J.-P. A. Immarigeon

Microstructural instabilities during superplastic forging of a nickel-base superalloy compact

The high temperature flow behavior of a nickel-base superalloy powder compact, prepared by hot isostatic pressing has been examined by means of uniaxial compression testing in terms of the microstructures developed during plastic flow. The tests were done isothermally at 1050 and 1100 °C and at constant true strain rates between 10-5 s-1 and 1 s-1. The fine grained compact exhibits some degree of superplasticity which always increases with compressive flow as the grain structure is refined. The faster the rate of deformation, the finer is the grain size produced at high strains, when steady state conditions of flow appear to develop. By deforming to different strains at a given strain rate or into the steady state regimes at various strain rates, grain sizes in the range 1 to 5 pun were produced. By unloading and restraining the test pieces in situ, the effect of grain size on the onset of plastic flow has been examined and the yield stress observed to increase with grain size. It is shown that, in this material, hardening or softening occurs during flow depending on the size of the initial grains. The changes in microstructure and flow stress observed during deformation are analyzed and the potential offered for control of the microstructure during isothermal forging is discussed.

Modelling of plastic flow in coarse grained nickel-base superalloy compacts under isothermal forging conditions

Abstract A constitutive relationship for predicting the flow stress and the evolution of microstructure during isothermal forging of coarse grained P/M Ni-base superalloy compacts, forged below their γ′ solvus temperatures, is derived. These coarse grained compacts gradually transform to microduplex γ−γ′ microstructures during forging and this leads to softening during plastic flow. To model the transformation and the resulting softening, the material is considered as a composite material consisting of hard (untransformed) and soft (transformed to the fine microduplex γ−γ′ grains) regions. A rate equation for this material is written in terms of that for the hard and the soft regions assuming that both carry the same stress but each region is subjected to different strain rates. This is consistent with the occurrence of flow localization in the transformed microduplex regions as observed experimentally. A single grain microstructural model and modified forms of established transformation kinetics relationships for grain boundary nucleated reactions are proposed for modelling the deformation dependent transformation. The constitutive relationship is consistent with necklace structure formation typically characteristic of these materials, and it is suggested that it can be used to predict the development of grain size gradients and shear instabilities in forgings.

Microstructural Changes during Isothermal Forging of a Co-Cr-Mo Alloy

Interest has evolved recently in thermomechanical processing of the cast Co-Cr-Mo surgical implant alloys such as Vitallium and Vinertia. Work has shown that the wrought forms of these alloys exhibit much improved properties over their as-cast counterparts. In this paper, the response of as-cast Vinertia to isothermal forging is examined by means of isothermal and isostrain-rate compression testing. The effects of temperature, strain rate, and strain on the breakdown of the as-cast micro-structure are examined in detail. The effects of prior heat treatment on plastic flow and microstructure achieved are also considered. It is shown that the interaction between the carbide phase and the recrystallization induced during hot working governs the degree of homogeneity that can be achieved in the forged product. Control of carbide volume fraction, size, and distribution by appropriate prior processing can lead to a fine grain equiaxed structure with uniformly distributed carbides. The potential offered by isothermal forging for control of the microstructure in this type of alloy is discussed, as well as the limits imposed on the process by the starting material and by the strain gradients expected during the forging of implants.

Flow behaviour of Mar M200 powder compacts during isothermal forging

AbstractThe high-temperature deformation behaviour of Mar M200 superalloy powder compacts, pressed below the γ′ solvus, has been examined. Compression tests were carried out at temperatures between 950 and 1200°C and at constant true-strain rates between 10−1 and 10−5 S−1. Superplastic behaviour was found at all the temperatures used and at strain rates below 10−2 S−1. The strain rate sensitivities at low strain rates are in the range 0.4–0.6 while at higher strain rates the exponents fall to between 0.1 and 0.3. In each range the exponent increases with temperature, indicating higher degrees of plasticity. Variations in strain-rate sensitivities are related to the various deformation mechanisms which are thought to contribute to flow. The effects of powder particle mesh size and size distribution on the flow properties of the compacts are also considered. Coarser mesh sizes produce the highest peak flow stresses at all strain rates and temperatures investigated. Compacts made predominantly from fine mesh...

