Origin Of Threshold Stress Research Articles

Coupling analysis on controlling mechanisms for creep of Al-Fe-Ni alloy

The dislocation creep of Al−Fe−Ni alloy at both 300 and 400 °C was described by a coupling modification on analyzing the creep constitutive equation after introducing both threshold stress and load transfer coefficient. The quantitative results and microstructure characterizations showed that the complex interaction between dislocation and eutectic phase was the origin of threshold stress, and the modulus difference between matrix and eutectic phase induced the load transfer effect. Furthermore, the temperature effects on the modulus of matrix and eutectic phase were different, leading to the decline of threshold stress and increase of load transfer coefficient at higher temperatures. This was further evidenced by the finite element modelling analysis. Finally, the controlling mechanisms during dislocation creep of eutectic alloys were discussed in an increasingly quantitative manner.

Tensile creep and rupture behavior along with evolution of microstructure in a Zr-2.5Nb alloy

Tensile creep behavior of the Zr-2.5Nb alloy has been studied through tests under constant load (stress range ~ 137–371 MPa) between 275 and 375 °C. The minimum creep rate of the alloy is found to vary with applied stress and temperature following a power-law relation. The values of stress exponent (n) obtained in the range of 5.3–6.8 in the temperature interval of 300–375 °C; whereas it is calculated as 1.2 and 7.0 under low and high stresses, respectively at 275 °C. The apparent activation energy of creep (QC) and stress exponent have been determined by analyzing the experimental data. The obtained QC (~198.5 ± 10.7 kJ/mol) is found to be higher than the lattice self-diffusion activation energy of pure zirconium (113 kJ/mol). Using this value, stress exponent ~5.6 ± 0.23 is obtained by the temperature-compensated power law. Microstructural characterization by transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS) analysis has confirmed coarsening of β-(Nb, Zr) precipitates with compositional changes during creep. The dislocation-precipitate interaction as evidenced by TEM observations is considered to be the origin of threshold stress, which decreases with increasing temperature. Considering the threshold stress, true activation energy of creep (Qt) and true stress exponent (nt) are found as ~160.4 ± 6.9 kJ/mol and ~4.8 ± 0.22, respectively. Analysis of creep data has confirmed the role of dislocation climb as the rate-controlling mechanism, along with validity of Monkman-Grant and modified Monkman-Grant relations. Scanning electron microscopy (SEM) of creep fracture surfaces has revealed evidence for prominent ductile fracture. Further, the obtained creep damage tolerance factor value of 1.9, indicating the predominance for cavitation during creep.

Is creep in discontinuous metal matrix composites lattice diffusion controlled?

Creep data for two discontinuous metal matrix composites, namely 3OSiC p-Al particulate and 26(Al 2O 3) f-Al-5Mg short-fibre composites, were analysed to establish whether their creep is matrix lattice diffusion controlled. It was found that ϵ ̇ m 1 n ( ϵ ̇ m is the minimum creep rate) varies with applied stress σ linearly when the true stress exponent n is set equal to 5. For both composites, the threshold stress σ TH was found to decrease rather strongly with increasing temperature. Good correlation of ϵ ̇ m /D L with ( σ- σ TH)/ G D L is the coefficient of lattice diffusion and G is the shear modulus) for both composites strongly suggests matrix lattice diffusion as the creep-rate-controlling process. However, the origin of threshold stress has not been identified and thus its temperature dependence has not been accounted for. Knowing σ TH as a function of temperature and applying a proper consititutive equation of creep, the apparent activation energy of creep Q c and the stress sensitivity parameter of creep rate m were calculated as functions of applied stress and temperature. These Q c calc and m calc were then compared with Q c exp and m exp following from ϵ ̇ = ϵ ̇ m (σ, T) relations as determined experimentally. While for the 3OSiC p-Al composite the agreement was found to be good, this was not the case for the 26(Al 2O 3) f-Al-5Mg composite.