Shape Memory Alloys (Part II): Classification, Production and Application

Slitine s prisjetljivosti oblika funkcionalni su “pametni” materijali s jedinstvenim svojstvom prisjetljivosti oblika. Prisjetljivost oblika karakteristična je sposobnost materijala koja je posljedica martenzitne fazne transformacije, a karakterizira je vraćanje slitine u oblik koji je imala prije deformacije. U radu su detaljno objašnjene slitine s prisjetljivosti oblika s gledišta njihove podjele, proizvodnje i primjene. U industrijskoj primjeni najčešće se upotrebljavaju slitine na bazi nikla i titanija, te slitine na bazi bakra. Slitine na bazi željeza i plemenitih metala još uvijek nemaju značajniju industrijsku primjenu. Slitine s prisjetljivosti oblika uglavnom se proizvode postupcima indukcijskog taljenja, brzog očvršćivanja (primjerice melt spinning) i kontinuiranog lijevanja.Shape memory alloys (SMAs) have been extensively investigated because of their unique shape memory behaviour, i.e. their ability to recover their original shape they had before deformation. Shape memory effect is related to the thermoelastic martensitic transformation. Austenite to martensite phase transformation can be obtained by mechanical (loading) and thermal methods (heating and cooling). Depending on thermomechanical conditions, SMAs demonstrate several thermomechanical phenomena, such as pseudoelasticity, superelasticity, shape memory effect (one-way and two-way) and rubber-like behaviour. Numerous alloys show shape memory effect (NiTi-based alloys, Cu-based alloys, Fe-based alloys etc.). Nitinol (NiTi) is the most popular and the most commonly used SMA due to its superior thermomechanical and thermoelectrical properties. NiTi alloys have greater shape memory strain and excellent corrosion resistance compared to Cu – based alloys. However, they are very costly. On the other hand, copper-based alloys (CuZn and CuAl based alloys) are much less expensive, easier to manufacture and have a wider range of potential transformation temperatures. The characteristic transformation temperatures of martensitic transformation of CuAlNi alloys can lie between −200 and 200 °C, and these temperatures depend on Al and Ni content. Among the Cu – based SMAs, the most frequently applied are CuZnAl and CuAlNi alloys. Although CuZnAl alloys with better mechanical properties are the most popular among the Cu-based SMAs, they lack sufficient thermal stability, while CuAlNi shape memory alloys, in spite of their better thermal stability, have found only limited applications due to insufficient formability owing to the brittle γ2 precipitates. The most important disadvantage of polycrystalline CuAlNi alloys is a small reversible deformation (one-way shape memory effect: up to 4 %; two-way shape memory effect: only approximately 1.5 %) due to intergranular breakdown at low stress levels. The technologies for production of shape memory alloys are induction melting, vacuum melting, vacuum arc melting, following hot and cold working (forging, rolling, wire drawing). In addition, rapid solidification methods, like melt spinning and continuous casting have been developed. These methods are characterized by high cooling rates. High cooling rates allow very short time for diffusion processes and may lead to extremely fine microstructure, better homogeneity etc. SMAs have found applications in many areas due to their thermomechanical and thermoelectrical properties (biomedical applications, engineering industry, electrical industry). In this paper, a review of shape memory alloys, properties and applications of mentioned materials is presented