Sun Keun Hwang

Effect of Mo on recrystallization characteristics of Zr–Nb–(Sn)–Mo experimental alloys

Abstract Feasibility of using Mo as a crucial alloying element in Zr-based alloys is addressed in terms of grain growth characteristics, texture and mechanical properties. In binary Zr–1Nb alloy, addition of Mo retarded recovery and recrystallization of cold worked microstructure and suppressed grain coarsening. Mo also prevented abnormal grain growth that occurred in the binary alloy. The grain refinement effect of Mo was explained in terms of β-phase forming at the interface of α-phase plates. Yield strength and ultimate tensile strength of heat-treated Zr–1Nb and Zr–1Nb–ySn-0.1Fe alloys were also enhanced by Mo addition due to solid-solution hardening and persistent cold-worked dislocation substructure. As cold-rolled Zr–1Nb alloys showed a rather high basal texture along the normal direction of plate surface and this was relaxed by Mo addition, which was attributed to grain refinement and promotion of β-phase. The present work indicates that Mo can be a valuable addition in Zr-based alloys particularly under extended high temperature exposure.

The effect of the applied strain range on fatigue cracking in lamellar TiAl alloy

Abstract Continuous low cycle fatigue behavior of a lamellar structured Ti–46.6Al–1.4Mn–2Mo (at.%) alloy was evaluated at the testing temperature of 800°C to investigate the effect of the applied strain range on fatigue cracking. It is observed that fatigue life is reduced with increasing applied strain range and the slope change of the Coffin–Manson plot is strongly related to the change of fatigue cracking mode from transgranular to intergranular with applied strain range. It is observed that the change of fatigue cracking mode cannot be explained by an environmental effect and a precipitation effect. In the present study, the α2→γ phase transformation phenomenon is observed and the experimental results clearly indicate that the phase transformation at the grain boundary is responsible for the change of fatigue cracking mode with applied strain range. This is confirmed by a low cycle fatigue test with an additionally heat-treated specimen in which a predominant γ phase is formed at the grain boundary.

Features of texture and structure development in zirconium under equal channel angular pressing

A detailed X-ray study of Zr rods, subjected to ECAP at 350oC by routes C and BC, was conducted by the new X-ray method of Generalized Pole Figures, combining texture measurement with registration of X-ray line profiles. The data analysis is based on conceptions of the texture formation theory, connecting features of grain reorientation with activated deformation mechanisms. A degree of reproduction of the same distinctive texture by successive ECAP passes with antecedent rotation of the rod reflects attendant structure changes in material.

Fabrication of Magnesium Alloy (AZ31) Sheet by Extrusion

In order to investigate the extrusion characteristics of magnesium alloy (AZ31), a computer simulation was attempted. Tensile properties of as-cast billet with different strain rates were incorporated into the simulation. The results showed a great change in distribution of stress and strain at near die region by the ram travel. It was found that the average gain size at longitudinal edge of the extruded sheet was smaller than that at center region, which might be attributed to recrystallization caused by severe plastic deformation.

The enhancement of low cycle fatigue life by carbon addition in lamellar TiAl alloy

Abstract Total strain range controlled low cycle fatigue tests with lamellar TiAl and TiAl–0.3at.% C alloys were conducted at 800xa0°C in order to investigate the effect of carbon addition on low cycle fatigue property. It is observed that carbon addition increases fatigue life with increased peak stress and decreased plastic strain range values. Because carbon addition has been reported to change the microstructure, the reason for the enhancement of low cycle fatigue life by carbon addition is investigated in terms of microstructural change. The experimental results clearly indicate that carbon addition inhibits the formation of grain boundary γ phase by reducing the lamellar spacing. Because the grain boundary γ phase is proved to be the region in which the plastic deformation is concentrated and intergranular crack is initiated, it is confirmed that carbon addition increases the fatigue life by inducing uniform plastic deformation and transgranular cracking.

