Manufacture, Characterization And Properties Of Oligocrystalline Cu-Based Shape Memory Foams

Copper-based shape memory foams(SMFs),or porous shape memory alloys(SMAs),have attracted many interests due to their multi-functional properties,such as lightweight,high specific strength,low cost,high damping,and special pseudoelastic effect,making them promising in application areas of civil,architecture and automobile engineering.However,they usually suffer from intergranular brittleness which limits their applications.In order to reduce the intergranular brittleness and improve the damping and pseudoelastic properties at the same time,this paper has tried to introduce an oligocrystalline microstructure into the Cu-based SMFs with a uniform spherical pore morphology and controllable pore structure,with the aim to reduce the negative effect of pores and grain constraints to the most.Uniform and spherical pore morphology of the Cu-Al-Mn SMFs have been guaranteed by upgrading the equipment and process of the silica-gel bead infiltration method and optimizing the foaming parameters including silica-gel bead density,heating temperature,and argon gas pressure.Cu-18.5Al-11Mn(at.%)foams with adjustable pore structure(pore diameter of 0.5~1.25 mm,porosity of 66%~81%)have been reliably manufactured,by taking a full advantage of the expanding feature of the silica-gel beads upon heating and controlling the size and density of the silica-gel bead space holders.The abnormal expanding of the silica-gel beads is inferred to be caused by steaming and expanding of the water reserved in the nano-sized pores,which may produce high pressure under the constraint of the complex pore structure to force the cell wall to deform and expand.Based on this inference,the microstructure,expanding process,and rules of the silica-gel beads can be well explained.The physical meaning and the quantitative characterization of the oligocrystalline degree(oligocrystallinity)of foams have been made clear for the first time as the ratio of surface area to grain boundary area(S_F/S_B)and grain size d over N(d/N)respectively.The oligocrystallinities of the foams are 3.09~4.59 and most the grain boundaries are bamboo-liked.However,due to the constricted effect of complex strut architecture on two-dimensional grain growth,the grain size d increases linearly with the strut length L,which leads to d/N depending on strut length and width ratio(L/N).The martensitic transformation,damping property and compression recovery of the Cu-Al-Mn SMFs have been found to be influenced by the coupling effect of strut architecture and oligocrystallinity.The thermal-induced martensitic transformation and low-amplitude(0.05%)martensite damping depends mainly on strut width(N)because the phase and martensite boundaries are prone to be pinned by quenched-in vacancies.The peak damping becomes less dependent on N but tends to increase with d/N,indicating the oligocrystalline structure favors the mobility of phase interface greatly.As the strain-amplitude increases,the vacancies unpin and the high-amplitude(1.90%)martensite damping improves linearly with d/N,indicating that the mobility of martensite is more favored by oligocrystalline structure at higher displacement.The compression recovery strain increases linearly with d/N to 4.92%under the porosity of66%,although higher porosity tends to compromise the favorable effect of higher d/N,demonstrating that higher d/N(oligocrystallinity)helps the stress-induced martensite to recover more easily.Stress-strain curves of all the SMFs are very smooth,indicating that the brittleness of the SMFs has been reduced by the uniform spherical pore morphology and the oligocrystalline structure.High oligocrystallinities up to 8.19 and remarkably improved properties have been achieved by applying long-time and cyclic heat treatments to the Cu-Al-Mn SMFs with the same porosity of 70%.The peak damping of the Cu-Al-Mn SMFs increases with d/N by 65%to a high value of 0.154,showing a great favorable effect of high oligocrystallinity to the mobility of phase interfaces.The low-amplitude martensite damping increases first to a peak value of 0.0627 and then decreases with increasing d/N because higher oligocrystallinity improves both the mobility and growth of the martensite plates.The high-amplitude martensite damping increases linearly with d/N to a high value of 0.260,despite the wider martensite plates.The maximum recovery strain increases linearly with d/N to a high value of 5.53%as the d/N increases to 7.88,despite the high porosity of 70%,indicating that the recovery process of stress-induced martensite is highly favored by higher oligocrystallinity.The property improvement with oligocrystallinity mainly results from the reduced constrained area of triple junctions and grain boundary area and enlarged inner and surface unconstrained areas that lower the grain constraints.A model to calculate the pore structure parameters such as pore size,porosity,and specific surface area has been developed according to the principle of the silica-gel beads expanding,squeezing and deforming processes.By using this model,the pore structure can be accurately calculated and forecasted with a small relative error of 0.003~0.052 by measuring the model parameters of primary size,expanding ratio,packing density and coordination number of the silica-gel beads.