발간논문

Home > KJMM 논문 > 발간논문

A study has been made to investigate the effects of various oxide particles on the nucleation and growth during strain induced dynamic transformation(SIDT) in Ti and Mg added steels. Compression tests have been carried out for steels with fine(20 ㎛), medium(90 ㎛) and coarse austenite(200 ㎛) microstructures at 750, 700 and 650℃ at a strain rate of 1s^-1. After the tests, specimens were water quenched and analyzed by OM, SEM and TEM. The results showed that the ferrite nucleation during SIDT was enhanced by Ti and Mg oxide particles as well as prior austenite boundaries via providing nucleation sites. On the other hand, the growth rate of ferrite formed by SIDT seemed to be more closely related with carbon in steel.

keyword :

The objective of this study is to analyze microstructures of multilayered Ti aluminide sheets fabricated by self-propagating high-temperature synthesis(SHS) reaction using hot rolling. Multilayered Ti/Al sheets were prepared by stacking thin Ti and Al sheets alternatively, and had a good Ti/Al interfacial bonding after pre-rolling at 500℃. When these sheets were heat-treated at 1000℃, spheroidal TiAl_3 phases were formed by SHS reaction at Ti/Al interfaces and inside Al layers. This formation continued until Al was completely consumed, and the Ti/Al multilayers turned into the Ti/TiAl_3 multilayers. Microstructural analysis on the hot-rolled, multilayered Ti/TiAl_3 sheets indicated that intermetallic phases such as TiAl_2, TiAl, and Ti_3Al were formed at Ti/TiAl_3 interfaces due to interaction between Ti and TiAl_3, and that pores formed in TiAl_3 layers were significantly reduced during hot rolling. These findings suggested that the SHS reaction using hot rolling was economical and useful for the continuous fabrication of the multilayered Ti/Ti aluminide sheets.

keyword :

The present study is to investigate the growth behavior of intermetallic phases in multilayered Ti/TiAl_3 sheets fabricated by hot rolling. When these sheets were heat-treated at 1000℃, interfacial phases such as Ti_3Al, TiAl, and TiAl_2 were grown consuming Ti and TiAl_3. As the heat treatment proceeded, TiAl was grown consuming TiAl_2, thereby finally fabricating multilayered sheets composed of Ti_3Al and TiAl. Conventional lamellar structure composed of Ti_3Al and TiAl could also be fabricated by heat treatment of multilayered Ti/TiAl_3 sheets at 1400℃. The growth rate and activation energy of interfacial phases were obtained from their growth behavior. The results indicated that the growth of intermetallic phases was mainly controlled by volume diffusion than by grain boundary diffusion. The formation process of the multilayered sheets and the formation mechanism of interfacial phases were elucidated, and an idea to fabricate multilayered or bulk Ti aluminide sheets by hot rolling and heat treatment was eventually suggested in this study.

keyword :

The heat affected zone(HAZ) and matrix of a welded STS 304 steel have been irradiated in an 1250 keV high voltage electron microscope at 673 K and up to 5.4 dpa(displacements per atom) to study the effect of electronbeam irradiation on the microstructure. In-situ observation showed that the voids formed by electron-beam irradiation coalescenced and grew to larger void with irradiation dose. Values of void size, void number density and void swelling percentage in HAZ were greater than those of matrix, and these were increased and then saturated gradually with irradiation dose. Non-equilibrium segregation phenomena such as Cr depletion and Ni enrichment were also observed in both the HAZ and matrix of welded STS304 steel at the grain boundary.

keyword :

It is understood that grain boundary cavitation is one of the detrimental processes for the degradation of austenitic stainless steels that reduces the creep-fatigue life at high temperatures. A new damage function based on a model for the creep-fatigue life prediction in terms of nucleation and growth of grain boundary cavities is proposed for austenitic stainless steel. This damage function is a combination of the fatigue and creep terms related to the cavitational damage (cavity nucleation and growth) in the life prediction equation and is found to be generally applicable to all the materials in which failure is controlled by the grain boundary cavitational damage. The cavity nucleation factor, P`, which is introduced in the creep-fatigue life model, is found to be closely related with the characteristics of grain boundary carbides acting as cavity nucleation sites. The modification of carbide characteristics through grain boundary serration is successfully made by the special heat treatment so that the modified carbides are favorable for cavitation resistance, resulting in a lowered P` value of material. It is observed that the creep-fatigue life is remarkably improved by the modification of carbide characteristics through grain boundary serration.

keyword :

