Abstract:The grain boundary misorientation distributions associated with the development of dynamic recrystallization were studied in a high-nitrogen austenitic stainless steel subjected to hot working. Under conditions of discontinuous dynamic recrystallization, the relationships between the grain or subgrain sizes and flow stresses can be expressed by power law functions with different grain/subgrain size exponents of about −0.76 (for grain size) or −1.0 (for subgrain size). Therefore, the mean grain size being much larger than the subgrain size under conditions of low flow stress gradually approaches the size of the subgrains with an increase in the flow stress. These dependencies lead to the fraction of high-angle boundaries being a function of the flow stress. Namely, the fraction of ordinary high-angle boundaries in dynamically-recrystallized structures decreases with a decrease in the flow stress. On the other hand, the fraction of special boundaries, which are associated with annealing twins, progressively increases with a decrease of the flow stress.
The deformation microstructures and mechanical properties were studied in a high-Mn steel subjected to hot compression. The deformation microstructures resulted from the development of dynamic recrystallization (DRX). Two DRX mechanisms, namely discontinuous and continuous, operated during warm-to-hot working. Under the conditions of hot working when the flow stresses were below 100 MPa, a power law function was obtained between the DRX grain size and the true flow stress with a grain size exponent of −0.8 owing to the discontinuous DRX. On the other hand, the gradual change in the operating DRX mechanism from a discontinuous to continuous one upon a transition from hot to warm working, when the true flow stress increases above 100 MPa, resulted in the grain size exponent of about −0.5 in the power law between the flow stress and the DRX grain size. The DRX microstructures developed by warm-to-hot working provide a beneficial combination of mechanical properties including high ultimate tensile strength in the range of 700–900 MPa and sufficient ductility with a uniform elongation well above 50%. The strengthening of the samples with DRX microstructures was attributed to the combined effect of the grain size and dislocation strengthening resulting in a rather high grain boundary strengthening factor of 570 MPa μm0.5 in the Hall-Petch-type relationship.
The grain refinement and strengthening of an austenitic stainless steel are studied under multiple multidirectional forging at 1073 K up to strains of 4. The structural changes are characterized by the development of discontinuous and continuous dynamic recrystallization (DRX) leading to the grain refinement down to submicrometer level. The new ultrafine grains develop primarily along original grain boundaries and deformation microbands, leading to heterogeneous necklace-like microstructures at intermediate strains followed by rapid expansion of the DRX grains occupying the whole worked sample at large strains. The change in the fraction of ultrafine grains is commonly characterized by a sigmoid-type dependence on strain and can be expressed by a modified Jonson-Mehl-Avrami-Kolmogorov equation. The DRX development is accompanied by a stepwise decrease in the flow stress at reloading in each subsequent forging pass especially in intermediate strains. This stepwise softening results from rapid growth of freshly nucleated grains. In contrast, after forging, both the yield strength and ultimate tensile strength at room temperature increase progressively with an increase in the number of forging passes. The strengthening during DRX development can be attributed to concurrent contribution of work-hardening and grain refinement and can be expressed by a summation of recovered and recrystallized fractions.
The effects of temperature and degree of tempforming deformation on the microstructure and impact toughness of high-strength low-alloy 25KhGMT steel have been considered. Tempforming forms a lamellar microstructure composed of grains and subgrains that are strongly elongated along the rolling direction. The average size of the grain section is 570-790 nm. Deformation texture includes 001 || ND and 111 || ND fibers. Tempforming increases the fracture work of this steel at lower test temperatures (KV -40°С ≥ 360 J) due to the delamination of the specimen perpendicular to the impact direction, which prevents crack propagation towards the direction of the impact.
The paper reports on the features of low-temperature superplasticity of the heat-treatable aluminum Al-Mg-Si alloy in the ultrafine-grained state at temperatures below 0.5 times the melting point as well as on its post-deformation microstructure and tensile strength. We show that the refined microstructure is retained after superplastic deformation in the range of deformation temperatures of 120–180 °C and strain rates of 5 × 10–3 s–1–10–4 s–1. In the absence of noticeable grain growth, the ultrafine-grained alloy maintains the strength up to 380 MPa after SP deformation, which considerably exceeds the value (250 MPa) for the alloy in the peak-aged coarse-grain state. This finding opens pathways to form high-strength articles of Al-Mg-Si alloys after superplastic forming.
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