The key microstructural factors affecting the resistance to edge cracking of two cold-rolled dual-phase (DP) steels, a DP600 and a DP800 DH, were investigated. The resistance to edge cracking was characterised by the hole-expansion ratio, λ, which was measured according to ISO 16630:2017. It was shown that for both steels λ could be greatly improved by adjusting the temperatures in the continuous annealing line. However, the adjustments also resulted in changes in the mechanical properties of the materials, in particular the yield strength, Rp0.2. For both materials the increase in λ correlated with a decrease in the proportion of martensite at the stamped edge, which was approximated by the total proportion of martensite and retained austenite in the bulk microstructure, and an increase in the proportion of bainite. There was no correlation between λ and the hardness of the ferrite and martensite in either steel. For the DP800 DH an increase in λ correlated with (i) a decrease in the intensity of the crystallographic texture, characterised by maximum orientation distribution function, f(g)max, and the proportions of the microstructure corresponding to the α and γ fibres, and (ii) a decrease in grain size resulting from a more complete austenitisation of the microstructure. For the DP600 there was no correlation between the crystallographic texture and λ.

RCF tests were conducted on bearings lubricated with three different lubricants: a gearbox oil, a MAC fluid and PFPE oil. The decomposition of the MAC fluid and PFPE was investigated by FTIR and NMR. Gearbox oil tests showed flaking and networks of subsurface cracks with partial intercrystalline crack growth. Cross-sectional analysis revealed the formation of WEAs. FEM correlated the damage with stress fields. The results of the MAC lubricated tests showed surface flaking and intercrystalline crack growth in the subsurface. The PFPE lubricated bearings showed no signs of hydrogen induced damage. Post-experimental hydrogen analysis of the bearings indicated an increased hydrogen concentration in the samples lubricated with the gearbox oil and MAC compared to those lubricated with PFPE.

A coupled diffusion-mechanical finite element simulation model was developed to study the diffusion of hydrogen in a cylindrical roller thrust bearing (CRTB). The simulations enabled obtaining qualitative information pertaining to stress-assisted diffusion in tribological loading and under the influence of residual stresses. The mechanical behavior of bearing steel was obtained from experiments and supported by literature. In parallel, rolling-contact fatigue (RCF) tests on CRTBs lubricated with a fully additivated transmission oil were conducted for 25 h and 50 h to investigate their premature failure modes. Post-RCF rheological analysis indicated decrease of the lubricant viscosity due to degradation. Carrier gas hot extraction analysis indicated a significant increase in hydrogen content in the bearings tested for 50 h. Whereas, serial cross-sectional analysis revealed the formation of subsurface White Etching Crack (WEC) networks associated with White Etching Areas (WEA); the cracks breached the surface on multiple positions causing flaking and eventual failure. On the other hand, no signs of damage were observed in the bearings tested for 25 h. The simulations revealed insignificant hydrogen accumulation due to stress-assisted diffusion in comparison to concentration gradient driven diffusion; hydrogen trapping was pronounced in zones undergoing plastic deformation due to the formation of deformation induced trapping sites. However, residual stresses had an evident influence on the subsurface accumulation of hydrogen. A comparison between the zones of elevated hydrogen concentration due to residual stresses and RCF induced subsurface damage yielded a good correlation.

The recently developed CH-W® 800 hot-rolled steel is specifically developed for automotive chassis applications that require both high strength and outstanding formability. A completely ferritic microstructure allows hole expansion ratios of 90% and more, which indicates the remarkable formability of the material. The tensile strength of at least 800 MPa is mainly due to its very fine-grained microstructure as well as titanium carbide nanoprecipitates.

The influence of Ti on hydrogen diffusion and trapping in various advanced high strength steels was investigated. Electrochemical hydrogen permeation tests were performed on various model steels, with and without Ti, with benchmark tests performed using a commercial steel variant. The hydrogen trapping parameters for each steel were determined by fitting the permeation curves with a finite element model based on the McNabb and Foster equations using least squares minimisation. The influence of Ti on the hydrogen trapping parameters was greatly dependent on microstructure, with ferrite-containing grades being most affected. The results are inconsistent with hydrogen trapping by TiC particles, but consistent with trapping by boundaries between neighbouring ferrite and martensite grains.

