Hyun-Joon Jun

Evolution of precipitates in the Nb–Ti–V microalloyed HSLA steels during reheating

Abstract 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.

High Temperature Deformation Characteristics of Zr76.11Ti4.20Cu4.51Ni3.16Be1.49Nb10.53 Bulk Metallic Glass Composite

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.

Formability Evaluation of both Monolithic and Multiphase Zr-Based Bulk Metallic Glasses

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.