Unraveling microstructure and mechanical response of Ni/Ti laminates via pass-resolved CARB and vacuum encapsulation

Ti/Ni multilayer composites were fabricated through the cross accumulative roll bonding (CARB) process at room temperature, followed by annealing at 800–900 °C for up to 10 h. SEM and EDS analyses confirmed uniform bonding without voids or delamination after rolling. Increasing the number of CARB passes from 4 to 7 led to reduced layer thickness, enhanced interfacial bonding, and the formation of wavy interfaces due to localised plastic instabilities and shear banding. After annealing, diffusion across Ti/Ni interfaces produced distinct intermetallic layers of Ni₃Ti, NiTi, and NiTi₂, whose thickness increased with temperature and time. Microhardness values of Ti and Ni layers increased from 148 and 170 HV (initial) to 201 and 221 HV after seven passes, while the NiTi and NiTi₂ intermetallics exhibited hardness values of 350 HV and 410 HV, respectively. The composite annealed at 800 °C for 8 h after seven passes achieved a maximum tensile strength of 560 MPa and elongation of 11%, increasing to 14.7% at 900 °C. Here, the maximum tensile strength refers specifically to the annealed condition, highlighting the effect of annealing on optimising the mechanical performance of the composites. These improvements are attributed to grain refinement, strain hardening, and the formation of ductile NiTi phases.