| Due to the unique properties like shape memory effect (SEM) and supperelasticity (SE), shape memory alloys (SMAs), as the important smart materials, interested people greatly in the past three decades. Especially, with the fast development in medical science and micro mechanical machining, many novel medical apparatus and micro mechanical devices based on SMAs were designed, fabricated, and widely used. By now, it is well known that the SE is due to the stress-induced martensic transformation, and the unique mechanical behavior is strongly related to the microstructural evolution. Moreover, the SE can be affected greatly by the loading rate. In order to fully utilize the materials and to optimize the novel devices based on the SMAs, it is crucial to study the relationship between microstructures and micro mechanical behavior during the stress-induced martensic transformation (MT).Cu based SMAs are an important kind of SMAs which had been well studied and widely used. In this experimental work, CuAlNi single crystal is selected as our study object. Uniaxial tensile tests were performed under quasi-static and multi strain rate conditions, and stress-strain curves, microstructures evolution and temperature distribution can be obtained during the whole stress-induced martensic transformation process.Under the quasi-static loading and unloading condition, the microstructures evolution of the whole sample was recorded in micro scale and well analyzed. Using a special photo processing technique and an uniformization method, we realized the quantitative characterization for the microstructural evolution. Most importantly, it is found that MT had three evolution periods during the loading:initial formation with dramatically stress drop, mixed formation and growth with steady stress plateau, and final merging due to the growth of martensite strips. Moreover, for the unloading process, many austenite strips might firstly happen near the edge of martensite areas. After that, as the reverse-transformation going, the austenite strips would merge with each other to be a large area; meanwhile, the martensite areas would decrease gradually until they totally disappeared.Under the dynamic loading and unloading condition, the influence of the strain rates on the stress-induced MT was studied. The main research focus was on the relationship between the stress-strain curve, the surface morphology and the temperature distribution of the whole sample in different strain rates. Experimental results illustrated that the surface temperature would increase during the loading but decrease during the unloading. The maximum temperature during the loading would increase as the strain rate increase, and then saturated at a steady value. However, the minimum temperature during the unloading would decrease and then increase to a saturating value. Because of the unstable surface temperature, as the strain rate increased, the transformation stress would increase gradually and then saturated at a certain value during the loading, but it would decrease gradually and also finally stay at a steady value during unloading. Moreover, the hysteresis loop increased firstly and then descended, as the increasing of the strain rates. Remarkably, a critical strain rate value was determined in this work, and the rate higher than this value could dramatically increase the maximum number of matensite bands. Through the comparison of the experimental results under the different strain rates in different materials and sample geometries, the influent factors on the thermo-mechanical coupling transformation, including transformation latent heat, relaxation time and critical strain rate, were discussed in details.Finally, through the experimental data, we successfully obtained the theoretical parameters in the existing SMA model. These results certainly give some hints for better understanding of the stress-induced MT. And it is believed that they can provide strong and original data to improve the multi-scale thermo-mechanical models. |
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