The effect of annealing treatment on mechanical properties has been investigated for a Ti-based bulk metallic glass (BMG). A series of compression tests as well as Vickers hardness tests has thus been performed at room temperature for the BMG specimens annealed in an Ar atmosphere at several temperatures near the glass transition temperature (Tg). The ductility under compression was found to increase up to about 42% without reducing the maximum strength, but with higher hardness values. Thermal properties were also analyzed by using a differential scanning calorimetry (DSC), while microstructures of annealed specimens were studied through X-ray diffraction (XRD) and also a high resolution transmission electron microscopy (HRTEM).

Deformation behavior of a Ti-based bulk metallic glass (BMG) has been investigated by performing a series of uniaxial compression tests at various temperatures within the supercooled liquid region (SLR). Non-Newtonian viscous flow characteristics of BMG alloys was found to change into a homogeneous Newtonian viscous flow under a proper combination of strain rate and test temperature by compression tests. These Newtonian and non-Newtonian viscous flows were analyzed by applying the Newtonian viscous flow theory and the transition state theory, respectively. A processing map could be also constructed based on a dynamic materials model to estimate the formability and the results were then compared with those of laboratory-scale extrusion tests performed under various punch jig speeds.

Macroscopic solid-to-solid hot formability of a Zr-based bulk metallic glass has been investigated in this study by applying a boss forming process within supercooled liquid region (SLR). The morphology and microstructures after boss forming were first examined by using an optical microscopy (OM), a field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), and transmission electron microscope (TEM). The variation of thermal properties before and after boss forming was also analyzed by using a differential scanning calorimeter (DSC). The glass transition temperature, Tg and the crystallization onset temperature, Tx, onset were found to decrease with increasing test time and temperature. Macroscopic extrusion formability was found to match well with the results predicted through a processing map based on a dynamic materials model (DMM). The specimens were found to form well under the conditions of high temperature and slow punch jig speed. A FEM simulation study has also been carried out to understand the causes of piping problem. A sound boss without a pipe could be formed by reducing the flow rate difference in the contact and the core sites.

The deformation behavior and macroscopic formability of a Zr-based bulk metallic glass composite (BMGC) has been investigated in this study by performing a series of compression and laboratory-scale extrusion tests under various deformation rates within the supercooled liquid temperature region. The morphology of Zr−Ti−Nb-rich dendrite precipitates after warm deformation was first examined by using optical microscopy (OM) and a field emission scanning electron microscope (FE-SEM). The extrusion of this BMGC alloy within the supercooled liquid temperature region was found more difficult than the extrusion of other Zr-based monolithic BMG alloys, possibly due to the existence of dendrite phases hindering the characteristic viscous flow generated in the amorphous phase. A FEM simulation has also been carried out by utilizing the stress-strain behaviors obtained from high temperature compression tests, and the results have been compared with the experimental extrusion test results. The FEM simulation results for the extrusion process as well as a processing map based on a dynamic materials model (DMM) were found to agree relatively well with the actual macroscopic extrusion formability.

The thermal properties of a Zr76.11Ti4.20Cu4.51Ni3.16Be1.49Nb10.53 bulk metallic glass (BMG) have been investigated by using a differential scanning calorimeter (DSC). The composition of dendrite phase was then subsequently analyzed by using an EPMA, XRD, and TEM. The glass transition and crystallization onset temperatures were determined as 339.7 °C and 375.8 °C for this BMG, respectively. The Zr-Ti-Nb dendrite phase was found to have a BCC structure. Mechanical properties have also been examined by conducting a series of uniaxial compression tests at various temperatures within supercooled liquid region under the strain rates between 10-4 /s and 3×10-2 /s. The hardness of matrix and dendrite was then measured separately. The glassy matrix appears to play major role on the elongation, while dendrite phase on the strength of this BMG composite at high temperatures within supercooled liquid region.

The formability of several Zr-based bulk metallic glasses in the supercooled liquid region has been estimated. Using the data obtained from compression tests, normalized processing maps based on a dynamic materials model (DMM) have been constructed to evaluate feasible forming conditions. Laboratory-scale hot extrusion of the Zr44Ti11Cu9.8Ni10.2Be25 BMG has also been carried out to clarify the effectiveness of the normalized processing maps established in this study. The influence of thermal properties and microstructural differences on the formability of BMGs is interpreted in terms of a normalized temperature within the supercooled liquid region.

The continuous-cooling-transformation (CCT) diagram and continuous cooled microstructure were investigated for low carbon (0.05wt.% C) high strength low alloy steels with/without boron. Microstructures observed in continuous cooled specimens were composed of pearlite, quasi-polygonal ferrite, granular bainite, acicular ferrite, bainitic ferrite, lower bainite, and martensite depending on cooling rate and transformation temperature. A rapid cooling rate depressed the formation of pearlite and quasi-polygonal ferrite, which resulted in higher hardness. However, hot deformation slightly increased transformation start temperature, and promoted the formation of pearlite and quasi-polygonal ferrite. Hot deformation also strongly promoted the acicular ferrite formation which did not form under non-deformation conditions. Small boron addition effectively reduced the formation of pearlite and quasi-polygonal ferrite and broadened the cooling rate region for bainitic ferrite and martensite.

