The functionality of shape memory alloys (SMAs) is a crucial property affecting the exploitation of these smart materials in biomedical devices and microactuators. Normally, their functionality degrades significantly within first limited cycles and is declined at smaller scales. Through a rational measurement of the cyclic superelastic behaviors in a phase-transforming bicrystal micropillar, we show that bicrystal Cu₆₇Al₂₄Mn₉ micropillar milled at a high-angle grain boundary (GB) exhibits enhanced reversibility under very demanding driving stress (about 600 MPa) over 10,000 transformation cycles despite its lattice parameters are far from satisfying any crystallographic compatibility conditions. Such lattice parameters of cubic austenite (a₀ = 5.8790Å) and specifically orthorhombic mart...[ Read more ]

The functionality of shape memory alloys (SMAs) is a crucial property affecting the exploitation of these smart materials in biomedical devices and microactuators. Normally, their functionality degrades significantly within first limited cycles and is declined at smaller scales. Through a rational measurement of the cyclic superelastic behaviors in a phase-transforming bicrystal micropillar, we show that bicrystal Cu₆₇Al₂₄Mn₉ micropillar milled at a high-angle grain boundary (GB) exhibits enhanced reversibility under very demanding driving stress (about 600 MPa) over 10,000 transformation cycles despite its lattice parameters are far from satisfying any crystallographic compatibility conditions. Such lattice parameters of cubic austenite (a₀ = 5.8790Å) and specifically orthorhombic martensite (a = 4.4320, b = 5.3453, c = 4.2631Å) phases with the respective space group of (Fm3̅m) and Pmmn are refined by precise characterization of crystal structures of both phases using the established crystallographic theory of derived crystal structure. we also report the first demonstration of superelasticity in single crystal custom-built tensile CuAl₂₄Mn₉ micro-samples oriented in different directions. Both tensile superelastic and functional fatigue behaviors are confirmed to be orientation-dependent at micron-scale. The micro-sample oriented in [223] (near [111] principal pole) shows the quite high driving stress for inducing phase transformation (∼500 MPa) compared to those of other three orientations (∼300 MPa). Two specific orientations of [205] and [325] exhibit the stable superelasticity by delivering a significant superelastic strains of ∼3.5% and ∼4% over 10,000 demanding cycles. Such orientational dependency of superelastic properties is analyzed by the systematic study of compatibility conditions by the crystallographic theory of martensite, and the Schmid law for twinning along the austenite/martensite interface.