Cu-Al-Mn Based Shape Memory Single Crystals Obtained Through One-step Annealing And Their Properties

After having substantial progress in the development of shape memory alloys(SMAs)in all kinds of application fields,The SMAs which now put into practical applications include Ni-Ti-based,Cu-based and Fe-based SMAs.Charcterized by both low cost and good machinability,the Cu-based SMAs have attract attentions of researchers.The Cu-based SMAs made by conventional solidification are usually polycrystalline,which brings forth polycrystalline brittleness.With the purpose of preparing single crystal,the fourth element which could cause bcc separation in the Cu-rich matrix was added into the Cu-Al-Mn based alloy so as to create a unique as-cast microstructure containing L21 parent phase and the A2 precipitates.Based on this method,the abnormal grain growth(AGG)could happen when directly annealing cast alloys,and single crystals of several centimeters were obtained.In this paper,three kinds of alloys in which bcc separation happened were designed,including Cu-Al-Mn-Mo,Cu-Al-Mn-Cr and Cu-Al-Mn-W.Several experiments methods,including OM,TEM,EBSD and compressive tests,were conducted for the analysis of the microstructure,phase transformation characteristics and the mechanical of abnormal grain growth,as well as SME of single crystals.The results obtained are as follows:(1)The microstructure of all Cu-Al-Mn-Mo cast alloys contained L21(Cu2AlMn)parent phase and the A2(Mo)precipitates,while the phase boundary in-between is coherent.The A2(Mo)precipitates would dissolve back into the matrix when annealing at 1173K,resulting in the formation of a continuous misorientation gradient with a deviation about 2°,which provided the key driving force for the migration of grain boundaries.In the end,single grains of several centimeters were obtained.The shape memory properties of Cu-Al-Mn-Mo single crystals were closely related to the crystallographic orientation alone deformation direction.For example,Cu-14Al-10Mn-1Mo single crystal with a orientation of[210]exhibited the best superelasticity,showing a full superelascticity of 11%and a superelastic strain(SE)of 6.7%.Cu-11Al-9Mn-1.5Mo single crystal with a orientation of[310]exhibited not only excellent superelasticity but also damping characteristic.In other case,like Cu-12Al-8Mn-1.5Mo single crystal with a orientation of[232],barely exhibited superelasticity.(2)The Cu-13.0Al-7.7Mn-0.6Cr cast alloy had a microstructure consisted of L21(Cu2AlMn)parent phase and the A2(Cr)precipitates that completely coherent with the parent phase.When annealing the cast alloy at 1173K,abnormal grain growth happened and single crystals of several centimeters were obtained.In this research,the dissolution of A2(Cr)nanoparticals was the key condition for the abnormal growth of grains.The Cu-13.0Al-7.7Mn-0.6Cr single possessed excellent superelasticity and fatigue resistance.When the Crystallographic orientation along the deformation direction was close to[100],the Cu-13.0Al-7.7Mn-0.6Cr exhibited full superelasticity of 12%and SE of 8.5%.During a cyclic deformation of 1200 times,the single crystal always showed full superelasticity.Though the SE and the critical stress decreased slightly,the rate of decrease was extremely low,thus most of the SE remained after cyclic deformation.(3)The Cu-14.2Al-10Mn-0.5W cast alloy had a microstructure containing L21(Cu2AlMn)parent phase and the A2(W)precipitates that completely coherent with the parent phase.Besides,?1(Cu9Al4)phase was observed in the cast alloy,which would dissolve back into the matrix during annealing process,and had no effect on the abnormal grain growth.The A2(Cr)nanoparticals would dissolve back into the matrix when annealing at high temperature,which was the key condition for the abnormal growth of grains.The Cu-14.2Al-10Mn-0.5W single crystal possessed excellent superelasticity and fatigue resistance.When the Crystallographic orientation along the deformation direction was close to[100],The single crystal exhibited full superelasticity of 12%and SE of 8.5%.During a cyclic deformation of 5000 times,the single crystal always showed full superelasticity.Though the SE and the critical stress decreased slightly,yet most of the SE remained after cyclic deformation.