Variation of stacking fault energy with austenite grain size and its effect on the MS temperature of γ→ε martensitic transformation in Fe–Mn alloy

It is increasingly important in the context of high-manganese steels of the kind that lead to twinning-induced plasticity to be able to estimate the temperature at which ε-martensite forms when austenite is cooled. We find that the thermodynamic method used in similar calculations for α ′ martensite cannot in many cases be implemented because of apparently imprecise thermodynamic data, a conclusion partly validated using limited first-principles calculations. Alternative, empirical methods are also evaluated. The austenite grain size dependence of the martensite-start temperature has also been rationalised in terms of existing theory for α ′ martensite. Experiments have also been conducted to show that the problem in dealing with the ε-martensite does not lie in the precision with which the transformation can be measured using dilatometry.

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“…There exist three sets of experiments where ε-martensite has been studied as a function of the austenite grain size [29,39,62]. Fig.…”

Section: Austenite Grain Size Effectmentioning

confidence: 99%

Critical assessment: Martensite-start temperature for the γ→ε transformation

Yang

Jang

Bhadeshia

et al. 2012

Calphad

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“…[1][2][3][4][5][6][7][8][9][10][11] The SFE of these steel grades increases intensively as a result of increases in the carbon [12] and aluminum [13,14] contents as well as the temperature, [15,16] while it decreases as a result of increases in the austenite grain size. [17,18] The effect of manganese (>15 wt pct) on increases in the SFE has been debated in several research articles. [2,5,16,19] The decrease in the fraction of martensite caused by decreasing the manganese content in as-quenched low-carbon steels, as described by Tomota et al, [20] indirectly confirms the aforementioned SFE variations.…”

Section: Introductionmentioning

confidence: 99%

“…[27][28][29][30][31] Although the empirical estimation of SFE values based on the chemical composition of steels has been used by Schramm and Reed, [32] Brofmann and Ansell, [12] Vercammen et al, [33] and Bracke et al, [9] the calculation of SFE through the regular solution thermodynamic model is still the most widely used method to judge the properties of high-manganese steels. [5,7,13,14,17,18,21,24,25,[34][35][36] In contrast to thermodynamics-based models, which do not consider the atomistic character of the stacking fault, ab-initio techniques such as density functional theory [37][38][39][40] have the ability to clearly simulate the irregularities in the stacking of atomic layers. [30] The next-nearest-neighbor model [41] has been successfully used for the austenitic stainless steels to decrease the computational complexity.…”

Section: Introductionmentioning

confidence: 99%

Derivation and Variation in Composition-Dependent Stacking Fault Energy Maps Based on Subregular Solution Model in High-Manganese Steels

Saeed-Akbari

Imlau

Prahl

et al. 2009

Metall Mater Trans A

639

272

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A subregular solution thermodynamic model was used to calculate the stacking fault energies (SFEs) of high-manganese (10 to 35 wt pct) steels with carbon contents of 0 to 1.2 wt pct. Based on these calculations, composition-dependent diagrams were developed showing the regions of different SFE values for the mentioned composition range. These diagrams were called SFE maps. In addition, variations in the SFE maps were observed through increasing the temperature, aluminum content, and austenite grain size. These changes were seen either as an increasing trend of SFE caused by raising the temperature and aluminum content, or as a decreasing behavior caused by increasing the grain size. The SFE value of 20 mJ/m 2 within these diagrams was introduced as the upper limit for the strain-induced martensite formation. The variations in this limit caused by increasing the temperature and aluminum content were mathematically evaluated to find out the minimum amount of manganese that was required to avoid the martensitic transformation. By introducing the isocarbon and isomanganese diagrams of the SFE, it was seen that both temperature and aluminum had a greater effect on the SFE when added to the steels with the lower manganese contents. Moreover, by adding more aluminum to the composition of the high-manganese steels, its influence on the SFE decreased continuously.

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“…Durante o resfriamento, houve a transformação de alguns grãos de austenita, os maiores, em bainita e martensita. Isto pode ser atribuído à estabilização química dos menores grãos de austenita, que possuem maior teor de carbono e, portanto, menor temperatura M S [38]. Além disso, a deformação moderada nesta região pode ter contribuído para a estabilização mecânica da austenita: grãos menores possuem maiores densidades de discordâncias, com maiores energias de falha de empilhamento associadas, resultando em menor força-motriz para a transformação martensítica [39].…”

Section: Resultsunclassified