Grain boundary and triple junction constraints during martensitic transformation in shape memory alloys

Abstract

We investigate the role of grain constraint upon martensitic transformation through in situ scanning electron microscope tensile experiments on shape memory microwires with a small number of grains and grain junctions. The martensite transformation morphology becomes more complex with increasing grain constraint: In unconstrained monocrystalline regions, the transformation is simple, single variant, and complete; near grain boundaries, the transformation is only partial, containing regions of untransformed austenite; near a triple junction, the morphology is complex, the transformation is partial and also multi-variant. These observations speak of transformationinduced stress concentrations that are more severe around triple junctions than around grain boundaries. Finite element modeling also provides an estimate for constraint effects on martensitic transformation yielding higher stresses near triple junctions than near grain boundaries. Towards the goal of developing polycrystalline Cu-based shape memory alloys that avoid intergranular fracture, our results support three design objectives: (1) Removal of triple junctions, (2) reduction of the total grain boundary area, and (3) geometry design containing unconstrained regions where the transformation can be most easily accommodated. V

Giant elastocaloric effect covering wide temperature range in columnar-grained Cu71.5Al17.5Mn11 shape memory alloy

“…5 Huge reversible strain can be achieved in single-crystalline SMAs, while it is difficult to be obtained in an ordinary polycrystalline one due to deformation incompatibility among separated grains. 12 The small reversible strain in polycrystalline SMAs strongly confines their applications in elastocaloric refrigeration. Fortunately, the reversible transformation strain can be significantly improved by microstructure tailoring in Cu-Al-Mn SMA with high cold-workability and machinability.…”

Characterization of Deformation Behavior of Individual Grains in Polycrystalline Cu-Al-Mn Superelastic Alloy Using White X-ray Microbeam Diffraction

“…The residual martensite would lead to incomplete strain recovery of the tensile sample (Figure 1). According to the modeling study by Ueland et al, the GB regions undergo severe grain constraint during deformation, which yields a high stress concentration, and therefore the GB area must be reduced for designing high-performing superelastic alloys [12]. …”

Surface roughness-controlled superelastic hysteresis in shape memory microwires

“…We estimate that the gauge sections of these two wires sample about 40 and 65 grains (rough and polished wires, respectively) and we therefore believe that factors such as grain size and orientation play only minor roles in affecting SMA properties [16].…”

“…1a shows an optical micrograph of such a glass-coated metallic fiber after drawing. The fibers were annealed at 800 °C in an argon atmosphere for 3 h and water quenched; during annealing the grains grow to span the wire cross section, forming a bamboo grain structure [16]. After annealing, the glass coating was removed by immersion in ∼10 % diluted aqueous hydrofluoric acid.…”