EFFECT OF ORIGINAL STRUCTURE ON AGING-INDUCED MICROSTRUCTURE AND TRANSFORMATION BEHAVIOR OF Ni-RICH NITI ALLOY USING VARIOUS AGING MODEs

Abstract

In the present study we discuss the evolution of aging-induced microstructure, critical temperatures of martensitic transformations (MTs) and their specific temperature ranges, latent heat and thermal hysteresis in Ni50.8Ti alloy with various original structures (OS) obtained using three different processing (including cold rolling/drawing, hot shear rolling and subsequent various solution treatment): fine-grained recrystallized structure (OS_1); hot deformation-induced structure (OS_2); coarse-grained recrystallized structure (OS_3). Subsequent aging was performed in a temperature range of 300−500 °C for 1−20h. After aging, OS_1 ensures two-stage MTs A→R and R→M upon cooling regardless of aging mode; OS_2 provides evolution from one-stage MT A→R to triple-stage MT at latest stage of aging; the quadruple-stage MT is revealed in OS_3 after aging at 430°C. Low-temperature aging at 300 °C ensures the most effective stabilization of R-martensite regardless of an original structure; high-temperature aging at 500 °C provides the least effective stabilization of R-martensite. Specific features of evolution of the latent heat and the width of hysteresis vs aging modes was explained by microstructural evolution. The correlation between values in the latent heats upon cooling/heating and hysteresis vs the aging duration were revealed. Hysteresis of an original structure is inherited after subsequent aging. The combined effect of an original structure and aging mode on the above characteristics is very complex and depends on a lot of competing factors; they are analyzed in detail. The revealed regularities permit precise tuning of the studied characteristics in a wide range to ensure the most favorable their combination for practical use.