In the present study the decomposition mechanisms of a multi-alkylated cyclopentane (MAC) lubricant on a steel surface were investigated in a ball-on-disk sliding test under vacuum. The lubricant was analyzed in-situ using a mass-spectrometer to study possible tribochemical reactions with the metal surface. The degradation of the lubricant was correlated with the formation of gaseous reaction products including hydrogen and various alkanes and alkenes. An increased carbon concentration was detected using XPS up to a depth of 1µm below the surface of the wear track. This was attributed to reactions between the lubricant and the metal surface involving carbon chain cracking and fragmentation of the lubricant. An oxidation reaction between the lubricant and metal oxides was also identified. From the sliding test results it was shown that the rate of formation of gaseous reaction products increased with increasing load and sliding speed. Flash temperatures were calculated according to the Kuhlmann-Wilsdorf model. Finally, a correlation between the frictional power density and the evolution of hydrogen was established.

Corrosion of high-purity Mg-Al alloys containing 3, 6 and 9 wt.% Al was investigated using electrochemical noise (EN) analysis. Changes in the EN signals were correlated with macroscopically observable events on the sample surfaces, which were recorded using time-lapse photography. Steady-state corrosion in all alloys was highly localised. For the Mg3%Al alloy, localised corrosion was preceeded by a period of general corrosion. The corrosion rate of the as-cast Mg9%Al alloy was less than those of the other materials but increased with solution annealing time. The results are consistent with strong galvanic coupling between Al-depleted and -enriched zones of the a-phase. (C) 2015 Elsevier Ltd. All rights reserved.

Hydrogen induced intergranular embrittlement has been identified as a cause of failure of aerospace components such as combustion chambers made from electrodeposited polycrystalline nickel. Accurate computational analysis of this process requires knowledge of the differential in hydrogen transport in the intergranular and intragranular regions. The effective diffusion coefficient of hydrogen may be measured experimentally, though experimental measurement of the intergranular grain boundary diffusion coefficient of hydrogen requires significant effort. Therefore an approach to calculate the intergranular GB hydrogen diffusivity using finite element analysis was developed. The effective diffusivity of hydrogen in polycrystalline nickel was measured using electrochemical permeation tests. Data from electron backscatter diffraction measurements were used to construct microstructural representative volume elements including details of grain size and shape and volume fraction of grains and grain boundaries. A Python optimization code has been developed for the ABAQUS environment to calculate the unknown grain boundary diffusivity. (C) 2015 Elsevier B.V. All rights reserved.

This chapter contains sections titled: Introduction Experimental Method Results Discussion Conclusions

The susceptibility to stress corrosion cracking (SCC) of hot-rolled AZ31 Mg alloy sheets with different levels of deformation were evaluated by constant extension rate tensile (CERT) testing in distilled water. The normalised macroscopic SCC susceptibilities of the materials, characterised in terms of the normalised UTS and elongation-to-failure, were relatively consistent; however, the predominant mode of SCC propagation was strongly dependent on the microstructure. For all materials, cracks were initiated by localised film rupture and dissolution. Crack propagation through fine-grained, recrystallised regions was intergranular, whereas crack propagation through the larger, original grains was transgranular and tended to follow {101¯2} twin boundaries. In both cases, the crack propagation mechanism was a form of hydrogen embrittlement.

Hydrogen embrittlement (HE) is a serious and costly industrial problem that affects many commonlyused structural metals.Given the wide range of service environments in which hydrogen may occur or be produced, this represents a very serious threat to the structural integrity of machinery and infrastructure in many industries.Despite having been studied for several decades, there is still little consensus regarding the underlying mechanisms for HE.Recently-developed theoretical and experimental methods, which enable the evaluation of the influences of hydrogen on the mechanical behaviour of metals at the nano-scale, are helping to elucidate new aspects of these mechanisms.However, there remains an urgent need to develop tools for the prediction of the reliability and lifetime of materials and components affected by hydrogen.Critical to achieving this goal is the development of accurate descriptions of hydrogen-microstructure interactions under conditions relevant to those occurring in service.Since these interactions occur at all length scales, this poses a true multiscale challenge.

Diffusion and trapping of hydrogen (H) in bodies undergoing combined rolling and sliding contact has been evaluated using finite element analysis. The elastic stress–strain conditions of the bodies were calculated in 3D for a single rotation. The stresses were then used to simulate H diffusion in a single plane close to the contact point over large numbers of cycles. The distribution of deformation-induced defects was approximated by relating an isotropic hardening model to the dislocation density. The influence of the defects on H diffusion was evaluated using the McNabb & Foster model assuming local equilibrium as per Oriani. The influence of residual stresses, such as those occurring in bearings after manufacturing, and frictional heating were also considered. The results show that slightly elevated H concentrations occur in the plastic zone conditions and that the increase in H concentration is due to trapping by deformation-induced defects. The influence of stress-assisted diffusion is small due to (i) the short period of time a point on the contact surface spends under load relative to the period of rotation; and (ii) the spatial separation of the hydrostatic and von Mises components of the contact stresses.

