Dissertation > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > The alloy learn with a variety of properties of alloys > Other special nature of the alloy > Shape Memory Alloys

Study on Influence of Preferred Orientation on Mechanical and Recoverable Properties of TN479 Shape Memory Alloy Sheet

Author ZhouTingJun
Tutor YanYing
School Northeastern University
Course Materials Physics and Chemistry
Keywords Ti44Ni47Nb9 shape memory alloy texture mechanical properties recoverable properties microstructure phase transformation
CLC TG139.6
Type Master's thesis
Year 2009
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The two important characteristics of pseudoelasticity and shape memory effect of shape memory alloys (SMAs) strongly depend on the orientation of single crystals. But generally in application, SMAs are typical polycrystalline in nature and usually processed by casting, followed by hot- or cold-working and suitable heat treatments. Thus, polycrystalline SMAs so processed have strong crystallographic texture. The investigation on texture formation in processing and the texture relation to properties of SMAs are very important for the use of SMAs in engineering application. Thus, in the paper, the microstructure, phase transformation, texture and influence of texture on tensile and recoverable strain and temperature of the hot-and cold-rolled TN479 SMA sheets were stuied in details by many experimental techniques. The purpose is to provide theoretic basis and reference date for application of the TN479 alloy with high properties in engineering.The microsturctural studies show that the hot-rolled TN479 sheet consists of TiNi matrix phase,β-Nb and (Ti, Nb)4Ni2O oxide, and in the cold-rolled sheet, there also exist martensite except TiNi,β-Nb and (Ti,Nb)4Ni2O oxide. TiNi andβ-Nb grains are in the shape of strip along the rolling direction. The grains become small equiaxed for the specimen heat treated at 600℃. When the heat treatment temperature reaches 850℃, recrystallized grains have occurred growth;Phase transformation results indicate that Ms and As points increase with the increase of heat treatment temperature at the same cooling rate. The phase transformation temperature and thermal hysteresis obviously rise with the increase of cooling rate at 850℃, and thermal hysteresis reaches the maximum for the annealed specimen.Texture analyses show that main textures of the hot-rolled sheet are{001} and{111} fibres. The strong components are close to{001}<010> in{001} fibre texture and are {111}<112> and{111}<165> in{111} fibre texture along the rolling direction (RD). The {001}<010> and{111}<132> are the strong components along the transverse direction (TD). In the cold-rolled sheet, main textures are{332} and{111} fibres. The strong components are {332}<110>and{lll}<110> along the RD. The{332}<113>,{111}<112> and{111}<123> are the strong components along the TD.Tensile test results reveal that at the same heat treatment temperature, the faster the cooling rate is, the lower the critical stress of stress-induced martensite transformation is; At the same cooling rate, the lower heat treatment temperature, the greater the critical stress. Generally, critical stress is the maximum along the RD and the minimum along 45°angle to the RD for the hot-rolled sheet heat treated at different conditions and the cold-rolled sheet quenched at less than 600℃and annealed at 850℃. The critical stress is the maximum along the TD and the second along the RD for the cold-rolled sheet quenched at more than 600℃.Tensile recovery test reveal that recoverable strains drop with the increase of heat treatment temperature for the quenched cold-rolled sheet. The preferred orientation has no obvious influence on recoverable strain for the hot- and cold-rolled sheets annealed or quenched at 850℃, the recoverable strain of about 7% for the annealed hot- and cold-rolled sheets and that of 5.3% for quenched cold-rolled sheet are obtained. The recoverable strain along the TD is greater than that along the RD for the cold-rolled sheet quenched at 500℃.

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