Jaimyun Jung

Microstructure and properties of titanium boride dispersed Cu alloys fabricated by spray forming

Abstract Dispersion strengthened Cu alloys have been manufactured by conventional spray forming and also by reactive spray forming, followed by hot extrusion of the spray deposited billets. In this work, we have systematically investigated the relationship between the microstructure and mechanical properties of the Cu alloys fabricated through two techniques by means of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and tensile testing. The size of dispersed particles in the reactive spray formed alloy was much finer than that in the conventionally spray formed alloy. That was because the dominant chemical reaction between Ti and B had occurred in Cu–Ti–B alloy melt in conventional spray forming while it had occurred after deposition of droplets in reactive spray forming. The yield strength of the reactive spray formed alloy was greater than that of the conventional spray formed alloy in spite of a lower volume fraction of reinforced particles. To understand the mechanism responsible for this experimental observation, the yield strength of the two Cu alloys were analyzed using the dislocation pile-up model and Orowan mechanism, which were fairly consistent with the experimental results. Increase in yield strength of reactive spray formed alloy compared with that of conventional spray formed alloy was largely attributed to nanoscale TiB dispersoids. This indicates that refining the reinforced particle size to nanoscale is of special importance for the development of high strength Cu alloys since the yield strength predominantly depends on the size and volume fraction of the reinforcement. From this point of view, reactive spray forming can be considered as a new promising process to develop high strength Cu alloys.

Factors governing hole expansion ratio of steel sheets with smooth sheared edge

Stretch-flangeability measured using hole expansion test (HET) represents the ability of a material to form into a complex shaped component. Despite its importance in automotive applications of advanced high strength steels, stretch-flangeability is a less known sheet metal forming property. In this paper, we investigate the factors governing hole expansion ratio (HER) by means of tensile test and HET. We correlate a wide range of tensile properties with HERs of steel sheet specimens because the stress state in the hole edge region during the HET is almost the same as that of the uniaxial tensile test. In order to evaluate an intrinsic HER of steel sheet specimens, the initial hole of the HET specimen is produced using a milling process after punching, which can remove accumulated shearing damage and micro-void in the hole edge region that is present when using the standard HER evaluation method. It was found that the intrinsic HER of steel sheet specimens was proportional to the strain rate sensitivity exponent and post uniform elongation.

Continuum understanding of twin formation near grain boundaries of FCC metals with low stacking fault energy

Deformation twinning from grain boundaries is often observed in face-centered cubic metals with low stacking fault energy. One of the possible factors that contribute to twinning origination from grain boundaries is the intergranular interactions during deformation. Nonetheless, the influence of mechanical interaction among grains on twin evolution has not been fully understood. In spite of extensive experimental and modeling efforts on correlating microstructural features with their twinning behavior, a clear relation among the large aggregate of grains is still lacking. In this work, we characterize the micromechanics of grain-to-grain interactions that contribute to twin evolution by investigating the mechanical twins near grain boundaries using a full-field crystal plasticity simulation of a twinning-induced plasticity steel deformed in uniaxial tension at room temperature. Microstructures are first observed through electron backscatter diffraction technique to obtain data to reconstruct a statistically equivalent microstructure through synthetic microstructure building. Grain-to-grain micromechanical response is analyzed to assess the collective twinning behavior of the microstructural volume element under tensile deformation. Examination of the simulated results reveal that grain interactions are capable of changing the local mechanical behavior near grain boundaries by transferring strain across grain boundary or localizing strain near grain boundary.Metals: grain neighbours influence twin formation during deformationGrains that should not favour twin formation exhibit twinning as a result of surrounding grains acting on their boundaries. A team led by HyoungSeop Kim at the Pohang University of Science and Technology in the Republic of Korea simulated the deformation of synthetic metallic microstructures with many grains of different orientations, based on steels that deform by both dislocation slip and twinning mechanisms. Twinning first started near grain boundaries and depended on initial grain orientation but, with further deformation, strong twin activity on one side of a boundary triggered strong twin activity on the other side of that boundary. This happened even when the grain on the other side of the boundary was unfavourable to twinning. Taking into account grain neighbourhood may therefore help in optimising twin-forming alloys.

Finite Element Analysis of Deformation Homogeneity During Continuous and Batch Type Equal Channel Angular Pressing

Equal channel angular pressing (ECAP) is the most promising and interesting process for refining the grain size to an ultrafine grain or nanosize by imposing severe plastic deformation into the workpiece and repeating the process while maintaining the original cross-section of the workpiece. In this paper, we simulated the batch type ECAP and the continuous type equal channel multi-angular pressing (ECMAP), which can impose large deformation by repeating the shear deformation, using the finite element method and investigated the similarity and difference of the two processes. In particular, modified die design of the continuous type ECMAP was proposed for strain uniformity.

