cubic (fcc) phase undergoes a diffusionless (martensitic) transition to the body-centered cubic (bcc) phase, rather than a diffusion-controlled chemical separation. The determined martensite start temperature ( <italic>M</italic> _s ) boundary exhibits a negative pressure dependence, expressed as \(\:T\:\left(\text{K}\right)\:=\:(-31\:\pm\:\:5)\:\times\:\:P\:\left(\text{G}\text{P}\text{a}\right)\:+\:(1060\:\mp\:\:10)\). The ~ 200 K discrepancy with previously reported low-pressure boundaries is successfully resolved as a natural thermal hysteresis loop between the forward and reverse martensitic reactions. While geological timescales permit slow chemical separation, laboratory experiments and dynamic planetary processes are governed by this diffusionless boundary. The addition of 10 mol% Ni significantly expands the low-temperature stability of the fcc phase compared to pure iron. We conclude that the physical responses of small planetary cores (e.g., Mercury, Mars, the Moon, and Ganymede) to dynamic events—such as convection, tidal stresses, and impacts—are dictated by the robustly expanded homogeneous fcc lattice or its martensitic derivatives mapped in this study. </p>