A.K. Mondal

On the comparative assessment of ratcheting-induced dislocation density in 42CrMo4 steel by X-ray diffraction profile analysis and hardness measurement

ABSTRACT The phenomenon of ratcheting occurs under the influence of non-zero mean stress during cyclic loading; it singificantly reduces the low cycle fatigue life of engineering structures. The present investigation deals with a detailed comparison on the estimation of dislocation densities in the 42CrMo4 steel induced by ratcheting using two different methods, i.e. X-ray diffraction (XRD) profile analysis and hardness. The dislocation densities in the ratcheted specimens were assessed using XRD profile analysis following the modified Williamson–Hall method as well as hardness measurements following the modified Nix and Gao model. The results showed that dislocation density increased in the ratcheted specimens as compared to the unratcheted ones and increase in accumulation of ratcheting strain was correlated with the increase in dislocation densities in the ratcheted specimens. It was established that both hardness and X-ray diffraction profile analysis methods can very effectively be used to assess the dislocation densities in the ratcheted specimens.

Correlation of Microstructure and Electrochemical Corrosion Behavior of Squeeze-Cast Ca and Sb Added AZ91 Mg Alloys

The effect of individual and mixed additions of calcium (Ca) and antimony (Sb) on the corrosion behavior of the squeeze-cast AZ91 alloy have been investigated. The corrosion resistance of all modified alloys is better as a result of the refined and reduced volume fraction of the β-Mg17Al12 phase as well as grain refinement. Individual additions are better than mixed additions. The AZ91-1.0Ca (AZX911) alloy comprising individual Ca addition with a continuous network of the Al2Ca phase reveals the lowest corrosion rate. Among the alloys comprising combined additions, the AZ91-2.0Ca-0.3Sb (AZXY9120) alloy exhibits the best corrosion resistance because of the higher and lower volume fractions of the Al2Ca and Ca2Sb phases, respectively.

Damage Assessment of A356 Al Alloy Under Ratcheting–Creep Interaction

The aim of this report was to examine the influence of asymmetric cyclic stress on the ratcheting behavior of A356 Al alloy with special emphasis on its postratcheting creep behavior. A series of A356 alloy specimens were deformed under asymmetrical cyclic loading with different combinations of mean stress and stress amplitude. These tests were carried out up to 2000 cycles. Followed by ratcheting, the specimens were subjected to impression creep tests under varied stresses and temperatures. It is revealed from the ratcheting tests that strain accumulation increases with increasing stress amplitude or mean stress. However, total accumulated ratcheting strain of the investigated alloy was significantly low compared to that reported for some other aluminum alloys. The results of creep tests indicated that predominantly dislocation climb–assisted creep occurred for the alloy. Postratcheted specimens exhibited higher creep rates compared to that of the as-received A356 alloy; this fact was attributed to the work softening of the specimens during the impression creep test. The extent of work softening was minimum in the specimen that accumulated the highest strain during ratcheting, leading to its lowest creep rate.