Copper-based shape memory alloys (SMAs) exhibit excellent shape memory properties in single crystalline form. However, when they are polycrystalline, their shape memory properties are severely compromised by brittle fracture arising from transformation strain incompatibility at grain boundaries and triple junctions. Oligocrystalline shape memory alloys (oSMAs) are microstructurally designed SMA structures in which the total surface area exceeds the total grain boundary area, and triple junctions can even be completely absent. Here it is shown how an oligocrystalline structure provides a means of achieving single crystal-like SMA properties without being limited by constraints of single crystal processing. Additionally, the formation of oSMAs typically involves the reduction of the size scale of specimens, and sample size effects begin to emerge. Recent fi ndings on a size effect on the martensitic transformation in oSMAs are compared and a new regime of heat transfer associated with the transformation heat evolution in these alloys is discussed. New results on unassisted two-way shape memory and the effect of loading rate in oSMAs are also reported.

In oligocrystalline shape memory alloys, the total grain boundary area is smaller than the surface area of the specimen, leading to significant effects of free surfaces on the martensitic transformation and related shape memory and superelastic properties. Here we study sample size effects upon the superelastic characteristics of oligocrystalline microwires after one loading cycle and after many. Cu-Zn-Al wires with diameters ranging from ~100 down to ~20 µm are fabricated by the Taylor liquid processing technique and characterized through both uniaxial cyclic tensile testing and mechanically constrained thermal cycling. The energy dissipated per superelastic cycle increases with decreasing wire diameter, and this size effect is preserved after extensive cycling despite a significant transient evolution of the superelastic response for early cycles. We also present fatigue and fracture data indicating that oligocrystalline wires of this normally brittle alloy can exhibit fatigue lifetimes two orders of magnitude improved over conventional polycrystalline Cu-Zn-Al.

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

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The supply of recycled material depends on historic consumption, i.e. what constitutes scrap available today originates from previously made products. Analytical tools, such as materials flow analysis, use this observation to estimate scrap metal flows. The supply of recycled metal also depends on changing economic conditions, e.g. metal consumption rates correlate with changes in gross domestic product. We use an autoregressive distributed lag approach to model the supply of recycled copper as a complementary approach to material flow analysis. We find that both industrial activity and world GDP correlate with total scrap supply, with limited dependence on copper price. We also develop independent models for direct remelt (higher quality) and refined (lower quality) scrap. A 1% increase in industrial production leads to a 2.1% increase in higher quality scrap quantity, while a similar increase in world GDP leads to a 1.4% increase in lower quality scrap. Based on this model dependence, we suggest that a recycling policy aimed at increasing recycling through the use of subsidies, taxes or price incentives should be directed towards the low-end segment of the scrap market and there it may still only have limited impact.