Papers by Keyword: Deformation Twin

Abstract: The microstructure evolution of the as-rolled Mg-3Zn-0.5Er alloy (wt.%) has been investigated. The results showed that the phase constituents of the sheet were composed of the α-Mg solid solution and the I-phase (Icosahedral quasicrystalline). The microstructure of sheets transformed from the deformation twinning into dynamic recrystallization (DRX) grains. Furthermore, the deformation twinning appeared again in the dynamic recrystallization grains during further hot rolling. It indicated that the twining was the main deformation mechanism and marginal dynamic recrystallization (DRX) occurred in the deformation twins at low strains. As the reduction reached 39.6%, the twin dynamic recrystallization (TDRX) regions cluster and widen by consumption of the initial deformation twins, then formed shear bands. After the reduction exceeded 66.3%, the microstructures of sheets were composed of equiaxed DRXed grains and fine twins. Moreover, compared with the RD-ND plane, the RD-TD plane had a smaller average grain size, ultimately reached ~15.8μm, which also led to a higher average value of the hardness.

Abstract: In this study, a Cu-Sn sintered bronze, used largely for con-rod bushing and automotive transmission, was treated by ultrasonic nanocrystalline surface modification (UNSM). Then, Vickers hardness and microstructural evolution of the treated region were investigated by using scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscope (TEM). The hardness of the treated surface doubled, which is attributed to the developed of nanoscale grains, deformation twins, and high density of dislocations induced by the UNSM. Microstructural modification beneath the UNSM treated surface was typically characterized with increase of the depth: (i) nanoscale grains (top surface), (ii) intersection of deformation twins (~30 μm), (iii) high density nanoscale twin/matrix lamellae (~50 μm), (iv) interception of micro band and deformation twins (~100 μm), (v) dislocation arrays (~200 μm), (vi) low density dislocations (~300 μm) and (vii) pre-existing coarse grains and annealing twins in unaffected region (400 μm ~deeper).

Abstract: The effects of temperature and strain rate on the stress–strain relationship and deformation twinning, during tensile tests, in polycrystalline Fe–5%Si alloy were studied. We performed tensile tests over a wide range of temperatures from 173 to 273 K, and strain rates ranging from 0.0001 to 0.1 /s, to clarify the relationship between total elongation and ratio of deformation twins, and the dependence of deformation twinning on grain orientation. All tensile specimens were fractured in a completely brittle manner. The total elongation decreased as the temperature decreased and the strain rate increased. The presence of deformation twins, in all fractured specimens, was confirmed by scanning electron microscope–electron backscattering diffraction analyses. The area fraction of the deformation twins increased as the total elongation decreased. However, a strong influence from the grain orientation on twinning activity was not observed for all temperatures and strain rates. A previous study on Fe–Si alloy single crystals showed that deformation twins form easily in <001>-oriented single crystals, but not in <111>-oriented single crystals. Our observations, on the dependence of deformation twinning on grain orientation in polycrystalline Fe–5%Si alloy, did not agree with those from single crystals. The present findings suggest that grain orientation does not play an important role in determining the occurrence of deformation twinning; not even in polycrystals. It is believed that the stress concentration, due to piled-up dislocations, during tensile deformation, cannot be relieved by the slip at low temperatures or high strain rates, and thus significantly affects deformation twinning.

Abstract: The effect of cold rolling reduction on shear band formation and crystal orientation within shear bands and annealing texture were investigated in Fe-3%Si {111}<112> single crystals. Several types of shear bands were observed with different angles to rolling direction, dependent on rolling reduction. As for shear band formation, those with smaller angles were formed earlier and those with larger angles were formed later. Regarding crystal orientation along shear bands after rolling reduction, orientation distribution from the initial became large in accordance with reduction and even exceeded Goss orientation when rolling reduction became larger than 40%. After annealing, however, recrystallized grains along shear bands were mainly Goss grains regardless of reduction. The speculated reason for the dominance of Goss after annealing is that Goss subgrains with less density of dislocations were surrounded by largely deformed areas.

Abstract: The fracture behavior transition due to the change of strain rate in 5%Si magnetic steel with dislocation microstructures was studied. The Si steel was multi-passed rolled at 800°C to a various reductions up to 50%. The room temperature tensile deformation was conducted at various strain rates from 10^-5/s to 10⁰/s. All rolled steels were fractured in ductile manners with local elongation (necking) at slower strain rate. When strain rate was faster, the local elongation disappeared and the fracture manner was turned to brittle. The strain rate at which fracture mechanism changed from ductile to brittle increased with the increasing of the reduction. On the other hand, the almost fully recrystallized Si steel was fractured in the brittle manner at any strain rate and the transition strain rate was not found. The fractured tensile specimen with no local elongations contains deformation twins; whereas these deformation twins were not observed in the fractured specimen with local elongations. This result indicates that dislocation structure evolved during rolling suppressed the twinning and that the dislocation structure is effective for the enhancement of toughness in Si steel.

