J. Mendez

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.

Influence of casting defects on the fatigue limit of nodular cast iron

The high cycle fatigue behaviour of a nodular cast iron has been investigated under tension loading. Casting defects are at the origin of crack initiation for all samples tested. Critical defects (shrinkage) are located either at the surface or within the bulk. The role of the size and position of natural defect on the fatigue limit has been established. Results show that, for a given size, internal defects are less damaging than surface located ones. SEM observations revealed the existence of non-propagating surface cracks below the fatigue limit. By means of a marking technique, the 3D shape of these cracks initiated around the defects was fully described. In an attempt to simulate the evolution of the fatigue limit with defect size, some existing approaches are compared to experimental data; it is shown that 2D analysis of defects (crack or notch) is conservative.

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.

Low cycle fatigue behavior in vacuum of a 316L type austenitic stainless steel between 20 and 600°C Part I: Fatigue resistance and cyclic behavior

Abstract In the present work, the fatigue behavior of a 316L type stainless steel was investigated in the temperature range 20–600°C. Fatigue experiments at constant plastic strain amplitude were carried out in vacuum in order to determine the intrinsic effect of temperature on fatigue lifetime and cyclic properties. A particular behavior has been pointed out at intermediate temperatures (200–400°C) at which dynamic strain ageing takes place. This behavior is characterized by a strong secondary cyclic hardening and at the same time by a significant increase of the fatigue resistance. In air the role of temperature is affected by the environment coupled effects.

Influence of environment on low cycle fatigue damage in Ti6Al4V and Ti 6246 titanium alloys

An experimental study of the initiation and propagation of short cracks naturally obtained on low cycle fatigue (LCF) smooth specimens has been carried out on β and αβ forged titanium alloys. Tests were done at room temperature in ambient air and in vacuum. Three different microstructures have been used. Regularly interrupted tests showed that crack initiation is delayed when tests are performed in vacuum. Surface crack densities were lower in vacuum than in air. Crack growth rates can be either strongly slowed down or unaffected by inert media, depending on the microstructure. Crack initiation sites were modified by air environment for the β forged alloy only. For one αβ forged alloy, an internal failure origin occurred in vacuum. The influence of environment on both crack initiation and short crack propagation is discussed in terms of effective slip length and stress-assisted hydrogen embrittlement.

On the effects of temperature and environment on fatigue damage processes in Ti alloys and in stainless steel

Abstract The low cycle fatigue resistance of different Ti alloys, in the temperature range 20–500°C, are presented and discussed as a function of their microstructure. The intrinsic effect of temperature and the role of environment are distinguished by comparing fatigue behaviour in air and in vacuum. Fatigue damage processes were characterized through qualitative and quantitative data on crack initiation and small crack growth. An intrinsic beneficial effect of temperature was observed in terms of cyclic plasticity. A significant effect of environment on fatigue life was noted at 300 and 500°C. In Ti alloys, this effect is mainly related to crack initiation, crack growth being weakly affected. However in some cases no difference was observed between tests performed in air and in vacuum. This behaviour is analysed by considering the role of each type of microstructural elements in the initation of the fatal crack. Similar effects of temperature and environment were obtained on a 316 L stainless steel in the range 20–600°C. In this case, environment fundamentally affects crack growth.

Effect of testing atmosphere (air/in vacuo) on low cycle fatigue characteristics of a duplex stainless steel

The effect of testing atmosphere on low cycle fatigue characteristics of a duplex stainless steel has been studied at room temperature. Fatigue tests have been conducted under fully reversed plastic strain control and constant plastic strain rate in two different environments: air and in vacuo. The material has been investigated under two distinct conditions: as annealed or unaged and as thermally aged, corresponding to different dominant cyclic deformation mechanisms at the plastic strain amplitudes chosen for the study. In vacuo testing resulted in longer fatigue lives, and consequently, higher cumulative plastic strain than in air experiences for both material conditions. Although prominent fatigue micromechanisms for a given plastic strain amplitude did not seem to be affected by testing atmosphere, for both unaged and aged conditions, strain localization and cracking phenomena were enhanced in air as compared to vacuum. The experimental results were finally discussed in terms of fatigue micromechanisms-environment interactions.

On the high cycle fatigue behavior of duplex stainless steels: Influence of thermal aging

Abstract In this investigation the high cycle fatigue (HCF) behavior of an austenite-ferrite duplex stainless steel (DSS) is studied as a function of aging at 475 °C. It is found that the HCF strength and the fatigue sensitivity of DSS increase with aging. The fatigue strength improvement results from the higher cyclic yield stress of the spinodally-hardened ferrite which induces an increasing difficulty for early propagation, into and through the ferritic matrix, of microcracks nucleated within austenite. The fatigue sensitivity impairment is suggested to be correlated to an easier microcrack nucleation stage in the aged DSS as compared to the annealed or unaged one.

Cyclic behaviour and surface slip features in 〈111〉 oriented copper single crystals

Abstract New information about cyclic behaviour and surface deformation features have been obtained on 〈111〉 oriented copper crystals cycled in air and in vacuum at different stages of fatigue life. In this way a secondary hardening stage has been observed and different types of slip events have been distinguished; in particular, evidence of {100} glide has been obtained which had never been observed before at room temperature in cyclically deformed copper crystals.

Determination of young's modulus by a resonant technique applied to two dynamically ion mixed thin films

Abstract A resonant frequency technique developed to determine the elastic constants and the internal friction of isotropic and anisotropic bulk materials from 300 K to 1500 K was extended to the determination of Youngs moduli of thin coatings. This paper presents the measurement technique and the associated calculations. Results for very thin films(approximately 2 μm) of SiC and NiTi obtained by dynamic ion mixing (DIM) are presented. This technique, which involves ion sputtering evaporation combined with a high energy ion implantation, gives well adherent and homogeneous coatings. It is shown that such films have very low Youngs modulus compared with the classically prepared crystalline materials. It is assumed that theselow values have their origin in the amorphous structure of the DIM films.