Precipitate microstructure in the B2 parent phase is known to have profound impacts on the properties of NiTi-based high temperature shape memory alloys (HTSMAs), including the martensitic transformation (MT) start temperature $M_s$, temperature- and stress-hysteresis, work output, dimensional stability and functional fatigue resistance. In order to understand the underlying mechanisms and hence to optimize aging heat treatments to achieve desired properties, we systematically investigate both the mechanical and chemical effects associated with nanoscale coherent precipitates on the behavior of MT. Using NiTi-Hf HTSMA as an example, we first study the equilibrium shape and stress and strain fields of an H-phase precipitate as a function of its size. We then determine quantitatively the elastic interaction energy between a precipitate and a nucleating martensitic particle consisting of either a single variant or multiple self-accommodating variants. In the meantime, we calculate the variation of concentration field around an H-phase precipitate during its growth. Finally, we quantify and compare the effects of the spatially inhomogeneous stress and concentration fields around an H-phase precipitate on $M_s$. The results indicate that the former is the dominant factor for long aging times while latter is the dominant factor for short aging times. Since the model predicts $M_s$ as a function of aging temperature and time, it can aid the design of aging heat treatment schedule to achieve desired $M_s$.

已知B2母相中的沉淀微观结构会对基于NiTi的高温形状记忆合金（HTSMA）的性能产生深刻影响，包括马氏体转变（MT）起始温度 ${\mathrm{中号}}_s$，温度和应力滞后，功输出，尺寸稳定性和抗疲劳性。为了了解潜在的机制，从而优化时效热处理以获得所需的性能，我们系统地研究了与纳米级相干沉淀物相关的机械和化学效应对MT行为的影响。以NiTi-Hf HTSMA为例，我们首先研究H相沉淀物的平衡形状，应力场和应变场，以及其大小的函数。然后，我们定量地确定沉淀物和由单个变体或多个自适应变体组成的成核马氏体颗粒之间的弹性相互作用能。同时，我们计算了H相沉淀物在生长过程中浓度场的变化。最后，${\mathrm{中号}}_s$。结果表明，前者是长时效时间的主导因素，而后者是短时效时间的主导因素。由于模型预测${\mathrm{中号}}_s$ 作为时效温度和时间的函数，它可以帮助设计时效热处理时间表，以达到所需的 ${\mathrm{中号}}_s$。