Laser melting technology, an ultra-high-temperature gradient direction solidification process, has been adopted on near equal atomic percent Ti-Al peritectic alloys with an effort to achieve the two-phase coupled growth structure. Both the alloy composition range and its local solidification parameters were determined by means of mathematical calculation of local solidification parameters in the melt pool. X-ray, SEM, TEM and optical microscopy techniques were carried out to investigate the microstructure and identify the phase composition. The two-phase (a and r) coupled growth morphology under conditions of high growth velocity and high-temperature gradient was first detected in the laser resolidified Ti-Al peritectic alloys. The aluminum composition range appearing in the coupled growth of a and g phases, lies in Ti-(51.0-54.0) at% Al, a little shift towards the left direction of the hypoperitectic plateau. Microstructural analysis showed that the coupled growth morphology changed from regular lamellar, irregular blocks and equiaxed structures in sequence, with the temperature gradient decreasing during growth. Energy spectrum analysis results showed that not only the solute diffusion but also the dissolution of phase g played an important role in the coupled structure evolution. Rapid eutectic growth KT model (Kurz, Trivedi, Metall Trans. A 22 (1991) 3051) could be used effectively to predict the characteristic lamellar spacing of two-phase coupled structures in Ti-Al peritectic alloys. The transformation from a peritectic, L+a→r, to metastable eutectic reaction, L→a+r, of near equal atomic percent Ti-Al peritectic alloys, induced the formation of two-phase coupled growth morphologies.

Stability analysis of a growing solid/liquid interface is the fundamental concept of modern solidification theory. Here, serial laser rapid solidification experiments were performed on a hypoperitectic Ti47Al53 alloy to explore the dendritic growth behavior near the limit of high-velocity absolute stability. SEM and TEM techniques were carried out to investigate the microstructure and identify the phase composition. By adopting an improved sampling method of TEM, the growth morphology evolution of the laser-resolidified layer was observed directly and high-velocity banding structure was firstly detected in Ti-Al peritectic alloys. The high-velocity banding structures are parallel to the solid/liquid interface (normal to the growth direction) and made of the oscillation structures grown alternatively in modes of cell and plane morphologies. In light bands with cellular growth mode, all dislocation assembles are parallel to the growth direction and forms the cell boundaries, while all dislocation distributes randomly in dark bands. The determined growth velocity range for the appearance of high-velocity banding structures is about 0.5B1.1ms 1 according to the rapid solidification experiments, and the origin of the banding agrees well with the prediction of the CGZK phenomenological model (Acta Metal. Mater. 40 (1992) 983).

The r→a phase transformation behaviours of Fe-Co and Fe-Mn alloys were systematically investigated by dilatometry and Differential Thermal Analysis (DTA). Two kinds of transformation kinetics, called normal and abnormal, were recognized for the first time and classified according to the variation of the ferrite formation rate. These transformation characteristics were observed for both isothermally and isochronally conducted annealing experiments. A transition, from abnormal to normal transformation kinetics, occurs for Fe-1.79at.%Co when successive heat treatment cycles are executed, which contrasts with Fe-2.26at.%Mn for which only normal transformation kinetics occurs after each of all successive heat treatment cycles. A possible mechanism for the appearance of abnormal transformation kinetics is given, which is based on the austenite grain size. Light microscopical analysis indicates a repeated nucleation of ferrite in front of the migrating g/a interface.

Primary cellular/dendritic spacing of the solidified structures plays an important role on the microstructure control under rapid solidification conditions. In order to clarify the phase evolution and growth mechanism of peritectic reaction, here laser resolidification experiments, with different growth velocity, have been performed on Ti–Al peritectic alloys. By taking both the transverse and longitudinal sections of the directional growth cells/dendrites in the laser traces, the primary phase formation and average primary cellular/dendritic spacing, as well as the corresponding solidification velocity are quantitatively determined by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). With the solidification velocity increasing, the separating primary phase evolves from single-phase a, two-phase (a+g), g, and the corresponding primary cellular/dendritic spacing decreases gradually. All experimental observations of primary spacing evolution, in all single phase regions, agree well with that predicted by rapid dendritic growth model.

The isochronal and isothermal austenite (r)→ferrite (a) transformation of pure iron was measured by high-resolution dilatometry and differential thermal analysis. Both abnormal and normal transformation kinetics were recognized for the first time in pure iron according to the variation in the ferrite formation rate. The occurrence of the type of r→a transformation strongly depends on the grain size; the transformation type changes from abnormal to normal with decreasing grain size. The abnormal transformation process involves the occurrence of additional peaks in the transformation rate for the first stage of the transformation. A phase transformation model, involving repeated nucleation (autocatalytic nucleation), interface-controlled continuous growth and incorporating correction for impingement, has been employed successfully to describe the observed kinetics of the abnormal transformation.