Abstract ▼ AI-Helper

A small amount of misch metal and/or Zr was added as a dopant to 70.5wt% Cu-26wt% Zn-3.5wt% Al shape memory alloy in order to study the effect of grain refinement and heat treatments on the transformation behavior, mechanical properties, thermal cycling, stabilization of martensite, and shape memory...

A small amount of misch metal and/or Zr was added as a dopant to 70.5wt% Cu-26wt% Zn-3.5wt% Al shape memory alloy in order to study the effect of grain refinement and heat treatments on the transformation behavior, mechanical properties, thermal cycling, stabilization of martensite, and shape memory ability. After solution treatment, the specimens were post-quench aged or step quenched at temperature range from 100 to 350%. This work has been performed by differential scanning calorimetry, X-ray diffraction, tensile test, hardness test, metallography, and energy dispersive X-ray spectroscopy. It was found that the addition of misch metal was very effective for reducing the grain size. The coarse grains over 1000㎛ have been refined to the size of 25 pm by the addition of 0.68wt% misch metal and 0.60wt% Zr. The rate of grain growth also decreased with the increase of misch metal content. However, the grain size of thermo-mechanically treated alloys was barely affected by cold working. The crystal structure of martensite was identified as monoclinic structure. The fracture strength and ductility have been significantly increased with the increase of misch metal content when tensile tested below Mi temperature, Also, the fracture strength has been more increased in the case of post-quench aging treatment than the direct quench aging treatment. The fracture mode has been changed from transgranular brittle fracture to ductile fracture with void formation and coalescence by the addition of misch metal and Zr. Whereas aging of the β-phase decreases the Ms temperature, that of the martensite phase increases the AS temperature. The change in AS with post-quench aging can be attributed to recovery of order in the β-phase. The hysteresis of transformation temperature △T(As-Ms) has an increasing tendency by grain refinement and thermal cycles. The shape memory ability has been reduced by the addition of misch metal and thermal cycling. The deterioration of shape memory ability by thermal cycling was due to the stabilization of martensite. As the grain size decreased, martensite stabilization more easily occured.

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