Defect strength and strain glass state in ferroelastic systems

It has been shown both experimentally and by computer simulations that stress-carrying defects can change a normal sharp first-order martensitic transformation (MT) to a continuous strain glass transition in ferroelastic systems and offer unique properties. However, strain glass state has been found only in limited ferroelastic systems with relatively small transformatoin strains such as the trigonal phase (1%) in NiTi-based alloys and the orthorhombic phase (2%) in TiNb-based alloys by means of impurity doping. We show in this paper that there exists a critical value of defect strength relative to the strength of MTs for creating a strain glass state. Using a typical strain glass system, Ti48.5Ni51.5 with symmetrical anti-site defects, as an example, we show that the equivalent Von Mises strain caused by anti-site defect with randomly distribution (with 3% concentration) is 7/8.9 times of that created by the stress free transformation strain of Martensitic phase in order to produce an R/B19 strain glass state respectively. The maximum interaction energy between the anti-site defects and the martensitic variant is estimated to be 16 times larger than that between different martensitic variants at this defect concentration. This finding may shed light on developing new strain glasses of much larger transformation strains for broader applications through defect engineering.