The effect of alloying additions on the superplastic properties of Ti-6 pct Al-4 pct V

Chang

1998

Metall Mater Trans A

A study has been made to investigate boundary sliding and its accommodation mode with respect to the variation of grain size and ␣/␤ volume fraction during superplastic deformation of a twophase Ti-6Al-4V alloy. A load relaxation test has been performed at 600 ЊC and 800 ЊC to obtain the flow stress curves and to analyze the deformation characteristics by the theory of inelastic deformation. The results show that grain matrix deformation (GMD) is found to be dominant at 600 ЊC and is well described by the plastic state equation. Whereas, at 800 ЊC, phase/grain boundary sliding (P/GBS) becomes dominant and is fitted well with the viscous flow equation. The accommodation mode for fine-grained microstructures (3 m) well agrees with the isostress model, while that for large-grained structures (11 m) is a mixed mode of the isostress and isostrain-rate models. The sliding resistance analyzed for the different boundaries is lowest in the ␣/␤ boundary, and increases on the order of ␣/␤ ϽϽ ␣/␣ Ϸ ␤/␤, which plays an important role in controlling the superplasticity of the alloys with various ␣/␤ phase ratios.

Section: Introductionmentioning

confidence: 99%

Quantitative analysis on boundary sliding and its accommodation mode during superplastic deformation of two-phase Ti-6Al-4V alloy

Chang

1998

Metall Mater Trans A

“…3 represents the flow stress and strain rate sensitivity (m) values as a function of strain rate at 97(FC for the three Ti3Al alloys. It should be noted that an increase in /3 phase causes an increase in flow stress and a decrease in the m-value at strain rates below 5 • 10 ~ sec L. As for the role of /3 phase in conventional (cz+[~) Ti alloys, it is generally believed that an increase in [J volume fraction of up 1~1 50-60% decreases the flow stress and increases the m-value [4][5][6][7]. However, in the present study of (c~2+/3) TidAl alloys, the reverse trend was observed.…”

Section: Methodsmentioning

confidence: 99%

“…The grain growth coefficient (C) in Table 3 increases with the increase in the volume fraction of 13, which implies that the extent of grain growth during the superplastic deformation is accelerated by an increase of the 13 volume fraction. We should be able to observe the enhanced grain growth in the alloy with higher 13 volume fraction since the diffusivity of Ti in [3 phase is about four orders higher than that in the c~2 phase at superplastic temperatures [5,11].…”

Section: Microstructural Evolution During Superplastic Deformationmentioning

confidence: 98%

Effect ofβ volume fraction on the dynamic grain growth during superplastic deformation of Ti3Al-based alloys

Nam

1998

Metals and Materials

The superplastic deformation behavior of 'FinAl based (~+/3) alloy was studied with respect to the volume fraction of ez2/fl. Three alloys containing 21, 50 and 77% in volume fractions of/3 exhibited large tensile elongations of over 500% at 970"C with a strain rate of 2.5 • 10 4 sec 1, The largest elongation was observed in the alloy with 21% of /3. As the volume fraction of ]3 phase increased, the flow stress and correspondingly, the strain-rate sensitivity values decreased. Due to the higher diffusivity of Ti in ,/3 phase than in ~h phase, the increase in ]3 volume fraction from 21% to 77% accelerated the dynamic grain growth, and degraded the superplasticity of the Ti3Al-based alloys. The strain-based grain growth behavior was quantitatively analyzed and incorporated into a constitutive equation. The calculated flow curves are in agreement with the experimental ones in the stable deformation region.

“…Especially, in the accommodation mode,, as well as the matrix deformation mode, two limiting cases or modes may be identified, each described by a rule of mixture relationship. One is an iso-strain rate mode [4], and the other is an iso-strain rate: mode [5]. The distinction between these two modes is particularly important in the case of (cx+13) titanium alloys because the [3 phase shows considerably lower flow stress than the phase.…”

Section: Deformation Modementioning

confidence: 99%

“…However, there has been a large controversy over the mechanisms [3] and deformation modes [4][5][6] of accommodation of GBS. Recently, an internal deformation theory involving a simple rheological model for grain boundary sliding and grain matrix deformation has been proposed by Chang et al [7], and successful in describing overall superplastic deformation behavior of A1 alloys and Zn-AI alloys [8,9].…”

Section: Introductionmentioning

confidence: 99%

Mechanical and microstructural analysis on the superplastic deformation behavior of two-phase ti-6a1-4v alloy

Chang

1998

Metals and Materials

Present investigation has been made to study the superplastic deformation behavior of Ti-6AI-4V alloy based on the theory of inelastic deformation, and m ana~.yze the boundary sliding characteristics by the use of transmission electron microscopy. The flow curves obtained by load relaxation tests at 600~ and 80(FC are fitted well with the inelastic deformation theory. It has been found that the deformation mode for the fine grained microstructures (3 Bin) is a type of iso-stress mode,-while that of coar,;e grained microstmctures (11 gm) is a mixed type of both iso-stress and iso-strain rate modes. The density of dislocations observed near the grain boundaries is higher along t~/[3 phase boundaries than the f~/~ grain boundavies, which confirms that the sliding resistance of a/~ is lower lhan that of c~/13.