Claude Esnouf

Chemical reactivity of aluminium with boron carbide

The chemical reactivity of boron carbide (B4C) with metallic aluminium (Al) was studied at temperatures ranging from 900 to 1273 K (627–1000 °C). Al–B4C powder mixtures were cold pressed, heated for 1–450 h under 105 Pa of purified argon and characterized by X-ray diffraction (XRD) optical metallography (OM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Whatever the temperature in the investigated range, B4C has been observed to react with solid or liquid Al. As long as the temperature is lower than 933 K (660 °C), i.e. as long as Al is in the solid state, interaction proceeds very slowly, giving rise to the formation of ternary carbide (Al3BC) and to diboride (AlB2). At temperatures higher or equal to 933 K, Al is in the liquid state and the reaction rate increases sharply. Up to 1141 ± 4 K (868 ± 4 °C), the reaction products are Al3BC and AlB2: at temperatures higher than 1141 K, Al3 BC is still formed while Al3B48C2 (β-AlB12) replaces AlB2. In the three cases, interaction proceeds via the same mechanism including, successively, an incubation period, saturation of aluminium in B and C, nucleation and growth by dissolution–precipitation of Al3BC and a C-poor boride and, finally, the passivation of B4C by Al3BC. These results are discussed in terms of solid–liquid phase equilibria in the Al–B–C ternary system, with reference to the binary invariant transformation: α-AlB12 + L ⇔ AlB2, which has been found to occur at 1165 ± 5 K (892 ± 5 °C).

Neutron powder diffraction studies of transition metal hemicarbides M2C1−x—II. In situ high temperature study on W2C1−x and Mo2C1−x

Abstract In the first part of this paper [Acta metall.36, 1891 (1988)], we have presented the experimental background for a high temperature neutron powder diffraction study of transition metal hemicarbides M2C1−x having various compositions. In this second part, we report the results: in Mo2C1−x, a first order transformation between an orthorhombic ζ-Fe2N-type superstructure and a hexagonal ϵ-Fe2N-type one has been identified in a middle temperature range (1100–1400°C); in the case of W2C1−x, the same ϵ-Fe2N-type phase appears to be the major constituent in our powders. In both carbides, the disordering of this ϵ-phase occurs through a second order transition at elevated temperatures. The ζ-Fe2N-type phase is also observed as a minor constituent of a W2C0.91 powder; a consistent discussion of these results points out a general scheme for the structural evolution of these M2C1−x compounds as a function of the temperature; this conclusion is also correlated to the refinement of the ζ and ϵ-type structures which has been performed at room temperature.

Accurate characterization of pure silicon-substituted hydroxyapatite powders synthesized by a new precipitation route

This paper presents a new aqueous precipitation method to prepare silicon-substituted hydroxyapatites Ca10(PO4)6-y(SiO4)y(OH)2-y(VOH)y (SiHAs) and details the characterization of powders with varying Si content up to y=1.25molmolSiHA(-1). X-ray diffraction, transmission electron microscopy, solid-state nuclear magnetic resonance and Fourier transform infrared spectroscopy were used to accurately characterize samples calcined at 400°C for 2h and 1000°C for 15h. This method allows the synthesis of monophasic SiHAs with controlled stoichiometry. The theoretical maximum limit of incorporation of Si into the hexagonal apatitic structure is y<1.5. This limit depends on the OH content in the channel, which is a function of the Si content, temperature and atmosphere of calcination. These results, particularly those from infrared spectroscopy, raise serious reservations about the phase purity of previously prepared and biologically evaluated SiHA powders, pellets and scaffolds in the literature.

TEM and EBSD investigation of continuous and discontinuous precipitation of CrN in nitrided pure Fe-Cr alloys

Pure Fe-Cr alloys (1 and 3 wt% Cr) were gas nitrided (NH3, N2, N2O mixture at 823 K). Two modes of CrN precipitation: continuous (fine disc-shaped precipitates) and discontinuous (lamellae-like precipitates) were identified and investigated using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Both types of precipitates presented a cubic NaCl-type structure and Baker-Nutting orientation relationship with respect to the ferritic matrix. A quantification procedure based on TEM images exploitation revealed that the size and the number of the fine precipitates vary inversely with nitriding depth. This result was compared to the profile of micro-hardness. SEM observations showed that only superficial regions in the Fe-3 wt% Cr were transformed by the discontinuous precipitation of CrN. Electron back-scattered diffraction (EBSD) studies of lamella revealed singular initiation and growth features. A qualitative mechanism of lamella initiation and growth is discussed.

Microstructural analysis of the stress-induced ε martensite in a Fe-Mn-Si-Cr-Ni shape memory alloy : Part I. Calculated description of the microstructure

Abstract The γ (f.c.c.)– e (h.c.p.) martensitic transformation is achieved by the introduction of stacking faults on each second compact plane of the f.c.c. structure. These stacking faults are created by the motion of Shockley partial dislocations. Depending on the Burgers vector of these dislocations, the martensite does not require a macroscopic shape change (self-accommodated martensite) or a homogeneous lattice shape change (monopartial martensite). Based on the monopartial nature of the stress-induced martensite, a model describing the martensitic morphology in the Fe–Mn–Si based shape memory alloys is presented. The theoretical results are compared with some observations in a Fe–Mn–Si–Cr–Ni shape memory alloy.

