Abstract
Tensile mechanical properties of fully recrystallized TWIP steel specimens having various grain sizes (d) ranging from 0.79 μm to 85.6 μm were investigated. It was confirmed that the UFG specimens having the mean grain sizes of 1.5 μm or smaller abnormally showed discontinuous yielding characterized by a clear yield-drop while the specimens having grain sizes larger than 2.4 μm showed normal continuous yielding. In-situ synchrotron radiation XRD showed dislocation density around yield-drop in the UFG specimen quickly increased. ECCI observations revealed the nucleation of deformation twins and stacking faults from grain boundaries in the UFG specimen around yielding. Although it had been conventionally reported that the grain refinement suppresses deformation twinning in FCC metals and alloys, the number density of deformation twins in the 0.79 μm grain-sized specimen was much higher than that in the specimens with grain sizes of 4.5 μm and 15.4 μm. The unusual change of yielding behavior from continuous to discontinuous manner by grain refinement could be understood on the basis of limited number of free dislocations in each ultrafine grain. The results indicated that the scarcity of free dislocations in the recrystallized UFG specimens changed the deformation and twinning mechanisms in the TWIP steel.
Highlights
High-manganese austenitic steels, having austenite single phase with face-centered cubic (FCC) structure at ambient temperature, have attracted a great attention as one of the advanced high strength steels for nextgeneration automotive applications, because of their good balance of high strength and large ductility
It is well known that coarse-grained metallic materials having FCC structure usually exhibit continuous yielding, whereas metals having body-centered cubic (BCC) structure show discontinuous yielding accompanying with yield-drop phenomena due to locking and unlocking of dislocations by interstitial solute elements, such as carbon and nitrogen[32]
We have tried to clarify the nature of discontinuous yielding in fully recrystallized high-Materials. A 31Mn-3Al-3Si steel (Mn) austenitic twinning induced plasticity (TWIP) steel through systematic investigations using digital image correlation (DIC) during tensile deformation, electron channeling contrast imaging (ECCI) in a scanning electron microscope (SEM), and in-situ synchrotron radiation X-ray diffraction during tensile tests
Summary
High-manganese austenitic steels, having austenite single phase with face-centered cubic (FCC) structure at ambient temperature, have attracted a great attention as one of the advanced high strength steels for nextgeneration automotive applications, because of their good balance of high strength and large ductility. It was noteworthy that the fully recrystallized UFG high-Mn steels exhibited very high yield strength as expected and unexpectedly discontinuous yielding accompanied with yield-drop phenomena in spite of their FCC structure. It is well known that coarse-grained metallic materials having FCC structure usually exhibit continuous yielding, whereas metals having body-centered cubic (BCC) structure (typically iron and carbon steels) show discontinuous yielding accompanying with yield-drop phenomena due to locking and unlocking of dislocations by interstitial solute elements, such as carbon and nitrogen[32]. When the average grain size is reduced below about 1–2 μm, even IF steels show the discontinuous yielding characterized by yield-drop and subsequent Lüders d eformation[42] These results indicate that the discontinuous yielding is a unique mechanical response universally occurring in UFG materials. We have tried to clarify the nature of discontinuous yielding in fully recrystallized high-Mn austenitic TWIP steel through systematic investigations using digital image correlation (DIC) during tensile deformation, electron channeling contrast imaging (ECCI) in a scanning electron microscope (SEM), and in-situ synchrotron radiation X-ray diffraction during tensile tests
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