L. Rémy

The interaction between slip and twinning systems and the influence of twinning on the mechanical behavior of fcc metals and alloys

The analysis of twin-slip and twin-twin interactions in fcc crystals is reviewed using the method proposed by Sleeswyk and Verbraak for the interactions between bcc twins. Slip or twinning dislocations impinging a twin boundary can generally be incorporated into the obstacle twin through often unfavorable dislocation reactions. Transmission electron microscopy observations are in agreement with the predicted mechanisms of stress relaxation. The link with Bollmann’s concepts for more general boundaries as applied to these simple Σ = 3 boundaries is emphasized. However the limitations of such analyses are pointed out particularly their inability to predict the influence of stress conditions. From the analysis and observations of interactions at twin boundaries the influence of twinning on work-hardening and fracture in fcc metals and alloys is tentatively outlined. In particular twinning can give rise to an increase of the flow stress particularly in polycrystals and it is actually responsible of the high mechanical properties of a number of commercial compositions. Experimental evidences of the role of twinning in the fast fracture and the fatigue fracture of fcc materials are reported which are in agreement with the inferences made from the study of various interactions.

Fatigue oxidation interaction in a superalloy—application to life prediction in high temperature low cycle fatigue

A study of the interaction between fatigue and oxidation has been carried out in the case of a cast cobalt base superalloy MARM 509 tested in laboratory air at 900 °C. The influence of fatigue cycling on oxidation of this alloy has been studied by quantitative metallography on polished specimens exposed to air in a furnace and on strain-cycled low-cycle fatigue specimens. The oxidation kinetics were determined by thickness measurements for matrix oxidation and by oxidized depth measurements for the preferential oxidation of MC carbides. In both cases the oxidation kinetics were found to be dramatically enhanced by cycling for the matrix oxidation according to a linear relationship with plastic strain amplitude and less dramatically for carbides according to an exponential relationship with the maximum cyclic stress. From these observations a damage equation which describes fatigue damage as a crack growth process has been proposed: the elementary crack advance is a summation of a mechanical contribution due to the fatigue process itself which is described by Tomkins’ equation and of an oxidation contribution which has been evaluated from metallographic measurements. Integration of this crack growth equation gives predicted fatigue lives which are in good agreement with experimental results within a factor of two.

Behavior of nickel-base superalloy single crystals under thermal-mechanical fatigue

The thermal-mechanical fatigue behavior of AM1 nickel-base superalloy single crystals is studied using a cycle from 600 °C to 1100 °C. It is found to be strongly dependent on crystallo-graphic orientation, which leads to different shapes of the stress-strain hysteresis loops. The cyclic stress-strain response is influenced by variation in Young’s modulus, flow stress, and cyclic hardening with temperature for every crystallographic orientation. The thermalmechanical fatigue life is mainly spent in crack growth. Two main crack-initiation mechanisms occur, depending on the mechanical strain range. Oxidation-induced cracking is the dominant damage mechanism in the lifetime of interest for turbine blades.

Low cycle fatigue damage in nickel-base superalloy single crystals at elevated temperature

Abstract Low cycle fatigue tests on AM1 nickel-base superalloy single crystals were conducted under axial strain control at 650, 950 and 1100 °C. The behaviour of the [001] orientation was investigated at the three temperatures, that of the [111], [101] and [213] specimens was studied at the two lower temperatures. The orientation dependence of fatigue life-total strain range curves was mainly due to variations in Youngs modulus with orientation. Most cracks grow in stege II mode whatever the temperature. Cracks nucleate at micropores and in the interior of specimens at low temperatures; surface cracks induced by oxidation are dominant at high temperatures and low strain ranges. Most of fatigue life is spent in microcrack growth.

High temperature, low cycle fatigue of IN-100 superalloy I: Influence of temperature on the low cycle fatigue behaviour

Abstract Low cycle fatigue tests on cast nickel-based superalloy IN-100 were conducted at various temperatures from 20 to 1000°C in air under continuous-strain cycling at a constant total strain rate. The fatigue life was found to decrease with increasing temperature for a given total strain range. Fatigue cracks are partly crystallographic at low temperatures and strongly oxidized at high temperatures. The reduction in fatigue life is discussed using potential drop measurements and observations on interrupted test specimens. Testing at 1000°C is shown to reduce drastically the crack initiation period and this behaviour is attributed to oxidation.

