Characterization of the TRIP/TWIP effect in austenitic stainless steels using Stress‐Temperature‐Transformation (STT) and Deformation‐Temperature‐Transformation (DTT) Diagrams
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Abstract Stress‐Temperature‐Transformation (STT) and Deformation‐Temperature‐Transformation (DTT) diagrams are suitable to characterize the TRIP (transformation‐induced plasticity) and TWIP (twinning‐induced plasticity) effect in steels. The triggering stresses for the deformation‐induced microstructure transformation processes, the characteristic temperatures, the yield stress and the strength of the steel are plotted in the STT diagram as functions of temperature. The elongation values of the austenite, the strain‐induced twins and martensite formations are shown on the DTT diagram. The systematics of STT and DTT diagrams and the method for their development are described. Especially, the correlations between the STT and DTT diagrams and the thermodynamics are explained in the present paper. The developed STT and DTT diagrams for a novel austenitic Cr‐Mn‐Ni (16%Cr, 6% Mn, 6% Ni) as‐cast steel and austenitic steel AISI 304 are presented. The Cr‐Mn‐Ni steel shows a deformation‐induced α′‐martensite and twin formation. In contrast, the AISI 304 shows a deformation‐induced ε‐ and α′‐martensite formation. The differences between both steel grades are based on thermodynamic pre‐conditions. Therefore the thermodynamic stability conditions of the phases and the kinetics of stress and deformation‐induced martensite and the twins formation are reflected in the developed STT and DTT diagrams.Keywords:
Twip
Diffusionless transformation
The deformation twinning behavior of Fe18Mn0.6C and Fe18Mn0.6C-2.5Al twinning-induced plasticity steels was compared by in-situ electron backscattering diffraction. Al suppressed deformation-induced twinning. A constitutive model considering the effect of Al on the twin formation kinetics was used to show that the work hardening and the ultimate tensile strength were lowered by the suppression of deformation twinning despite the pronounced solution hardening contribution of Al.
Twip
Work hardening
Hardening (computing)
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Isothermal process
Carbon fibers
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Twip
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Matrix (chemical analysis)
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In this paper we investigated the microstructure and phase transition of atomized Fe-(1.3, 7.9, 11.7 wt.%) Mn alloy powders. The results show that the main phases of Fe-1.3Mn, Fe-7.9Mn and Fe-11.7Mn powders are ferrite, α’-martensite+austenite, α’-martensite+ε-martensite+austenite, respectively. The δ-ferrite in the Fe-1.3Mn powder is the high-temperature δ-ferrite directly formed from liquid, companying by a small number of nanometer sized austenite particles precipitated from the ferrite matrix. In the Fe-11.7Mn powder, the γ-austenite, ε-martensite and α’-martensite are found in the same region and have the K-S orientation relationship, suggesting phase transitions of γ-austenite → ε-martensite → α’-martensite and γ → α’-martensite.
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The stabilization of austenite during isothermal holding above Ms has been investigated using 1%-carbon, 5%-nickel steel. The stabilization increases with the amount of martensite present, but it is established that the martensite is not an important factor in stabilization. It has been found that actually the presence of martensite is not even necessary for the stabilization to occur. Hence stabilization is possible without prior decomposition of austenite either into bainite or martensite and seems to reflect some internal rearrangement occuring in the parent phase during retarded cooling or isothermal holding.
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Deformation twinning behavior in Fe-17Mn-0.6C, Fe-17Mn-0.8C, and Fe-18Mn-1.2C (wt.%) twinning-induced plasticity (TWIP) steels was investigated by atomic force microscopy (AFM) and electron backscatter diffraction pattern (EBSD) analyses. The AFM-based surface relief analysis combined with the EBSD measurements was employed to determine active twinning direction as well as deformation twin fraction in specific crystallographic orientations. A carbon addition is known to increase the stacking fault energy; however the deformation twin fraction in the <144> tensile orientation did not change against carbon concentration. On one hand, the <111> tensile orientation grains showed suppression of deformation twinning with increasing carbon concentration. These results imply that another factor in addition to the stacking fault energy-based criteria is required to interpret the deformation twinning behavior of carbon-added TWIP steels.
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Stacking-fault energy
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두 種類의 martensite合金, 즉 Fe-C基 martensite鋼(Fe-1.7%C)과 Fe-Ni基 martensite合金(Fe-27% Ni-0.14%C)을 마련해서, 이 두 martensite 조직 중에 含有된 잔류 austenite의 tempering 擧動을 X-線的으로 調査하여 다음과 같은 結論을 얻었다.
1. Fe-1.7%C martensite 鋼의 잔류 austenite는 約 150℃×1hr tempering에서 分解하기 시작하였으며 280℃×1hr tempering에서는 거의 大部分 分解하였다.
2. Fe-1.7%C martensite 鋼中의 殘留 austenite의 (111)γ 回折線의 積分幅은 tempering 溫度와 더불어 增加하였다.
3. Fe-27% Ni-0.14%C martensite 鋼은 430℃까지 tempering하여도 殘留 austenite가 分解하지 않았다.
4. Fe-27% Ni-0.14%C martensite 鋼에 있어서 austenite의 積分幅은 360℃ tempering 까지는 거의 一定한 값을 나타내다가 그 以上 溫度가 增加함에 따라 減少하였다.
Tempering
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Twip
Work hardening
Hardening (computing)
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