Stress corrosion cracking (SCC) of Mg alloys is intergranular (IGSCC) or transgranular (TGSCC). A continuous or nearly continuous second phase, typically along grain boundaries, causes IGSCC by microgalvanic corrosion of the adjacent Mg matrix. IGSCC is expected in all such alloys, typical of most creep resistant alloys, because each known second phase has a more positive corrosion potential than the matrix a-Mg; the degree of severity depends on the electrochemical properties of the second phase; these electrochemical properties need to be studied. Nearly continuous second phases can be avoided by Mg alloy design. TGSCC is most likely caused by an interaction of hydrogen (H) with the microstructure. A study of H-trap interactions is needed to understand this damage mechanism, and to design alloys resistant to TGSCC. Understanding is urgently needed if wrought alloys are to be used safely in service, because prior research indicates that many Mg alloys have a threshold stress for SCC of about half the yield stress in common environments including high-purity water.

Variable polarity, gas-tungsten arc welds were made on roll-cast AZ31 magnesium sheet using AZ61 filler metal. Weld and base metal coupons were evaluated in air and buffered saline solution to produce S-N fatigue curves. When tested in air, both welds and base metal demonstrated a fatigue limit, with the weld failing in the heat-affected zone (HAZ) at a reduced fatigue life. In saline solution at low cycle-high stress, there was a notable reduction in fatigue life attributed to crack initiation at corrosion pits. At high cycle-low stress, failure of welds shifted to the base metal where there was severe corrosive attack and reduced load bearing cross-section. Controlled immersion tests confirmed this shift in attack from the HAZ and weld metal to the base metal at long times. Although polarization curve measurements showed the weld metal to be the most reactive (i.e. more negative corrosion potential), the gradual formation of a stable passive film eventu-Bally protects the weld metal and HAZ from further attack. By alloying more Al in the anodic weld metal (e.g. increasing AZ61 filler dilution), its corrosion potential is shifted closer to that of the base metal and corrosive attack is minimized. Higher Al content also results in higher amounts of eutectic β, and smaller grain size and higher hardness in the weld metal, matching that of the base metal.

Stress corrosion cracking (SCC) of Mg alloys is intergranular (IGSCC) or transgranular (TGSCC). A continuous or nearly continuous second phase, typically along grain boundaries, causes IGSCC by micro-galvanic corrosion of the adjacent Mg matrix. IGSCC is expected in all such alloys, typical of most creep resistant alloys, because each known second phase has a more positive corrosion potential than the matrix α-Mg; the degree of severity depends on the electrochemical properties of the second phase; these electrochemical properties need to be studied. Nearly continuous second phases can be avoided by Mg alloy design. TGSCC is most likely caused by an interaction of hydrogen (H) with the microstructure. A study of H-trap interactions is needed to understand this damage mechanism, and to design alloys resistant to TGSCC. Understanding is urgently needed if wrought alloys are to be used safely in service, because prior research indicates that many Mg alloys have a threshold stress for SCC of about half the yield stress in common environments including high-purity water.

AZ31Mg alloy sheet was welded using a gas-tungsten arc (GTA) process over inserts containing 2.3–9.3wt.% Al. The welded specimens were susceptible to SCC in distilled water, with susceptibility increasing with decreasing weld metal Al (or β particle) concentration. Primary stress corrosion cracks initiated at the weld metal–HAZ interface by stress-assisted localised dissolution and propagated through the weld and base metals by transgranular and intergranular H-assisted fracture (TG-HAF and IG-HAF) respectively. The IG fracture mode may be intrinsic to the texture imparted upon the base metal by rolling. The increase in SCC susceptibility with decreasing weld metal Al concentration is contrary to the purported roles of β particles in promoting localised corrosion and as crack nucleation sites, but corresponds with increases in weld – base metal galvanic current density and weld metal localised corrosion susceptibility.

Fracture mechanics based techniques can be used to study the phenomena of stress corrosion cracking and hydrogen embrittlement and to model the degradation of metallic materials caused by the uptake of atomic hydrogen. Constant extension rate tensile experiments were performed on both smooth and pre-cracked specimens of Mg alloys at various strain rates in a corrosive environment and, for reference, in laboratory air. The experiments show the embrittlement caused by theuptake of atomic hydrogen which is generated in the corrosion reaction of the magnesium . It is discussed whether these experimental findings can be simulated and to some extent rationalised by applying a mesoscale model which has previously been used to mimic the effect of hydrogen embrittlement in steels.