Phase Changes in Ni-ti under Laser Shock Loading

Near-equiatomic Ni-Ti, known for its shape memory behavior, can decompose to martensitic phases and/or second phase compounds. This phase competition is investigated in NixTi100-x (49<x<55) in as-quenched (AQ) and shock-loaded states. Dynamic loading was done with direct drive laser pulses of nanosecond duration; shocked samples were recovered for analysis. Unexpectedly, the lowest-Ni alloy (x = 48.9) showed reversion of B19’ martensite (present in the AQ state) to the B2 parent phase upon loading, attributed to shock-induced heating. Higher-Ni alloys (x = 50.7, 52.6, 54.5, 54.6) showed Ni4Ti3 but no martensite in the AQ state, consistent with resistivity and dilatometry results which showed no hysteresis indicative of first order phase changes over 1.9 K<T<1273 K. Shock loading these alloys formed no martensite, but the Ni4Ti3 amounts (present in the AQ state) did change upon loading, indicating the importance of shock-induced heating. A thermodynamically complete equation of state (EOS) for NiTi in its B2 (CsCl) structure was generated by ab initio quantum mechanical calculations. This was tested by performing laser-launched flyer experiments, which showed consistency with the theoretical EOS, validating its use in the prediction of dynamic loading histories in the samples during direct drive loading.