Universal Scaled Strength Behaviour for Micropillars and Nanoporous Materials
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1185-II07-03
Universal Scaled Strength Behaviour for Micropillars and Nanoporous Materials Brian Derby and Rui Dou School of Materials, University of Manchester, Grosvenor Street, Manchester, M1 7HS, UK. ABSTRACT The strength of submicron fcc structure metal columns, σ, fabricated by FIB machining or electrodeposition, shows a strong correlation with specimen diameter, d, with σ/µ = A(d/b)-0.63, where A is a constant, µ is the single crystal shear modulus resolved onto the slip system and b is the Burgers’ vector. The strength of bcc structure metals does not follow such a well defined correlation with size across different metals but the data occupies the same region of parameter space as with the fcc metals. Nanoporous gold specimens show a similar size-correlated behaviour but with an exponent of -0.5. This may indicate different mechanisms operating in each case.
INTRODUCTION The strength of metal pillars, columns or wires with diameter in the range of 30 nm – 30 µm shows a pronounced size effect, with yield strength in excess of 1 GPa for the smallest pillars tested [1-5]. A similar behavior is seen with the deformation of nanoporous metals, with similar strength values reported when the ligament diameter approaches 10 nm [6-10]. Previous work has investigated this behavior and proposed a simple scaling relation for the behaviour of face centred cubic (fcc) structure metals [11,12]. Here we explore this scaling relation and extend it to materials with other crystal structures. We also investigate whether nanoporous gold scales in the same manner. A large number of experiments have been reported on micropillar compression studies of a range of metals including Au, Ni, Cu, Al, Mo and the semiconductor GaAs. [1-5, 13-21] In most cases the specimens were small single crystal specimens with aspect ratios (height/diameter) ≈ 3. In the majority of the reports, the deformation of individual micropillars follows an erratic stress/strain history with deformation occurring in bursts of strain at almost constant stress interspersed by regions of elastic deformation. In these samples deformation appeared to be highly localized with clearly visible shear offsets along the deformed pillar. In a few cases deformation showed a continuous increase in deformation stress with increasing plastic strain, analogous to the deformation behavior of polycrystals and in these cases deformation was not localized.. At small length scales conventional mechanisms for the generation of dislocations are severely constrained by the small dimensions of the specimens, which either physically limit the length of dislocation segments available for dislocation multiplication or remove mobile dislocations through the close proximity of free surfaces and consequent attractive mirror forces. [14] This “dislocation starvation” strengthening mechanism is supported by experimental observations on
the behavior tested in compression, where deformation is observed occurring in bursts and a deformed column shows extensive stepping on the column surface. Tr
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