Authors: H Watanabe T Mukai K Higashi
Publish Date: 2008/07/11
Volume: 39, Issue: 10, Pages: 2351-
Abstract
The parametric dependencies of threshold stress for superplastic flow were characterized over a wide range of temperatures using a finegrained magnesium alloy with various grain sizes The threshold stress was proportional to the reciprocal of the square root of the grain size at 473 K whereas it was independent of the grain size at 598 K The threshold stress decreased with increasing deformation temperature more quickly than the shear modulus did It was suggested that the temperature dependence of the threshold stress is associated with the interaction between the solute atoms and dislocations The origin of the threshold stress probably resulted from the dislocations breaking away from the solute atmospheres formed in the grain boundaries and in the lattice at low temperatures and at high temperatures respectivelyGrain growth during superplastic flow caused both by the exposure at high temperatures and by welldocumented straininduced grain growth has a visible effect on the stressstrain curves because the flow stress during superplastic flow increases with increasing grain size work hardening occurs as a result of grain growth In fact due to the ease of grain growth in several magnesium alloys a graingrowth effect on the flow stress is suggested to be the origin of threshold stresslike behavior in the lowstrainrate range66 The authors however assumed that the grain sizes after static annealing before straining remain unchanged during the tensile deformation up to ε = 01 for this reason the effect of grain growth during deformation to ε = 01 on the calculated thresholdstress values was neglected throughout that articleIn the present study the linearextrapolation method was adopted to calculate the thresholdstress values Inspection of Figures 5 and 7 revealed that the n = 2 provided a linear extrapolation even in the lowstrainrate range If the threshold stress varies with exposure time or strain rate the data will not provide a linear extrapolation The linearity over a wide range of strain rates observed in the present study indicates the absence of significant grain growth even in the lowstrainrate rangeIn order to experimentally confirm the grainsize stability of the materials used in the present study both the strainhardening behavior and graingrowth behavior in the gage portion were examined under several extreme conditions the slowest strain rates or the highest temperatures The arrows in Figures 2 and 4a show the deformation conditions examined in Appendix A The microstructures of the gage portion were observed after plastic deformation to ε ≈ 01 at which the flow stress was determinedTrue stresstrue strain curves up to ε = 02 for a the asextruded alloy deformed at 523 K and 1 × 10−5 s−1 b the asextruded alloy deformed at 623 K and 1 × 10−3 s−1 and c the annealed alloy deformed at 473 K and 3 × 10−6 s−1 True stresstrue strain curve for the asextruded AZ31 alloy deformed at 623 K and 1 × 10−3 s−167 is also shown in bThe average grain sizes d and distribution of grain size in the PM ZK61 before deformation upper rows and after deformation to ε ≈ 01 lower rows a and b for the asextruded alloy deformed at 523 K and 1 × 10−5 s−1 c and d for the asextruded alloy deformed at 623 K and 1 × 10−3 s−1 and e and f for the annealed alloy deformed at 473 K and 3 × 10−6 s−1It is beneficial to examine the elastic modulus of the present alloy at elevated temperatures The material used for measurement was ZK60 Mg6 mass pct Zn05 mass pct Zr alloy assuming similar elastic properties between ZK61 and ZK60 This will be confirmed from the fact that the slight difference in chemical composition does not generally affect the elastic moduli Hotextruded ZK60 which was annealed at 723 K for 5 hours before the measurement was machined into a rectangular specimen The specimen had a length of 60 mm a width of 10 mm and a thickness of 15 mm The annealing treatment led to some sort of bimodal grain mixture with fine grains 15 μm in size and coarse grains 100 to 500 μm in size The dynamic shear modulus G was measured up to 493 K in every 10 K by employing the method of resonant vibration in cantilever specimen holdingThe measured dynamic shear modulus of ZK60 decreased linearly with increasing temperature up to ∼450 K This is in contrast to the behavior of AZ91 Mg9 mass pct Al1 mass pct Zn alloy with a grain size of 70 to 300 μm68 the temperature dependence of the measured modulus did not deviate from linearity dG/dT = −00082 GPa K−1 at least up to 623 K The deviation in ZK60 may be ascribable to the relaxation from the occurrence of GBS65 which results from the existence of fine grains 15 μm in size The dynamic unrelaxed shear modulus of ZK60 which was derived by fitting the data from RT to 450 K was calculated to be G = 1833 − 00090 T where the units of G and T are GPa and K respectively The shear modulus from 473 to 623 K used for the present analysis was estimated by extrapolation
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