Role of Deformation Twinning in Cold Rolling and Recrystallization of Titanium

The evolution of microstructure and texture during cold rolling and recrystallization annealing of commercial-purity Ti (CP-Ti) was established. Cold rolling to 40% reduction activated mechanical twinning- mostly > 3 2 11 < } 2 2 11 { compressive twins and > 1 1 10 < } 2 1 10 { tensile twins. The formation of twins resulted in an inhomogeneous microstructure, in which only the localized regions containing twins were refined and the regions deformed by slip remained coarse. The twinned grains, containing high stored energy and numerous high-angle grain boundaries, became the preferential sites of nucleation during subsequent recrystallization. During recrystallization heat treatment at 500~700°C, the cold-rolling texture (ϕ1=0°, Φ=35°, ϕ2=30°) diminished in intensity, whereas a recrystallization texture component (ϕ1=15°, Φ=35°, ϕ2=35°) appeared. The recrystallization heat treatment temperature affected the rate of recrystallization but not the texture characteristics per se. During the subsequent grain growth stage, the recrystallization texture component increased. This behavior was attributed to the growth of larger-than-average grains of this particular crystal orientation.The evolution of microstructure and texture during cold rolling and recrystallization annealing of commercial-purity Ti (CP-Ti) was established. Cold rolling to 40% reduction activated mechanical twinning- mostly > 3 2 11 < } 2 2 11 { compressive twins and > 1 1 10 < } 2 1 10 { tensile twins. The formation of twins resulted in an inhomogeneous microstructure, in which only the localized regions containing twins were refined and the regions deformed by slip remained coarse. The twinned grains, containing high stored energy and numerous high-angle grain boundaries, became the preferential sites of nucleation during subsequent recrystallization. During recrystallization heat treatment at 500~700°C, the cold-rolling texture (ϕ1=0°, Φ=35°, ϕ2=30°) diminished in intensity, whereas a recrystallization texture component (ϕ1=15°, Φ=35°, ϕ2=35°) appeared. The recrystallization heat treatment temperature affected the rate of recrystallization but not the texture characteristics per se. During the subsequent grain growth stage, the recrystallization texture component increased. This behavior was attributed to the growth of larger-than-average grains of this particular crystal orientation.

The Forming Characteristics of Simultaneous Radial-Forward Extrusion Processes

Numerical simulations are applied to investigate the simultaneous radial-forward extrusion process in a combined extrusion such as subsequent radial-forward extrusion after radial extrusion. Design factors for the process such as gap height, deflection angle into annular gap and frictional condition are employed in the analysis. The analysis is focused to see the influence of design factors on the maximum force requirement for the forming process. One of the selected simulation results is compared with the experiments in terms of load-stroke relationships. The pressure distributions exerted on the die-wall interfaces are also investigated to reveal if the tooling system is safe, especially the die set. The plastic stress-strain relationship is derived analytically from the material constants used in elastic deformation analysis. It is revealed from the simulation results that the influence of the deflection angle on the maximum force requirement for the process is greatest among design parameters. AA 6063 alloy is selected as a model material for the analyses in this study.

Mechanical Property of Single Phase Co-Ni-Cr-Mo Based Superalloy Produced by Cold Working and Recrystallization Heat Treatment

Mechanical property of carbon-free Co-Ni-Cr-Mo based superalloy was investigated at room temperature and at 943 K. Cold working and subsequent recrystallization heat treatment was carried out to produce an equiaxed grain structure. The average grain size was controlled to 28 and 238 µm in order to understand the effect of grain size on mechanical property and flow behavior. At room temperature and 943 K, 0.2% offset yield strength increased with decreasing grain size to exhibit grain size dependence. At 943 K, a specimen with the average grain size of 28 µm showed higher yield strength than that obtained with a specimen having the average grain size of 238 µm. A steady-state like behavior leading to an extensive ductility was observed in the fine-grained specimen. {111}<112> deformation twinning was found to correlate with the steady-state like behavior in the true stress-true strain curve. It was suggested that grain refinement is favor to enhance the fracture strength and allow the deformation twinning to occur.

Improvement in Mechanical Properties of Magnesium Alloy (AZ31) Sheet Fabricated by Casting and Subsequent Plastic Working

In order to enhance the mechanical properties of magnesium alloy (AZ31) sheet, an integrated sheet-making process including billet casting, extrusion and rolling was attempted. Microstructural analysis was carried out and the mechanical properties at each processing step were also investigated by tensile tests. By an extrusion process of ingot-processed billet, average grain size of billet was reduced from 91.2 to 28.2 μm, and a further reduction in grain size to 16.9 μm was obtained by subsequent rolling of extruded 5 mm thin slab. In the final step, a rolled sheet of 1mm showed a remarkable refinement in grain size down to 8.8 μm. Regarding mechanical properties, tensile strength and elongation for as-cast billet, as-extruded thin slab and as-rolled sheet increased from 189, 258 to 234 MPa, and from 13, 16.5 to 23 %, respectively.

Monte-Carlo Modeling of Recrystallization Kinetics of Cold-Rolled Titanium

The recrystallization behavior of cold-rolled, commercial-purity titanium was studied experimentally and with Monte-Carlo (MC) modeling. Utilization of EBSD-OIM as input for MC modeling resulted in realistic predictions of recrystallization kinetics, microstructure and texture, which were in good agreement with experimental results. MC modeling of recrystallization kinetics predicted that the non-uniform stored energy distribution, heterogeneous nucleation of recrystallization and recovery in combination leads to a negative deviation from linear JMAK kinetics. It was found that concurrent recovery that takes place during recrystallization is an important process that controls both the overall recrystallization kinetics and the deviation of linear JMAK kinetics. On the other hand, the non-uniformly distributed stored energy itself has little effect on the negative deviation from JMAK kinetics but intensifies the deviation when heterogeneous nucleation is combined. Modeling results also revealed that heterogeneous nucleation of recrystallized grains and their early impingement in local areas of high deformation are essential for producing a log-normal distribution of grain size and a typical recrystallization texture of rolled titanium.