Characterization Of Training-free Fe–Mn–Si Shape Memory Alloys Containing VN Precipitates

A training-free Fe-17Mn-5Si-10Cr-4Ni-0.9VN shape memory alloy with trace V,N elements(mass 0.9%) has been designed so that mechanical properties and shape recovery are high enough to be applied to engineering application. Shape memory effect(SME),alloy strength,hardness, corrosion resistance effects has been studied. Through aging treatment at 800℃,precipiates of VN are gained as the second-phase ptical microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffraction analysis and other equipment are used to study particle precipitation and distribution of VN.SME tests shows that:aging treatment greatly enhance the SME of shape memory alloys, deformated 4% at room temperature, SME of 24h specimen is up to twice as SME of specimen without aging. the low-temperature deformation remarkable improves the SME of alloy,6h specimen deformed at-38℃achieve a shape recovery of up to 80%.By the impact of VN precipitation,Ms point of alloy declines. Decrease the deformation temperature, SME of long-time specimens perform a drop of fall. Aging treatment effectively improved the mechanical properties of alloys, VN precipitation greatly improves the hardness and yield strength. Electrochemical impedance spectroscopy showed that the design of adding V, N elements to alloy greatly improves the corrosion resistance of the alloy, as aging time extending, the resistance of alloy gradually decreases.Optical microscopy shows that as the aging progressing, rolling and other grain deformation caused by heat treatment process, organizational heterogeneity, twin crystal and some dislocation defects, gradually reduced.εmartensite microstructure photographs show as the different orientations of different grains and a wide range of internal dislocations and crystal defects such as twin, martensite lath is not very evenly distributed. Characterized by SEM, showing VN precipitates and agglomerates with the aging progresses, precipitates on the grain boundary are more than that in the grain, distribution of precipitates in 24h specimen is not continuous, but gatheres into larger size particles embeding in the matrix. EDX analysis also supports the view of particles reunion. X-ray diffraction of specimens with 10% tensile deformation shows that aging effects inducedεmartensite volum of specimens when deformed, andεmartensite content curves approximate the shape memory effect curve. X-ray diffraction patterns show that low-temperature deformation proves the strong impact ofεmartensite content. Deformed at temperature as low as -27℃,εmartensite content of 2h,8h specimens significant increases,whileεmartensite content of 24h specimen increases only from 17.2% to 22.5%. Situ- X-ray diffraction patterns of 6h specimen heat recovery process explain thatεmartensite to austenite transformation occurs in the range of 125℃～200℃.Compare with shape memory effct curves, the joint action ofεmartensite and sub-structure impels a good shape memory effect. Second phase particles of alloy is characterized by electrolytic extraction and X-ray diffraction, the particles are confirmed to be VN, the difference of matrix and precipitate lattice constant is 14.2%.