S.K. Hwang

Effects of equal channel angular pressing temperature on deformation structures of pure Ti

Abstract A single equal channel angular pressing at various temperatures (473 to ∼873 K) was conducted on commercially pure titanium in order to investigate the effects of the pressing temperature on the microstructures. Optical and scanning electron microscopy showed that fine shear traces, elongated along the direction of approximately 25° inclined to the longitudinal axis of a sample, were well developed over the whole temperature range. Transmission electron microscopy revealed that the shear traces were composed of parallel bands having a width of a submicrometer scale. It was also revealed that the bands of a sample pressed at 473 K resulted from slip but those of samples pressed above 523 K resulted from {1011} deformation twinning. For the twinned samples, the twin density initially increased with temperature up to 623 K and then decreased at higher temperatures. Contrary to the changes of the twin density, the dislocation density decreased with temperature up to 623 K and then increased. At a pressing temperature of 873 K, fine recrystallized grains were also observed to appear in the vicinity of the shear traces. These anomalous microstructural changes with the pressing temperature were discussed in terms of the characteristics of {1011} twinning produced by equal channel angular pressing.

Effects of nitrogen flow rates on the growth morphology of TiAlN films prepared by an rf-reactive sputtering technique

Abstract Over the last decade titanium aluminum nitride (TiAlN) has been under active investigation as a promising alternative to binary coating materials for cutting and forming tools because of its superior oxidation resistance and lower thermal conductivity. However, the effects of nitrogen flow rates on the reactive sputtering of TiAlN films remain relatively untapped despite their importance in studying the growth morphology to acquire valuable knowledge for the selection of optimum coating conditions. In this paper we report the results of our study regarding the implications of nitrogen flow rate on the growth morphology of the TiAlN films reactively sputtered from a stoichiometric target of TiAl. Scanning and transmission microscopy (SEM, TEM), X-ray diffraction (XRD) and atomic force microscopy (AFM) have been used for film characterization, the latter of which is yet to be fully exploited as a tool for analyzing the effects of N 2 as a sputtering parameter.

Refinement of the lamellar structure in TiAl-BASED intermetallic compound by addition of carbon

Intermetallic compounds based on TiAl are under extensive studies for structural materials in aerospace applications. Recently the effects of interstitial elements on the mechanical properties have been reported, which can be summarized as the enhancement of tensile strength and creep resistance. Appel reported that the creep resistance of TiAl alloy could be increased by carbon addition. Considering the important effect of carbon on the creep resistance of TiAl, it is important to gather more information on the basic role of carbon on the microstructure. In ingot alloys, there is an indication that the details of the fully lamellar structure were affected by carbon or nitrogen. This effect, however, has not been addressed in the intermetallic compound processed by powder metallurgical method. In this respect, the TiAl compound made by EPM (Elemental Powder Method) is of particular interest since this process is a potential alternative to ingot metallurgy for fabricating parts of complex shape. Recently the authors reported that a TiAl-Mn-Mo alloy could be successfully produced by EPM. In the present study, therefore, it was intended to study the effect of carbon on the phase transformation of this alloy of a FL (Fully Lamellar) microstructure.

A grain boundary engineering approach to promote special boundaries in Pb-base alloy

Abstract A grain boundary engineering approach was made to improve the microstructure of a commercial Pb-base alloy for better performance in automobile battery application. For the purpose of enhancing the resistance against intergranular corrosion, cracking and creep, a thermomechanical processing route was sought to promote special boundaries, specifically coincidence site lattice (CSL) boundaries. Through a combination of cold working and recrystallization heat treatment, it was possible to increase the fraction of low Σ CSL boundaries more than 91%. Annealing twins, produced during the thermomechanical processing, were identified as a key factor for regeneration of CSL boundaries. Multiplication of the CSL boundaries was interpreted in terms of the ‘ Σ 3 regeneration’ model proposed by previous authors.

A new strain gage method for measuring the contractile strain ratio of Zircaloy tubing

Abstract An improved strain gage method for determining the contractile strain ratio (CSR) of Zircaloy tubing was developed. The new method consists of a number of load-unload cyclings at approximately 0.2% plastic strain interval. With this method the CSR of Zircaloy-4 tubing could be determined accurately because it was possible to separate the plastic strains from the elastic strain involvement. The CSR values determined by use of the new method were in good agreement with those calculated from conventional post-test manual measurements. The CSR of the tubing was found to decrease with the amount of deformation during testing because of uneven plastic flow in the gage section. A new technique of inscribing gage marks by use of a YAG laser is discussed.

Microstructure evolution in Zr under equal channel angular pressing

AbstractPure polycrystalline Zr was deformed by equal channel angular pressing (ECAP), and the microstructural characteristics were analyzed. By repeated alternating ECAP, it was possible to refine the grain size from 200 to 0.2 µm. Subsequent annealing heat treatment at 550 °C resulted in a grain growth of up to 6 µm. Mechanical twinning was an important deformation mechanism, particularly during the early stage of deformation. The most active twinning system was identified as 85.2 deg {10

The effect of yttrium addition on the oxidation resistance of EPM TiAl-based intermetallics

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Monte Carlo simulation of microstructure evolution based on grain boundary character distribution

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