M. Gerland

Evolution of dislocation structures and cyclic behaviour of a 316L-type austenitic stainless steel cycled in vacuo at room temperature

Abstract Dislocation structures formed during cyclic deformation at room temperature in vacuo of a 316L-type austenitic stainless steel are presented. It has been shown that for this material having a rather low stacking fault energy of about 28 mJ m−2, dislocation structures exhibit a planar slip or a wavy slip character depending on the cyclic plastic strain amplitude. In particular the existence of wall and channel, labyrinth or ladder structures has been shown; these structures have been observed to evolve progressively into cells during extensive cycling in vacuo. The volume fraction of each type of structure in the specimen has been evaluated quantitatively as a function of the cyclic plastic strain level and the number of cycles throughout the fatigue life in vacuo.

Low cycle fatigue behaviour in vacuum of a 316L-type austenitic stainless steel between 20 and 600°C—Part II: Dislocation structure evolution and correlation with cyclic behaviour

In this second part, we have reported the evolutions of the dislocation structures of an austenitic stainless steel cycled in vacuum for different temperatures between 20 and 60°C. For each temperature, constant plastic strain amplitudes Δɛp/2 ranging from 6 × 10−4 to 5 × 10−3 have been considered. The dislocation evolutions have been correlated to the cyclic behaviour described in Part I. The slip character appears to be more wavy at 20 and 600°C than at intermediate temperatures, with dislocation structures composed, in the main, of cells and walls-and-channels. Between 200 and 500°C, the number of cycles to failure is higher than at 20 and 600°C and, at the same time, the dislocation arrangements are more planar from the first cycles, up to failure, but cells, walls-and-channels and tangles are less numerous. Instead, another structure appears, which we have called the corduroy structure, composed of alignments of very small defects such as loops, debris and cavities. This structure develops progressively with cycling and is all the more extended as cycling is performed at low strain amplitudes. The optimum temperature for its formation is 400°C. It is shown that the extension of the corduroy structure can be directly correlated to the amount of secondary hardening observed in vacuum between 200 and 500°C. In contrast, the degree of formation of corduroy structure is not directly correlated with fatigue-life evolution. Fatigue lifetime results from the competition between a beneficial effect (planar slip) and a detrimental one (the cyclic stress level). The planar behaviour has been associated with dislocation interactons with C and N solute atoms, which also determine dynamic strain ageing at intermediate temperatures.

Microstructural study of two LAS-type glass-ceramics and their parent glass

The two glass-ceramics studied here derive from the complex system (MgO,ZnO,Li2O)-Al2O3-SiO2 and are obtained by controlled devitrification of the same parent glass. Although they have the same chemical composition, one is a “β-quartz” (or “β-eucryptite”) type while the other one is a “β-spodumene” glass-ceramic. A detailed microstructural analysis of these materials has been performed at different scales by several complementary characterization methods (SEM, TEM, DTA, XRD and FTIR). This extensive study has shown the great microstructure difference (grain distribution, grain size, nature of vitreous and crystalline phases) between these two glass-ceramics obtained from the same parent glass.

Comparison of two new surface treatment processes, laser-induced shock waves and primary explosive: application to fatigue behaviour

Abstract Two surface treatment processes, the one by laser-induced shock waves, the other by primary explosive, have been used on a 316 L type stainless steel in order to improve fatigue behaviour. In spite of very different shock characteristics (18 GPa, 0.6 ns for the laser treatment and 1.5 GPa, 1 μs for the explosive treatment), the microstructures and the microhardness profiles are rather similar. The significant increase of microhardness in the first 50 μm in depth is associated with a high twin density. The fatigue behaviour of the two treated samples is characterized by very high cyclic stresses during the whole cycling and slightly shorter fatigue lives than in the untreated material. The surface of the treated samples exhibits more considerable secondary microcrack density than the untreated material because of the high twin density of the near surface treated layers.

