Application of a Composite Model in the Analysis of Creep Deformation at Low and Intermediate Temperatures
The creep behaviors of TA2 and R60702 at low and intermediate temperature were presented and discussed in this paper. Experimental results indicated that an apparent threshold stress exhibited in the creep deformation of R60702. Meanwhile, the primary creep phase was found as the main pattern in the room temperature creep behavior of TA2. Compared with exponential law, the power law has been proved to be a proper constitutive model in the description of primary creep phase. It also showed that θ projection method had its significant advantage in the evaluation of accelerated creep stage. Thus, a composite model which combined power law with θ projection method was applied in the creep curves evaluation at low and intermediate temperature. Based on the multiaxial creep deformation results, the model was modified and discussed. A linear relationship existed between composite model parameters and applied load. Finally, the creep life could be accurately predicted and the composite model method is suitable for application in low and intermediate temperature creep life analysis.
Creep behaviours of Cr25Ni35Nb and Cr35Ni45Nb alloys predicted by modified theta method
Abstract Creep behaviours of Cr25Ni35Nb and Cr35Ni45Nb alloys were investigated under 22–50 MPa at 1173 K, 1198 K and 1223 K. The stress dependence of minimum creep rate and rupture life obeyed power law, and the stress exponent values were similar for both alloys indicating similar deformation and fracture mechanism. These alloys also obeyed Monkman–Grant and Modified Monkman–Grant Relations, and Cr25Ni35Nb exhibited higher MG and MMG constants. It can be concluded that Cr35Ni45Nb has higher creep strength than Cr25Ni35Nb, whereas the creep ductility is not as good as Cr25Ni35Nb alloy. A modified theta equation was proposed to estimate the creep behaviours, where the random errors of experimental data from the fitted creep curves were smaller than traditional theta equation. The obtained theta parameters revealed good linear relations as a function of stress, and the predicted creep behaviours were in good agreement with experimental ones.
Submicron polymer structures with X-ray lithography and hot embossing
This article describes the process chain for replication of submicron structures with varying aspect ratios (AR) up to 6 in polymethylmethacrylate (PMMA) by hot embossing to show the capability of the entire LIGA process to fabricate structures with these dimensions. Therefore a 4.7 μm thick layer of MicroChem 950k PMMA A11 resist was spin-coated on a 2.3 μm Ti/TiOx membrane. It was patterned with X-ray lithography at the electron storage ring ANKA (2.5 GeV and λc ≈ 0.4 nm) at a dose of 4 kJ/cm3 using a Si3N4 membrane mask with 2 μm thick gold-absorbers. The samples were developed in GG/BDG and resulted in AR of 6–14. Subsequent nickel plating at 52°C resulted in a 200 μm thick nickel tool of 100 mm diameter, which was used to replicate slit-nozzles and columns in PMMA. Closely packed submicron cavities with AR 6 in the nickel shim were filled to 60% during hot embossing.