THESIS
2021
1 online resource (xiii, 90 pages) : illustrations (some color)
Abstract
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 Cu
67Al
24Mn
9 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 (a
0 = 5.8790Å) and specifically orthorhombic
mart...[
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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 Cu
67Al
24Mn
9 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 (a
0 = 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 CuAl
24Mn
9 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.
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