In this study, the rate of dislocation accumulation in the tensile strained twinning induced plasticity (TWIP) steel was calculated via the X-ray diffraction (XRD) measurements and compared with ...other fcc metals and alloys. The results indicated that the XRD technique is an alternative method to estimate the dislocation density. Moreover, flow stress analysis of Fe–31Mn–3Al–3Si TWIP steel with the grain size of about 18
μm indicated that, beside a direct effect of the dislocation interactions on the flow stress, another strengthening mechanism is also required to describe the flow behavior. For this reason, the strengthening contribution due to the formation of mechanical twins was considered as a reduction of dislocation mean free path. Interestingly, the estimated flow stress equation consisting of the strengthening effects of both dislocation interactions and dynamic microstructure refinement due to mechanical twinning (i.e., the dynamic Hall–Petch effect) are in good agreement with the experimental data and equation proposed by Ludwigson for low SFE materials.
The present study investigated the tensile deformation behavior of quenched and tempered martensite steels at various austenitization and tempering temperatures using in situ neutron diffraction ...experiments. Phase lattice strains in the bcc and cementite (θ) phases and dislocation structures in the bcc phase were analyzed. The phase lattice strain in bcc (εphasebcc) became almost stagnant after yielding as the tempering temperature increased. The phase lattice strain in θ increased linearly with an increase in the flow stress, independent of the austenitization and tempering temperatures. The stress partitioning between bcc and θ was confirmed after the yielding of bcc, which contributed to the work hardening. The phase stresses of bcc and θ and their stress partitioning improve the mechanical properties of martensite steels, which can be summarized by the systematic changes in phase lattice strain and dislocation properties due to the austenitization and tempering temperatures.
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High manganese austenitic steels with various mean grain sizes (1.8, 7.2, 49.6
μm) were tensile tested and their twinning behaviors were studied. The fine-grained steel (
d
=
1.8
μm) showed high ...strength with adequate ductility. Deformation twinning was strongly inhibited by grain refinement. These observations suggest that the reason for the large ductility lies on not only the twinning but also in the suppressed dynamic recovery due to low stacking fault energy.
Simple cold-rolling and annealing of martensite starting structure can produce ultrafine grained structure in carbon steel. The microstructural evolution during the process was studied in a 0.13%C ...steel. The ultrafine lamellar dislocation cells (LDCs) with mean thickness of 60 nm were mainly observed in a 50% cold-rolled specimen as well as the irregularly bent lamellas (IBLs) and the kinked laths (KLs). The LDCs and the IBLs had large local misorientations. The specimens annealed at temperatures from 723 to 773 K showed the multiphased ultrafine structure composed of equiaxed ultrafine ferrite grains with the mean grain size of 180 nm, nano-carbides distributed uniformly and small blocks of tempered martensite. The formation of the ultrafine grained structure was discussed from the viewpoint of characteristics of the martensite starting structure. It was concluded that the fine grained structure of martensite play an important role for ultrafine grain subdivision during plastic deformation.
Submerged arc weld metal of a high strength low alloy steel was subjected to friction stir welding (FSW) at a higher rotation rate of 400rpm (FSW-a) and a lower rotation rate of 125rpm (FSW-b), ...respectively. The microstructures and mechanical properties of three typical phase structures, coarse bainite phase in the weld metal, refined bainite phase and ferrite phase in the nugget zones (NZs) of FSW joints were investigated. Compared to the weld metal, enhanced mechanical properties were achieved in the NZs of both FSW joints. Large cracks apparently propagated along the bainite lath in the coarse grains of the weld metal, which would cause the brittle quasi-cleavage fracture. However, large crack propagation was inhibited in the refined bainite phase structure in the NZ of FSW-a joint, and enhanced strength and toughness with dimple fracture were achieved. Meanwhile, enhanced mechanical properties, including strength and ductility, as well as toughness, were obtained in the NZ of FSW-b joint, because of the refinement of the ductile ferrite structure.
Nanoscale crystallographic features of ultrafine grained interstitial free steel fabricated by accumulative roll-bonding (ARB) process have been studied by electron back-scattered diffraction in ...field-emission type scanning electron microscope. This work has clearly indicated that most of the elongated ultrafine grains in the ARB processed sheet are surrounded by high-angle grain boundaries. The characteristic textures in the ARB processed sheet were also clarified.
Strain-rate and temperature dependences of deformation behavior of ultrafine-grained (UFGed) AZ61Mg alloy were examined. For this purpose, AZ61Mg alloy specimens were subjected to multi-directional ...forging (MDFing) under decreasing temperature conditions to have various grain sizes. The average grain sizes attained by MDFing to 1, 3, 6 and 9 passes were approximately 8, 2, 0.5 and 0.3
μ
m, respectively. A superior balance of the mechanical properties of strength and ductility at room temperature was achieved by MDFing to three passes and over. The strain-rate sensitivity of mechanical properties of the MDFed specimens increased with decreasing grain size. The specimens with grain sizes of 0.5 and 0.3
μ
m (0.5 or 0.3 specimen) exhibited stronger strain-rate and temperature dependences of total elongation than those with grain sizes of 8 and 2
μ
m (8 or 2 specimen). This can be partially ascribed to grain-boundary sliding, since an AFM observation revealed the occurrence of room-temperature grain-boundary sliding in the 0.3 specimen. The activation volume
V
* for the 8, 2 and 0.5 specimen increased with increasing temperature, while the 0.3 specimen exhibited an inverse temperature dependence of
V
*. This suggests a change in deformation mechanism with decreasing grain size as well as the occurrence of grain-boundary sliding.
The effects of cold rolling reduction and annealing temperature on the mechanical properties of twinning induced plasticity (TWIP) steel have been investigated. The results indicated that the ...strengthening effect of unrecrystallized areas with a high density of nano-scale mechanical twins increased with increasing cold rolling reduction. In addition, the ductility also increased with increasing annealing temperature. Therefore, utilization of large cold rolling reduction and subsequently annealing treatment in the partial recrystallization region was suggested as an effective method to obtain submicron grained TWIP steel with an excellent combination of strength and ductility.
70/30 (Cu/Zn) brass plates with a 2 mm thickness were jointed by repeated rapid cooling friction stir welding. The joints from each FSW cycle showed the typical construction of a microstructure which ...includes the stir zone and the thermo-mechanically affected zone, but the morphology and boundary characteristics in these zones changed with the different cycles. In the stir zone, the grain size decreased and the number fraction of the high angle boundaries increased with the increasing number of FSW cycles. The texture analysis suggested that the post-annealing effect, which frequently occurred after the FSW process, was remarkably restricted by the liquid CO
2
cooling, which accelerated the refinement of the microstructure. As a result, a joint with an ultrafine grained structure (0·8 μm) and an excellent strength ductility matching (548 MPa and 34% respectively) can be achieved by multi-pass rapid cooling FSW process.
A new process to obtain ultrafine grained bulk steel was developed. Plain low-carbon steel sheet with martensite starting microstructure was simply cold-rolled by 50% and annealed. The specimens ...annealed at intermediate temperatures such as 773 K revealed the multiphased nano-structure and showed superior mechanical properties.
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