Transformation behavior of TRIP steels
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True-stress (σ), true-strain (ε) and volume fraction martensite(f) were measured during both uniform and localized flow as a function of temperature on TRIP steels in both the solution-treated and warm-rolled conditions. The transformation curves(f vs ε) of materials in both conditions have a sigmoidal shape at temperatures above Ms σ (maximum temperature at which transformation is induced by elastic stress) but approach initially linear behavior at temperatures below Ms σ where the flow is controlled by transformation plasticity. The martensite which forms spontaneously on cooling or by stress-assisted transformation below Ms σ exhibits a plate morphology. Additional martensite units produced by strain-induced nucleation at shear-band intersections become important above Ms σ. Comparison of σ-ε andf-ε curves indicate that a “rule of mixtures” relation based on the “static” strengthening effect of the transformation product describes the plastic flow behavior reasonably well above Ms σ, but there is also a dynamic “transformation softening” contribution which becomes dominant below Ms σ due to the operation of transformation plasticity as a deformation mechanism. Temperature sensitivity of the transformation kinetics and associated flow behavior is greatest above Ms σ. Less temperature-sensitive TRIP steels could be obtained by designing alloys to operate with optimum mechanical properties below Ms σ.
KeywordsAustenite Martensite Metallurgical Transaction Trip Steel Stable Austenite
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