Zhou Yigang

A Comparative Study of Various Flow Instability Criteria in Processing Map

Abstract Four instability criteria, namely Murty, Gegel, Malas and Prasad criteria, were compared, and the physical significance of parameters was analyzed in this paper. It is indicated that the instability map developed by Murty criterion is similar to Prasad criterion, showing an unstable flow at high strain rates. Murty criterion exhibits a little narrower unstable region. The instability maps developed by Malas criterion and Gegel criterion have similar shapes, and the unstable regions are wider than those Prasad criterion and Murty criterion. In addition to the unstable flow at high strain rates as predicted by Murty criterion and Prasad criterion, Malas criterion and Gegel criterion have a good ability for predicting unstable flow at high temperatures and low strain rates. The processing maps were validated by hot compression tests of Ti-22Al-25Nb alloy, and the possible causes of various instability were discussed.

Quantitative Analysis on Microstructural Evolution of Lamellar α Phase in Titanium Alloy during Hot Working

Abstract To quantitatively research the microstructural evolution of lamellar α phase in titanium alloys, large size cakes of Ti-17 alloy with initial lamellar microstructure were isothermally-forged at different strains in α+β phase field followed by solid-solution and aging treatment. The changes of α thickness and morphology in different locations (center, midradius, edge) of the cakes deformed to different strain levels were measured and the imposed strain was estimated using finite element analysis. It is found that increasing of forging strains causes an increase in thickness of α phase. The modification of α morphology depends strongly on the forging strains and the locations in the cakes. The distribution curves of α morphology parameter Feret Ratio were characterized by no peak at lower strains, and a single peak appears at Feret Ratio 1.5-2.5 at larger strains. Moreover, the peak values increased with increasing of the height reduction, and the distribution curves at the center have the largest peak values. Increasing of forging strains will cause the high-Feret-Ratio lamellae to decrease but low-Feret-Ratio α to increase noticeably. Sufficiently imposed strains also can improve the microstructure homogeneity in different locations of the cakes. The effect of the effective strains on the percentage of α phase with different morphologies is also quantificationally summarized. The average strains for initiation of globalization are of the order of 0.4 and those for completion are of the order of 1.0, which are lower than that of conventional α+β titanium alloys.

Effect of beta flecks on low-cycle fatigue properties of Ti-10V-2Fe-3Al

The effect of beta flecks on low-cycle fatigue (LCF) properties was investigated at room temperature for a high-temperature (α + β)-processed and also a β-processed Ti-10V-2Fe-3Al alloy. For both the (α + β)-processed and the β-processed material, beta flecks had a significant influence on LCF and tensile properties. The materials with beta flecks showed a loss in LCF life and ductility. Larger beta flecks resulted in lower LCF life and ductility. It seemed that β-forged material contained much smaller beta flecks but had a similar detrimental effect on LCF properties as the (α + β)-processed one. Based on the diffusion calculation, it could be concluded that chemical composition inhomogenieties could not be reduced by using beta forging in the 820 °C β region. Extensive light microscopy and scanning electron microscopy (SEM) observations showed that beta flecks were susceptive to crack nucleation and propagation.