The Fracture Behavior of Tungsten Wire Reinforced Superalloy Composites during Isothermal Forging

The isothermal forging behavior of a wire reinforced superalloy powder composite has been examined. The material consisted of a Mar-M200 matrix containing 40 pct by volume of tungsten wire and was prepared by hot isostatic pressing. Specimens were deformed by uniaxial compression at constant temperature in the range 1050 °C to 1180 °C, and at constant true strain rates between 10-5 s-1 and 10-1 s-1. Loading was normal to the direction of wire alignment. Microstructural defects existing in the as-pressed composites are compared with defects in the forged materials. An upper bound forming limit occurs when fibers come into contact. However, microstructural damage occurs at lower strains which depends on temperature and strain rate. Observed and calculated values of peak flow stress are used to define practical forming conditions for the material which should avoid the formation of internal damage at low strains.

Flow and fracture behaviour of wire-reinforced superalloy composites during isothermal forging

AbstractThe forging behaviour of hot isostatically pressed nickel-base superalloy composites reinforced with 40% tungsten wire has been examined. Specimens were deformed by isothermal compression at temperatures from 1050 to 1180°C and at constant true strain rates of between 10−5 and 10−1 s−1. The loading direction was normal to the direction of fibre alignment. Peak flow stresses for the composites were up to four times higher than those for the non-reinforced matrix under similar working conditions. These differences are explained in terms of variations in microstructure and differences in directionality of flow in the two types of material. A forming limit of a geometric nature is described whereby damage is induced beyond a critical strain which depends on the volume fraction of fibres and their stacking arrangement. Damage in the form of voids and cavities was introduced at strains well below this critical strain. This damage occurred as a result of stresses at the tensile poles of the fibres normal...

Spread and fracture patterns in forging superalloy fibre-reinforced composites

Abstract Fibre-reinforced superalloy-matrix composite materials combine superior properties of oxidation resistance, high strength coupled with ductility and toughness at elevated temperatures. However, they are extremely difficult to machine or form by conventional metal working processes due to the strongly anisotropic nature of their properties. Isothermal forging, where the billet and dies are maintained at the forging temperature during deformation, is one of the most important processes evolved in recent years and by combining this process with powder fabrication, highly alloyed casting-type materials can be processed into homogeneous products which are extensively used in the aerospace industry. In the present investigation, the open-die isothermal forging of square-section billets of fibre-reinforced superalloy composites (hot isostatically pressed nickel-base superalloy compacts reinforced with 40% volume fraction of tungsten wires) at constant strain-rates was considered, with the aim of providing useful information concerning the formability of the composite material and, therefore, its industrial applicability, i.e. in the shaping of complex forms. The mode of deformation, the macroscopic fracture behaviour of the composite material and the spread encountered for the various forging parameters has been given. It is concluded that deformation in composites proceeds mainly under conditions of plane-strain, that formability is limited at low strains and that a forging limit criterion of geometric nature is to be expected.

Dynamic recrystallization during creep in a 45 Pct Ni-35 pct Fe-20 pct Cr alloy system

A combined 3.5 wt pct Mo + 1.2 wt pct Ti imparted dynamic recrystallization in a 35 wt pct Fe-45 wt pct Ni-20 wt pct Cr alloy system during creep at 700 °C, whereas 3.5 wt pct Mo addition alone did not initiate recrystallization. Dynamic recrystallization substantially increased the creep elongation and produced a high ductile fracture topography in the present alloy system. A subgrain coalescence nucleation mechanism for dynamic recrystallization mechanism was operative during creep. The critical initiation strain requirements are also discussed.