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The current work studies the correlations between microstructure and retained austenite (RA) transformation, in a single-quenched and partitioned (Q&P) 1180 steel microstructure, through in situ tensile tests combined with electron backscatter diffraction (EBSD) analysis. This allows the study of RA stability across a limited range of morphological characteristics to be studied in the absence of confounding factors introduced by varying the entire steel microstructure. Among the microstructural attributes of interest, RA grain aspect ratio is found to have the largest influence on transformation rate, where globular-shaped grains transform more slowly than those with a more lenticular shape. Furthermore, by tracking individual grains during deformation, it is apparent that larger grains transformed more slowly than smaller grains; a purely statistical study of grain size vs strain might conclude that smaller grains are more stable, but in reality, the smaller grains transform faster and are simply statistically replaced by partially transformed larger grains. These conclusions are in contrast to relationships that might be inferred from previous studies where the entire steel microstructure was varied, along with the morphology of the RA.
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Effect of Strain Path on Retained Austenite Transformation Rates and Material Ductility in Transformation-Induced Plasticity-Assisted Advanced High-Strength Steel
TBF 1180 steel was plastically deformed under different strain paths in order to study both the ductility and RA transformation rates. Specimens were prepared from a 1 mm thick sheet and then tested incrementally under uniaxial tension, plane-strain tension, and biaxial tension. The retained austenite (RA) levels were measured, as a function of the plastic strain, using electron backscatter diffraction (EBSD). The plane-strain tension specimens had the fastest rate of RA transformation as a function of strain, followed by uniaxial tension, and then biaxial tension. The forming limits were measured for each strain path, yielding major limit strains of 0.12 under uniaxial tension, 0.09 under plane-strain tension, and 0.16 under biaxial tension. These results were compared to prior work on a 1.2 mm Q&P 1180 steel sheet, which had a similar yield and ultimate tensile strength, but exhibited slightly greater forming limits than the TBF material. The visual inspection of the micrographs appeared to show an equiaxed RA morphology in the Q&P 1180 steel and a mixture of equiaxed and lamellar RA grains in the TBF 1180 steel. However, the statistics generated by EBSD revealed that both alloys had RA grains with essentially the same aspect ratios. The average RA grain size in the Q&P alloy was found to be about three times larger than that of the TBF alloy. As such, the small but consistent formability advantage exhibited by the Q&P 1180 alloy along all three strain paths can be attributed to its larger average RA grain size, where larger RA grain sizes correlated with a more gradual transformation rate.
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- Award ID(s):
- 2147126
- PAR ID:
- 10627288
- Publisher / Repository:
- MDPI
- Date Published:
- Journal Name:
- Journal of Manufacturing and Materials Processing
- Volume:
- 9
- Issue:
- 3
- ISSN:
- 2504-4494
- Page Range / eLocation ID:
- 75
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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