Fatigue Oxidation Interaction in IN 100 Superalloy

The interaction between fatigue and oxidation has been studied in IN 100,* a cast nickel based su-peralloy tested in laboratory air at 1000 °C. The effect of fatigue cycling on oxidation was studied by quantitative metallography on polished specimens which were oxidized in a furnace and on strain cycled low cycle fatigue specimens. Thickness measurements have shown that matrix oxidation obeys a parabolic kinetics and is strongly enhanced by fatigue cycling. Measurements of oxide spikes have shown that interdendritic oxidation obeys at1/4kinetics and is weakly affected by cyclic straining. A phenomenological equation was proposed which accounts for this interaction through fatigue induced fracture events in the oxide scale. Analysis of the distribution histograms shows that the actual life to crack initiation is identical to the number of cycles to break the matrix oxide scale in the most critical areas.

High temperature low cycle fatigue and thermal-mechanical fatigue behaviour of an oxide-dispersion-strengthened nickel-base superalloy

Abstract The high temperature low cycle fatigue (LCF) and thermal-mechanical fatigue (TMF) behaviour of the oxide-dispersion-strengthened (ODS) nickel-base superalloy MA 760 was investigated. LCF tests were carried out at 950 and 1050 °C. TMF tests were performed using two different temperature ranges, 550–1050 °C and 600–1100 °C. Material was tested parallel to the extrusion direction. Experimental results indicate the influence of strain rate, temperature and shape of the TMF cycle on cyclic stress-strain behaviour, fatigue life, damage and microstructure stability. TMF lives were compared with LCF lives. Metallographic observations were used to study the initiation sites and crack paths as well as the stability of the γ′ strengthening phase. At very high temperature, MA 760 exhibits delamination at longitudinal grain boundaries parallel to the applied stress and reduction in size of the γ′ precipitates within a factor of 10–30. The effects of such a microstructure change are discussed in terms of a cyclic stress-strain behaviour and lifetime. Finally, a comparison between the ODS alloy and the directionally solidified alloy CM 247-DS is undertaken.

Strain localization phenomena under cyclic loading: application to fatigue of single crystals

Kinematic hardening plays an important role in strain localization phenomena under cycling loading. A single crystal constitutive model including non-linear kinematic hardening is presented. Using a heterogeneous distribution of kinematic hardening variable in a single crystal plate, a continuum model for the formation of intrusion/extrusion is proposed based on FE simulation. The attention is focused on ratchetting phenomena taking place in the localization band. A second example of strain localization is shown by the strain field at the crack tip in single crystals. 2D and 3D finite element computations of the crack tip field in a CT specimen are provided. They are compared to analytical solutions. The 3D computations show that slip activity is different in the bulk or at the surface of the specimen. The evolution of the crack tip field subjected to cyclic loading is investigated. Ratchetting phenomena are shown to take place in some of the localization bands.

An oxidation fatigue interaction damage model for thermal fatigue crack growth

Abstract Thermal fatigue of MAR-M509, a typical cast superalloy for vanes in jet engines, was previously shown to involve mainly crack propagation which is strongly influenced by oxidation. Crack propagation was so modeled using damage equations which are based on local stresses in a volume, element at the crack tip. The volume element at the crack tip was taken as the secondary dendrite size, a material constant. Local stresses ahead of the crack were estimated from Traceys finite element analysis of plane strain small scale yielding which was adapted to a cyclic loading using Rices hypothesis. A fatigue damage equation was fitted to fatigue crack growth data measured at high frequency on CT specimens, which accounts for load ratio effects. High temperature oxidation of precracked CT specimens induced embrittlement at medium temperatures ahead of the oxidized precrack. The critical stress to fracture was found to increase with increasing distance from the oxidized crack tip and to depend on oxygen diffusion and on interdendritic oxide depth, which can be easily measured by quantitative metallography on oxidized specimens. Damage equations were then derived for isothermal fatigue and thermal fatigue crack growth; all the involved parameters were fitted to the experiments on virgin and preoxidized CT specimens. These equations were used to compute the isothermal fatigue life at 900°C to 0.3-mm crack depth and the propagation rate of thermal fatigue cracks in wedge specimens and they were found to give reasonably good predictions in most cases.

Fatigue failure probability in a powder metallurgy Ni-base superalloy

Abstract A probabilistic model of fatigue failure of PM superalloy is presented which takes into account the size, shape and location of the defects which initiate fatigue cracks. The fatigue failure probability is linked with the growth rate of cracks initiated from inclusions, which is described using fracture mechanics. The risk of fatigue failure from surface, sub-surface and internal particles is evaluated and discussed. The model predictions are found in fairly good agreement with experimental results.