Room-temperature nanoindentation creep behavior of ultrafine-grained Ti-6Al-4V alloy processed by multi-directional isothermal forging

Ultrafine-grained (UFG) Ti-6Al-4V alloy (referred to as Ti64) was processed by multi-directional isothermal forging (MDIF). The average grain size of the as-received Ti64 was refined from 3.66μm to 0.15μm and 0.14μm after 9 and 12 deformation passes, respectively. Nanoindentation creep tests were performed at room temperature under peak loads of 9, 18, and 27 mN to systematically investigate the creep behavior of both the as-received and UFG conditions. Both alloys exhibited creep deformation under all tested loads, with maximum creep displacement increasing with load. The creep response was dominated by the primary stage. Stress-exponent analysis of the steady-state stage indicated that dislocation motion governs creep in both alloys. After threshold-stress correction, the effective stress exponent n eff (4.293–4.431) falls within the characteristic range for dislocation creep, whereas the uncorrected apparent exponent n app (13.8–113.1) is anomalously high, confirming that the threshold-stress correction removes the artifact of inflated apparent exponents. The n eff values remain within the dislocation-creep regime across all conditions; their minor variations correlate with the high fraction of high-angle grain boundaries (HAGBs > 80%) and the increased number of grains sampled within the plastic zone of the UFG alloy, suggesting that dislocation–grain-boundary interactions may play a more pronounced role in the UFG condition.