D. Ma

Optimum glass formation at off-eutectic composition and its relation to skewed eutectic coupled zone in the La based La–Al–(Cu,Ni) pseudo ternary system

Glass formation and its relation to the dendritic and eutectic growth has been investigated for the La100−x[Al0.412(Cu,Ni)0.588]x (x=30–56.3) and La86−yAl14(Cu,Ni)y (y=16–29) alloy systems. The experimental results show that in the La-rich pseudo ternary La–Al–(Cu,Ni) system, optimum glass formation actually occurs at an off-eutectic composition. A nearly fully amorphous rod with 12 mm in diameter can be obtained at an off-eutectic composition near La62Al15.7(Cu,Ni)22.3, while only a 1.5 mm diameter rod can be obtained fully amorphous for the eutectic alloy La66Al14(Cu,Ni)20. A strong dependence of GFA on the composition is observed for these alloys. In addition, formation of a composite (i.e. αLa dendrite reinforced glass matrix) in 12 mm diameter cast rods is observed over a wide range of composition, including the eutectic composition. It has been found that the GFA does not correlate well with the extent of the undercooled liquid region (ΔTx) and is not even closely related to the reduced glass transition temperature (Trg). These unusual observations in glass formation and its relation to the skewed eutectic coupled zone have been explained in terms of the competition between the growth of crystalline phases (i.e. eutectic and dendritic phases) and the formation of the amorphous phase.

Effect of weak convection on lamellar spacing of eutectics

Abstract An asymptotic expansion method is proposed to solve the convection–diffusion equation which governs the directional solidification of eutectic alloys under the condition of weak convection at low Peclet number. The so-called weak convection is defined here as the condition for which the velocity of convective flow and its gradient ahead of the S/L interface are both relatively small. Our analysis shows that such weak convective flow will give rise to an increase of eutectic lamellar spacing since the convection produces a slight shift of solute concentration profile at the interface and a decrease of eutectic growth undercooling. Our analytical results are compared with other models and have been applied to some practical cases such as weak forced convection induced by a rotating disk and natural convection induced by solutal and thermal differences in solidification processing. Calculations for two typical eutectic alloys, Pb–Sn and Al–Cu, are performed in order to reveal schematically the effect of weak convective flow on eutectic growth.

Unidirectional solidification of a Zn-rich Zn–2.17 wt%Cu hypo-peritectic alloy

Abstract Unidirectional solidification of a Zn-rich Zn-2.17 wt%Cu hypo-peritectic alloy has been carried out to investigate the microstructure evolution over the growth velocity range 0.02–4.82 mm/s ata temperature gradient of 15 K/mm by means of the Bridgman technique. Regular and plate–like two-phase cellular structures were observed in samples grown at growth velocities V above 0.48 and 2.64 mm/s,respectively. The dominant microstructure in samples grown below 0.22 mm/s was dendrites of primary sin a matrix of secondary η. Intercellular spacing ι decreased ε with increasing growth velocity V such that ιV1\2 is a constant of 316 ± 55 µm3/2/s1/2. Secondary dendrite arm spacing λ2 of primary decreased with increasing V such that λ2V1/3 isa constant of 14.9 ± 0.9 µm4/3/s1/3. The observed transition from regular cells to plate-like cells of η is discussed on the basis of competitive growth and crystallographic effect.

OBSERVATION OF LAMELLAR STRUCTURE IN A Zn-RICH Zn-6.3at.% Ag HYPER-PERITECTIC ALLOY PROCESSED BY RAPID SOLIDIFICATION

According to the classical description, solidification of a peritectic alloy involves one solid phase reacting with a liquid phase on cooling to produce a second solid phase [1]. Recently, peritectic solidification has attracted more attention in experimental and theoretical studies [2–6]. The usual product of peritectic solidification is primary phase surrounded by peritectic phase and remaining liquid, due to the difficulty of diffusion in the solid primary phase. However, there are some reports on the possibility of coupled growth in peritectic systems [2,6–9]. In 1959, Chalmers [10] has suggested the possibility of simultaneous (coupled) growth of primary and peritectic phases in peritectic alloys if the growth of the primary phase can be suppressed by a high temperature gradient. In 1974, Flemings [11] also supported the idea of coupled growth in peritectic alloys. Later, however, Boettinger [2] solidified Sn-Cd alloys and predicted, by an analysis similar to the Jackson-Hunt theory [12] of lamellar eutectic growth, that coupled growth in peritectic systems was unstable. Laraia and Heuer [7] have suggested for some peritectic systems, particularly those in which the peritectic phase is a compound, that capillarity effects acting on the primary phase may lower its liquidus and promote a metastable eutectic reaction instead of the equilibrium peritectic reaction. Boettinger [13] has suggested that it may be possible to suppress the formation of the primary phase by applying rapid solidification. Based on this idea, rapid solidification by melt-spinning was carried out on a Zn-rich Zn-6.3at.(10 wt.)% Ag hyper-peritectic alloy and evidence of a resulting eutectic-like lamellar structure is reported. The possibility of coupled growth in the present peritectic system under rapid solidification conditions is also discussed.