An explicit, finite difference scheme has been used to simulate the influence of coherency stresses and interface kinetic barriers on the phase evolution of a binary, thin-film diffusion couple. Thin-films, initially consisting of alternating layers of two terminal phases α and γ, were held at a temperature where the formation of an intermediate phase β at α/γ interface was thermodynamically probable. In the absence of both the coherency stresses and interface kinetic barriers, β phase nucleates to grow at the beginning of the phase evolution and all phase compositions at interfaces are constant during the evolution. When either the coherency stresses or interface kinetic barriers are present, however, the interfacial compositions become time-dependent and thus the following characteristics of phase evolution are possible: (1) thermodynamically stable phase β does not form at the early stages of the evolution but nucleates to grow in the middle of the evolution, and (2) thermodynamically unstable phase β can initially nucleate to grow and then shrink to disappear during the evolution. Even if the initial states of α/γ multilayers have the same overall composition, the presence of coherency constraint can result in different final equilibrium states depending on either the initial phase compositions of α and γ or the magnitude of interface kinetic barriers.

keyword :

Co-Cr-Mo(ASTM F75) alloy is normally used for hip and knee prostheses. In this study, the effects of heat treatments on the mechanical and corrosion properties of Co-Cr-Mo alloy were studied. Solid solution treatment was performed at 1230℃ for 2 h to increase the ductility of the as-cast alloy, and the alloy was aged 700℃ for 1 h to increase the strength. Tensile and corrosion tests were carried out in each heat-treated alloy. Chemical composition change and phase transformation were investigated by X-ray diffractometry(XRD) and energy dispersive spectroscopy (EDS). Morphologies of each as-cast and heat-treated alloy were observed by optical microscopy(OM) and scanning electron microscopy(SEM). Tensile strength and elongation of Co-Cr-Mo alloy were observed as 940 MPa and 18%, respectively, when the alloy was aged at 700℃ for 1 h after solid solution treatment and quenching. The mechanical properties of Co-Cr-Mo alloy were improved without retrogression of corrosion properties by solid solution treatment.

keyword :

The cyclic deformation features of low stacking fault energy materials are not clearly understood and the available reports up to date are not consistent and sometimes even contradictory to each other. Some controversies about the cyclic hardening behavior in planar slip f.c.c metals partly originated from extremely low work hardening rate and frequent strain bursts, which maintain the stress level close to the critical resolved shear stress for a long period of time. In this paper, the effects of stacking fault energy and alloying elements on the cyclic deformation behavior and slip mode of f.c.c metals are reviewed. An attempt is also made, using available data, to develop a quantitative criterion which predicts the slip mode from wavy to semi-planar to planar slip with alloying in cyclic deformation of alloys. Since the repeated reversal of dislocation motion with cycling increases the chance of mutual trapping and annihilation, the solute content above which only planar slip behavior is observed usually increases dramatically in cyclic deformation. The low energy clustered dislocation structures such as cells, loop patches and/or ladder structure can be gradually formed from the initially planar microstructure if the strain amplitude is high enough to induce more active to-and-pro motion of dislocations during cycling of semi-planar slip alloys. The prediction on the transition of slip mode is compatible with most of the data on fatigue of Cu-Al and Cu-Zn alloys.

keyword :

Embrittlement in Fe-36 wt.%Ni alloy (invar alloy) was investigated to prevent the surface cracks formed during continuous casting operation. High temperature tensile tests were carried out in the temperature range of 600-1200℃ with in-situ solidified samples. The ductility trough was found in the temperature range of 600-850℃ in which the intergranular fracture mode was observed. This might be caused by the cavities formed along the grain boundary by diffusion and condensation of vacancies during solidification and cooling. Below about 600℃ the hot ductility increases due to insufficient time for diffusion of vacancies to the grain boundary and increased flow stress. The hot ductility above about 850℃ might result from the formation of recrystallized microstructure reducing the stress concentration at the cavities along the grain boundaries. It was also found that the embrittlement strongly depends on the strain rate and cooling rate. The increase in strain rate and cooling rate greatly improved the embrittlement.

keyword :

Total strain range controlled low cycle fatigue tests have been conducted in lamellar Ti-46.6Al;-1.4Mn-2Mo (at.%) alloy at 800℃ in order to investigate the effect of compressive hold time on high-temperature low cycle fatigue fracture behavior. It is observed that fatigue life decreases with imposing compressive hold time. The experimental results clearly indicate that the imposed compressive hold time accelerates α_2→γ phase transformation at the grain boundary to induce the formation of grain boundary γ phase. Because grain boundary γ phase is proved to be the region where plastic deformation is easily concentrated, it is confirmed that imposed compressive hold time decreases low cycle fatigue life by inducing grain boundary weakness and intergranular cracking.

keyword :