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<p>Nanoscale precipitates have been proved to affect the transformation behavior of shape memory alloys (SMA) significantly. In this work, the effects of Heusler precipitates (Ni<sub>2</sub>TiAl) on the Ni<sub>50</sub>Ti<sub>24.5</sub>Hf<sub>22.5</sub>Al<sub>3</sub> SMA were systematically investigated. Density functional theory (DFT) calculations showed that coherent Heusler precipitates can mechanically stabilize the austenite phase, indicating the decrease of transformation temperatures. This trend was further confirmed experimentally. The depression of transformation temperatures with the increase of heat treatment time was observed as nanoscale coherent precipitates nucleated and grew. This was followed by an abrupt increase in transformation temperatures when aging time exceeded 200&#xa0;hours at 700&#xa0;°C. We attribute this increase to the loss of coherency between the Heusler precipitates and matrix which weakened their mechanical coupling. Detailed microstructure characterization by high-resolution transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) were provided to support the finding.</p>

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Effect of Heusler Phase Precipitation on the Transformation Behaviors of a NiTiHf-Based Shape Memory Alloy

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年1月6日

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  • Ching-Chien Chen,
  • Shivam Tripathi,
  • Shukai Yao,
  • Alejandro Strachan,
  • Michael S. Titus

摘要

Nanoscale precipitates have been proved to affect the transformation behavior of shape memory alloys (SMA) significantly. In this work, the effects of Heusler precipitates (Ni2TiAl) on the Ni50Ti24.5Hf22.5Al3 SMA were systematically investigated. Density functional theory (DFT) calculations showed that coherent Heusler precipitates can mechanically stabilize the austenite phase, indicating the decrease of transformation temperatures. This trend was further confirmed experimentally. The depression of transformation temperatures with the increase of heat treatment time was observed as nanoscale coherent precipitates nucleated and grew. This was followed by an abrupt increase in transformation temperatures when aging time exceeded 200 hours at 700 °C. We attribute this increase to the loss of coherency between the Heusler precipitates and matrix which weakened their mechanical coupling. Detailed microstructure characterization by high-resolution transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) were provided to support the finding.