Shape Memory Alloys (Part I): Significant Properties

U ovom radu naveden je pregled termomehaničkih svojstava slitina s prisjetljivosti oblika i opća obilježja martenzitne transformacije. Slitine s prisjetljivosti oblika pripadaju skupini relativno novih metalnih materijala te se od drugih slitina (npr. dentalnih slitina) razlikuju po pseudoelastičnosti i efektu prisjetljivosti oblika. Pseudoelastičnost je povezana s nastankom martenzita transformacijom austenita. Austenitno-martenzitna transformacija može se inducirati mehaničkim (naprezanje) ili toplinskim (zagrijavanje i hlađenje) metodama. Tijekom martenzitne transformacije ne odvijaju se procesi difuzije, nego dolazi do neelastične deformacije kristalne strukture. Austenitno-martenzitnu transformaciju karakteriziraju temperature početka te završetka austenitne i martenzitne transformacije (As i Af, Ms i Mf). Također su detaljno objašnjeni jednosmjerni, dvosmjerni i višestruki efekt prisjetljivosti oblika, te je uočeno da je jednosmjerni efekt prisjetljivosti oblika još uvijek najčešće primjenjivan efekt.Shape memory alloys (SMAs) belong to a group of functional materials with the unique property of “remembering” the shape they had before pseudoplastic deformation. Such an effect is based on crystallographic reversible thermo-elastic martensitic transformation. There are two crystal phases in SMAs: the austenite phase (stable at high temperature) and the martensite phase (stable at low temperature). Austenite to martensite phase transformation can be obtained by mechanical (loading) and thermal methods (heating and cooling). During martensitic transformation, no diffusive process is involved, only inelastic deformation of the crystal structure. When the shape memory alloy passes through the phase transformation, the alloy transforms from high ordered phase (austenite) to low ordered phase (martensite). There are two types of martensite transformations. First is temperature-induced martensite, which is also called self-accommodating (twinned) martensite. The second is stress-induced martensite, also called detwinned martensite. The entire austenite to martensite transformation cycle can be described with four characteristic temperatures: Ms – martensite start temperature, Mf – martensite finish temperature, As – austenite start temperature, and Af – austenite finish temperature. The main factors influencing transformation temperatures are chemical composition, heat treatment procedure, cooling speed, grain size, and number of transformation cycles. As a result of martensitic transformation in SMAs, several thermomechanical phenomena may occur: pseudoelasticity, shape memory effect (one-way and two-way SME) and rubber-like behavior. Pseudoelasticity occurs when the SMA is subjected to a mechanical loading at a constant temperature above Af. The second thermomechanical behaviour that can be observed in SMA is the shape memory effect (SME), mainly one-way SME, which is the most commonly used SME. When the sample is subjected to a mechanical loading, the stress reaches a critical value and the transformation of twinned martensite into detwinned martensite begins and finishes when the loading process is finished. When the loading-unloading process is finished, the SMA presents a residual strain recoverable by alloy heating, which induces the reverse phase transformation. As a result, the alloy recovers to its original shape. In this paper, a review of thermomechanical properties of shape memory alloys and general characteristics of martensite transformations is shown