Effects of cooling rate and cooling finish temperature (CFT) on the microstructure and mechanical properties have been investigated in B-added low carbon (0.05wt%C) HSLA steels aiming for the use of linepipes. At first, continuous cooling transformation (CCT) diagram of the present steel has been established by using thermo-mechanical process simulation in a Gleeble system at various cooling rates from 0.1 degrees C/s to 50 degrees C/s, followed by microstructure identification based on metallographic observations and micro-hardness measurement. The microstructure identified in the CCT experiments could be classified as granular bainite, acicular ferrite, bainitic ferrite and lath martensite as increasing the cooling rate. The microstructure as well as mechanical property also depended upon cooling finish temperature. As cooling finish temperature increased, the fraction of acicular ferrite increased without a decrease of hardness value. This was due to the softening retardation effects caused by good carbide forming elements of V, Cr, and Mo. In addition, lower bainite microstructure was newly developed when the accelerated cooling stopped at temperature below 450 degrees C. The present simulation results have been well compatible to pilot plant scale rolling. That is, the increasing cooling finish temperature could result in enhanced upper shelf energy without a decrease of tensile strength, which was attributed to the decrease of dislocation density in addition to the softening retardation effects of V, Cr and Mo.

Effects of microstructural parameters on fatigue resistance (σFL) of the steel tire cords have been investigated experimentally from microscopic points of view. At first, microstructural parameters depending on carbon content have been identified by using transmission electron microscopy (TEM). The fatigue resistance of the steel tire cords depending on carbon content has been measured by using the Hunter rotating beam tester under the bending stress of 900 to 1500 MPa. The fatigue resistance was improved with increasing the carbon content from 0.7, 0.8 to 0.9 wt. % C, due to variations of microstructural parameters, such as lamellar spacing (λp), cementite thickness (tc), and volume fraction (Vc) of cementite. As the carbon content increased, the lamellar spacing and the cementite thickness decreased, while the volume fraction of cementites increased. The effects of microstructure on fatigue resistance have been discussed in terms of the microstructural parameters mentioned above.

In as-cast slab steel, dendritic Nb-rich (Ti,Nb)(C,N) carbonitrides were observed which have a thermodynamically stable chemistry at lower than 1000 °C. These dendritic carbonitrides were dissolved and then re-precipitated to two kinds of carbonitrides, Ti- and N-rich and Ti- and C-rich (Ti,Nb)(C,N) carbonitrides during reheating.

Effects of cooling rate and isothermal holding on the precipitation behavior during solidification have been investigated in 0.063C-0.017Ti-0.056Nb HSLA steels. The precipitates identified in as-cast slab were semi-dendritic, dendritic or rod-like Nb-rich (Nb,Ti)(C,N). The morphology and chemistry were quite different from these precipitates formed after reheating and/or hot rolling processes. No precipitation has been observed at the end of the solidification and the continuous cooling down to 800 °C. ICP and TEM analyses indicated that most Nb and Ti were mainly precipitated into carbonitrides by the isothermal holding at temperature range from 900 to 1000 °C. In case of continuous casting process, the isothermal holding region corresponds to a certain flat cooling region probably due to the latent heat of solidification. The Nb-rich carbonitrides formed during solidification are associated with the micro-segregation of Ti and Nb in interdendritic region.

The precipitates identified in as-cast slab were semi-dendritic, dendritic or rod-like Nb-rich (Nb,Ti)(C,N). The morphology and chemistry were different from these precipitates after reheating and/or hot rolling processes. Most Nb and Ti were mainly precipitated into carbonitrides during the isothermal holding at the temperature range from 900 to 1000 °C.

The retained austenite in hot rolled TRIP steels can decompose into other phases during the coiling process and cooling process, which can produce serious inhomogeneity of the TRIP sheet steels. In order to evaluate the decomposition behavior of retained austenite, the simulation of hot rolling and coiling processes and in-situ heating observations have been made for the hot-rolled C-Mn-Si TRIP steels. It was found that retained austenite decomposes into various phases depending on coiling temperatures ranging from 350 to 500degreesC, such as carbide-free bainitic ferrite, ferrite associated with cementite and pearlite. The step by step sequences of the decomposition behavior are discussed in terms of the carbon concentration variation and the thermal stability of retained austenite depending on coiling temperatures and holding times.

It is well known that Fe3Al intermetallic compound shows an anomalous peak of the yield strength at about 500 degrees C. That is, the yield strengths increase with increasing deformation temperature in the range of 300 degrees C-500 degrees C, and then decrease at higher temperatures. The dislocation structure was examined by transmission electron microscopy, and high temperature mechanical properties was examined by tensile and load relaxation tests. The flow stress curves obtained from load relaxation tests were then analyzed in terms of internal variable deformation theory. It was found that the flow curves consisted of three micro-deformation mechanisms -i.e. inelastic deformation mode, plastic deformation mode and dislocation creep deformation mode, depending on both dislocation structure and deformation temperature. The flow curves could be well described by the constitutive equations of these three micro-deformation mechanisms based on the internal variable deformation theory.