A study of Gas Nitriding of TiNiCr ternary shape memory alloys

• Main Authors: Chen-Yu Lin, 林振宇
• Other Authors: Shy-Feng Hsieh
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/88938582842126488571

碩士 === 國立高雄應用科技大學 === 模具工程系碩士班 === 96 === Ti50Ni50-XCrX, Ti51Ni49-XCrX and Ti50-XNi50CrX (X=0.5 at.%) ternary shape memory alloys (SMAs) were gas nitrided to modify the surface conditions in this research, in which Ni-rich Ti49.5Ni50Cr0.5 alloy was solid-soluted at 900C, and then aged at 300C for 1 hr (short-aging) and 85 hrs (long-aging). The surface layer, phase transformation, microstructure and mechanical properties of the nitrided surface were also investigated. Experimental results show that the nitrided layer thickness (a few microns ~ 90 ) increases with increasing the nitrided temperature (600C~900C). The nitrided compound layer of these alloys consists of TiN, CrN and Ti2NiH0.5, in which the layer thickness and compounds can affect surface hardness markedly. The specimens nitrided at 600C can be obtained the maximal surface hardness, but those nitrided at 800C can not be effectively increased the surface hardness owing to surface cracks appearing in nitrided layers. Among these non-gas-nitrided alloys, Ni-rich Ti49.5Ni50Cr0.5 alloy exhibits the lowest steady state friction coefficient, Ti50Ni49.5Cr0.5 alloy the next largest and Ti-rich Ti51Ni48.5Cr0.5 alloy the highest due to the premier has the highest hardness and pseudoelasticity in B2 phase. The wear characteristics of these nitrided alloys can be markedly improved, especially at 600C gas-nitrided alloys, i.e., the nitrided alloys can exhibit excellent wear resistance and a low friction coefficient. Martensitic transformation temperatures are depressed slightly to lower ones due to the constraint stress originating from the nitrided layers, and/or the penetration of N and H into the TiNi matrix during the gas nitriding process. The shape recovery ability is also slightly reduced because the nitrided layer can not exhibit a shape memory effect and its constraint effect will also depress the shape recovery of the TiNi matrix. However, some short-aging Ni-rich TiNiCr alloys have the better shape recovery than the others after gas-nitriding process.