A Study of the effects of strain on NiTi shape memory alloy

Shape memory alloys are materials that are capable of returning to an original shape after large deformations in response to a stimulus. Shape memory alloys can be metals, ceramics, polymers and gels. This thesis studies the changes of various properties of NiTi metal as samples are prestrained various amounts. After conducting various characterization techniques, relationships were developed for the changes in transformation temperatures, Knoop hardness number, and crystal lattice parameters. Through the use of differential scanning calorimetry and dynamic mechanical analysis the transformation temperatures were determined. It was observed in both experiments that the initial austenite start temperature(As) increased parabolically while austenite finish temperature (Af)increased linearly with prestrain. Also, DSC showed that Martensite start temperature (Ms) increased linearly with prestrain, however, DMA did not show any trend for Ms. Knoop hardness tests as well as deformation tests were performed to investigate changes in the mechanical properties of NiTi with increased prestrain. It was found that the Knoop hardness number increased linearly with increased prestrain. Additionally, from tensile data collected during the deformation testing, it was found that the maximum flow stress at each strain amount follows a parabolic trend. Also from the stress-strain data, it was found that NiTi has a higher strain hardening coefficient than most materials. When Knoop hardness numbers were compared to the true stress values, it was found that the hardness number increased following a cubic trend with increasing stress. Using X-Ray diffracton, the crystal structure of the NiTi was found to be monoclinic with lattice parameters, a = 2.884, b = 4.637, c = 4.101, y = 97.7. It was also observed that the volume of the unit cell remained constant regardless of prestrain. Using an optical microscope, large grains with stripes perpendicular to the prestrain direction were observed. The occurrence and size of these grains increased with the amount of prestrain. Surface scans using atomic force microscopy showed that these stripes were all approximately the same depth and regularly spaced. These grains appeared to be self-accommodated martensite regions but could not be fully characterized using these experiments.