Effects of interphase boundary anisotropy on the three-phase growth dynamics in the β(In) – In 2 Bi – γ(Sn) ternary-eutectic system

International audienceWe present an experimental investigation on the effects of the interphase energy anisotropy on the formation of three-phase growth microstructures during directional solidification (DS) of the β(In)-In2Bi-γ(Sn) ternary-eutectic system. Standard DS and rotating directional solidification (RDS) experiments were performed using thin alloy samples with real-time observation. We identified two main types of eutectic grains (EGs): (i) quasi-isotropic EGs within which the solidification dynamics do not exhibit any substantial anisotropy effect, and (ii) anisotropic EGs, within which RDS microstructures exhibit an alternation of locked and unlocked microstructures. EBSD analyses revealed (i) a strong tendency to an alignment of the In2Bi and γ(Sn) crystals (both hexagonal) with respect to the thin-sample walls, and (ii) the existence of special crystal orientation relationships (ORs) between the three solid phases in both quasi-isotropic and anisotropic EGs. We initiate a discussion on the dominating locking effect of the In2Bi-β(In) interphase boundary during quasi steady-state solidification, and the existence of strong crystal selection mechanisms during early nucleation and growth stages

Surface effects in nucleation and growth of smectic B crystals in thin samples

We present an experimental study of the surface effects (interactions with the container walls) during the nucleation and growth of smectic B crystals from the nematic in free growth and directional solidification of a mesogenic molecule ($C_4H_9-(C_6H_{10})_2CN$) called CCH4 in thin (of thickness in the 10 $\mu$m range) samples. We follow the dynamics of the system in real time with a polarizing microscope. The inner surfaces of the glass-plate samples are coated with polymeric films, either rubbed polyimid (PI) films or monooriented poly(tetrafluoroethylene) (PTFE) films deposited by friction at high temperature. The orientation of the nematic and the smectic B is planar. In PI-coated samples, the orientation effect of SmB crystals is mediated by the nematic, whereas, in PTFE-coated samples, it results from a homoepitaxy phenomenon occurring for two degenerate orientations. A recrystallization phenomenon partly destroys the initial distribution of crystal orientations. In directional solidification of polycrystals in PTFE-coated samples, a particular dynamics of faceted grain boundary grooves is at the origin of a dynamical mechanism of grain selection. Surface effects also are responsible for the nucleation of misoriented terraces on facets and the generation of lattice defects in the solid.Comment: 15 pages, 24 figures, submitted to PR

La dynamique de solidification des eutectiques lamellaires : des échantillons minces aux systèmes massifs

Nous rappelons quelques résultats expérimentaux récents concernant la dynamique des fronts eutectiques lamellaires en solidification directionnelle d'échantillons minces. Ces résultats ont permis à la fois de répondre à des questions théoriques relevant de la phénoménologie des fronts de propagation unidimensionnels (1D) et de résoudre quelques problèmes métallurgiques. Nous nous proposons d'aborder maintenant la dynamique des fronts de solidification eutectique lamellaire en échantillons massifs (fronts 2D). Nous esquissons un tableau des phénomènes nouveaux que nous nous attendons à observer dans de tels échantillons

Lamellar eutectic growth with anisotropic interphase boundaries

International audienceWe present a numerical study of the effect of a free-energy anisotropy of the solid-solid interphase boundaries on the formation of tilted lamellar microstructures during directional solidification of nonfaceted binary eutectic alloys. We used two different methods – phase-field (PF) and dynamic boundary-integral (BI) – to simulate the growth of periodic eutectic patterns in two dimensions. For a given Wulff plot of the interphase boundary, which characterizes a eutectic grain with a given relative orientation of the two solid phases, the lamellar tilt angle depends on the angle between the thermal axis z and a reference crystallographic axis. Both PF and BI results confirm the general validity of a recent approximate theory which assumes that, at the trijunctions, the surface tension vector of the interphase boundary is parallel to z. In particular, a crystallographic locking of the lamellae onto a direction close to a deep minimum in the Wulff plot is well reproduced in the simulations

Formation of locked-lamellar grains in a slightly hypoeutectic Al-Al2Cu alloy during thin-sample directional solidification

We studied the formation and growth of locked-lamellar microstructures in a thin sample of a slightly hypoeutectic Al-Al2Cu alloy. The coupled-growth dynamics, including early stages and steady-state regimes, was observed optically in real time during directional solidification. The orientation of the a (Al) and q (Al2Cu) crystals was measured ex situ in a series of eutectic grains by X-ray Laue microdiffraction. A nucleation event of a q crystal on a pre-existing a crystal, and the subsequent growth of a eutectic grain with a type-C orientation relationship, that is, with a coincidence of {123}-a and {100}-q planes, were observed in situ. In type-C eutectic grains, lamellar locking occurred parallel to the low-energy coincidence plane. A regular (floating) coupled-growth dynamics was observed in misoriented eutectic grains

Formation of locked-lamellar grains in a slightly hypoeutectic Al-Al2Cu alloy during thin-sample directional solidification

We studied the formation and growth of locked-lamellar microstructures in a thin sample of a slightly hypoeutectic Al-Al2Cu alloy. The coupled-growth dynamics, including early stages and steady-state regimes, was observed optically in real time during directional solidification. The orientation of the a (Al) and q (Al2Cu) crystals was measured ex situ in a series of eutectic grains by X-ray Laue microdiffraction. A nucleation event of a q crystal on a pre-existing a crystal, and the subsequent growth of a eutectic grain with a type-C orientation relationship, that is, with a coincidence of {123}-a and {100}-q planes, were observed in situ. In type-C eutectic grains, lamellar locking occurred parallel to the low-energy coincidence plane. A regular (floating) coupled-growth dynamics was observed in misoriented eutectic grains

Formation of locked-lamellar grains in a slightly hypoeutectic Al-Al2Cu alloy during thin-sample directional solidification

We studied the formation and growth of locked-lamellar microstructures in a thin sample of a slightly hypoeutectic Al-Al2Cu alloy. The coupled-growth dynamics, including early stages and steady-state regimes, was observed optically in real time during directional solidification. The orientation of the a (Al) and q (Al2Cu) crystals was measured ex situ in a series of eutectic grains by X-ray Laue microdiffraction. A nucleation event of a q crystal on a pre-existing a crystal, and the subsequent growth of a eutectic grain with a type-C orientation relationship, that is, with a coincidence of {123}-a and {100}-q planes, were observed in situ. In type-C eutectic grains, lamellar locking occurred parallel to the low-energy coincidence plane. A regular (floating) coupled-growth dynamics was observed in misoriented eutectic grains