EFFECTS OF STOICHIOMETRY ON TRANSFORMATION TEMPERATURES AND ACTUATOR-TYPE PERFORMANCE OF NITIPD AND NITIPDX HIGH-TEMPERATURE SHAPE MEMORY ALLOYS

摘要

Ternary NiTiPd and quaternary NiTiPdX (X=Au, Pt, Hf) high-temperature shape memory alloys (HTSMAs) were produced with titanium-equivalent (Ti+Hf) compositions from 50.5 to 49.0 at. percent . Thermomechanical testing in compression was used to evaluate the transformation temperatures, transformation strain, work output, and permanent deformation behavior of each alloy to study the effects of quaternary alloying and stoichiometry on HTSMA behavior. Microstructural evaluation showed the presence of second phases for all alloy compositions. No-load transformation temperatures in the stoichiometric alloys were relatively unchanged by Au substitution, slightly increased by substitution with Pt, and severely depressed by substitution of Hf for Ti. Transformation temperatures were highest in the Ti-rich and stoichiometric compositions, and lower in the compositions with less than 50 at. percent Ti (+Hf). During constant-load thermal cycling, transformation strain, and therefore work output, increased with increasing stress. Transformation strain was relatively unchanged by composition in the NiTiPdHf and NiTiPdPt alloys, while it decreased with decreasing Ti content in NiTiPd and increased with decreasing Ti content in the NiTiPdAu alloys. Permanent strain associated with the constant-load thermal cycling was lowest for alloys with Ti-equivalent-lean compositions. Based on these results, basic rules for optimizing the composition of NiTiPd alloys for actuator performance are discussed.