The R-phase transformation in NiTiFe-based alloys and thermal switches

Addition of Fe to NiTi introduces an intermediate trigonal R-phase and suppresses the monoclinic B19' martensitic transformation. The R-phase transformation in NiTiFe offers a useful window for actuator operation as it exhibits reduced hysteresis with a favorable fatigue response. This study establishes correlations between compositional and thermo-mechanical processing parameters and the formation of the R-phase in ternary NiTiFe alloys.  Furthermore, two sets of neutron diffraction measurements in NiTiFe shape memory alloys during mechanical loading at cryogenic temperatures were performed at Los Alamos National Laboratory. First, with the objective of examining NiTiFe in cyclic, low-stroke, actuator applications (such as in cryogenic thermal switches), the R-phase was strained to 1% at 92 K and subsequently heated to complete strain recovery under a load. Second, with the objective of examining NiTiFe in one-time, high-stroke, actuator applications (such as in safety valves), martensite that was stress-induced from the R-phase was strained to 8% at 92 K and subsequently heated to complete strain recovery under a load. Lastly, the design, construction and testing of an improved low temperature thermal conduction switch is presented. Such a switch integrates the sensor and actuator elements and can be used to create a variable thermal sink to other tanks for liquefaction, densification, and zero boil off systems for advanced spaceport applications.