Study On Compressive Properties Of Columnar Crystal Growth Direction Perpendicular To CuAlBe Alloy

In the fabrication process of shape memory alloy(SMA)fasteners,the fasteners are required to be radically compressed to reduce their diameters.In this project,directionally solidified columnar grain CuAlBe SMA for fastener specimens were transversely compressed(perpendicular to the growing direction of the columnar grains)to study the effects of compression on the performance of the CuAlBe SMA.The purpose of this study is to provide technical support for the diameter reducing process of the fasteners.In this paper,the alloy rod of dia.6mm was prepared by heated mold continuous casting process.The structure of the alloy is columnar crystal growing along the axis of the rod and its composition is Cu-11.9wt%Al-0.54wt%Be.Four phase transformation characteristic temperatures M_S?M_f?A_S?A_f were measured by DSC.It was found that the alloy withA_f temperature of -81.7? exhibited superelasticity at room temperature.2.5mm×2.5 mm×5 mm alloy compression specimens were cut along the transverse direction of the alloy rod.The mechanical properties of the alloy compression specimens were tested along the transverse direction,including room temperature compression,cyclic compression and single cycle compression.The results of compression test at room temperature show that the fracture strength and strain of CuAlBe columnar alloy specimens under transverse compression are 1453MPa and 44%respectively,and the induced stress of martensite is 403 MPa.The compressive properties of the specimens are obviously better than those of ordinary polycrystalline specimens(equiaxed grains).Scanning electron microscopy(SEM)was used to observe the fracture surface of compressed specimens.It was found that the fracture surface of the equiaxed grain specimens was mainly brittle fracture,while that of columnar specimens was mainly plastic fracture.It also shows that the equiaxed grain specimens is easier to fracture than the columnar specimens.The experimental results of multi-cyclic compression,single cyclic compression at room temperature and heating recovery show that the maximum residual strain of CuAlBe columnar crystals is about 10% when the cyclic compression strain is 14%.The actual residual strain is about 8.4%.The superelastic strain is about 4%.The actual residual deformation of 1.1%-1.4% compression specimens after single cycle compression can only be recovered to 0.3%-0.6% when heated to 250?.This is probably due to the formation of relatively stable?'martensite during compression.Columnar Crystalline Alloy Specimens at temperatures much higher than room temperature A_f should have exhibited complete superelasticity,but they exhibited shape memory properties during transverse compression.This is a manifestation of anisotropy.The transverse in-situ compression tests of CuAlBe columnar alloy samples were carried out by a self-designed manual compression device.The deformation of individual grain in the sample was measured by metallographic microscope and microhardness tester.The stress-strain curves of each grain were drawn and the phase transformation process was observed.Meanwhile,the orientation of each grain in the sample was determined by EBSD.The results show that when the nominal strain is 14%,the residual strain is 2%.Among the six observed grains,five showed superelasticity and one showed memory performance.The characteristics of stress-strain curves of grains with<111> and <211> orientation are:greater slope of the loading section,that is,greater elastic modulus;lack of distinct transformation plateau;less strain hysteresis;less grain deformation.On the other hand,the characteristics of stress-strain curves of grains with<310> and <210> orientation are:less elastic modulus;distinct transformation plateau;greater strain hysteresis and greater grain deformation.Integrating the?-?curves and the observation of metallographic structure,it is determined that the martensitic inducing stress in<111> and <211> orientation is about 370MPa,while in<310> and <210> orientation about 270MPa.Based on the experiments,it is concluded that when compressed perpendicular to their growing direction,columnar grains exhibit strong anisotropic in the following aspects:elastic modulus;martensitic transformation induing stress and transformation amount and speed;elastic effect and/or shape memory effect.Due to the anisotropic effect,columnar grain structure with superelastic effect can exhibit shape memory effect simply by deforming in different orientations that is to say,the phase transition lag is enlarged.This effect can be applied to the fabrication of SMA fasteners,providing new ideas for the further development of the project.