Effects Of Grain Refinement On The Structure And Properties Of A CuAlMn Shape Memory Alloy

Because of superiority in performance and price advantage,Cu-based shape memory alloys（SMAs）have important application prospects.However,intergranular fracture is prone to take place in these alloys because their coarse grains.Thus,in the present study the Cu-11.9Al-2.5Mn（wt.%）SMA was taken as the research object,and its grains were refined by using the modification effect of inoculants.In addition,the microstructure observation,phase analysis,mechanical properties as well as damping property tests on the resultant specimens were also conducted.The results show that:1.The Cu₅₁Zr₁₄ inoculants have obvious refining effect on the grains of the CuAlMn SMA.When the adding amount of the Cu₅₁Zr₁₄ inoculant reaches 0.9 wt%an optimum refining effect can be obtained.The average grain size can be reduced from 184μm of the original alloy to about 35μm,and the average grain size of the quenched CuAlMn SMA can be reduced from 1050μm of the original alloy to about 37μm.We believe that because the melting point of Cu₅₁Zr₁₄ inoculant is much higher than that of the CuAlMn SMA,Cu₅₁Zr₁₄ can distribute in the melt of the CuAlMn SMA in the form of tiny solid particles,and act as effective nucleus during the solidification of the alloy.Therefore,increasing the number of nucleus in unit volume can certainly increase the degree of grain refinement.However,when the amount of inoculants is too high,due to the agglomeration of them their refinement effect on the CuAlMn SMA decreases,i.e.the grain size increases.2.Quenching treatment can eliminate the non-martensite phases present in the as-cast CuAlMn SMA,while aging treatment after quench can efficiently decrease the concentration of quenched-in vacancies,accordingly decreasing the tendency of martensite stabilization.In this paper,the final determination of the optimal heat treatment process is:quenching afterβrising at 900^oC for 15 min,and then rapidly heating up to 350^oC and aging for 15 min.3.The average hardness of the refined CuAlMn SMA increases firstly and then decreases with the increase of adding amount of inoculant.When the adding amount of inoculant is 0.9 wt%,the hardness of the CuAlMn SMA increases to its maximum of 251HV from the original 175 HV of the alloy without inoculant addition.When the adding amount of inoculant exceeds 0.9 wt%,the hardness begins to decrease.The tensile strength of the CuAlMn SMA exhibits the same change rule with increasing the adding amount of inoculant.When the adding amount of inoculants is 0.9 wt%,the tensile strength of the CuAlMn SMA increases to its maximum of 720 MPa from the original 425 MPa of the alloy without inoculant addition.4.Damping properties of refined CuAlMn SMA exhibit complex changes with increasing the adding amount of inoculant,which can be ascribed to the integrated result of the increase in the number of various interfaces in unit volume and the decrease of the mobility of interfaces.In the low temperature region,due to the refinement of twins,the damping capacity of the CuAlMn SMA increases with the decrease of grain size,but the net height of the internal friction（IF）peak（P1）arising from the refinement of twins gradually decreases because of the enhancement of the internal friction（IF）background.The net height of the IF peak（P2）arising from the reverse martensitic transformation（MT）increases firstly and then decreases with the decrease of grain size.