S. Qiu, O. Benafan, R. Vaidyanathan, University of Central Florida, Orlando, FL; S. Padula, R. Noebe, NASA Glenn Research Center, Cleveland, OH
The objective of this work is to understand fundamental deformation mechanisms in shape memory alloys when they are subjected to external thermal and mechanical loading in actuator applications. We report on in situ neutron diffraction measurements performed on NiTiPd and NiTiPt shape memory alloys during selected combinations of heating, cooling and mechanical loading at Los Alamos National Laboratory. The choice of alloy systems is motivated by the elevated temperature range at which they exhibit phase transformations that result in the shape memory effect. Compared to x-rays from a conventional source, neutrons can penetrate deeper, making surface stress effects negligible and hence neutron diffraction measurements are more representative of bulk behavior in polycrystalline samples. The micromechanical and microstructural changes, i.e., texture, strain and phase volume fraction evolution, were quantified during heating and cooling in unloaded samples (i.e., free recovery testing). The results from free recovery experiments were compared with measurements made during heating and cooling of samples subjected to constant loads (i.e., constrained recovery testing as in actuator applications). This comparison helped assess the effect of external loads on the internal strain, texture and phase volume fraction evolution. The dimensional stability of these alloys was also assessed through in situ thermo-mechanical cycling experiments. The volume fraction and texture of the retained martensite and the associated internal strains were quantified in the aforementioned constrained recovery experiments. Comparisons were also made with similar experiments in binary NiTi. The ability to quantitatively follow the micromechanical and microstructural changes during the phase transformation under external load and heating/cooling has provided valuable information for the future engineering of these high temperature shape memory alloys. Financial support from NASA GRC (NNX08AB51A) is acknowledged.
Summary: The objective of this work is to understand fundamental deformation mechanisms in shape memory alloys when they are subjected to external thermal and mechanical loading in actuator applications. We report on in situ neutron diffraction measurements performed on NiTiPd and NiTiPt shape memory alloys during selected combinations of heating, cooling and mechanical loading at Los Alamos National Laboratory. The choice of alloy systems is motivated by the elevated temperature range at which they exhibit phase transformations that result in the shape memory effect. Compared to x-rays from a conventional source, neutrons can penetrate deeper, making surface stress effects negligible and hence neutron diffraction measurements are more representative of bulk behavior in polycrystalline samples.
The micromechanical and microstructural changes, i.e., texture, strain and phase volume fraction evolution, were quantified during heating and cooling in unloaded samples (i.e., free recovery testing). The results from free recovery experiments were compared with measurements made during heating and cooling of samples subjected to constant loads (i.e., constrained recovery testing as in actuator applications). This comparison helped assess the effect of external loads on the internal strain, texture and phase volume fraction evolution. The dimensional stability of these alloys was also assessed through in situ thermo-mechanical cycling experiments. The volume fraction and texture of the retained martensite and the associated internal strains were quantified in the aforementioned constrained recovery experiments. Comparisons were also made with similar experiments in binary NiTi. The ability to quantitatively follow the micromechanical and microstructural changes during the phase transformation under external load and heating/cooling has provided valuable information for the future engineering of these high temperature shape memory alloys.
