Concurrent operation of ⟨c + a⟩ slip and twinning under cyclic loading of Ti-6Al-4V

International audienceDeformation processes operating in hard/soft grain pairs were characterized in Ti-6Al-4V specimens with a bimodal microstructure submitted to fatigue and dwell fatigue loadings. Concurrent operation of 1st order ⟨c + a⟩ pyramidal slip and slip-stimulated deformation twinning was observed in hard grains, resulting from multi-scale strain incompatibilities. The former constitutes a direct experimental evidence of stress redistribution under cyclic loadings while the latter, which is triggered by local stress concentrations due to dislocations pile ups, is detected for the first time. Insights into the crack initiation processes are then discussed

Deformation processes near a crack initiation site under dwell-fatigue loading of Ti-6Al-4V

The present article reports an investigation of the mechanism of surface crack initiation of a dwell fatigue tested Ti-6Al-4V alloy with a bi-modal microstructure. Interactions between slip bands and grain boundaries were characterized in order to obtain insights into the crack initiation process and discuss the similarities with models described in the literature. Twinning and local lattice rotation occurred as a result of the slip band blocking at the interface and suggests high local stress concentrations. Nevertheless, crack initiation happened to be intergranular and not transgranular. The crack opened up the basal plane that was located at the interface between two nodules poorly oriented for slip and having a common c axis of the hexagonal unit cell

Analysis of deformation mechanisms operating under fatigue and dwell-fatigue loadings in an α/β titanium alloy

International audienceThis paper investigates the deformation processes operating in α grains of Ti-6Al-4V submitted to fatigue and dwell-fatigue loadings at room temperature. With this aim, mechanical tests were interrupted after a given number of cycles or cumulated plastic strain. Slip traces analysis combined with electron back-scattered diffraction enabled to evidence the operation of <a> and <c + a> slip, twinning and interface sliding. The main parameters controlling their activity were analyzed, in particular regarding the crystallographic orientation and the load hold duration. Based on this statistically sound investigation, the differences between fatigue and dwell-fatigue behaviors are finally discussed

Why titanium alloys withstand more strain under dwell-fatigue than under fatigue?

Experimental observations reveal that crack initiation proceeds at a reduced number of cycles if a load hold at maximum stress is introduced during cyclic testing. Although this feature was extensively investigated and stems from the occurrence of room temperature creep, other differences between fatigue and dwell-fatigue behaviors are still to be clarified. In particular, a higher plastic strain is cumulated prior to failure if the load hold is present. This observation highlights differences in the deformation behavior depending on the applied loading. The present article reports an investigation of this point using Fast-Fourier transforms based crystal plasticity simulations of the stress and strain field. Significant differences were evidenced and discussed in light of experimental results reported in the literature. In particular, the occurrence of crack initiation for different cumulated plastic strain depending on the loading conditions is elucidated

Why titanium alloys withstand more strain under dwell-fatigue than under fatigue?

Experimental observations reveal that crack initiation proceeds at a reduced number of cycles if a load hold at maximum stress is introduced during cyclic testing. Although this feature was extensively investigated and stems from the occurrence of room temperature creep, other differences between fatigue and dwell-fatigue behaviors are still to be clarified. In particular, a higher plastic strain is cumulated prior to failure if the load hold is present. This observation highlights differences in the deformation behavior depending on the applied loading. The present article reports an investigation of this point using Fast-Fourier transforms based crystal plasticity simulations of the stress and strain field. Significant differences were evidenced and discussed in light of experimental results reported in the literature. In particular, the occurrence of crack initiation for different cumulated plastic strain depending on the loading conditions is elucidated

In-Situ SEM and EBSD Analysis on Plastic Behavior of Laser Beam Welding Ti-6Al-4V Alloy [激光焊接Ti-6Al-4V钛合金塑性行为的原位SEM/EBSD分析]

cited By 0EBSD and in-situ SEM were used to study the plastic behavior of laser beam welding Ti-6Al-4V. Due to the high weld temperature(exceeding the β-transus) and high cooling rate, β→α' martensitic transformation occurs. Within the fusion zone, only acicular morphology exists, while in 1st heat affected zone(HAZ) and 2nd HAZ, these acicular laths globalize, which show an obvious microstructural gradient. The result of the in-situ plastic deformation shows the deformation can be divided into 2 stages. In the first stage of the deformation, either slipping or twinning can occur. It depends on whether the initial orientation can respect <120>//TA(tensile axis) by rotating the grains or not. While in the second stage, only slipping can be found. © 2019, The Editorial Board of Materials China. All right reserved

Clinicopathological study of 13 cases of squamous cell carcinoma complicating hidradenitis suppurativa.

International audienceBACKGROUND: To the best of our knowledge, only 52 cases of squamous cell carcinoma (SCC) complicating hidradenitis suppurativa (HS) have been reported since 1958. We describe 13 new cases. METHODS: We propose a clinical and histological analysis of our cases. We include these results in a review of previously reported cases to analyze a total of 65 patients. In our series of 13 cases, we also investigate the presence of human papillomavirus (HPV) in tumor samples, by polymerase chain reaction (PCR) on paraffin-embedded material. RESULTS: Malignant transformation affects mainly men with a long-term history of genitoanal HS. Although our cases were 7 well-differentiated carcinomas and 6 verrucous carcinomas, lymphatic and visceral metastasis occurred in 2 and 3 cases, respectively. With PCR, we demonstrated presence of HPV in genitoanal tumoral lesions, principally HPV-16. CONCLUSION: SCC complicating HS evolves poorly, despite a good histological prognosis. Our results sustain the implication of HPV in the malignant transformation of HS