H. W. Kerr

On equilibrium and non-equilibrium peritectic transformations

Abstract There is a lack of systematic experiments on equilibrium and non-equilibrium effects around peritectic compositions in different alloys, despite the existence of a large number of peritectic compositions in binary systems. Peritectic reactions, requiring contact between the three phases, are contrasted to the more general peritectic transformations. The literature on the kinetics of peritectic transformations is reviewed. The possibilities of; 1. (i) the equilibrium peritectic reaction, 2. (ii) suppression of the peritectic transformation, or 3. (iii) nonequilibrium peritectie reactions, are illustrated by experiments on Al-Ti alloys. The observations are explained with reference to the expected free energy curves and the phase diagram. The requirements for similar observations in other systems are given. A classification of peritectics is given in appendix, in terms of the free energy curves of the three phases.

Grain refinement in magnetically stirred GTA welds of aluminum alloys

The mechanisms of grain refinement have been examined for magnetically stirred gas tungsten arc (GTA) welds completely penetrating thin sheets of several aluminum alloys. Grain refinement in unstirred welds may be brought about by adding sufficient titanium to produce heterogeneous nucleation by Ti-rich particles. In some alloys magnetic stirring is shown to extend the range of welding conditions which produce a partially equiaxed structure, and to widen the equiaxed fraction of partially equiaxed welds. This is attributed to magnetic stirring lowering the temperature gradient, allowing nucleation and growth of Al-rich grains further ahead of the columnar interface growing in from the fusion boundaries. In alloys with low Ti levels, magnetic stirring may cause refinement by sweeping grains from the partially molten zone ahead of the advancing solidification interface. This mechanism requires that the partially molten zone be sufficiently wide, and that the grain size in this zone remain small.

Crystallographic orientations between aluminum grown from the melt and various titanium compounds

Abstract The nucleation of aluminum by titanium has been attributed to many different reactions, but the only relationship thoroughly investigated seems to be that between Al and the peritectic compound Al3Ti. We have now studied both the heterogeneous nucleation of Al by TiC and the solidification of Al in contact with massive pieces of TiC. The relationship, (001)Al//[001]TlC and [001]Al//[001]TlC, seems to hold well even under conditions of varying carbide pre-oxidation. Such momplete epitaxy allows only one nucleus variant per compound particle in contrast to a greater number of variants in relationships (of partial epitaxy) accepted for Al3Ti. With this result one can discuss the changing nucleation effectiveness of titanium at various concentrations and discover some important features influencing nucleation: In particular, we propose two apparently new and very simple rules that must apply every time that crystallographic relationships have to be considered in nucleation problems.

The nucleation and solidification of Al-Ti alloys

A series of hyperperitectic Al-Ti alloys at 0.35, 0.5, 0.7 and 0.8 wt pct Ti has been frozen at rates varying from less than 1°C/s to in excess of 100°C/s. Cooling-curve analyses, metallographic and microprobe examinations, taken altogether, allow identification both of the nucleants and the solidification modes acting in this important alloy system. Two sets of conclusions are drawn, one in general about low concentration peritectic systems like Al-Ti, and the other about particular interactions in Al-Ti. For example, it is revealed that AlxTi compounds exist; Al3Ti, AlxTi and Al are nucleated by TiC; and AlxTi and TiC are both nucleants for aluminum.

The effects of process variables on pulsed Nd:YAG laser spot welds: Part I. AISI 409 stainless steel

The weldabilities of AA 1100 aluminum and AISI 409 stainless steel by the pulsed Nd:YAG laser welding process have been examined experimentally and compared. The effects of Nd:YAG laser welding parameters, including laser pulse time and power intensity, and material-dependent variables, such as absorptivity and thermophysical properties, on laser spot-weld characteristics, such as weld diameter, penetration, melt area, melting ratio, porosity, and sur-face cratering, have been studied experimentally. The results of this work are reported in two parts. In Part I, the weldability of AISI 409 stainless steel by the pulse laser welding process is reported. In Part II, the weldability of A A 1100 aluminum under the same operating con-ditions is reported and compared to those of the stainless steel. When welding AISI 409 stainless steel, weld pool shapes were found to be influenced most by the power intensity of the laser beam and to a lesser extent by the pulse duration. Conduction mode welding, keyhole mode welding, and drilling were observed. Conduction mode welds were produced when power in-tensities between 0.7 and 4 GW/m2 were used. The initial transient in weld pool development occurred in the first 4 ms of the laser pulse. Following this, steady-state conditions existed and conduction mode welds with aspect ratios (depth/width) of about 0.4 were produced. Keyhole mode welds were observed at power intensities greater than 4 GW/m2. Penetration of these keyhole mode welds increased with increases in both power intensity and pulse time. The major weld defects observed in the stainless steel spot welds were cratering and large-occluded gas pores. Significant metal loss due to spatter was measured during the initial 2 ms of keyhole mode welds. With increasing power intensity, there was an increased propensity for occluded gas pores near the bottom of the keyhole mode welds.