Key factors of stretch-flangeability of sheet materials

Abstract Stretch-flangeability evaluated using hole-expansion testing represents the ability of sheet materials to resist edge fracture during complex shape forming. Despite a property imperative for automotive part applications of advanced high-strength steels, factors governing stretch-flangeability are not yet well understood. In this study, the mechanical properties of a selected group of materials with different microstructures were investigated using tensile, fracture toughness, and hole-expansion tests to find the factor governing the stretch-flangeability that is universally applicable to a variety of metallic materials. It was found that the fracture toughness of materials, measured using the fracture initiation energy, is a universal factor governing stretch-flangeability. We verified that fracture toughness is the key factor governing stretch-flangeability, showing that the hole-expansion ratio could be well predicted using finite element analysis associated with a simple ductile damage model, without explicitly taking into account the microstructural complexity of each specimen. This validates the use of the fracture toughness as a key factor of stretch-flangeability.

Finite Element and Experimental Analysis of Closure and Contact Bonding of Pores During Hot Rolling of Steel

The closure and contact bonding behavior of internal pores in steel slabs during hot rolling was studied using experiments and the finite element method (FEM). Effects of pore size and shape were investigated, and three different cases of pore closure results were observed: no closure, partial closure, and full closure. The FEM results well reproduced various closure events. Bonding strengths of unsuccessfully closed pores, measured by tensile tests, showed critical effects. Also, there was a difference in bonding strengths of several fully closed pores. Fracture surfaces showed that welded regions could be divided into three (not, partially, and perfectly) welded regions. The pressure–time curves obtained from the FEM results indicate that pore surface contact time and deformed surface length are important parameters in pore welding. Pore size, pore shape, time of pressure contact, and deformed surface length should be considered to completely eliminate pores in final products.

Deep Drawing Behavior of CoCrFeMnNi High-Entropy Alloys

Herein, the deep drawability and deep drawing behavior of an equiatomic CoCrFeMnNi HEA and its microstructure and texture evolution are first studied for future applications. The CoCrFeMnNi HEA is successfully drawn to a limit drawing ratio (LDR) of 2.14, while the planar anisotropy of the drawn cup specimen is negligible. The moderate combination of strain hardening exponent and strain rate sensitivity and the formation of deformation twins in the edge region play important roles in successful deep drawing. In the meanwhile, the texture evolution of CoCrFeMnNi HEA has similarities with conventional fcc metals.

Microstructural evolution during reactive spray forming of dispersion strengthened Cu alloy

Dispersion strengthened Cu alloy was fabricated by injecting Cu-B alloy powders into the spray of Cu-Ti droplets. The microstructures of over-sprayed powders and spray deposited billet were observed by optical microscopy, scanning electron microscopy, and transmission electron microscopy. The over-sprayed powders were composed of not only Cu-B and Cu-Ti alloy powders but also small amounts of Cu-B alloy powders surrounded by Cu-Ti droplets. Fine dispersoids of TiB were observed in the Cu-B powders surrounded by Cu-Ti, indicating that very rapid reaction of Ti and B had occurred during the flight of the droplets. TiB dispersoids of ∼10 nm having an orientational relationship with the Cu matrix were distributed in the Cu-B alloy powder region and coarser TiB dispersoids of ∼50 nm were observed in the circumferencial Cu-Ti region. The spray deposited billet consisted of the regions showing a fine microstructure of round shape, presumably originating from the injected Cu-B alloy powders, and a relatively coarse cellular microstructure. TiB2 and TiB of ∼200 nm were observed along the grain and cell boundaries. Fine TiB dispersoids of ∼10 nm having an orientational relationship with the Cu matrix were observed in both regions. The solidification behavior, with special interest in (he formation of dispersoids, was examined based on this observation.

Strengthening mechanism of a spray-formed Cu-TiB2 composite

Cu-TiB2 composite has been manufactured by spray forming process. The microstructures of over-sprayed powder, spray-formed billet, and extruded rod have been characterized by optical microscopy and x-ray diffractometry. The tensile strength of extruded rod was measured by tensile testing. TiB2 particles were formed in a Cu melt of 1500°C by chemical reaction between Ti and B. The TiB2 volume fraction within the spray-formed billet varied by the density difference between TiB2 and Cu melt. The strengthening of Cu-TiB2 alloy was well described by dislocation pile-up model. The results indicate that yielding of this composite was subject to reinforced particle fracture by local stress concentrations caused by dislocation pile-up between obstacles. Reinforcement fracture also has an effect to decrease the work hardening rate with the increase of reinforcement volume fraction during plastic deformation. We believed that this is because of the strengthening loss due to the increase of fractured particles with increasing TiB2 volume fraction.