Laser resolidification of a Zn–3.37 wt.% Cu peritectic alloy

Abstract Laser rapid solidification of a Zn–3.37 wt.% Cu alloy that span a peritectic reaction in its phase diagram, i.e. e + L → η , was performed at various scanning velocities between 12.7 and 23.3 mm s −1 . Solidification microstructures were characterized through both longitudinal and cross sections, showing three typical microstructures that were located in upper remelted zone, bottom remelted zone and partially-remelted zone of the laser pool, respectively. The observed microstructures containing plate-like cellular η with or without dendritic e are consistent with our previous observations for the same alloy grown at velocities between 2.64 and 4.82 mm s −1 using Bridgman solidification technique. The critical velocity of the transition from e dendrite-free (fully cellular η ) to e dendrite-containing microstructure is evaluated on the basis of competitive growth, indicating an excellent agreement with our experimental results in Zn-rich Zn–Cu alloys processed by both Bridgman solidification and laser surface remelting techniques. The intercellular spacing ( λ ) of η cells decreases from 8.0 to 1.5 μm with increasing growth velocity from 1.0 to 19.1 mm s −1 such that λV 0.59 is constant in parametric agreement with the predictions of Hunt–Lu model. Vickers microhardness measurements were carried out along cross section of the laser remelted zone, showing that the highest value of about 140 HV was obtained in comparison with that of the original substrate alloys of 90 HV.

Evaluation of composition region for peritectic coupled growth

The composition region for stable coupled peritectic growth is evaluated by means of extending Boettingers analysis. Our present analysis demonstrates that there exists a critical composition between Cα (composition of primary α at peritectic equilibrium) and Cβ (compostion of peritectic β at peritectic equilibrium), i.e. Ccrit, separating the peritectic plateau (between Cα and Cβ) into two regions where coupled and banded growth regimes dominate between Ccrit and Cβ and between Cα and Ccrit, respectively. The calculated critical composition, Ccrit, is consistent with the observation in Ni–Al, Ti–Al and Sn–Cd peritectic systems. Effects of interfacial energies of constituent phases and convection on the separation of these two growth regimes are also discussed.

Unidirectional solidification of Al–Cu eutectic with the accelerated crucible rotation technique

The accelerated crucible rotation technique (ACRT) is applied to Al-Si eutectic to reveal the effect of forced convection on the unidirectional solidification structure. Experimental results show that if the crucible rotation is started from the beginning of the unidirectional solidification, irregular eutectic with short bars and chunks of silicon will be formed. But if a normal Bridgman process is experienced before the crucible rotation is applied, a more satisfactory directional needle-like silicon structure where the silicon is straight, long, and more regular will be formed during the ACRT process. In order to get a satisfactory unidirectional needle-like silicon structure, ACRT must be used with suitable solidification parameters (temperature gradientG and growth rateR). The rotation method of the crucible is also important.

Discontinuous precipitation initiated at interphase boundaries in a Zn-rich Zn-6.3 at.% Ag alloy

Discontinuous precipitation in a two-phase Zn-rich Zn-6.3at.%Ag alloy has been studied. It has been shown that the eta/epsilon interphase boundaries have the ability to initiate discontinuous precipitation as do the eta/eta grain boundaries. The back-polishing method is used to show the interconnection between a discontinuous precipitate colony and the eta/epsilon interphase boundary. The reaction front velocity of discontinuous precipitation initiated at the eta/eta grain boundary is 12-90% higher than that of discontinuous precipitation initiated at the eta/epsilon interphase boundary under various ageing temperatures.