Microscopic mechanism of martensitic stabilization in shape-memory alloys: Atomic-level processes

Aging in martensite, which is accompanied by a gradual change in physical properties, has been observed in most shape-memory alloys for more than half a century. However, its microscopic mechanism has remained controversial due to a lack of experiments that can probe the atomic-level processes. By using a method which combines molecular-dynamics and Monte Carlo simulations, we clarify the atomic mechanism for one of the well-observed martensitic aging effects, martensitic stabilization. We successfully reproduce the observed effects using our method. Quantitative analysis of the atomic configurations during aging reveals that martensite stabilization is not associated with a change in the average martensite structure. 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. Our simulation results are consistent with the symmetry-conforming short-range order model [X. Ren and K. Otsuka, Nature (London) 389, 579 (1997)].

Figure 1

(Color online) Schematic illustration of the present simulations: (a) Well-aged parent phase with B2 structure is obtained by the combination of MD and MC methods, which contains $5\text{at}\text{.}\mathrm{\%}$ ASDs. (b) Martensitic phase transformation takes place during the cooling process by MD simulation, the fresh martensite with $\text{L}{1}_0^{″}$ (orthorhombic) structure is formed. (c) Aging for different MC steps to produce a group of different-time-aged martensite samples. (d) The reverse martensitic phase transformation occurs during heating process by using MD and martensitic stabilization behavior is investigated.

Figure 2

(Color online) Martensitic stabilization behavior: reverse martensitic transformations from $\text{L}{1}_0^{″}$ (orthorhombic) structure to B2 structure take place during the heating process by MD simulation [corresponding to the processes shown in Figs. 1c to 1d] and the $A_{\text{f}}$ temperature increases with aging time.

Figure 3

(Color online) Variation in (a) $A_{\text{f}}$ temperature, (b) LRO, and (c) SRO parameters during aging in martensite [corresponding to the processes shown in Figs. 1b to 1c].

Figure 4

(Color online) Changes in SRO of point defects during the processes shown in Figs. 1a, 1b, 1c. For simplicity, the (010) plane of our model is shown: (a) defect symmetry around a known defect in the parent phase; note the essentially equal probability for the presence of a defect at the neighboring sites. (b) Defect symmetry around a known defect in fresh $\text{L}{1}_0^{″}$ martensite. (c) Defect symmetry around a known defect in well-aged $\text{L}{1}_0^{″}$ martensite; note the change in defect probability.