E. Gautier

Influence of stresses on the kinetics of pearlitic transformation during continuous cooling

Abstract The effect of stress/strain on the kinetics of pearlitic transformation is reviewed. Our results on the pearlitic transformation of an eutectoid carbon steel under applied uniaxial tensile stresses are analyzed. From these results, we have modelized the effect of the internal stresses, generated during cooling in a solid specimen, on the transformation kinetics. This model is based on a shifting in time of the IT curves of the steel as a function of the internal stress condition (deviatoric part of the stress tensor). A coupled thermal, phase transformation, stress calculation, which includes this model, is applied to the cooling of an eutectoid carbon steel cylinder. The calculated results show that the internal stresses have an important effect on the kinetics of transforamtion and on the temperature evolutions. The calculated cooling laws are compared with those obtained by experiment and the validity of the model is discussed.

Study of the α/β phase transformation of Zy-4 in presence of applied stresses at heating: analysis of the inherited microstructures and textures

Abstract The α→β→α phase transformation of Zy-4, as well as the inherited microstructures and textures have been investigated, in presence (or not) of applied tensile stress during the anisothermal α→β transformation. Without applied stress, the β treatment led to an inherited α texture sharper than the initial one. A detailed analysis of this texture change revealed a strong variant selection in the β→α transformation. With applied stress, transformation plasticity as well as creep of the β phase were clearly identified on the dilatometric curves. Moreover, the sharpness of the inherited α texture was significantly reduced. This has been related to a strong reduction of variant selection in the β→α transformation.

Role of Internal Stress State on Transformation Induced Plasticity and Transformation Mechanisms During the Progress of Stress Induced Phase Transformation

Results of transformation plasticity deformation are reported for a martensitic transformation in a Fe-20Ni-0.5C alloy. Specimens were tested under different mechanical variations. Based on transformation plasticity deformation variations versus the progress of the transformation and microstructural observations, the mechanisms of transformation plasticity are analysed. The role of internal stresses generated during the transformation is pointed out. At least, it is shown that straining during the transformation modifies transformation mechanisms.

Morphology transitions of deformation-induced thin-plate martensite in Fe–Ni–C alloys

The morphology and crystallography of deformation-induced martensites formed during isothermal tensile tests in Fe-30Ni-0.34C and Fe-25Ni-0.66C alloys were investigated by means of optical, transmission electron and scanning electron microscopy. Transitions in morphology from thin plate to coupled plate, to lenticular coupled-plate martensite and from thin plate to lenticular to compact martensite have been observed with increasing deformation. Stress favours the growth of martensite when concurrent plastic strain allows accommodation of macroscopic transformation strains and the change of the Bain strain accommodation mechanism. Mobile dislocations and emission dislocations are directly related to the change of the Bain strain accommodation mechanism from twinning to slip

FINITE ELEMENT CALCULATION OF THE MICROMECHANICS OF A DIFFUSIONAL TRANSFORMATION

A unit cell of the material with suitable boundary conditions is modelled by a 3D F.E. mesh, over which the transformation will develop in a successive way. We use two different descriptions of the transformation, namely a spherical growth of nuclei of the new phase and a random progression model. An external stress state is applied to the cell, resulting in a transformation plastic strain, the evolution of which we study versus the progress of the transformation. We have chosen the example of an isothermal pearlitic transformation of a steel

Modeling of Precipitation Coupled with Thermodynamic Calculations

A precipitation model is developed based on three physical mechanisms, ie nucleation, growth and coarsening of the precipitates. The model follows the particle size distribution by using a finite volume method. Inputs of thermodynamic data are required, such as the driving force for nucleation and equilibrium concentrations. An ideal dilute solution, an ideal solution and a model fully coupled with the thermodynamic calculation software Thermo-Calc®[l] are used to describe the Gibbs energy of the different phases. The influence of using these different thermodynamic approximation on the prediction of the precipitation model is studied.

Phase transformations and generation of heat treatment residual stresses in metallic alloys

Today, a better understanding of the development of microstructures and internal stresses during heat treatment is achieved through modelling and numerical simulations.The key point is to take into account the phase transformations and the induced couplings with the thermomechanical behaviour of the material.These phenomena and their present modelling are reviewed for different metallic alloys, mainly steels, aluminium and titanium alloys.

MORPHOLOGY TRANSITION OF DEFORMATION-INDUCED LENTICULAR MARTENSITE IN FE-NI-C ALLOYS

The morphology and habit planes of deformation-induced lenticular martensite were investigated by optical and transmission electron microscopy in Fe-30Ni and Fe-30Ni-0.11C alloys. Transitions in morphology were observed with progressive deformation levels going from lenticular to butterfly and to compact martensite for the Fe-30Ni alloy and lenticular to butterfly and to small butterfly martensite for the Fe-30Ni-0.11C alloy. The habit planes changed from {225}f or {259}f for the thermal lenticular martensite to {111}f for the strain-induced martensite. The morphology and crystallography of the small butterfly martensites was also investigated. A change in the orientation relationships from K-S to N-W relations was also observed. These changes were attributed to the contribution of mobile dislocations which modified the shear mode from twinning to slip, and to a plastic accommodation of transformation strains.

Deformation of Eutectoid Steel During Pearlitic Transformation Under Tensile Stress

ABSTRACT The effect of tensile stress (0 to 80 MPa) on the isothermal pearlitic transformation of a 0, 8% C steel has been studied in the case of a transformation between 663 and 683°C. The variations of strain versus time (dilatometry) and the transformation kinetics (variation of electric resistance of samples under tensile stress) were obtained using a dilatometer generating fast thermal and mechanical cycles. The main phenomena observed and measured are as follows : pearlitic transformation rate increases with applied stress. The total strain ΔLT during pearlitic transformation increases linearly with stress above a critical value. This critical value and the slope of the curve ΔLT=f (δ) depend on the transformation temperature. Comparison of deformation and transformation kinetics reveals that the plastic deformation under tensile stress is not a linear function of the progress of the transformation. The plastic deformation is considerable during the first 20–30% of transformation and increases slowly with further transformation.

Modeling of β→α Transformation in Complex Titanium Alloys

A model has been developed which is able to predict the kinetics of β → α transformation in industrial multicomponent titanium alloys during complex heat treatments. It isbased on (i) analytical nucleation and growth laws based on simple geometric representationsof the di erent morphologies commonly observed in these alloys; (ii) the assumption of localequilibrium at interfaces, handled within the CalPhaD framework; (iii) averaged solute balancesin each morphology. The potentialities of the model will be demonstrated on the Ti17 industrialalloy upon isothermal holdings and cooling from the β phase field.