IntroductionAustenitic stainless steel has good corrosion resistance and oxidation resistance, but its strength is less than 300 MPa, which greatly limits the application of austenitic stainless steel in industry. At present, it is an effective measure to strengthen austenitic stainless steel by plastically straining the grain size to submicrons or even nanometers. However, strain hardening and strength uniformity are greatly reduced due to high density dislocations accumulating at the twin boundaries and within small grains. At present, the mechanism of fracture toughening produced by nano twin bundles is still unclear.Recently, Professor Lu Lei (Corresponding author) of the Shenyang Institute of Metals published the latest research result “Fracture behavior of heterogeneous nanostructured 316L austenitic stainless steel with nanotwin bundles” in Acta Materialia. In this article, the researchers tested the fracture toughness of nano twinned 316L stainless steels annealed at different temperatures and different plastic strains, revealing the toughening mechanism of nano twinning in nanocrystalline matrices against damage and finding the most suitable heat treatment process. , so that its strength and toughness get the best match.Figure 1 Schematic diagram of specimens used for fracture toughness and tensile testsFigure 2 TEM image of DPD 316L stainless steel(a) DMD 316L stainless steel cross section TEM image with ε = 1.6(b) Nano-sized deformed twins(c) Elongated nano twin matrixFig.3 Cross-section TEM image of DPD 316L stainless steel with ε=1.6 for 20 min annealing at 720°CFigure 4 fracture toughness(a) Load-displacement curves of untreated DPD 316L stainless steel at different plastic strains(b) Load-displacement curves of DPD 316L stainless steel annealed at different temperatures for ε = 1.6(c) The corresponding J-integral-crack opening curve in Fig. (a)(d) The corresponding J-integral-crack opening curve in Fig. (b)Figure 5 SEM image of fracture surface of DPD 316L stainless steel specimen(a) ε=0.4(b) ε=1.6(c) ε = 1.6, 710 °C annealing 20minFigure 6 fracture morphology analysisWhen (a,b)ε=1.6, the fracture surface of two parts of fractured part is in the same position.(c,d) CLSM diagram corresponding to (a,b)Fig.7 Crack tip appearance of DPD 316L stainless steel with ε=1.6(a) Morphology of crack tip of DPD 316L stainless steel with ε = 1.6(b) Enlarged view of box b in Figure (a)(c) Enlarged view of box c in Figure (a)Fig. 8 Schematic diagram of crack propagation(a) Nucleation of vacancies and growth in nanocrystalline matrices(b) Cracks surround the nanotitanium beam and the nanotwinned beam obstructs crack propagation(c) Nano twin bundles are pulled, and vacancies nucleate at their tip(d) Produce shear cracks at a distance from the nano twin bundles and eventually leave the nano twin bundles(e) Dimple-shaped section where the fracture surface is concave and convexFig. 9 Fracture toughness and yield strength curvesSummaryThe nano twin strands play an important role in suppressing the formation of vacancies in the nanocrystalline matrix and improving the mechanical properties. At the same time, the nano twin strands can suppress crack propagation and greatly increase the fracture resistance. Through the annealing treatment, the variable coarse nanocrystal grains transform into recrystallized grains or recrystallized grains, and the resulting nano twin beam can improve the toughening effect. The yield strength of nano twinned steel can reach 1 GPa, and the fracture toughness is about 140 MPa m1/2.Reference: Fracture behavior of heterogeneous nanostructured 316L austenitic stainless steel with nanotwin bundles (Acta Materialia, 2018, doi.org/10.1016/j.actamat.2018.02.065).
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