In-situ study of the deformation-induced rotation and transformation of retained austenite in a low-carbon steel treated by the quenching and partitioning process
“…Moreover, interface plasticity driven by sliding of substructure boundaries has been discussed in recent studies 14–16 , and while the corresponding microstructural mechanisms (in particular dislocation-interface interactions) are not fully identified yet, it has been argued 14 that (film-like) austenite-martensite interfaces between individual martensite laths can also contribute significantly to an accommodation of plastic strains in the context of interface-plasticity. It was also recently shown that retained austenite at triple junctions transforms into martensite at an early deformation stage, whereas interlath retained austenite remains stable up to larger plastic strains 25 . The contribution of interlath retained austenite to strain-rate sensitive ductility, as first observed in the present study, is further discussed in the remainder of this paper.Figure 3Microstructure of Q&P heat treated steel.…”
We investigate an extraordinarily high ductility in a low alloy carbon steel at an elevated temperature after a quenching and partitioning (Q&P) treatment. The conventional (quenched and tempered) reference material does not show similar behavior. Interestingly, the Q&P treated material’s ductility is considerably reduced at increasing strain rates while strength remains almost constant. These results indicate the presence of a diffusion-controlled deformation mechanism at elevated temperatures. Our research shows that interlath retained austenite is more stable during deformation at higher temperatures, resulting in a delayed transformation to martensite and therefore to a more pronounced contribution to plastic deformation at (and in the vicinity of) the many interfaces inherently present in this multi-phase steel.
“…Moreover, interface plasticity driven by sliding of substructure boundaries has been discussed in recent studies 14–16 , and while the corresponding microstructural mechanisms (in particular dislocation-interface interactions) are not fully identified yet, it has been argued 14 that (film-like) austenite-martensite interfaces between individual martensite laths can also contribute significantly to an accommodation of plastic strains in the context of interface-plasticity. It was also recently shown that retained austenite at triple junctions transforms into martensite at an early deformation stage, whereas interlath retained austenite remains stable up to larger plastic strains 25 . The contribution of interlath retained austenite to strain-rate sensitive ductility, as first observed in the present study, is further discussed in the remainder of this paper.Figure 3Microstructure of Q&P heat treated steel.…”
We investigate an extraordinarily high ductility in a low alloy carbon steel at an elevated temperature after a quenching and partitioning (Q&P) treatment. The conventional (quenched and tempered) reference material does not show similar behavior. Interestingly, the Q&P treated material’s ductility is considerably reduced at increasing strain rates while strength remains almost constant. These results indicate the presence of a diffusion-controlled deformation mechanism at elevated temperatures. Our research shows that interlath retained austenite is more stable during deformation at higher temperatures, resulting in a delayed transformation to martensite and therefore to a more pronounced contribution to plastic deformation at (and in the vicinity of) the many interfaces inherently present in this multi-phase steel.
“…To understand and elucidate these micro-mechanisms, in situ straining experiments were performed where EBSD diffraction was used to probe select regions of specimens at incremental strains. This technique allows us to probe the evolution of crystallographic features associated with plastic deformation and unravel the strain hardening mechanisms 13 , 34 , 35 .…”
While there exists in nature abundant examples of materials with site-specific gradients in microstructures and properties, engineers and designers have traditionally used monolithic materials with discrete properties. Now, however, additive manufacturing (AM) offers the possibility of creating structures that mimic some aspects of nature. One example that has attracted attention in the recent years is the hierarchical structure in bamboo. The hierarchical architecture in bamboo is characterized by spatial gradients in properties and microstructures and is well suited to accommodate and survive complex stress states, severe mechanical forces, and large deformations. While AM has been used routinely to fabricate functionally graded materials, this study distinguishes itself by leveraging AM and physical metallurgy concepts to trigger cascading deformation in a single sample. Specifically, we have been successful in using AM to fabricate steel with unique spatial hierarchies in structure and property to emulate the structure and deformation mechanisms in natural materials. This study shows an improvement in the strength and ductility of the nature-inspired “hierarchical steel” compared with conventional cast stainless steels. In situ characterization proves that this improvement is due to the sequential activation of multiple deformation mechanisms namely twinning, transformation-induced plasticity, and dislocation-based plasticity. While significantly higher strengths can be achieved by refining the chemical and processing technique, this study sets the stage to achieve the paradigm of using AM to fabricate structures which emulate the flexibility in mechanical properties of natural materials and are able to adapt to in-service conditions.
“…The deformation behaviors of RA and BF were dissimilar because of their different properties. The strain at the RA and BF can be well characterized by kernel average misorientation (KAM) [ 22 , 23 , 24 , 25 ]. A high KAM value represents a large deformation.…”
The microstructure of carbide-free bainitic steel has been widely reported because of its excellent mechanical properties. However, no work has explored the deformation behavior of each phase in the microstructure, namely, retained austenite (RA) and bainitic ferrite (BF). In this paper, the deformation behavior of RA and BF during tension was investigated through the in situ electron backscatter diffusion method. The results showed that the RA surrounded by BF with a high Schmid factor (SF) was accelerated towards being transformed into martensite, but the surrounding BF with low SF value retarded the transformation. The kernel average misorientation (KAM) findings revealed that the strain of the RA and BF rapidly increased initially, slowed down after the critical points of 0.25 and 0.54, respectively, and then increased again. The calculated result for the KAM fraction indicated that the contribution of RA to the total plastic deformation gradually decreased throughout the total tensile process, whereas that of BF increased in the presence of RA.
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