Microstructural analysis of the stress-induced e martensite in a Fe-Mn-Si-Cr-Ni shape memory alloy. Part II : Transformation reversibility

Abstract The shape memory effect exhibited by some Fe–Mn–Si based alloys is related to the γ(f.c.c.)-e(h.c.p.) martensitic transformation. In these alloys, the shape memory effect, incomplete even for low deformation rates, drastically decreases when the deformation rate increases. The evolution of the microstructure of the martensite, and particularly the interaction of martensite plates and grain boundaries, is studied in a Fe–Mn–Si–Cr–Ni alloy to determine the origin of the loss of shape memory. The influence of the back-stress created by the dislocations located at the tip of the martensite bands is pointed out. The reduction of the shape memory is attributed to the accommodation of this back-stress when the deformation rate, i.e. the martensite band width, increases. The same kind of analysis is used to analyse the problem of superelasticity.

Neutron powder diffraction studies of transition metal hemicarbides M2C1−x—I. Motivation for a study on W2C and Mo2C and experimental background for an in situ investigation at elevated temperature

Abstract The crystallography of hexagonal hemicarbides of the sixth group transition metals (Mo2C, W2C) has been investigated by many workers, and an up to date review is presented herein (Section I); it appears that controversies concerning the various ordered structures (i.e. C6, ϵ-Fe2N and ζ-Fe2-N-type phases) exist in the case of W2C, while the crystallography of Mo2C seems to be more consistently established, although one can be surprised that it is significantly different from that of the similar W2C carbide (the ϵ-Fe2N-type phase was not observed in the system MoC). These remarks show that a high temperature (up to 2200°C) in situ powder neutron diffraction study on these compounds is needed in order to confirm their high temperature forms. In view of the peculiarities of such an experiment, we have concentrated our attention to detail in this first part the different aspects of our experimental approach (Section 2). The results will appear as part 2 of the present article.

Contribution of advanced microscopy techniques to nano-precipitates characterization: case of AlN precipitation in low-carbon steel

Abstract Aluminum nitride (AlN) precipitation in a Fe–Al–N alloy was studied by Transmission Electron Microscopy (TEM). Crystallographic and chemical properties of precipitates were investigated using electron diffraction, High Resolution TEM, EDX microanalysis, EELS spectroscopy and Energy-Filtered TEM. Observations were carried out on extraction replicas and thin foils. Principally, two varieties of AlN precipitates have been observed: fine cubic (NaCl-type) precipitates with a platelet-like morphology exhibiting a Bain orientation relationship with respect to the α-iron matrix in the case of 3 h annealing at 923 K, and coarse cuboidal-shape hexagonal (wurtzite) precipitates having a particular orientation relationship with respect to the ferrite in the case of 5 h annealing at 973 K. A cubic–hexagonal transformation of AlN nitrides activated by the heterogeneous germination on iron sulfide (FeS) was established. The two AlN varieties were clearly recognized by EELS spectroscopy and the quantification of both populations of precipitates from EFTEM images is discussed.

Characterization of the stress-induced ε martensite in a Fe–Mn–Si–Cr–Ni shape memory alloy: microstructural observation at different scales, mechanism of formation and growth

Abstract The martensitic transformation induced by traction at room temperature in a Fe–16Mn–9Cr–5Si–4Ni (%mass) has been studied by optical microscopy, scanning electron microscopy, transmission electron microscopy and scanning tunneling microscopy. The samples were previously submitted to a thermomechanical treatment which increases the shape memory properties. The martensitic microstructure and the fine structure of the stacking faults are both studied to clarify the nucleation and growth mechanisms of martensite. The band structure of the martensite is pointed out; these bands correspond to a mixture of thin martensite plates and extremely thin austenitic zones. Inside a grain, the monopartial nature of the martensite has been demonstrated from the elementary plate to all the martensite bands. From all the observations, the pole mechanism appears to be the main mechanism of martensite nucleation.

Growth Twinning and Generation of High-Frequency Surface Nanostructures in Ultrafast Laser-Induced Transient Melting and Resolidification

The structural changes generated in surface regions of single crystal Ni targets by femtosecond laser irradiation are investigated experimentally and computationally for laser fluences that, in the multipulse irradiation regime, produce sub-100 nm high spatial frequency surface structures. Detailed experimental characterization of the irradiated targets combining electron back scattered diffraction analysis with high-resolution transmission electron microscopy reveals the presence of multiple nanoscale twinned domains in the irradiated surface regions of single crystal targets with (111) surface orientation. Atomistic- and continuum-level simulations performed for experimental irradiation conditions reproduce the generation of twinned domains and establish the conditions leading to the formation of growth twin boundaries in the course of the fast transient melting and epitaxial regrowth of the surface regions of the irradiated targets. The observation of growth twins in the irradiated Ni(111) targets provides strong evidence of the role of surface melting and resolidification in the formation of high spatial frequency surface structures. This also suggests that the formation of twinned domains can be used as a sensitive measure of the levels of liquid undercooling achieved in short pulse laser processing of metals.