Kwangjun Euh

Microstructure and hardness improvement of TiC/stainless steel surface composites fabricated by high-energy electron beam irradiation

Abstract Stainless steel-base surface composites reinforced with TiC particles were fabricated by high-energy electron beam irradiation. Three kinds of powder/flux mixtures: TiC, (Ti+C), and (Ti+SiC) powders with 40 wt.% of CaF 2 flux were deposited evenly on an AISI 304 stainless steel substrate, which was then irradiated with an electron beam. TiC agglomerates of partially remelted particles and pores were found in the surface composite layer fabricated by irradiation of TiC powders. In the composite layer fabricated by irradiation of Ti and C powders having lower melting points than TiC powders, a number of primary TiC particles were precipitated without forming TiC agglomerates or pores. This indicated the more effective TiC precipitation obtained from the melting of Ti and C powders, instead of TiC powders. The hardness and wear resistance of the surface composite layer were directly affected by the hard primary TiC particles, and thus were two to three times greater than those of the stainless steel substrate.

Microstructural modification and hardness improvement in boride/Ti–6Al–4V surface-alloyed materials fabricated by high-energy electron beam irradiation

The present study is concerned with the fabrication and microstructural analysis of boride/Ti-6Al-4V surface-alloyed materials using the irradiation of a high-energy electron beam. Mixtures of TiB2 or MoB powders and CaF2 flux were placed on a Ti-6Al-4V alloy substrate and subsequently irradiated using a high-energy electron beam. Specimens processed with a flux mixing ratio of 40 wt pct showed that the melted region of 1.1 to 1.5 mm in thickness was homogeneously formed without defects and contained a large amount of titanium borides (TiB). The formation of TiB in the melted region greatly improved the Vickers hardness, high-temperature Vickers hardness, and wear resistance to levels 2 or 3 three times higher than the those for the Ti alloy substrate. Also, the addition of MoB powders into the mixtures made possible the fabrication of surface-alloyed materials with various properties by controlling the kind, size, and volume fraction of TiB and the characteristics of the matrix. These findings suggested that surface alloying using high-energy electron-beam irradiation was economical and useful for the development of boride/Ti-6Al-4V surface-alloyed materials with improved properties.

Effect of tempering on hardness improvement in a VC/steel surface-alloyed material fabricated by high-energy electron-beam irradiation

Abstract The present study is concerned with the tempering effect in improving the hardness of a vanadium carbide (VC)/carbon steel surface-alloyed material fabricated by high-energy electron-beam irradiation. The mixture of VC powders and flux (50%MgO–50%CaO) was placed on a plain carbon steel substrate, and then electron beam was irradiated. The surface-alloyed layer of 1.8 mm in thickness was homogeneously formed without defects. The microstructural analysis indicated that coarse VC particles were formed along solidification cell boundaries, and the matrix inside cells was mostly composed of lath-type martensite and fine cuboidal VC particles. A large amount of these VC particles in the lath-type martensitic matrix provided hardness four times greater than that of the substrate. When the VC/steel surface-alloyed material was tempered, fine VC particles precipitated in the tempered martensitic matrix, thereby leading to additional hardness increase. In addition, reduction of residual stress and an increase in fracture toughness could be expected.

Grain-structure evolution of brazing-treated A4343/A3003/A4343 aluminum brazing sheets rolled with different reductions

The grain-structure evolution of three-layered A4343(clad)/A3003(core)/A4343(clad) aluminum brazing sheets was studied by means of electron backscatter diffraction. Three different samples were prepared by cold rolling with 17%, 22%, and 44% reduction, and the brazing treatment was carried out in form of differential scanning calorimetry. Orientation maps of the clad and the core suggested that the grains in the melted and re-solidified clad did not grow epitaxially from the core in case of the 17% and 22% cold-rolled samples. In contrast, the grains in the 44% cold-rolled sample seemed to have been formed by epitaxial growth from the core. Kernel average misorientation (KAM) analysis revealed that the samples cold rolled with 17% and 22% reduction retained a deformed microstructure in the core even after the brazing treatment, but the core of the 44% cold-rolled sample was composed of coarse elongated grains, free from any substructure. The dissolution depth corresponding to the thickness of the core affected by the brazing treatment was proportional to the average KAM of the core.

Effects of Ti addition and heat treatments on mechanical and electrical properties of Cu-Ni-Si alloys

The mechanical and electrical properties of Cu-5.98Ni-1.43Si and Cu-5.98Ni-1.29Si-0.24Ti alloys under heat treatment at 400 and 500 °C after hot- and cold-rolling were investigated, and a microstructural analysis using transmission electron microscopy was performed. Cu-5.98Ni-1.29Si-0.24Ti alloy displayed the combined Vickers hardness/electrical conductivity value of 315.9 Hv/57.1%IACS. This was attributed to a decrease of the solution solubility of Ni and Si in the Cu matrix by the formation of smaller and denser δ-Ni2Si precipitates. Meanwhile, the alloyed Ti was detected in the coarse Ni-Si-Ti phase particles, along with other large Ni-Si phase particles, in Cu-5.98Ni-1.29Si-0.24Ti.