Abstract: This paper describes the fatigue crack propagation behavior of extruded AZ31B magnesium alloys (average grain size: approximately 15 and 119 μm, respectively). Fatigue crack propagation tests were performed on center cracked tension (CCT) specimens at a stress ratio of R=0.1 and a frequency of 10 Hz at room temperature. Loading axis was parallel to the extrusion direction; crack face was perpendicular to basal plane of each grain. The crack growth rate (da/dN) of the coarse-grained specimen was approximately 5 times higher than that of the fine-grained specimen. Fracture surfaces of the fine-grained and coarse-grained specimens showed various directional steps independent of macroscopic crack growth direction.

Abstract: Two-phase single-crystal intermetallic alloys composed of Ni₃Al (L1₂) and Ni₃V (D0₂₂) with some orientations were compressed at various temperatures, and their deformation microstructures were observed by transmission electron microscopy (TEM). The deformation at room temperature was governed by the glide motion of dislocations in the primary Ni₃Al precipitates and the activation of the microtwins in the Ni₃V variant structures in the channel regions. The interfaces between the primary Ni₃Al precipitates and the Ni₃V variant structures are suggested to work as the barriers to the dislocation motion. While, at temperature above the peak temperature (873 K), the deformation microstructures of the two-phase intermetallic alloy exhibited the ribbon-like deformation microstructures penetrating the constituent phases i.e. through the interfaces between primary Ni₃Al precipitates and the Ni₃V variant structures in the channel regions. It was also suggested that the superior strength in the two-phase intermetallic alloys is due to the high flow strength of the Ni₃V phases and to the interfacial hardening receiving when the dislocations activated in the primary Ni₃Al precipitates propagate to the channel regions.

Abstract: Deformation-induced martensite preferentially nucleates at the twin boundary between matrix austenite and deformation twin in 316 type stainless steel. In the cold-rolled specimen, the martensite formed at the twin boundary has K-S relationships with both of the austenite matrix and the deformation twin, that is, “double K-S relationship” is realized. While in the case of cold-drawn specimen, two kinds of twins with different twin planes are typically observed, and therefore, the deformation-induced martensites are formed at the intersections of the two deformation twin boundaries, satisfying “triple K-S relationship” among austenite matrix and two deformation twins, although there is a small misfit from the perfect K-S relationship. The complicated crystallographic orientation relationship leads to a strong variant restriction for deformation-induced martensites. Due to the difference in the number of nucleation sites, martensitic transformation is greatly promoted in cold-drawn specimen rather than cold-rolled one.

Abstract: Dual-phase lamellar precipitates can be easily occurred in the austenitic steel valve in heat treatment and use. They affect seriously the performance of the valve at high-temperature, which is one important reason leading to valve failure. The morphology and precipitation location of lamellar precipitates of the 5Cr21Mn9Ni4N (21-4N) exhaust valve were investigated by scanning electron microscopy, and the influencing factors and precipitating mechanisms were analyzed. Experimental results show that, the lamellar precipitation is very sensitive to using or aging temperature, the higher temperature the more precipitates. The lamellar precipitation consists of continuous and discontinuous precipitation. The latter mainly appeared at the locations with larger deformation. Slip bands and deformation twins at high-temperature deformation and grain boundaries are important locations for precipitation.

Abstract: Fracture behaviour transitions due to change in the strain rate in steels with various Si content ranging from 2% to 5 wt% were studied. Room-temperature tensile tests were conducted over wide range of strain rates ranging from 10-3 s-1 to 103 s-1. Concerning of the steels with low Si content (no more than 3%), the nominal stress - nominal strain curves represented both uniform and local elongations at all strain rates. On the other hand, in 4% Si steel at a strain rate higher than 101 s-1, the tensile sample broke down without local elongation (necking). The stress at breaking was found to be nearly equal to its work hardening rate. The strain rate at which fracture behaviour transition took place in 5% Si steel (10-1s-1) was lower than that in 4% Si steel. TEM observations clarified the existence of deformation twins in the sample that fractured without necking. These results indicated that Si addition is subject to the brittle fractures and that the fracture mechanism transition is closely related with the deformation twinning behaviour.