Papers by Author: Philippe Bocher

Abstract: Ultra fine grained (UFG) pure titanium fabricated by severe plastic deformation techniques has been recently considered for biomedical applications. In this study, the effects of grain size and crystallographic orientation on the biocompatibility of commercially pure titanium have been evaluated. Samples having significant differences in terms of average grain size (from 0.4 to 20 mm) and crystallographic textures have been produced using equal channel angular pressing (ECAP) and compared. X-ray diffraction and electron back scattered diffraction (EBSD) were used to document the texture and microstructural properties. Cell attachment tests were done to study the biocompatibility of the samples using MC3T3 pre-osteoblast cells. The number of attached cells was found to be higher on the samples having more (0002) plane parallel to the surface regardless of their grain sizes. It was concluded that the texture plays a more significant role than the grain size in the biocompatibility of pure titanium.

Abstract: It is well known that induction surface heating followed by rapid quenching generally increases the fatigue life of steel components subjected to bending loads by significantly postponing the micro-crack nucleation and propagation processes. The phase transformation volume change combined with severe thermal gradients leave a hard surface layer under relatively high and deep compressive residual stresses. In this paper, residual stress measurements are done on induction hardened AMS6414 martensitic steel (aerospace grade of AISI4340) cylinders using two techniques: the so-called contour method and X-ray diffraction. For both methods, induction hardened parts raise many challenges. The contour method hardly describes high stress gradients near the surface while the diffraction technique accuracy appears limited considering the strong microstructural variation and the high depth of the stresses to be measured. For the contour method, a CMM and an optical pen using the confocal chromatic imaging principle were used to measure the surface after precision WEDM cutting. The effect of data filtering and smoothing on the calculated stresses are discussed. For X-ray analysis, the effect of stress relaxation during layer removal and analysis technique is explained. The difference between the residual stress measurements done with the two techniques is discussed with emphasis on both the surface and the in-depth measurements.

Abstract: Mathematical model, based on Fick’s second law of diffusion, was used to predict the time required to complete isothermal solidification and to determine the effect of process variables during the transient liquid phase bonding of Inconel 625 and 718 superalloys with nickel based brazing filler alloy BNi-2. Experimental investigations were carried out in the range of 1325 – 1394K to verify the model and the predicted times were in excellent agreement with the experimentally determined values. The obtained activation energies for diffusion of boron were very close to the ones reported for other nickel base polycrystalline superalloys; however, it was observed that the time required for complete isothermal solidification are significantly less than that of other nickel based superalloys with different nickel based brazing filler alloys. Because of this advantage, these combinations of base and filler alloys are expected to replace other currently used ones. Further, significant reduction of holding time was observed with increasing brazing temperature and with decreasing joint gap. The composition of the joints at the end of holding period, when the holding time was not sufficient to complete isothermal soldification, has been determined in order to predict the amount of brittle eutectic phases in the final joint microstructures.

Abstract: Induction heating is a case hardening process used to improve performance of machine components by producing a hard martensitic microstructure and high compressive residual stresses at the surface layer. A reliable numerical model able to predict the hardness profile would shorten process development. However, the accuracy and the efficiency of the model are restricted by the coupling complexity between the electromagnetic and thermal fields, and the nonlinear behaviour of the material properties. The paper analyzes the sensitivity of the material properties values and of the finite element meshing onto the predictive modeling of the case hardening profiles. The material used is SAE-4340 low-alloy steel. The simulations are done using a computer-modeling software (Comsol) and the sensitivity analysis is conducted by using an experimental design method.

Abstract: Regions with sharp local textures, called macrozones, have been characterised in a bimodal IMI834 billet, containing 30% of primary αp grains surrounded by secondary αs colonies. It is shown that the αs colonies have been inherited according to a strong variant selection during the β→αs phase transformation. In each observed macrozone, the favoured variants have in average their c-axes in the same macroscopic direction as the αp grains. A detailed analysis of neighbouring αp grains and αs variants clearly shows that the variants favoured at β/αp boundaries are those able to share their c-axes with a neighbouring αp grain. The sharpness of such a variant selection mechanism is strongly related to the local orientation distribution of neighbouring αp/β grains at high temperature. This explains the differences in variant selection sharpness observed from one macrozone to the other.

Abstract: The presence of hcp regions with grains having relatively close orientations has been reported in commercial near alpha titanium billets (IMI 834, Ti 6246, etc). The size of these textured regions (called macrozones) is significantly larger than the average grain size of the microstructure observed after thermomechanical processing. The elongated shape of these large hcp regions suggests that they are eventually related to large prior b grains that pancaked during the ingot break down process. In this contribution, Orientation Image Microscopy was used to study the relationship between the hcp local microtexture heterogeneities and the prior b orientations. Specifically, the orientations of the primary (equiaxed) ap grains and the secondary (lamellar) as colonies produced after the transformation of the b phase were discriminated from OIM maps. Furthermore, from the as inherited OIM map, it was possible to reconstruct the corresponding b OIM map over large regions. The analysis showed that the large hcp macrozones observed in the as received material are not related to corresponding bcc macrozones. However, within an hcp macrozone, various clusters of b grains with similar orientations can be found. In such coherent regions, randomly orientated b grains were also observed, which could be related to microstructural changes during deformation (continuous dynamic recrystallization) as suggested by hot deformation results.