The effect of hydrogen on the cyclic deformation behavior of Waspaloy in two heat treatment conditions was assessed via low cycle fatigue tests. H-charged and non-charged samples of both heat treatments were cycled to saturation to facilitate a comparison of stress amplitude, back stress, effective stress and shear bands behavior. For both aging conditions (weak coupling domain), H charging leads to a significant reduction in the stress amplitude, which is supported by the decrease of the effective stress and an increase of the back stress. Atomic force microscopy and transmission electron microscopy of these samples revealed a decrease of the shear bands spacing, with a modification of the shear bands widths and the dislocation density in bands. This result suggests that hydrogen impacts the dislocations-precipitates interaction.

This study is focused on a through characterization of the microstructure and chemical composition of dissimilar linear friction welded joints. First, Ti-6Al-2Sn-4Cr-2Mo (Ti6242) and Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti17) joints are described to identify the mechanisms behind the successful cohesion of the material surfaces. It was found that continuous dynamic recrystallization and cohesion took place through grain-boundary migration in a shared single phase β-domain obtained through high temperature and high strain conditions. Then, the possibility of assembling Ti6242 and Inconel 718 is examined. Chemical results combined with microstructure and crystal orientation analyses showed that the intermixing zone formed between the materials was particularly heterogeneous, with the development of an intermediate bimodal microstructure, mainly composed of Ti-β phase and Ti2Ni intermetallic phase.

Understanding the effects of wheel-rail contact on the microstructure of rails is an important issue for railway management. The impact of wheel-rail contact and surface preparation on the microstructure of rails is studied using a rolling contact bench. Microstructure changes are characterized by coupling microhardness measurements and scanning electron microscopy combined with electron backscattering diffraction. This analysis led to a complete description of the sub-surface microstructure in link with the contact conditions. It was found that the use of a corroded layer on the material surface led to a considerable strain-hardening decrease. Lower surface strain-hardening was also found for sliding conditions compared to pure rolling conditions. EBSD characterizations using different indicators highlighted the importance of the scale of investigation: the use of Kernel Average Misorientation led to the identification of larger impacted depths than the Inverse Pole Figures.

The prediction of stress level during Resistance Spot Welding at high temperature is very useful to reduce and/or avoid Liquid Metal Embrittlement phenomenon. However, the experimental estimation of stress level into metal sheets during a Resistance Spot Welding process is impractical. Therefore, a numerical methodology to predict mechanical stresses, using more accessible experiments, is proposed in this work.

This study presents a successful attempt to reproduce White Etching Layer (WEL) under pure mechanical conditions, with the use of a new experimental test bench with tests conditions representative of the wheel-rail contact. The effect of the presence of a run-in or corrosive fuse layer at the rail surface has a great effect on the microstructure evolution. First, a tribological analysis of the effect of the surface preparation on material flows in the contact, hardness evolution, and formation of WEL is presented. Then a multiscale characterization approach combining optical microscopy, scanning electron microscopy and near surface EBSD characterization at the sub-mu m level has been carried out to understand the formation mechanism of such mechanically formed WEL. The different scenarios of the wear behavior and microstructural transformation were drawn. The influence of the conditions of preparation of the sample surfaces, in particular of the presence of a tribological fuse layer (either a run-in layer or a corrosive layer) on the wear fatigue competition and the WEL formation, were also studied.

Galling resistance of different stainless steels was investigated using the ASTM G98 standard. Galling resistance is often only addressed via galling threshold but an increasing number of studies nowadays focus on galling severity. During these studies, three galling categories have recently been identified in stainless steel, based on surface topography evolution, SEM observation, and local chemical analyses. These three categories of galling, namely tolerant, moderate galling, and severe galling have been depicted but still poorly understood. The objective of this work is to determine the relationships between the microstructure, its evolution, and the galling response of the different materials. The authors aim to clarify these relationships and propose an explanation of the consequences of galling on the microstructure of the galled samples. A correlation between the galling severity and the subsurface plastic behaviors is proposed. In particular, the mobility of dislocations in close surface is investigated as a plausible parameter determining galling severity.

This study focused on the comparison of the microstructure changes induced in two pearlitic rail steels subjected to full-scale wheel-rail contact rig tests (DB Systemtechnik, Germany): a conventional R260 steel grade and a heat-treated premium R370CrHT rail grade (also known as MHH400). The total loading of the wheel/rail contact rig test was computed to be 2.4 million gross tons but aimed at reproducing large deformations caused by loadings around 100 million gross tons into the materials. The description of the microstructure showed the ability of the rig to reproduce large loadings. Through the coupling of optical and SEM observations, EBSD measurements and hardness tests along the depths of the tested rails, it was shown that the plastic deformation was much more confined in R370CrHT than in R260. This thorough comparison of the microstructure changes will help the building of wear scenarios.

