Si Cr Ni Shape Memory Research Articles

Microstructural analysis of the stress-induced ε martensite in a Fe–Mn–Si–Cr–Ni shape memory alloy : Part II: Transformation reversibility

The shape memory effect exhibited by some Fe–Mn–Si based alloys is related to the γ(f.c.c.)- ε(h.c.p.) martensitic transformation. In these alloys, the shape memory effect, incomplete even for low deformation rates, drastically decreases when the deformation rate increases. The evolution of the microstructure of the martensite, and particularly the interaction of martensite plates and grain boundaries, is studied in a Fe–Mn–Si–Cr–Ni alloy to determine the origin of the loss of shape memory. The influence of the back-stress created by the dislocations located at the tip of the martensite bands is pointed out. The reduction of the shape memory is attributed to the accommodation of this back-stress when the deformation rate, i.e. the martensite band width, increases. The same kind of analysis is used to analyse the problem of superelasticity.

Microstructural analysis of the stress-induced ε martensite in a Fe–Mn–Si–Cr–Ni shape memory alloy: Part I—calculated description of the microstructure

The γ(f.c.c.)– ε(h.c.p.) martensitic transformation is achieved by the introduction of stacking faults on each second compact plane of the f.c.c. structure. These stacking faults are created by the motion of Shockley partial dislocations. Depending on the Burgers vector of these dislocations, the martensite does not require a macroscopic shape change (self-accommodated martensite) or a homogeneous lattice shape change (monopartial martensite). Based on the monopartial nature of the stress-induced martensite, a model describing the martensitic morphology in the Fe–Mn–Si based shape memory alloys is presented. The theoretical results are compared with some observations in a Fe–Mn–Si–Cr–Ni shape memory alloy.

Characterization of the stress-induced ε martensite in a Fe–Mn–Si–Cr–Ni shape memory alloy: microstructural observation at different scales, mechanism of formation and growth

The martensitic transformation induced by traction at room temperature in a Fe–16Mn–9Cr–5Si–4Ni (%mass) has been studied by optical microscopy, scanning electron microscopy, transmission electron microscopy and scanning tunneling microscopy. The samples were previously submitted to a thermomechanical treatment which increases the shape memory properties. The martensitic microstructure and the fine structure of the stacking faults are both studied to clarify the nucleation and growth mechanisms of martensite. The band structure of the martensite is pointed out; these bands correspond to a mixture of thin martensite plates and extremely thin austenitic zones. Inside a grain, the monopartial nature of the martensite has been demonstrated from the elementary plate to all the martensite bands. From all the observations, the pole mechanism appears to be the main mechanism of martensite nucleation.

HREM study of self-accommodated thermal ε-martensite in an FeMnSiCrNi shape memory alloy

An FeMnSiCrNi shape memory alloy containing 70% of thermal ε-martensite has been studied using high resolution electron microscopy. Images recorded along a 〈011〉 γ//〈2110〉 ε axis show, at the atomic level, that large ε-bands have a layered substructure and consist of thin ε-layers separated by layers of untransformed austenite. The structure of the ε (h.c.p.)/γ (f.c.c.) interface at the ε-layer tips has been determined by using both the analysis of numerically filtered images and isotropic elasticity calculations: it can be described as a periodic array of 30° and 90° Shockley partials in a 2:1 proportion. This model illustrates the self-accommodation properties of thermal ε-martensite at the finest scale and explains the easy reversion of the hexagonal phase.

Effect of thermomechanical cycling in an FeMnSiCrNi shape memory alloy

Previous studies on the effect of cycling in Fe-Mn-Si alloys have shown an increase in reversibility with increasing number of thermal cycles. In Fe-Mn-Si-Cr-Ni and Fe-Mn-Si-Co-Ni alloys, the shape recovery strain during thermomechanical cycling increased when the strain amplitude for each successive cycle is kept constant. Where the total strain (i.e., the sum of the transformation and permanent strain) is held constant, a small decrease in strain recovery with increasing number of cycles occurs. Nonetheless, an increase in reversibility with increasing number of thermomechanical cycles is a common feature for most studies. While previous studies have shown that training or thermomechanical transformation cycling may improve the shape recovery in these alloys, the mechanism is not still clear. Characterization of the transformation behavior during cycling is an important prerequisite for practical use of such alloys. In this paper the authors report microstructural changes during thermomechanical cycling of an Fe-Mn-Si-Cr-Ni shape memory alloy.