A. Pineau

Martensitic transformations induced by plastic deformation in the Fe-Ni-Cr-C system

Martensitic transformations induced by plastic deformation are studied comparatively in various alloys of three types: Fe-30 pct Ni, Fe-20 pct Ni-7 pct Cr, and Fe-16 pet Cr-13 pct Ni, with carbon content up to 0.3 pct. For all these alloys the tensile properties vary rapidly with temperature, but there are large differences in the value of the temperature rangeMs toMd, which strongly increases with substitution of chromium for nickel or with carbon addition. Using the node method, it is found that the intrinsic stacking fault energy in the austenite drastically increases with temperature in all the chromium-bearing alloys investigated. This variation is consistent with the observed influence of temperature on the appearance of twinning or ε martensite during plastic deformation. Very different α’ martensite morphologies can result from spontaneous and plastic deformation induced transformations, especially in Fe-20 pct Ni-7 pct Cr-type alloys where platelike and lath martensites are respectively observed. As in the case of ε martensite, the nucleation process is analyzed as a deformation mode of the material, using a dislocation model. It is then possible to account for the morphology of plastic deformation induced α’ martensite in both Fe-20 pct Ni-7 pct Cr and Fe-16 pct Cr-13 pct Ni types alloys and for the largeMs toMd range in these alloys.

Twinning and strain-induced F.C.C. → H.C.P. transformation in the FeMnCrC system

Abstract Twinning and the strain-induced f.c.c. → h.c.p. martensitic transformation were studied in various alloys of the FeMnCrC system: binary FeMn alloys, ternary FeMn5Cr and FeMn0.2C alloys, and quaternary Fe20Mn4Cr0.5C alloy. The tensile properties of these alloys were shown to be dependent on both composition and temperature. In a certain number of alloys, the work-hardening rate strongly increases with decreasing temperature and a maximum in elongation versus temperature was noted. This behavior was shown to be related to the occurrence of either twinning or the strain-induced f.c.c. → h.c.p. phase transformation. The influence of temperature on both twinning and the f.c.c. → h.c.p. martensitic transformation is discussed. The beneficial effect of these two deformation modes on the work hardening rate is emphasized.

Influence of uniaxial stress on the morphology of coherent precipitates during coarsening—elastic energy considerations

Abstract The theory of inclusions and inhomogeneities by Eshelby is applied to predict the influence of uniaxial applied stress on the morphology of coherent precipitates during coarsening. The case where the misfit between the matrix and the precipitates is a pure dilatation is considered. The elastic energy is calculated as a function of the particle shape, the applied stress and the ratio between the Youngs modulus of the precipitates and that of the matrix. Three shapes are considered: spheres, plates perpendicular to the stress axis and needles aligned with the stress axis. The calculations are used to construct a map which gives the conditions leading to the lowest elastic energy. The results are compared with observations in Ni base alloys which have shown that a stress applied during coarsening can alter the shape of γ particles.

Low cycle fatigue behavior of inconel 718 at 298 K and 823 K

The cyclic stress-strain response and the low cycle fatigue life of conventionally heat treated Inconel 718 were studied. Fully reversed strain-controlled tests were performed at room temperature and at 823 K. Optical and electron microscopy were used to study the development of deformation and cracking during cycling. A power-law relationship between life-time and plastic strain amplitude was obtained. A substantial decrease in fatigue life occurred as the temperature was increased from 298 to 823 K and as the cycling frequency was lowered from 3 cyclesJmin to 0.3 cyclesJmin at 823 K. At 298 K, for all the strain amplitudes investigated, an initial rapid hardening was followed by softening, while at 823 K only softening occurred. Electron microscopy showed that the precipitates were sheared in the course of cyclic straining and that plastic deformation proceeded by the propagation of planar bands. These bands were identified as twins. Twinning was found to be more abundant at elevated temperatures than at room temperature, especially at lower frequencies. Cracking was generally initiated along the interfaces between these twin bands and the matrix but, at elevated temperatures and low strain rates, intercrystalline cracking took place, as well. The influence of particles shearing and twinning on the cyclic stress-strain response of the material are discussed. The importance of planar deformation and twinning on intergranular cracking is emphasized.

Temperature dependence of stacking fault energy in close-packed metals and alloys

The experimental data currently available concerning the temperature dependence of the stacking fault energy (SFE, γ) observed in the study of the size variation of stacking fault nodes and ribbons in thin foils on a heating or a cooling stage in an electron microscope are reviewed. It is shown that in spite of the experimental difficulties useful information can be obtained by this technique. In pure cobalt and a number of transition metal alloys (CoNi, CoFe, FeCrNi and FeMnCr) a marked increase of the SFE with increasing temperature has been unambiguously demonstrated even though the observed changes in node size are partly irreversible. In the case of silver-base and copper-base alloys the experimental data are often contradictory and, except in the case of AgSn alloys which show a small reversible increase in SFE with temperature, the quantitative estimates of the temperature dependence of SFE are less reliable. The influence of temperature on SFE can be related to the stability of the face-centred cubic phase with respect to the hexagonal close-packed phase at constant composition. Fairly good agreement is observed between the measured value of dγ/dT and that estimated from thermodynamic data, at least in the case of transition metal alloys. The possibility of explaining the observed temperature dependence of SFE in terms of recent calculations based on electron theory is discussed.

