Jacques Lacaze

An Assessment of the Fe-C-Si System

Precise knowledge of the Fe-C-Si system has been the subject of numerous previous studies because of the importance of this system for steelmaking processes and cast iron foundry. How-ever, the most recent articles on this subject still reveal uncertainties or insufficient information. The purpose of this work was to assess this system and to give it a precise description, but one which was simple enough for practical applications. An evaluation of the parameters describing the thermodynamic properties of the phases involved in the binary Fe-Si system has been achieved, mainly based on values of the silicon activity in the body-centered cubic (bcc) phase and on knowledge of the phase diagram. The A2-B2 ordering reaction of the bcc phase has been included. It has been shown that this reaction is of paramount importance for the description of the thermodynamic properties of this phase and its field of stability. This preliminary work and previous similar studies of the Fe-C and Si-C systems were used as a base for the evaluation of the ternary stable and metastable Fe-C-Si systems. Optimization of the parameters describing the properties of the phases involved in both of these systems was achieved using experimental thermodynamic data and the critical assessment of the phase diagrams.

Influence des conditions de solidification sur le déroulement de la solidification des aciers inoxydables austénitiques

Abstract The effects of the composition and rate of solidification on mode (δ or γ) o austenitic stainless steels are studied on the bases of former experimental results and of a theoretical analysis of constrained dendrite growth. The position of the eutectic (laδ+γ) line in the Fe-Cr-Ni equilibrium phase diagram is ascertained by differental thermal analysis for 10.5

Dendritic growth and crystalline quality of nickel-base single grains

Abstract It is a usual observation that subgrains exist in nickel-base single grain components solidified by the lost wax process. The associated misorientations are generally small, but they can eventually lead to casting defects in the case of highly complex mold shapes. This work presents an attempt to relate the formation of subgrain boundaries with the development of the dendritic solidification microstructure. Experimental investigations have been undertaken on cast components made of AM1 nickel-base superalloy designed for high temperature turbine blades. Single grains were obtained by means of a grain selector at the bottom of each part. Metallographic observations have been made to characterize the dendritic array, together with gamma diffraction to measure the crystalline quality of the material and X-ray topography for mapping of misorientations on a dendritic scale. Small misorientations between dendrite stems have been found at the upper end of the selector which lead to the formation of subgrains. Moreover, during the growth process, the total mosaicity of the material increases, firstly as a consequence of an increase in the misorientations between subgrains, and secondly because of a decrease of the internal quality of each subgrain. It is proposed that misorientations are due to thermomechanical stresses which build up during λ′ precipitation at temperatures slightly below the solidus temperature of the alloy.

Experimental investigation of the development of microsegregation during solidification of an AlCuMgSi aluminium alloy

Abstract Chemical heterogeneities which build up during dendritic solidification of metallic alloys are of great importance to their service properties. In nominally single-phase alloys, off-equilibrium eutectic often appears which is associated with large compositional changes in the primary phase at the scale of the solidification microstructure. In order to obtain a precise description of solute heterogeneities, it has become common practice to draw distribution curves obtained from numerous microprobe point counts. The experimental distribution curves show differences with respect to the usual predictions, these differences being related to the sign of curvature at low solid fraction and to the amount of highly segregated material at high solid fraction. The present study is an attempt to investigate experimentally microsegregation build-up during solidification, and was carried out on an aluminium alloy quenched during directional solidification.

Modelling of microporosity formation in aluminium alloys castings

Abstract The solidification of Al-7wt.%Si alloy in sand mold geometries has been simulated using a 2D-finite volumes model which takes full account of coupled heat, mass and momentum transport phenomena. The shrinkage boundary condition used in this model is detailed, and results of the simulation are presented in terms of pressure drops which are viewed sometimes as one of the major causes of micropore formation in aluminium castings. Solidification of plates of different thicknesses in sand mold has been investigated. Calculations give a maximum absolute pressure drop of 27 000 Pa in the 7.5 mm thick casting whereas this maximum is only 300 Pa in the 30 mm thick casting. Considering the condition of microporosity formation, these results point out that pressure drops are far too low to account for the formation of micropores which are observed experimentally. Hydrogen segregation has to be considered and integrated in a model to give a good prediction of micropore appearance.

Growth conditions at the solidification front of multicomponent alloys

Over the last twenty years, much progress has been made in the description of the growth conditions at a cellular or dendritic solidification front. Several models to predict the tip radius have been proposed for binary alloys, while important experimental studies were pursued on transparent alloys to check the theoretical predictions. On the contrary, very little experimental data is available for metallic systems, particularly in multicomponent alloys. The recent measurements of Tian and Stefanescu on a Fe-C-Si-Mn alloy during cellular or dendritic growth are thus of great interest. These authors compared predictions made with models for binary alloys with their measurements of tip radius during cellular and dendritic growth. The application of these models is first discussed; then a model for multicomponent alloys is presented and applied to these experimental data.