Existence Of Threshold Stress Research Articles

Creep Behavior and Microstructural Evolution of Al–Cu–Mg–Ag Alloys with Various High Cu Contents

The creep behavior and microstructural evolution of three Al–Cu–Mg–Ag alloys with Cu content around its solid solubility limit in Al (5.65 wt %) were investigated at 180–240 °C and applied stress of 150–300 MPa. The creep resistance of aged alloy, which is mainly determined by the number density of Ω phase, is the best for 6.00 wt % Cu, better for 5.30 wt % Cu, and the worst for 5.65 wt % Cu. After solid-solution treatment, the lowest Cu content in the Al matrix for the alloy with 5.65 wt % Cu is observed due to the existence of more residual phases. It results in the lowest number density of Ω phase the following aging and poor creep resistance. Increasing temperature from 180 to 240 °C at the same stress (225 MPa), the steady creep rate of alloys increases by 225 times, which is apparently larger than that (26 times) for increasing stress from 225 to 300 MPa at the same temperature (180 °C). It indicates that the coarsening of the Ω phase with increasing temperature should be more serious than that with increasing stress. The creep mechanism of Al–Cu–Mg–Ag alloy can be attributed to the dislocation climb with the existence of threshold stress.

Creep Deformation Behavior of Inconel 617 Alloy in the Temperature Range of 650 °C to 800 °C

Creep behavior of the Inconel 617 alloy has been investigated through tests carried out in the temperature range of 650 °C to 800 °C under 95 to 350 MPa. Creep curves obtained from tests on the alloy exhibit a non-classical nature, with the primary creep rate decreasing to a minimum value, followed by a typical increase, which is either continuous at temperatures ≤ 700 °C or has an intermediate steady-state regime at 750 °C or 800 °C. The variation of minimum creep rate (έmin) with stress (σ) obeys the power law relationship (έmin = Aσn), with the stress exponent, n, increasing with temperature to a peak value at ~ 700 °C. The value of n > 5 has been rationalized by considering the existence of threshold stress, which like n has been found to decrease sharply with temperature beyond 700 °C. Investigation of the post-creep microstructures by transmission electron microscope has revealed the formation of γ′ and M23C6 precipitates, which obstruct dislocation motion during the primary stage. Microstructures of the samples creep-tested at ≥ 750 °C have exhibited relatively smaller amounts of γ′. The existence of threshold stress and the steady-state regime in the tertiary creep stage has been ascribed to obstruction of dislocation motion by M23C6 and γ′ precipitates.

Superplastic Deformation Behavior of the Y-TZP Tested in Torsion

Constant torque torsional creep tests for 3Y-TZP have been conducted in an ambient atmosphere at 1623K to 1773K in the torque range of 0.53 to 1.51Nm. While there are no traces of primary and tertiary creeps, the creep-rate decreases with the increase in strain due to concurrent grain growth. In the shear stress versus shear strain-rate relations obtained in the present experimental range, there exist two regions; one in which m-value is 0.43 to 0.45 and the other in which m-value is 0.21 to 0.22 above and below the shear stress of around 14MPa, respectively. In the former, i.e., high m region, grain size exponent, p ≃ 2 and activation energy, Q=443KJ/mol, whereas in the latter, i.e., low m region, p ≃ 3 and Q=490KJ/mol. The former region corresponds to the superplastic region II. Regarding the latter region, it seems to be relating to the existence of threshold stress.

Harper-Dorn creep in alpha-zirconium

Harper-Dorn (H-D) creep in alpha-zirconium has been investigated at homologous temperatures 0.35 to 0.48 (773 to 1023 K) and stresses ranging from 4.10 −6 to 9.10 −5 G ( G is the shear modulus) by the helicoid specimen technique. It has been shown that H-D creep takes place at intercept grain sizes larger than about 125μ, while at smaller grain sizes Coble creep operates under the same external conditions. The H-D creep is most probably dislocation core diffusion controlled. A threshold stress for steady state creep has been detected increasing with decreasing temperature. The existence of threshold stress has been qualitatively accounted for by high dislocation density (~10 12m −2) in the specimens tested. The conservative motion of jogs on screw dislocations dependent on dislocation core diffusion has been suggested as the creep rate controlling mechanism. The energy of jog formation has been estimated to which a mean distance between jogs ~ 5 b corresponds at 1000 K, where b is the Burgers vector. Also the observed transient creep has been briefly discussed. The transient components of the creep strain cannot be accounted for exclusively by anelastic bowing out of links of dislocation network.