This paper focuses on the galling mechanisms occurring in stainless steels and aims to provide a better comprehension of the effects of microstructure on galling resistance. Five stainless steels are studied in this paper, namely Nitronic60, AISI660, 316L, 316LN (austenitic stainless steels) and Uranus45 N (duplex austenite-ferrite). Both surface topography and in-depth microstructure are characterized in order to determine the consequences of galling apparition. Experimental investigations at macroscopic and microscopic scales show that galling can occur following several mechanisms. Galling leads to either adhesive wear spots randomly distributed on the surface (tolerant galling), adhesive wear initiated on the periphery of the pin (moderate galling) or abrasive wear and smearing (severe galling). Depending on these categories, the galling threshold and severity are highly variable. Studying these specific mechanisms can help us predict and eventually increase galling resistance for a given material couple. Thus, several microstructural investigations have been performed in order to discuss about the possible origins of these galling categories.

The mechanical behavior of two mono-material Ti-alloy joints obtained by Linear Friction Welding (LFW) were investigated through micro-tensile testing combined with full-field measurements; the resulting tensile behaviors were examined in dependence of the local microstructures and crystallographic features. The near-α Ti–6Al–2Sn–4Zr–2Mo (Ti6242) and the β-metastable Ti–5Al–2Sn–2Zr–4Mo–4Cr (Ti17) welds were tested up to failure in both as-welded (AW) and heat treated (PWHT) configurations. In the AW Ti17 joint, the dissolution of the hardening α precipitates in favor of a βmetastable phase resulted in a remarkable loss in stiffness and strength compared to the as-received Widmanstätten α+β base material (BM). The gradients of microstructure led to significant heterogeneities in the local mechanical behaviors and subsequent strain incompatibilities between the refined weld center line (WL) and the neighboring coarse-grained thermo-mechanically affected zone (TMAZ). These features caused an early and intense strain localization in the joint ultimately causing failure by quasi-cleavage at the WL/TMAZ border. Transforming the AW microstructures through β annealing followed by α+β ageing permitted to form back a Widmanstätten α+β microstructure in the whole assembly resulting in homogenized elastic and plastic behaviors. In the joint core of the AW Ti6242, the precipitation of α’/α” martensitic laths or αsecondary fragments upon cooling the high temperature βtransformed microstructure caused a noteworthy refinement and a consistent weld overmatch; a slight loss of stiffness was also noted. The microstructures neighboring the refined joint did not exhibit singular microstructural changes but the dissolution of αsecondary platelets initially hardening the β matrix and a probable resorption of hardening particles. These phenomena appeared to have weakened these heat-affected zones (HAZ) leading to an intense slip occurrence in the latter and a quasi-cleavage fracture. The heat treatment consisting of an α+β annealing followed by ageing permitted to homogenize the stiffness values through the reprecipitation of equilibrium α+β microstructures across the joint. The formerly softened HAZ was hardened back and exhibited a mechanical behavior similar to the neighboring BM. A gradual microstructure refinement was preserved across the joint ensuring a remaining slight weld overmatch.

Linear friction welding (LFW) of near-α Ti-6Al-2Sn-4Zr-2Mo (Ti6242) and β-metastable Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti17) was studied through eight sets of process parameters. Varying the main LFW process parameters revealed that: (1) increasing the ratio between the normal pressure and local flow stress shortens the duration of friction phase (III); this ratio is influenced by the normal pressure and/or heat generated by the longitudinal deformation conditions, the latter being driven by the oscillation parameters (amplitude and frequency); (2) the joint and PAZ extents can be drastically lowered by favoring the extrusion of the heated material through higher normal pressures; (3) the presence of defects is mostly due to contaminant layers initially present on the contact surfaces; these defects can be dissipated into the bulk with the help of an enhanced recrystallization and/or material blending; (4) the two-component {110}〈111〉 β texture intensity is mostly influenced by the amplitude and degree of recrystallization. Subjecting three significantly different Ti17 joints to β-annealing resulted in similar homogenized microstructures and defect dissolution. The different material responses of Ti17 and Ti6242 to LFW showed the necessity of defining optimized sets of process parameters depending on the welded materials and initial microstructures.

Linear Friction Welding (LFW) is a solid-state joining process producing narrow joints mainly developed for the aircraft industry. The thermo-mechanical loads involved in LFW lead to significant local microstructural changes. This study aimed at identifying the mechanisms impacting these changes in order to develop a Post-Weld Heat Treatment (PHWT) optimizing the joint microstructure. The temperature fields showed that a zone of 1 mm on either side of the weld center line experienced thermo-mechanical processing in the beta-domain for 2 s followed by a rapid cooling to 400 degrees C. Inspection of the weld by Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) revealed a strongly affected microstructure characterized by a sharp microstructural refinement and the presence of defects at the interface. The joint consists of: 1) the Welding Line (WL) which underwent a complete alpha -> beta transformation accompanied by the recrystallization of the prior-beta grain and the development of a (110}<011> texture followed by intragranular precipitation of textured alpha' Hexagonal Close-Packed (HCP) martensitic laths; 2) the Thermo-Mechanically Affected Zone (TMAZ) characterized by a partial alpha -> beta transformation resulting in a microstructure refinement by alpha variant selection upon cooling. A third zone, the Heat Affected Zone (HAZ), was revealed as having a microstructure indistinguishable from the base material (BM) but being slightly harder. The texture analysis of the reconstructed beta phase in the joint core showed that the local deformation conditions were asymmetrical between the forging and the oscillating part and that the WL may have experienced a complex material stirring with turbulent flow. These microstructural changes generate an increase in hardness in the joint with a maximum increase of HV0.3 by 40% in the WL. The PWHT consisting of an alpha + beta annealing followed by ageing resulted in an alpha' -> alpha + beta decomposition and alpha globularization in the TMAZ leading to a gradual microstructure refinement from the BM to the WL. A rather homogenous hardness was obtained across the assembly after the PWHT.