Twinning and strain-induced f.c.c. → h.c.p. transformation on the mechanical properties of CoNiCrMo alloys

Abstract Co, CoNi15Cr and CoNi15Cr5Mo alloys have been used to examine the influence of various deformation modes on the tensile mechanical properties. Transmission electron microscopy and X-ray diffraction analysis have been used to identify the deformation modes introduced by plastic deformation applied in the temperature range between 77 and 620 K. In a certain number of alloys it has been found that low temperature plastic deformation induces the γ (f.c.c.) → ϵ (h.c.p.) martensitic transformation. At higher temperatures, twinning occurs whereas, at still more elevated temperatures, the plastic deformation proceeds only by the propagation of undissociated dislocations. The γ → ϵ strain-induced transformation and twinning drastically affect the mechanical properties. These deformation modes lead to a large increase in the work-hardening rate and, in some alloys, a maximum in homogeneous tensile elongation as a function of temperature was observed. Measurements of SFE (stacking fault energy) at room temperature have been performed using the dissociated nodes method. The mechanical properties and the deformation modes of the various alloys are discussed in relation to the influence of SFE on twinning and the strain-induced γ → ϵ phase transformation.

The effect of microstructure and environment on the crack growth behaviour of Inconel 718 alloy at 650 °C under fatigue, creep and combined loading

Abstract The crack growth properties of various microstructures developed in one heat of Inconel 718 alloy were investigated at 650 °C under air and vacuum environments. The microstructures included fine-grained material (ASTM grain sizes 6–8), coarse-grained material (ASTM grain sizes 3–4) and material of a necklace structure (ASTM grain sizes 3–4 and 8–10). The effect of grain boundary β (Ni 3 Nb) phase precipitation was also studied. Continuous fatigue, creep and creep-fatigue conditions were examined. For continuous fatigue the influence of frequency was investigated over the range between 5 × 10 −2 and 20 Hz. For creep-fatigue conditions, hold times of 10 and 300 s were superimposed on a 5 × 10 −2 Hz triangular wave shape signal. It was shown that the grain boundary microstructure had a very strong effect when the fatigue crack propagation behaviour was essentially time dependent. This effect is associated with the occurrence of brittle intergranular fracture and dramatic increases in crack growth rate. The microstructure had no effect under vacuum testing.

Coalescence-Controlled Anisotropic Ductile Fracture

The anisotropic ductile fracture of rolled plates containing elongated inclusions is promoted by both the dilational growth of voids and the coalescence process. In the present article, the emphasis is laid on the latter process. The effects of void shape and mainly of inter-particle spacings are investigated. Two types of coalescence models are compared: a localization-based model and plastic limit-load models. The capabilities of both approaches to incorporate shape change and spacing effects are discussed. These models are used to predict the fracture properties of two low alloy steels containing mainly manganese sulfide inclusions. Both materials are characterized in different loading directions. Microstructural data inferred from quantitative metallography are used to derive theoretical values of critical void volume fractions at incipient coalescence. These values are used in FE-calculations of axisymmetrically notched specimens with different notch radii and loading directions.

Influence of strain-induced martensitic transformations on fatigue crack growth rates in stainless steels

The fatigue crack growth rates (FCGR) of two unstable austenitic stainless steels (Fe-16 Cr-13Ni) and (Fe-18Cr-6.5Ni-0.19C) were determined in theMs-Md temperature range where a strain induced μ → α′ martensitic transformation occurs near the crack tip. These FCGR were compared to the rates measured in the stable austenitic phase of a Fe-31.5Ni and a Fe-34 Ni alloy and in the martensitic phase obtained by quenching the Fe-31.5 Ni alloy below Ms. In the Fe-31.5 Ni, the FCGR are an order of magnitude higher in the martensitic than in the austenitic structures for ΔK ≤ 40 ksi in. The FCGR of the stainless steels decrease markedly when the test temperature approachesMs in theMs - Md range. The FCGR for the alloy Fe-18Cr-6.5 Ni-0.19 C in a warm-worked condition are consistently higher than for the same alloy in the annealed condition for ΔK ≤ 40 ksi √in.. The results are discussed in terms of the influence of phase structures, stacking fault energy and work hardening exponent on the FCGR.

Experimental study of cavity growth in ductile rupture

Abstract Composite materials made of a steel matrix and spherical alumina particles were prepared to study the growth of cavities nucleated from Al2O3 inclusions during deformation at room temperature. Two materials containing different volume fractions of Al2O3 particles, i.e. ƒ = 0.5% and 2% , were investigated. Axisymmetric notched specimens were employed to determine the effect of stress triaxiality on cavity growth rate. These specimens were calculated by finite element method. They were predeformed at room temperature and subsequently broken at − 196°C. The experimental results are in broad agreement with the theoretical results derived from Rice and Tracey model. In particular it is observed that the cavity growth is proportional to the local strain obtained from the finite element calculations. Moreover the effect of stress triaxiality intervenes exponentially as predicted by the Rice and Tracey model. However, the theoretical proportionality factor in front of the exponential term is lower than the experimental one. The reasons for this discrepancy, especially the effect of interactions between neighbouring inclusions which are not taken into account in this model, are briefly discussed.