Cellular morphologies in rapidly solidified Al-Al2Cu and Al-Al3Ni eutectic alloys

Abstract The cellular structures found in impure specimens of Al-Al 2 Cu and Al-Al 3 Ni eutectic alloys solidified at 0.44 to 2.88 mm/sec have been investigated. In Al-Al 2 Cu elongated cells containing parallel traces of mismatch surfaces were found at the lowest solidification rate, but the traces were arranged more randomly and the cells became more equiaxed as the rate of solidification increased. In Al-Al 3 Ni the cells displayed a specific shape within each eutectic grain at the lowest solidification rate, but were less crystallographic in nature at the faster rates. The crystallographic orientation relation between eutectic phases has been determined for both eutectics and compared with data for the same systems solidified more slowly.

The mechanism of grain refinement during solidification of Zn-Ti base alloys

Through cooling curve determinations, metallography, and electron microprobe analysis the mechanism of grain refinement in Zn-Ti and Zn-Ti-Cu alloys was determined. It is shown that zinc-titanium-oxide particles in the melt act as nucleants. Through probe analysis and considerations of matching of crystallographic planes it appears that the nuclei are the spinel Zn2TiO4. Additions of lead to Zn-Ti melts drastically reduces the effectiveness of the nucleant.

Unidirectional solidification of monovariant eutectic Cu-Mg-Ni alloys: I. Planar interface stability criterion

Alloys lying in composition along the monovariant trough of the Cu-Mg-Ni phase diagram were unidirectionally solidified, to obtain aligned microstructures. The composition range extended from the binary Cu-Mg eutectic to 10 at.% Ni. It is shown that for these alloys the simple constitutional undercooling criterion G/R ≤ mC0(1−k)/Dk, holds for planar/non-planar interface breakdown if the true distribution coefficients k and liquidus slope m are applied for all alloy compositions C0, assuming D to be constant. For this alloy system the use of single values of k and m estimated from alloys low in nickel would seriously misjudge the interface breakdown conditions at higher nickel concentrations.

Interface stability and microsegregation in bismuth-tin alloys

Abstract Unidirectional solidification experiments with the semimetal bismuth containing up to 8.35 at % tin have been carried out to investigate the interface stability, preferred orientations and microsegregation patterns. The solid-liquid interface appeared to be more stable than predicted by theory. When the interface did break down the microsegregation pattern was related to the observation that {100} facets were present at the interface, and were observed to project into the liquid as either parallel ridges or pyramids. The growth direction preferred for any set of growth conditions appeared to be the one having an interface morphology which could satisfactorily redistribute the rejected solute.

Crystallographic relationships and morphologies of the Bi-Zn and Bi-Ag eutectic alloys

Abstract The Bi-Ag and Bi-Zn eutectic alloys solidified at 0.06 cm/hr form a “broken-lamellar” morphology. Electron diffraction has determined preferred interface planes and growth directions for both eutectics. The observation of growth twins in all Ag lamellae indicates their growth via a twin re-entrant edge mechanism, in agreement with the prediction of faceted solid-liquid interfaces for Ag. The Bi phase is expected to solidify in a non-faceted manner for both alloys. Coupled growth of the Zn phase in the Bi-Zn eutectic is achieved by a different mechanism. The Zn growth axis is variable but is always such that the non-faceted solid-liquid interface planes are non-basal. The Bi-Ag eutectic alloy solidified at high rates (6 cm/hr) with a regular (broken-lamellar) regions which lead irregular regions. This behaviour remains unexplained because of inadequate crystallographic data.