This work is aimed to understand the mechanisms for evolution and reversed evolution of dislocation structure under variable strain amplitudes, using automotive-grade interstitial-free steels (IF steel) under strain ratio (R) = 0 condition. The microstructures were mainly examined by the SEM under BEI/ECCI mode and TEM were used for this study. Near the endurance limit, the dislocation cells smaller than 2£gm develop preferably along grain boundaries and triple junctions among the grains. Within grain interiors, it is hardly observed these small dislocation cells and cyclic hardening even at £`max =0.2%. When strain amplitudes were controlled at a range from £`max = 0.25% to 0.6%, a secondary cyclic hardening occurs prior to fatigue failure and less than 2um dislocation cells rapidly developed thoroughly. The secondary hardening rates were found to be directly proportional to the strain amplitudes. For high-low strain fatigue tests, while the maximum strain was decreased from 1.2% to 0.2% or 0.15%, dislocation cells were collapsed first and re-grouped into loop-patch structures due to the gliding behavior of dislocations changing from multiple-slips to single-slip. However, once the strain range is further reduced to 0.1%, dislocation cells would persist, showing no signs of collapse. Moreover, the reversal development of dislocation structures is independent of strain ratio. Furthermore newly developed loop patches are usually confined within dislocation domains with very condensed dislocation cell walls with high boundary misorientation.

Fretting phenomenon occurs when two bodies in contact undergo small repetitive relative motion such that the localized surface and subsurface material properties are altered leading to damage or failures. Fretting conditions are obtained by controlling externally applied parameters such as load, frequency of displacement, displacement amplitude. Material properties which influence fretting behavior include hardness, ductility, hardening behavior. External parameters like surface roughness, temperature also play a role in deciding the extent of damage. Based on fretting conditions and specimen geometry, various fretting modes can be classified. Rotational fretting is one such damage mode, observed in industrial applications such as cable ropes under tension used for support in construction industry and variable stator vanes (VSVs) in compressors of turbines. In spite of industrial and engineering relevance, rotational fretting has received little attention. In the present work, rotational fretting of self-mated AISI 4340 material pair was studied, with the objective of characterizing subsurface damage induced by fretting. AISI 4340 (EN 24) is a low alloy martensitic steel with an excellent combination of strength, ductility and toughness. It is widely used in high strength cyclic loading applications like gears, bearings, automobile pistons and aircraft landing gears as well as in low corrosion, high strength offshore applications. It can be readily machined and surface hardened which makes it useful for wear related applications. A novel rotational fretting test set up, capable of operating under various test loads, frequencies, displacement amplitudes and temperatures was used to perform experiments. Specimens were subjected to a combination of normal load and rotational displacement and caused to mutually contact on non-conformal curved surfaces which simulate a bearing or bushing geometry. Fretting results were primarily determined by the frictional torque versus angular displacement plots. The running condition response was linked to the fretting material response regime. Surface and subsurface characterization studies of fretted regions were conducted using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). TEM studies revealed varying levels of fretting induced plastic deformation within the fretted contact zone. Good correlation with available literature relating to formation of dislocation cells and presence of high dislocation density in the fretting damaged regions was established. Although quantifying the dislocation density as a damage indicator is a challenge, it is proposed that a microstructural feature based approach has the potential to be developed into a useful tool for life assessment and life prediction studies.