The resistance of metal alloys to fatigue can be classified into four major regimes: low-cycle fatigue (or short life), limited resistance (between 10 5 and 10 6 cycles), high-cycle fatigue (between 10 6 and 10 7 cycles) and gigacycle (more than 10 7 cycles).This chapter introduces the basic concepts of cyclic mechanical behavior, crack initiation and propagation in these different regimes.

This work presents a computational protocol, based on a variance analysis integrated with the bootstrap method, to investigate surface damage resulting from localization of plastic deformation in slip bands of a Ni-base superalloy. For assessment of fatigue crack initiation, surface roughness is introduced to quantify the degree of slip irreversibility. The contribution of this study is to provide a most relevant roughness parameter that can extract the salient features of slip bands within damaged grains, and thus enabling the discrimination of damaged and undamaged grains for description of fatigue-induced surface damage. Results suggest that the maximum peak height (Sp) is the most relevant parameter. And a very pronounced variability of Sp values is observed for damaged grains.DOI: http://dx.doi.org/10.5755/j01.ms.24.1.17684

Galling mechanisms have been investigated in several stainless steels following ASTM G98 test method. Galled samples have been studied by both surface and in depth analysis. This characterization leads to the determination of new galling categories.

Linear Friction Welding (LFW) is a solid-state joining process producing narrow joints mainly developed for the aircraft industry. The Thermo-Mechanical Processing (TMP) loads imposed on the material cause complex microstructure transformations. This study aimed at characterizing the local microstructures and crystallographic configurations to identify the mechanisms impacting the stability and the mechanical properties of a Ti17 LFW joint with the objective of establishing an innovative homogenizing Post-Weld Heat Treatment (PWHT). The process caused a narrow zone of +/- 2 mm from the friction interface to reach the isothermal a -> beta transformation temperature under a heating rate of 500 K/s followed by quenching (-100 K/s). The TMP resulted in the formation a +/- 3 mm wide soft Process Affected Zone (PAZ) delimited by three characteristic zones: the Welding Line (WL width approximate to 400 mu m), The Thermo-Mechanically Affected Zone (TMAZ width approximate to 1 mm) and the Heat Affected Zone (HAZ). beta TMP resulted in the total dissolution of the alpha phase in the joint core (WL + TMAZ) and formed strongly textured {110}< 011 > beta recrystallized grains in the WL by continuous dynamic recrystallization of squeezed prior-beta grains in the TMAZ. alpha + beta TMP led to a gradual loss of hardness in the HAZ whose microstructure is similar to the one of the base material (BM) but is subject to gradual a laths dissolution due to the local temperatures experienced during LFW. A PWHT consisting in a beta annealing followed by an alpha + beta ageing resulted in a uniformly hardened weld with a homogenized Widmanstatten microstructure in the whole assembly.

The opportunity to define a microscopic law of fatigue crack initiation using Manson-Coffin law formulated in terms of cyclic slip irreversibility deduced from AFM measurements is discussed for a polycrystalline superalloy with different grain sizes and precipitate sizes. The results show that the modified Manson-Coffin law, relating cyclic slip irreversibility parameter to fatigue crack initiation life, is sustained through a two-parameter power law: ε ′ f and c. The analysis suggests that the exponent c-value can be related to the degree of plastic strain incompatibility between grains, and the cumulative irreversible cyclic plastic strain to crack initiation is a relevant damage indicator for crack initiation. Consequently, our approach allows giving a physical base of engineering law.

The goal of this work is to increase understanding of plastic deformation of the pearlitic microstructure in the wheel/rail contact. The tridimensional gradient of microstructure below the running band of a worn R260 rail is investigated using multi-scale approach based on microstructural observations by optical and Scanning Electron Microscopy (SEM-FEG), microindentation and Electron BackScatter Diffraction (EBSD) investigations. Due to severe plastic deformation, work hardening is progressively experienced by the rail. In the middle of the running band, the pearlitic colonies are fragmented by accumulation of severe shear strain up to 3 mm in depth. At the rail surface, the resulting lamellar structure is elongated and aligned in the shear direction. At a transition depth of 3–4 mm, both fragmented and unaffected pearlitic colonies are observed. In these fragmented colonies, cementite lamellae are heavily bent and partly broken. Correspondingly, a strong increase of large angle grain boundaries (LAGB) is measured. The interlamellar spacing progressively decreases from this transition depth to the near surface. This quantitative analysis of the tridimensional gradient of microstructure will contribute to improve modeling of rail plasticity and crack propagation by RCF by including anisotropy of the running band and effect of in-depth microstructure evolution.