Atomistic Simulations Of Aging Effects In Shape Memory Alloys

Shape memory alloys are well known as new functional materials due to their unique shape memory effect and superelasticity. These unusual properties result from the martensite phase transformation and the reverse martensite phase transformation. So far, the shape memory alloys had been used for many applications such as sensors, actuators, MEMS (Micro-electromechanical Systems) and so on.Variety of the shape memory alloys had been found in the past decades, such as Au-Cd, Au-Cu-Zn, Cu-Zn-Al, Cu-Al-Ni, Ti-Ni-Hf, Ni-Mn-Ga, Ti-Ni, Ti-Ni-Fe and so on. However, only a few of them can be used in commercial applications. One of the reasons is that the most of shape memory alloys subject to the aging effects, which is accompanied by a gradual change in physical properties of shape memory alloys. These aging effects strongly affect the reliability of devices using shape memory alloys. However, their microscopic mechanism has remained controversial due to lack of experiments that can probe the atomic level processes. Therefore, in the present work, the aging effects in shape memory alloys and related exotic physical behavior were systematically investigated by using atomistic simulations.The aging effects actually involve two key characteristics: 1) short-time scale martensite phase transformation or reverse martensite phase transformation; and 2) long-time scale aging process. To capture these key points, a novel atomistic approach was developed, which combines the molecular dynamic method and the Monte-Carlo method. The former was used to simulate the"short-time"scale phase transformation and the latter was used to simulate the"long-time"scale aging process. By using this approach, the atomic process of aging effects in shape memory alloys is able to be obtained.First, one well-observed martensite aging effects was reproduced successfully by our approach. It is the martensitic stabilization, which refers to the reverse martensite phase transformation temperature will increase after martensite aging. Quantitative analysis of the atomic configurations during aging reveals that martensitic stabilization is not associated with a change in average martensite structure (long-range order). It involves instead a gradual change in the short range order of point defects so that the defect short range order tends to adopt the same"symmetry"as the crystal symmetry of the host martensite lattice. It suggested that the most reasonable microscopic mechanism of martensitic stabilization is the symmetry-conforming short-range-ordering model (i.e. when in equilibrium state, the symmetry of short-range configuration of point defects will follow the crystal symmetry).Second, the mechanical response to martensite aging was studied by atomistic simulations. It is found that the critical stress for the reorientation of a martensite variant increase with aging time, which is consistent with experimental results. Atomic probing reveals that it results from the reconfiguration of point defects within short-range driven by the symmetry-conforming short-range ordering model as well. Semi-quantitative formulas were developed to calculate the change in free energy of system after martensite aging, which is responsible for the increment of critical stress. Further analysis implies that the famous"rubber-like behavior"(RLB) also results from the energy difference induced by short-range-ordering change of point defects.Third, it is experimentally known that martensite aging effects can be quickly eliminated once the aged martensite is brought into the parent phase even for a very short time, i.e., the annihilating effect of martensite aging (AEMA). By using similar atomistic method, it is found that the origin of AEMA is related to atomic diffusion in the parent phase, not merely the reverse phase transformation. Through atomic diffusion, the configuration of point defects within short range recovers back to the fresh martensite state so that the martensite aging effects were eliminated. The open structure in the parent phase and high-temperature result in a significantly higher diffusivity of point defects in the parent phase compared with that in the martensite; this explains the ultrafast annihilation of martensite aging. Moreover, the driving force of AEMA is attributed to the symmetry-conforming short-range ordering tendency of point defects.Finally, using the experiments and atomistic simulations, a time-related behavior was found during the AEMA (deaging) process. Experimentally, Au-49.5at%Cd shape memory alloy was investigated, and it is found that a shorter time holding/aging in the parent phase results in a higher Ms as compared with the fully deaged case. Atomistic simulations reproduce the experimental observation and suggest that the origin of this time-dependent behavior arises from the gradual change in short-range configurations of point defects, being the same as that of the martensite aging. As conjugated results, one parent phase aging effect that the Ms temperature decrease with aging time in parent phase has been found.In conclusion, through the systematical investigation in this work, the microscopic mechanism of aging effects was clarified both in martensite and in parent phase for shape memory alloys. Using atomistic simulation, the exotic behaviors of aging effects were reproduced and explained very well. The establishment of the semi-quantitative formulas of aging effects leads to the deep understanding of these unusual phenomena. Therefore, the present study was most significant in fundamental research, and provided a direction to control the aging effects in shape memory alloys for practical applications.