In this research, the effect of heat treatment temperature on the recovery of function of TiNi shape memory alloy (SMA) is investigated. The chemical composition of specimen is Ti50.4 at%Ni, and cold working ratio is 30 %. At first, the specimen is heattreated at 673 K for 3.6 ks. And then, the specimen is heattreated for recovery of function after isothermal 1000 times loadingunloading test under constant strain. The heattreatment temperature for recovery of function is performed at 573, 673 and 773 K for 0.6 ks. The functional deterioration process of specimens heattreated for recovery of function is investigated by the isothermal loadingunloading tests under constant strain. Recovery strains of samples which heattreated at 673 K and 773 K are recovered to the same level of initial condition by heattreated for recovery of function. However, when heattreated at 573 K, recovery strain does not recover. On the other hand, plastic strains of all samples recover to the same level of initial condition irrespective of heattreatment temperature. Nevertheless, when heattreated at 773 K, the increment of plastic strain is larger than other cases. From these results, the optimal heattreatment temperature for recovery of function is 673 K. Key words: TiNi alloy, shape memory alloy, actuator, function deterioration process Shape memory alloys (SMAs), especially TiNi alloy systems, are applied to various fields; for example, engineering and medicine. They show superior properties; not only shape memory effects but also ductility, toughness, fatigue resistance, corrosion resistance and abrasion resistance [14] . Shape memory properties and the superelasticity are repeatedly used in some applications of SMA. However, the functions of SMAs are deteriorated by the repeated usage. Several authors reported the effect of cold working ratio on function deterioration process of TiNi shape memory alloy, by using repeated loadingunloading cycle tests with constant strain condition [58] . From these studies, it became clear that the function deterioration process is caused by the increasing of dislocation density during the repeated usage. It is expected that the function of SMAs should be recovered if the dislocation density of SMAs is decreased by heartreatments.In this study, the effect of the heattreatment temperature on recovery of function for TiNi shape memory alloy was investigated by using repeated loadingunloading cycle tests with constant strain condition. The chemical composition of alloys used in this study is Ti50.4 at%Ni. The specimen shape is a wire with the diameter of about 1mm and the gage length of 60 mm.The specimen was processed in the following manner; the TiNi shape memory alloy ingot was made by using a high frequency induction vacuum furnace, and then was hot forged and hot extruded followed by cold drawing and intermediate annealing. Cold working ratio (CW) of the specimen is 30.1 %. To investigate the effect of heattreatment temperatu...
The phase transformation behavior of polycrystalline shape memory alloys is investigated by a constitutive model under multi-axial stress states of tension and shear. The constitutive model used here was developed by the authors to describe the accommodation mechanism acting on the phase transformation phenomenon. The transformation occurs in microscopic transformation systems of crystals and is calculated by the constitutive model. The transformation stress is introduced here as an indicator to describe the macroscopic transformation start. Transformation stresses are obtained from calculated stress-strain curves for various loading paths. The interaction surface of macroscopic transformation stresses obtained is revealed to be very similar to the Mises yield surface in the stress space. Discussions are also made on the relation between the equivalent stress and the equivalent transformation strain of Mises type. , etc. which can successfully describe transformation behavior of shape memory alloys. But most of them are phenomenological ones with internal variables through which accommodation mechanism is reflected in the models indirectly and there are few models that can directly treat the accommodation mechanism which controls transformation behaviors of micro structures of shape memory alloys. The accommodation mechanism automatically chooses transformation planes and directions which should be active so that the internal stress caused by transformation becomes smallest. One of such models that can treat accommodation mechanism directly was developed by the authors [5] [6] where the equal strain assumption is introduced to describe the accommodation behavior in microscopic transformation systems of crystals.In the present paper, the outline of the constitutive model proposed by the authors is described first and results from sample calculations by the constitutive model are given next in order to demonstrate the ability of the model for describing the deformation behavior of shape memory alloys. Though it is demonstrated from results of sample calculations that the model is applicable for describing deformation behavior of shape memory alloys, a great
In this research, the effect of heat treatment time on the performance recovery of Ti-Ni shape memory alloy (SMA) is investigated. The chemical composition of specimen is Ti-50.4 at%Ni, and cold working ratio is 30 %. At first, the specimen is heat-treated at 673 K for 3.6 ks. And then, the specimen is heat-treated for recovery of function after isothermal 1000 times loading-unloading test under constant strain. The heat-treatment temperature for performance recovery is 573 K, and heat-treatment time is varied from 0.6 to 3.6 ks. The functional deterioration process of specimens heat-treated for performance recovery is investigated by the isothermal loading-unloading tests under constant strain. Recovery strain and critical stress for inducing martensite increase with increasing heat-treatment time, and are saturated when heat-treatment time is more than 1.8 ks. Moreover, the number of cycle to failure increases with increasing heat-treatment time up to 1.8 ks, has a maximum at 1.8 ks, and then followed by a decrease. These tendencies are caused by a variation of the dislocation density owing to the variation of the heat-treatment condition. From these results, the optimal heat-treatment time for performance recovery is 1.8 ks when heat-treatment temperature is 573K.
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