Effect of pressure on the microstructure of an austenitic stainless steel shock-loaded by very short laser pulses

Irradiation of metallic targets by a high-energy pulsed laser can generate in materials shock waves with pressure amplitudes of the same order as with conventional shocks from explosives, flyer plate impact etc., but with much shorter pulse durations. Experiments were performed with a 0.6 ns pulsed laser on 304 austenitic stainless steel samples. The effects of induced pressure on the microstructure were investigated by transmission electron microscopy in addition to microhardness measurements and are compared with the conventional results. The twin density and the presence of α-phase are particularly studied. In spite of the very short pulses, twins were present in the observed areas whatever the pressure, while α-phase embryos were only present in the pressure range 15–25 GPa.

Explosive cladding of a thin Ni-film to an aluminium alloy

A thin Ni-film has been cladded to an aluminium alloy by the use of an explosive. Two film thicknesses (50 and 100 μm) and four explosive compositions have been used. The cladding has been characterised by surface topography, microhardness measurements, optical and scanning electron microscopy, and energy dispersive spectroscopy; residual stresses through the film and the bonding have been determined by X-ray diffraction. In all cases, a good quality bonding is obtained, with a low roughness on the film surface. Compressive residual stresses have been measured from the surface to the bonding.

Deformation mechanisms in a TiNi shape memory alloy during cyclic loading

The deformation mechanisms governing the cyclic stress-strain behaviour of a TiNi shape memory alloy were investigated in this work. To understand the development of these mechanisms during cyclic loading, three low-cycle fatigue tests were performed and stopped at different stages. The first test was stopped after the first cycle; the second one was stopped after 40 cycles, corresponding to the beginning of the stabilisation of the cyclic strain-stress behaviour; and the last one was carried out to failure (3324 cycles). Submitted to fatigue loading, the response of the TiNi shape memory alloy presents a classical pseudoelastic response. Two deformation mechanisms, identified by TEM observations, are highlighted, the first one by twins and the second by dislocation slip and its interaction with precipitates. These two mechanisms evolve without competition during cyclic loading. The nanomechanical properties of the alloy were also examined, and the evolution of the microhardness or indentation modulus was monitored.

Fatigue crack propagation resistance of a FeAl-based alloy

The fatigue crack growth behavior of a powder metallurgically produced iron aluminide of composition Fe-40 at.%Al is investigated. The material is processed by mechanical alloying followed either by extrusion or by hot isostatic pressing. The resulting microstructures are examined by transmission electron microscopy. The test results indicate that the processing route has almost no influence on the resistance to fatigue crack propagation. Special emphasis is given to the roles of crack closure and environment. The fatigue crack growth resistance is shown to be significantly reduced in ambient air. The results, including closure measurements, indicate that this is due to an environmental enhancement of the growth process in conjunction with a lack of complementary shielding from oxide-induced closure. This enhancement appears to be mainly governed by the moisture content of the test atmosphere. Oxygen adsorption is proved to be efficient in preventing this embrittling effect. The mechanisms of embrittlement are discussed and a limited influence of hydrogen on the fracture process is suggested.

Dislocation structure and corduroy contrast in a 316L alloy fatigued at (0.3–0.5) Tm

Abstract An austenitic stainless steel 316L was fatigued under vacuum at intermediate temperatures. At 300 °C the fatigue life of this alloy is much higher than at room temperature or at 600 °C. This behaviour is associated with the occurrence of the Portevin-LeChatelier effect and with the presence of a corduroy contrast and a corduroy dislocation microstructure. These different features are described and a coherent explanation is proposed.

Surface treatment of a 316L type stainless steel by explosive: microstructural characterization and monotonic tensile behaviour

A new surface-treatment process using a thin layer of primary explosive was applied to a 316L type stainless steel. The induced microstructural modifications and the residual mechanical properties of the treated material have been evaluated. The surface roughness quality and the microhardness increase are higher than after usual shot-peening treatments. The near-surface microstructure, observed by transmission electron microscopy, is composed of numerous mechanical twins the density of which decreases with increasing depth. The yield strength (0.2% offset) of the treated layer has been evaluated and related to the mean value of the microhardness in this layer.