Corrosion Resistance and Corrosion Fatigue Strength of Carbon Steel Coated with Chromium Nitride by Multistage PVD Method

Chromium nitride(CrN) exhibits good corrosion resistance in addition to excellent electrical and mechanical properties. A decrease in the number of defects such as pinholes or voids formed during deposition is required for improvement of corrosion resistance and corrosion fatigue strength of coated materials. A multistage physical vapor deposition(PVD) method was proposed in order to fabricate dense thin-coating films. In an electrochemical anodic polarization test in aerated 2%H_2SO_4 solution, 0.37 wt%C steel that was coated with CrN thin film using the multistage PVD method showed better corrosion resistance than a specimen coated by a conventional method. This is due to a decrease in the number of small defects in the coating film deposited using the multistage PVD method. Evaluation of the corrosion fatigue strength of multistage-coated steel was performed by means of a cantilever-type rotating-bending fatigue test in 3%NaCl solution. The corrosion fatigue strength of multistage-coated steel was improved compared with that of the uncoated steel;however, the increase in fatigue strength was less in the multistage-coated specimen than in the specimen coated by the conventional method, contrary to the experimental results obtained from the anodic polarization test. It was pointed out that corrosion fatigue strength is dependent on not only the number of defects but also the shape of large defects in the coating film

Crack Propagation behavior under Low-Cycle Corrosion Fatigue of A7003-T6 Aluminum Alloy

Crack propagation behavior has been observed in A7003- T6 aluminum alloy under low cyclic loading with hold time over the frequency range 4×10^~1 Hz in 3.0% saline solution. From the experimental results, the effect of cyclic frequency on crack propagation rates was found to be exemplified by two different regimes. One has a positive dependency on the frequency below a critical frequency, f_, at which point maximum environmental attack occurs in terms of da/dN, and the other is negative above f_. The behavior of crack propagation below f_ was explained by the concept of stress assisted dissolution which tends to inhibit mechanical failure by crack blunting and microbranching. The number and depth of secondary cracks occurring under the fracture surface were measured by metallographic examination, and the actual crack tip stress intensity factor was estimated

Effect of Microstructure on Creep and Creep-Fatigue behavior in Ti-6Al-4V Alloy at Elevated Temperature

The effect of microstructure on creep and creep-fatigue behavior at 773K was studied in the Ti-6Al-4V alloy having three different microstructures. The three types of microstructures prepared using different heat treatment conditions included the equiaxed α structure, lenticular α structure and bimodal(composed of equiaxed α and lenticular α)structure. Creep tests were carried out under constant load conditions at 773K in air. Creep-fatigue tests were carried out under total strain controlled conditions using a trapezoidal waveform with hold times of 2min and 10min at 773K in air. Creep rupture strength of the alloy with equiaxed α structure was similar to that of the alloy with lenticular α structure and was higher than that of the alloy with bimodal structure. The number of cycles to failure under creep-fatigue condition of the alloy with lenticular α structure was lower than that of the other two structures. The effect of microstructure on crack propagation life was small as compared with crack initiation life under creep-fatigue conditions. The fracture mode of the alloy with equiaxed α and the bimodal structures was transgranular under creep-fatigue condition. On the other hand, the crack of the alloy with lenticular α structure was initiated and propagated at the interface between the α layer precipitated at the grain boundary and the lenticular α structure