Study of Micro / Nanoscale Mechanical Behavior of NiTi Shape Memory Alloys

• Main Authors: Po-Hsien Sung, 宋柏賢
• Other Authors: Te-Hua Fang
• Format: Others
• Language: zh-TW
• Online Access: http://ndltd.ncl.edu.tw/handle/75439696820531327156

碩士 === 國立高雄應用科技大學 === 機械與精密工程研究所 === 101 === This study analyzes micro-and nanoscale mechanical behavior of NiTi shape memory alloy. A simulation study on the behavior of slip system, strength, effect of shape recovery, stress and energy distribution using nanobending test and torsion test by molecular dynamics. The deformation process of nanowires with engagement of the AFM tip. Young’s modulusand yield strength was then calculated by force-distance curvesin the elastic deformation process. A pair of torque is applied to the two ends of the wire in opposite directions during the torsion test. Critical torsion angle, Shear modulus and slip system has been observed by torsion test. A experiment study of fracture mechanics of NiTi wires and hardness of NiTi alloy after annealing by nanoindeatation. The nanobending test simulation result shows the adhesion force between the AFM tip and the nanowire and adhesion deformation depends on the nanowire diameter. The force between AFM tip and nanowire increases with increasing nanowire diameter. The Young’s modulus, yield strength, shear modulus and critical torsion angle decrease with decreasing slenderness ratio of nanowire. The shape recovery in nanowire after loading on the (100) surface is faster than that on the (110) surface. The torsional test simulation result shows the square corss-section nanowire will carry the larger allowable torsional deformation then circular corss-section when they have the same length and corss-sectional area. The superelasticity are observed when stress is released and torsional angle not reaches the failure angle. The experiment result shows the critical torsion angle increase with increasing slenderness ratio of nanowire. The SEM image showing fractograph area with parabolic-shaped dimples characteristic and microvoids. The hardness of the NiTi alloy decreasing with the number of cycles due to a decrease in the penetration depth. Hardness decreases with annealing temperature until 500 °C. At and above 600 °C, the hardness increases with increasing annealing temperature due to the partial dissolution of Ti3Ni4 precipitates, which prevent dislocation.