Abstract
NiTiHf alloys are emerging as promising candidates for high-temperature shape memory actuator applications. The H-phase precipitation in these alloys significantly influences their mechanical strength and transformation temperatures. In this paper, we report a complementary study using time-resolved Small-Angle Neutron Scattering (SANS) along with transmission electron microscopy, to probe the H-phase evolution as a function of aging conditions in the Ni50.3Ti29.7Hf20 alloy. While, electron microscopy provided direct structural information about the precipitates, SANS analysis allowed in probing quantitatively the temporal evolution growth and coarsening of H-phase. It is demonstrated that while the diffusion of Ni atoms in the matrix predominantly governs the growth regime, the coarsening kinetics is primarily controlled by Hf diffusion. Further, relatively sluggish coarsening of the H-phase in this alloy, as compared to Ni4Ti3 precipitates in Ni-rich binary NiTi alloy, has been attributed to the slow migration of Hf in the matrix.
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