Cross-sectional analysis of the graded microstructure and residual stress distribution in a TiNi alloy treated with low energy high-current pulsed electron beam

Abstract

A Ti-(55.08wt.%)Ni shape memory alloy (SMA) having a grain size of 30 μm, was subjected to low-energy high-current pulsed electron beam (HCPEB) irradiation with parameters: pulse duration τ = 2−2.5 μs, maximum electron energy 25 keV, energy density ES = 3.8 ± 0.7 J/cm2; beam diameter of ∼ 60 mm, number of pulses n = 32. Microstructure modifications, a value and distribution of residual stress induced by HCPEB treatment were investigated in detail. XRD and cross-sectional TEM/EDS/SAED analysis were used for microstructural characterization of the TiNi samples before and after HCPEB treatment. As a result, the HCPEB-modified TiNi surface layer consists of four successive sublayers differing in phase composition, type of texture and defect substructure. In total, the thickness of the HCPEB-modified TiNi surface layer is, at least, 10 μm. Based on the XRD and nanoindentation data, the dependences of residual stress values on the depth from the irradiated surface in the HCPEB-modified TiNi surface layer were estimated and plotted. The values and sings of the stress components in perpendicular (σI⊥) and parallel (σI||) directions to the TiNi sample surface were calculated: σI⊥ <0, σI||>0. The gradient of residual stress values is preserved in the HCPEB-modified TiNi surface layer: from ∼ 800 MPa (in absolute value) near the irradiation surface to ∼ 220 MPa at a depth of about ∼ 10 μm. The residual stress fields are localized in the near-surface layer I and sublayer III with different type of B2-structure. Their relaxation occurs in the sublayer II by means of the stress-induced В2 → R martensitic transformation.