Young Gun Ko

Effects of temperature and initial microstructure on the equal channel angular pressing of Ti–6Al–4V alloy

Abstract Equal channel angular pressing of Ti–6Al–4V alloy was successfully carried out isothermally above 600 °C. The equiaxed microstructure presented more uniform material flow than the Widmanstatten microstructure, which was discussed in relation to flow softening behavior of the two microstructures.

High-temperature deformation behavior of ELI grade Ti-6Al-4V alloy with martensite microstructure

High-temperature deformation behavior and microstructural evolution process of ELI Ti-6Al-4V alloy having martensite microstructure were investigated with the variation of strain, strain rate and temperature. A series of hot compression tests was carried out isothermally for martensite microstructure at the true strain range of 0.6 to 1.4, strain rate range of 10-3 s-1 to 1 s-1 and temperature range of 700 oC to 950 oC. The processing maps for martensite microstructures were constructed on the basis of dynamic materials model (DMM). At the strain rate higher than 10-2 s-1 and the temperature lower than 750 oC regions of flow instability such as adiabatic shear band and micro-cracking were observed. Also, after imposing an effective strain of ≈ 1.4, deformed microstructure showed the significant kinking/bending behavior of lamellae resulting in the dynamic globularization associated with the fragmentation of beta-phase. The effects of strain, strain rate and temperature for dynamic globularization were discussed based on the microstructure and efficiency of power dissipation.

Cavitation Behavior of Ultra-Fine Grained Ti-6Al-4V Alloy Produced by Equal-Channel Angular Pressing

Cavitation behavior during superplastic flow of ultra-fine grained (UFG) Ti-6Al-4V alloy was established with the variation of grain size and misorientation. After imposing an effective strainup to 8 via equal-channel angular pressing (ECAP) at 873 K, alpha-phase grains were markedly refined from 11 μm to ≈ 0.3 μm, and misorientation angle was increased. Uniaxial-tension tests were conducted for initial coarse grained (CG) and two UFG alloys (ε = 4 and 8) at temperature of 973 K and strain rate of 10-4 s-1. Quantitative measurements of cavitation evidenced that both the average size and the area fraction of cavities significantly decreased with decreasing grain size and/or increasing misorientation. It was also found that, when compared to CG alloy, cavitation as well as diffused necking was less prevalent in UFG alloys, which was presumably due to the higher value of strain-rate sensitivity. Based on the several theoretical models describing the cavity growth behavior, the cavity growth mechanism in UFG alloys was suggested.

Superplastic Behavior of Ultrafine Grained Al Alloys Fabricated by Severe Plastic Deformation

Ultrafine grained (UFG) 5083 Al and 5154 Al alloys were prepared by equal channel angular pressing (ECAP) with an effective strain of ~ 4 or ~ 8. This investigation was aimed at examining the effect of the ECAP strain and post-rolling inducing different microstructure in these alloys on the deformation mechanisms at low temperature superplastic (LTS) and high strain superplastic (HSRS) regimes. The sample after 4 passes (a strain of ∼ 4) did not exhibit LTS, but superplastic elongations were obtained in the sample after 8 passes (a strain of ∼ 8). An analysis of the mechanical data in light of the standard deformation mechanisms revealed that deformation of the sample after 4 passes was governed by dislocation climb while grain boundary sliding attributed to LTS of the sample after 8 passes. In addition, the 5154 Al alloy processed by ECAP and postrolling was capable of enhancing HSRS elongation significantly. An analysis revealed that the deformation mode was changed from dislocation viscous glide to grain boundary sliding by additional ECAP strain and post-rolling.

Load Relaxation Behavior of Ultra-Fine Grained Ti-6Al-4V Alloy

In this study, superplastic deformation behavior of ultrafine-grained Ti-6Al-4V alloy was investigated on the basis of the inelastic deformation theory which consists of grain matrix deformation and grain boundary sliding. Specimens with coarse equiaxed grains (11μm in diameter) were significantly refined (≈0.3μm in diameter) with high angle boundaries after 4 times of isothermal equal channel angular (ECA) pressing at 600°C. Load relaxation test was performed at 600 ∼ 700°C to enlighten the deformation mechanisms operating at specific temperature and to find optimum superplastic forming conditions for ultrafine-grained structures. Main efforts were devoted to analyze quantitatively the relative amount of each deformation mode, i.e., dislocation glide and grain boundary sliding operating at specific temperature. Introduction Superplastic forming/diffusion bonding (SPF/DB) method is widely applied in the aerospace industry due to significant benefit of weight and cost savings as compared to the conventional forming methods. Accordingly, considerable attempts have been made to investigate the superplastic deformation behavior of aerospace structural materials such as Ti-6Al-4V alloy. In contrast to its commercial success, however, superplastic forming of Ti-6Al-4V alloy is limitedly applied under the conditions of high temperatures (above 850°C) and low strain rates (below 10/s) [1] with the grain size above 1μm. To date, a number of grain refining processes are extensively reported to fabricate ultrafine-grained (UFG, below 1μm) materials, which lead to achieve superplasticity at low temperatures/high strain rates. Equal channel angular (ECA) pressing, one of the severe plastic deformation (SePD) processes, has turned out to be an effective method for producing large bulk UFG materials without inducing any internal porosity and dimensional changes [2], and has been investigated for Ti-6Al-4V alloy [3, 4]. Recently, Ko et al. [4] examined superplastic elongation (≈400%) of UFG Ti-6Al-4V alloy at relatively low temperatures (600 ~ 700°C) and established a general framework for characterizing superplastic deformation behavior associated with strain rate sensitivity. However, detailed mechanisms of superplasticity in UFG Ti-6Al-4V alloy were not clearly understood. Recently, an inelastic deformation theory involving constitutive relation for dislocation glide and grain boundary Materials Science Forum Online: 2005-01-15 ISSN: 1662-9752, Vols. 475-479, pp 2955-2960 doi:10.4028/www.scientific.net/MSF.475-479.2955