No abstract

No abstract

No abstract

OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.

No abstract

A set of tensile tests has been carried out under air environment in the temperature range [450°C-700°C] in order to characterize the interactions between oxidation assisted intergranular cracking and Portevin -Le Chatelier effect. It is shown that the occurrence of jerky flow stops the intergranular damaging mechanism. The construction of a Dynamic strain aging-Intergranular fracture-PLC instabilities map is then proposed. The surprising relation between crack initiation disappearance and PLC instabilities of type C in the temperature range explored is discussed. Finally the assumption of the occurrence of such phenomenon at the tip of a propagating crack is addressed. IntroductionJerky flow also known as Portevin-Le Chatelier effect has been first shown on Al-Cu alloys [1] and, afterwards, has been studied intensively in other dilute metallic alloys especially in steels, aluminium and nickel based alloys [2][3][4][5]. Physical processes involved in this type of flow instability as well as the associated localization of the deformation have been modeled and simulated [6][7][8]. Recent improvements of measuring techniques for deformation fields [9] offer the possibility to check those simulations under several aspects (localization, spatial coupling between bands, bands width). Nevertheless, a few studies are dealing with the role of grain boundaries on PLC plastic instabilities [10][11][12] and consequently, there is a lack of knowledge on the effect of this phenomenon on the evolution of intergranular internal stresses. An interesting way to study this particular point can be found in the scientific field dealing with oxidation assisted intergranular cracking (OAIC) because, in most of the cases, this damaging process requires to operate an intergranular oxidation process together with a high level of intergranular internal stresses. Fournier et al [13] have clearly shown that the occurrence of PLC instabilities in alloy 718 during slow strain rate tests at 400°C stops immediately the intergranular crack propagation. The aims of the present paper are firstly, to bring new experimental results completing at higher temperature the work undertaken by Fournier et al and then, to evaluate the interactions between crack initiation and localization of plastic instabilities. As strain rate controlled tensile tests have been carried out on thin strip, both crack initiation and propagation sensitivity to dynamic strain aging (DSA) have been investigated. Materials and Experimental ProcedureThe material used in this study was obtained through a double melting process: vacuum induction melting plus vacuum arc remelting. The nominal composition of the alloy is given in Table 1. The cast ingot was hot and cold rolled down to a thickness of 1 mm, followed by a solution annealing heat treatment at 1000°C for one hour ended by air quenching.

No abstract

Alloy 718 samples were oxidized in air under experimental conditions close to those experienced during high-temperature heat treatments (shaping: about 1000°C) or in service (turbomachine disks: 650°C). The main objective of the study was to evaluate the impact of these treatments on oxygen penetration into the grain boundaries and, hence, to assess the harmfulness of these penetrations in terms of defect initiation and propagation. The method relies mainly on the SIMS-based imaging technique. This technique, which supplements those applied in previous studies, uses a series of basic maps to locate within the volume the oxygen in free or combined state, in relation with the microstructural features (grain boundaries, grains, delta phase, etc). The results point out to intergranular oxidation for both exposure conditions without clearly demonstrating the existence of atomic oxygen penetrations ahead of the intergranular oxidation front. The affected depths seem sufficient both to form preferential initiation sites (oxidation at 1000°C) and to assist intergranular propagation of defects and/or cracks (oxidation at 650°C).