Shape memory alloys (SMAs) are the materials that have a capability toexhibit a large amount of strain, restore the original shape and perform mechanicalwork at various temperatures (Jiang et al., 2009; Mitwally & Farag, 2009; Zurbitu etal., 2009). The most common SMAs are Cu-based, Fe-based and Ti-based alloys(Sari & Kirindi, 2008). For example Cu-Al-Ni SMAs are the most used alloys forhigh temperature engineering applications.
However Ti-based alloys are used notonly for engineering applications but also applicable in the biomedical fields.Among any other Ti-based alloys, Ti-Ni is the most widely used SMAs.The first Ti-Ni shape memory alloys (SMAs) was discovered by WilliamBuehler and Frederick Wang in 1962 (Otsuka & Ren, 2005; Mccormick, 2006;Foroozmehr et al., 2011; Mohd Jani et al., 2014).
This alloy is well known asNitinol which is stand for Ni-Ti and Naval Ordnance Laboratory where the alloy wasfound. Since then, Ti-Ni SMAs have been used in various applications includingengineering and medical fields (Gonzalez et al., 2009; Delville et al.
, 2011; Vojt?chet al., 2010; Sawaguchi et al., 2004; Mohd Jani et al., 2014). The unique ability ofthis alloy is an appearance of the shape memory effect and superelasticity(pseudoelasticity) behaviour which is associated with its thermo elastic martensitictransformations from B2 austenite as high temperature phase to B19′ martensite as2the low one (Miyazaki et al., 1986; Bönisch et al., 2013; Jiang et al.
, 2009;Mantovani, 2000; He et al., 2005; Gonzalez et al., 2009; Filip & Mazanec, 1995). Incontrast to martensitic transformation in ordinary material, such as steel,transformation in shape memory alloy can be induced either by temperature, which iscalled a thermally-induced martensitic transformation, or by applied stress, alsoknown as stress-induced martensitic transformation (He et al., 2005).Transformation behaviour and the mechanical properties of this alloy is significantlyinfluenced by chemical composition, manufacturing processing, andthermomechanical cycling (Jiang et al., 2009; Miller & Lagoudas, 2001)Among Ti-Ni SMAs, Ti-Ni alloys with near-equiatomic composition are themost technologically important shape memory alloy due to the combinations ofexcellent functional properties, shape memory effect and superelasticity, withoutstanding mechanical properties, good corrosion resistance and biocompatibility(Nishida & Wayman, 1987; Kolomytsev et al., 1997; He & Liu, 2011; Zhou et al.
,2005). More specifically for Ti-Ni alloys containing ? 50.6 at.% Ni, which are oftenreferred to Ni rich near-equiatomic alloys, have attracted to be developed into manyapplications due to the phase transition temperature can be controlled through the Nicontent (Michutta et al.
, 2006; Khalil-allafi et al., 2002; Dlouhy et al., 2003).Moreover, appropriate combination of the manufacturing technology, subsequentthermal treatment regimes and/or the addition of third element can improve theproperty parameters of these alloys (Kolomytsev et al., 1997).In order to obtain superelasticity behaviour, the critical stress required fordislocation slip should be higher than stress required inducing martensite. Therefore,any technique to increase critical stress for dislocation slip, such as introducing moredislocations and precipitation hardening, would be beneficial to attain superelasticity(Miyazaki et al.
, 1986; Mitwally & Farag, 2009).Ti-Ni SMAs exhibit different superelasticity behaviour according to chemicalcompositions. For Ti-Ni alloys with < 50.
4 at.% Ni (near-equiatomic alloys),superelasticity is displayed by cold worked specimens which annealed attemperatures lower than their recrystallization temperature (Miyazaki et al., 1982).The introduction of random dislocations in the material due to cold work improves3the strength and hardness of Ti-Ni alloy but causing recoverable strains to decrease(Lin & Wu, 1994).
However, the strength will be reduced again and shape memoryeffect will be restored due to rearrangement of dislocations during annealing(Mitwally & Farag, 2009; Miller & Lagoudas, 2001)