Strengthening And Toughening Of A Cu-Al-Ni Shape Memory Alloy And Its Effect On The Damping Property

Cu-Al-Ni shape memory alloys(SMAs)have attracted?wide?attention in recent yearsdue to their excellent shape memory properties and high damping capacity.However,these alloys still have the problems of intergranular brittle fracture and relatively low mechanical properties due to their coarse grain size,which has become the main bottleneck hindering their further applications.For this reason,the grain size of the Cu-13.2Al-4Ni(wt.%)SMA was refined effectively by adding Cu-Zr inoculant and Ti element in order to achieve the purpose of strengthening and toughening.In this paper,metallographic microscopy(MM)and field emission scanning electron microscopy(FESEM)were used to observe the microstructure of the obtained materials;microhardness tester and universal material testing machine were used to test the hardness and tensile mechanical properties of materials,respectively;differential scanning calorimeter(DSC)and dynamic thermomechanical analyzer(DMA)were used to test the martensitic transformation behavior and damping properties of materials,respectively;self-designed bending test was used to test the shape memory effect(SME)of the material.It is found that with increasing the adding amount of the inoculant,the average grain size of the alloy decreases first and then increases.When the addition amount is 0.8 wt.%,the best refining effect can be achieved: the average grain size of the alloy can be reduced from the original 1400 μm to 31 μm.On this basis,if adding 1.0 wt.% Ti element,the average grain size of the alloy can be further reduced to 22 μm.After hot rolling,the average grain size of the alloy without refinement is 240 μm,while that of the alloy with the best refining effect is 29 μm.With the decrease of grain size,the mechanical properties of Cu-Al-Ni SMA are gradually improved due to the principle of fine grain strengthening.The finest grain size corresponds to the bestmechanical properties: the hardness,tensile strength and elongation can be increased to 306.56 HV,725 MPa and 8% from the original 249.22 HV,438 MPa and 4.8%,respectively.After hot rolling,they can be further increased to 342.38 HV,952.53 MPa and 9.76%,respectively.Tensile fracture analysis shows that the intergranular brittle fracture of the alloy is gradually solved with the decrease of grain size.With the decrease of grain size,the martensitic transformation(MT)temperature of Cu-Al-Ni SMA moves to low temperature,while the damping capacity in martensite state and the height of internal friction(IF)peak increase gradually due to the refinement of martensite and the increase of interfacial density.After hot rolling,the damping of the alloy can be further improved due to the elimination of casting defects.With the decrease of grain size,the SME of the alloy decreases gradually due to the combined effect of the increases of grain boundary affected zone and strength.After hot rolling,the SME of the alloy is further reduced.It can be found that the strengthening and toughening of the Cu-Al-Ni SMA by inoculant refining method can also improve the damping property due to the refinement of the structure.