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T. G. Nieh

Bio: T. G. Nieh is an academic researcher. The author has contributed to research in topics: Strain rate & Superplasticity. The author has an hindex of 1, co-authored 1 publications receiving 15 citations.

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TL;DR: In this article, the superplastic properties and microstructure evolution of a 0.15%Zr and 0.7%Cu modified 6061 aluminium alloy were examined in tension at temperatures ranging from 475 to 600°C and strain rates ranging from 7 × 10-6 to 2.8× 10-2 s-1.
Abstract: The superplastic properties and microstructure evolution of a 0.15%Zr and 0.7%Cu modified 6061 aluminium alloy were examined in tension at temperatures ranging from 475 to 600°C and strain rates ranging from 7 × 10-6 to 2.8 × 10-2 s-1. The refined microstructure with an average grain size of about 11 μm was produced in thin sheets by a commercially viable thermomechanical process. It was shown that the modified 6061 alloy exhibits a moderate superplastic elongation of 580% in the entirely solid state at 570°C and ϵ = 2.8 × 10-4 s-1. Superior superplastic properties (elongation to failure of 1300% with a corresponding strain rate sensitivity coefficient m of about 0.65) were found at the same strain rate and a temperature of 590°C, which is higher than the incipient melting point of the 6061 alloy (~575°C). The microstructural evolution during superplastic deformation of the 6061 alloy has been studied quantitatively. The presence of a slight amount of liquid phase greatly promotes the superplastic...

19 citations


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TL;DR: In this article, an Al-Li-Mg-Sc alloy with an initial grain size of ∼60μm was processed by equal-channel angular extrusion (ECAE) at 300°C up to a total strain of 12.
Abstract: An Al–Li–Mg–Sc alloy with an initial grain size of ∼60 μm was processed by equal-channel angular extrusion (ECAE) at 300 °C up to a total strain of 12. Transmission electron microscopy (TEM) and orientation imaging microscopy (OIM) were employed to establish the mechanism of grain refinement. It was found that new ultrafine grains evolved by a strain-induced continuous process, which is termed continuous dynamic recrystallization (CDRX). At ɛ ∼ 1, a well-defined subgrain structure had developed. Upon further straining the average mis-orientation of deformation-induced boundaries increased; low-angle boundaries (LAB) gradually converted into true high-angle boundaries (≥15°) (HAB). At ɛ ∼ 4, arrays of boundaries with low and high angle mis-orientations were observed. At ɛ ∼ 12, a structure dominated by HAB with an average grain size of ∼0.9 μm was formed. This size is roughly similar to that for subgrains developed at preceding strains. It was shown that CDRX occurs homogeneously; the formation of new grains takes place both along initial boundaries and within interiors of original grains as well.

197 citations

Journal ArticleDOI
TL;DR: Superplasticity in crystalline solids was studied in this article, where the authors showed that the superplastic properties of solids can be inferred from the properties of the solids themselves.
Abstract: (1989). Superplasticity in crystalline solids. International Materials Reviews: Vol. 34, No. 1, pp. 312-312.

39 citations

Journal ArticleDOI
TL;DR: In this paper, an Al-Li-Mg-Sc-Zr alloy was fabricated by an ingot metallurgy technique and subjected to intense plastic straining through equal channel angular extrusion at three different temperatures, 240, 325 and 400°C.
Abstract: An Al–Li–Mg–Sc–Zr alloy was fabricated by an ingot metallurgy technique and subjected to intense plastic straining through equal channel angular extrusion at three different temperatures, 240, 325 and 400°C. The superplastic properties and microstructure evolution of the alloy were examined in tension in the temperature interval 250–500°C at strain rates ranging from 1·4 × 10−5 to 1·4 s−1. Superior superplastic properties (elongation to failure of 3000% with a corresponding strain rate sensitivity coefficient m of ∼0·6) were attained at 450°C and ɛ=1·4 × 10−2 s−1 in samples processed at 400°C to a total strain of ∼16. The alloy in this state had an average grain size of ∼2·6 μm and the recrystallisation fraction was about 90%. It was shown that the highest superplastic ductility appears in samples with more uniform microstructure containing the highest portion of high angle boundaries. It was established that the uniformity of structure and its stability under superplastic deformation is more impo...

39 citations

Journal ArticleDOI
TL;DR: In this article, a 7055 aluminum alloy subjected to intense plastic straining through equal channel angular extrusion (ECAE) was studied in tension over a range of strain rates from 1.4 × 10-5 to 5.6× 10-2 s-1 in the temperature interval 300 - 450 °C.
Abstract: Superplasticity in a 7055 aluminum alloy subjected to intense plastic straining through equal channel angular extrusion (ECAE) was studied in tension over a range of strain rates from 1.4 × 10-5 to 5.6 × 10-2 s-1 in the temperature interval 300 - 450 °C. The alloy had a grain size of ~ 1 μm. A maximum elongation to failure of ~750% occurred at a temperature of 425 °C and an initial strain rate of 5.6 × 10-4 s-1, with a strain rate sensitivity coefficient m of about 0.46. The highest m value was ~0.5 at a strain rate of 1.4 × 10-3 s-1 and T≥ 425 °C. Moderate superplastic properties with a total elongation of about 435% and m of ~0.4 were recorded in the temperature interval 350 - 400 °C; no cavitation was found. It was shown that the main feature of superplastic behaviour of the ECAE processed 7055 aluminum alloy is a low yield stress and strong strain hardening during the initial stages of superplastic deformation. Comparing the present results with the superplastic behaviour of the 7055 Al subjec...

29 citations

Journal ArticleDOI
TL;DR: In this paper, the superplastic behavior of an Al-Mg-Sc-Zr alloy with a grain size of 0.7µm produced by equal channel angular pressing (ECAP) was examined in the temperature interval 150-500°C at strain rates ranging from 10−5 to 10−1 s−1.
Abstract: The superplastic behavior of an Al–Mg–Sc–Zr alloy with a grain size of ~0.7 µm produced by equal channel angular pressing (ECAP) was examined in the temperature interval 150–500 °C at strain rates ranging from 10−5 to 10−1 s−1. No significant grain coarsening occurs under static annealing up to 450 °C because of the strong pinning effect of Al3(Sc, Zr) dispersoids. The alloy showed superior ductility of 365 pct at 175 °C, 1200 pct at 275 °C and ~3300 pct at 450 °C and strain rates of 1.4×10−4 s−1, 5.6×10−3 s−1 and 5.6×10−1, with corresponding strain rate sensitivities of 0.3, 0.49 and 0.2, respectively. Analysis of the superplastic behavior in terms of threshold stress and surface observations showed that grain boundary sliding (GBS) controlled by grain boundary diffusion is the dominant deformation mechanism under all of the conditions. The strong pinning effect of coherent Al3(Sc, Zr) particles leads to a high dislocation density within grains after superplastic deformation that leads to initial strain hardening at low temperatures and high threshold stress. Analysis of the superplastic behavior showed that the strong temperature dependence of the threshold stress is most likely attributable to the interaction between dislocations and the coherent dispersoids, and the effect of temperature on the optimal strain rate of superplastic deformation associated with the highest values of elongation-to-failure is attributable to absorption of a lattice dislocation by a grain boundary rather than the climb of this dislocation along the boundary.

23 citations