Chang-Young Hyun

Mechanism of surface modification of a porous-coated Ti-6Al-4V implant fabricated by electrical resistance sintering

A porous-coated Ti-6Al-4V implant was fabricated by electrical resistance sintering, using 480 μF capacitance and 1.5 kJ input energy. X-ray photoelectron spectroscopy (XPS) was used to study the surface characteristics of the implant material before and after sintering. There were substantial differences in the content of O and N between as-received atomized Ti-6Al-4V powders and the sintered prototype implant, which indicates that electrical resistance sintering alters the surface composition of Ti-6Al-4V. Whereas the surface of atomized Ti-6Al-4V powders was primarily TiO2, the surface of the implant consisted of a complex of titanium oxides as well as small amounts of titanium carbide and nitride. It is proposed that the electrical resistance sintering process consists of five stages: stage I – electronic breakdown of oxide film and heat accumulation at the metal-oxide interface; stage II – physical breakdown of oxide film; stage III – neck formation and neck growth; stage IV – oxidation, nitriding, and carburizing; and stage V – heat dissipation. The fourth stage, during which the alloy repassivates, is responsible for the altered surface composition of the implant.

Creep deformation behaviour of AZ31 magnesium alloy over wide range of temperature and stress

Abstract The creep deformation behaviour of coarse grained AZ31 magnesium alloy was examined in the temperature range from 423 to 673 K (0·46–0·73Tm) under various constant stresses covering low strain rate range from 4×10−9 to 2×10−2 s−1. Most shape of the creep curve was typical of class II behaviour. However, only at low stress and low temperature, the shape of the creep curve was typical of class I behaviour. At very low stress at 673 K, the stress exponent for the secondary creep rate was ∼2. At low stress level, the stress exponent was ∼3 and the present results were in good agreement with the prediction of Takeuchi and Argon model. At high stress level, the stress exponent was ∼5 and the present results were in good agreement with the prediction of Weertman model. The transition of deformation mechanism from solute drag creep to dislocation climb creep could be explained in terms of solute atmosphere breakaway concept.

Grain size dependence of flow stress in ECAPed Ti with constant texture

Abstract The effect of grain size on the flow stress in an ECAPed Ti with a constant texture was investigated, assuming that 2, 4, 5 and 6 passes microstructures have a similar texture. The average size of recrystallized grains decreased to 0.5 μm, 0.4 μm, and 0.3 μm with respect to the ECAP pass number of 2, 4, and 6, respectively. The ultimate tensile strength (UTS) and yield strength (YS) increase with an increase in the number of pressing. The UTS and YS of the 6 passes ECAPed sample were found to be 740.2 MPa and 580.3 MPa, respectively. An equation for the flow stress of an ECAPed Ti with a constant texture as a function of the strain and grain size was derived for the ECAPed metal. The following equation was finally obtained: σ(σ)=103.9+1825σ–9.6σ 1/2 · d −1/2 +8.3 d −1/2 .

A study on the creep cavitation in an Ni3Al alloy using the hydrogen embrittlement technique

Abstract In this study, the hydrogen embrittlement technique was employed in order for the direct observation of entire cavities on exposing grain boundary facets in a Ni3Al alloy. Also, stereo pair SEM micrographs were used to investigate the effects of inclined angle of grain boundary on cavity growth, by comparing relative positions of features on the stereo pair. Many of cavities at 65% of the rupture time are on the verge of coalescing with other cavities. Cavities on inclined boundaries had grown for longer time than those on transverse boundaries. Thus, the time to cavity coalescence on transverse facets are lower bound of the material lifetime. And, overall damage was found to have the value of about 0.6 at rupture.