Microstructural modification and property improvement of boride/Ti-6Al-4V surface-alloyed materials fabricated by high-energy electron-beam irradiation

The present study is concerned with the fabrication and microstructural analysis of boride/Ti-6Al-4V surface-alloyed materials using the irradiation of a high-energy electron beam. Mixtures of TiB2 or MoB powders and CaF2 flux were placed on a Ti-6Al-4V alloy substrate and subsequently irradiated using a high-energy electron beam. Specimens processed with a flux mixing ratio of 40 wt pct showed that the melted region of 1.1 to 1.5 mm in thickness was homogeneously formed without defects and contained a large amount of titanium borides (TiB). The formation of TiB in the melted region greatly improved the Vickers hardness, high-temperature Vickers hardness, and wear resistance to levels 2 or 3 three times higher than the those for the Ti alloy substrate. Also, the addition of MoB powders into the mixtures made possible the fabrication of surface-alloyed materials with various properties by controlling the kind, size, and volume fraction of TiB and the characteristics of the matrix. These findings suggested that surface alloying using high-energy electron-beam irradiation was economical and useful for the development of boride/Ti-6Al-4V surface-alloyed materials with improved properties.

Microstructure and Mechanical Properties of a Cu-Fe-P Copper Alloy Sheet Processed by Differential Speed Rolling

The microstructure and mechanical properties of a Cu-Fe-P copper alloy processed by differential speed rolling (DSR) were investigated in detail. The copper alloy, with a thickness of 3 mm, was rolled to 50% reduction at ambient temperature without lubrication with a differential speed ratio of 2.0:1 and then annealed for 0.5h at various temperatures ranging from 100 to 800°C. Conventional rolling was performed under the same rolling conditions for comparison. The shear strain introduced by the conventional rolling process showed positive values at the positions of the upper roll side and negative values at the positions of the lower roll side. However, the result was zero or positive values at all positions for samples rolled by DSR. The effects of DSR on the microstructure and mechanical properties of the as-rolled and subsequently annealed samples are discussed. (Received March 27, 2010)

Microstructural analysis of vanadium carbide/steel surface-alloyed materials fabricated by high-energy electron-beam irradiation

This study is concerned with the microstructural analysis of vanadium carbide (VC)/steel surface-alloyed materials fabricated by high-energy electron-beam irradiation. The mixtures of VC powders and MgO-CaO flux were deposited on a plain carbon steel substrate, and then electron beam was irradiated on these mixtures using an electron-beam accelerator. Microstructures of the irradiated surface regions were examined by optical microscopy, scanning electron microscopy, and transmission electron microscopy. Residual pores were found in the specimen processed without flux, but hardly found in the specimens processed with a considerable addition of flux. As a result of irradiation, vanadium content was homogeneously maintained throughout the melted region, and fine vanadium carbides were formed in the melted region. These microstructural modification including the formation of vanadium carbides greatly improved hardness, especially high-temperature hardness up to 500 °C.

Tensile and electrical properties of direct aged Cu-Ni-Si-x%Ti alloys

In this study, the tensile and electrical properties of aged Cu-4Ni-Si-Ti (C4NS-Ti) alloys, with and without solution heat treatment (SHT) prior to aging, were examined. The C4NS specimens, without and with 0.09 and 0.18 wt% of Ti, were prepared and either SHT + aged or DAed (directly aged) at 450 °C for various durations ranging from 0 to 6 h, and the tensile and electrical properties were measured. It was demonstrated that the combined tensile strength/electrical conductivity was substantially greater for the DAed C4NS specimens as compared to the SHT + aged counterparts, and the addition of Ti provided further improvement. The aging responses of DAed C4NS specimens were also strongly affected by added Ti. The effect of SHT on the tensile and electrical properties of aged C4NS alloys and the change in aging behavior with the addition of a small amount of Ti were discussed as a function of aging time based on the micrographic and fractographic analyses.

Microstructural analysis of TiC reinforced ferrous surface composites processed by accelerated electron beam irradiation

The present study aims to analyze microstructures of TiC reinforced ferrous surface composites processed by accelerated electron beam irradiation. Two kinds of powder/flux mixtures, i.e., TiC and (Ti + C) powders with 40 wt% of CaF2 flux, were deposited evenly on an AISI 304 stainless steel substrate, which was then irradiated with electron beam. TiC agglomerates and pores were found in the surface composite specimen processed by irradiation of TiC powders because of insufficient melting of TiC powders. In the specimen processed by irradiation of Ti and C powders having lower melting points than TiC powders, a lot of large TiC carbides were precipitated in the melted region, together with a few TiC agglomerates or pores. This indicated the more effective TiC precipitation obtained from the melting of Ti and C powders, instead of TiC powders. The hardness of the surface composite layer was about two times higher than that of the AISI 304 substrate mainly due to the precipitation of TiC carbides. 2001 Elsevier Science B.V. All rights reserved.