Rian J Dippenaar

In situ study of dynamic recrystallization and hot deformation behavior of a multiphase titanium aluminide alloy

Hot-compression tests were conducted in a high-energy synchrotron x-ray beam to study in situ and in real time microstructural changes in the bulk of a β-solidifying titanium aluminide alloy. The occupancy and spottiness of the diffraction rings have been evaluated in order to access grain growth and refinement, orientation relationships, subgrain formation, dynamic recovery, and dynamic recrystallization, as well as phase transformations. This method has been applied to an alloy consisting of two coexisting phases at high temperature and it was found that the bcc β-phase recrystallizes dynamically, much faster than the hcp α-phase, which deforms predominantly through crystallographic slip underpinned by a dynamic recovery process with only a small component of dynamic recrystallization. The two phases deform to a very large extent independently from each other. The rapid recrystallization dynamics of the β-phase combined with the easy and isotropic slip characteristics of the bcc structure explain the ex...

Phase transition and ordering behavior of ternary Ti–Al–Mo alloys using in-situ neutron diffraction

Abstract Neutron diffraction has been used for in-situ investigations to elucidate the phase transformation behavior of two Mo-containing TiAl alloys with compositions of Ti-44Al-3Mo and Ti-44Al-7Mo (in at.%). Five different phases are present in these alloys. These include three ordered phases at room temperature, namely α2, β0 and γ and two disordered phases, and, which occur at higher temperatures. The sequence of the three phase transformations in each alloy has been determined. The phase transformation and disordering/ordering temperatures were determined on heating and cooling from the diffracted peak intensities. The neutron experiments are particularly sensitive to the order–disorder transitions in TiAl alloys, which are compared with the overall phase fractions obtained from previous high energy X-ray diffraction. Hysteresis and undercooling effects are observed for the various phase transformations and depend on the nature of atomic rearrangements.

In-Situ Observation of Crystallization and Growth in High-Temperature Melts Using the Confocal Laser Microscope

This review discusses the innovative efforts initiated by Emi and co-workers for in-situ observation of phase transformations at high temperatures for materials. By using the high-temperature confocal laser-scanning microscope (CLSM), a robust database of the phase transformation behavior during heating and cooling of slags, fluxes, and steel can be developed. The rate of solidification and the progression of solid-state phase transformations can be readily investigated under a variety of atmospheric conditions and be correlated with theoretical predictions. The various research efforts following the work of Emi and co-workers have allowed a deeper fundamental understanding of the elusive solidification and phase transformation mechanisms in materials beyond the ambit of steels. This technique continues to evolve in terms of its methodology, application to other materials, and its contribution to technology.

Austenite grain growth and the surface quality of continuously cast steel

Austenite grain growth does not only play an important role in determining the mechanical properties of steel, but certain surface defects encountered in the continuous casting industry have also been attributed to the formation of large austenite grains. Earlier research has seen innovative experimentation, the development of metallographic techniques to determine austenite grain size and the building of mathematical models to simulate the conditions pertaining to austenite grain growth during the continuous casting of steel. Oscillation marks and depressions in the meniscus region of the continuously casting mold lead to retarded cooling of the strand surface, which in turn results in the formation of coarse austenite grains, but little is known about the mechanism and rate of formation of these large austenite grains. Relevant earlier research will be briefly reviewed to put into context our recent in situ observations of the delta-ferrite to austenite phase transition. We have confirmed earlier evidence that very large delta-ferrite grains are formed very quickly in the single-phase region and that these large delta-ferrite grains are transformed to large austenite grains at low cooling rates. At the higher cooling rates relevant to the early stages of the solidification of steel in a continuously cast mold, delta-ferrite transforms to austenite by an apparently massive type of transformation mechanism. Large austenite grains then form very quickly from this massive type of microstructure and on further cooling, austenite transforms to thin ferrite allotriomorphs on austenite grain boundaries, followed by Widmanstätten plate growth, with almost no regard to the cooling rate. This observation is important because it is now well established that the presence of a thin ferrite film on austenite grain boundaries is the main cause of reduction in hot ductility. Moreover, this reduction in ductility is exacerbated by the presence of large austenite grains.

Defect dynamics in polycrystalline zirconium alloy probed in situ by primary extinction of neutron diffraction

After α + β-zirconium has fully transformed into β-phase upon heating, the intensities of all β-Zr Bragg reflections decrease simultaneously as a function of time. It is shown that this effect represents a transition from the kinematic to the dynamic theory of diffraction due to the ever increasing crystal perfection driven by thermal recovery of the system. The best fitting coherent crystallite size of 30 μm and other microstructural features are verified by in situ laser scanning confocal microscopy. This effect of primary extinction in neutron diffraction has been employed to further investigate the crystal perfection kinetics. Upon further heating, crystal recovery is identified as a process of dislocation annihilation, suffering from lattice friction. Upon cooling, precipitating α-Zr induces strain into the perfect β-crystallites, re-establishing the kinematic diffraction intensities. An Avrami analysis leads to the estimations of nucleation time, consumption of nucleation sites and lower-dimensional growth. Such technique bears great value for further investigation on all metal systems annealed close to the melting temperature.

In situ observations of early oxide formation in steel under hot‐rolling conditions

A technique was developed to study in situ the early stages of the reaction between iron and air. Using a high‐temperature microscope, we observed at temperatures between 1000°C and 1050°C and within the first 30 s of reaction, the formation of iron‐oxide layers on the surface of low‐carbon steel. We observed the nucleation and growth of a first layer of iron oxide and the consecutive formation in sequence, of higher iron oxides sweeping over the surface of the former oxide. The grain boundaries of the steel substrate remain visible for quite some time following exposure to an oxidizing atmosphere indicating that diffusion through steel grain boundaries may have a determining influence on the formation of oxides. These findings emphasize the importance of conducting further studies to better understand the kinetics and mechanisms by which iron‐oxide layers form in the early stages of oxidation.

TEM characterization of precipitates in the segregated regions of a low-carbon, low-manganese, titanium-added steel.

A concentric solidification technique has been employed to simulate sulphide precipitation at the centreline of a continuously cast low‐carbon, low‐manganese, titanium‐added steel slab. Selected precipitates were identified using transmission electron microscopy following sample preparation by focused ion beam milling techniques. FeTiS2 and hexagonal MnS containing iron atoms form in close proximity to each other in super‐saturated areas of the concentrically solidified sample. The presence of FeTiS2 precipitates in low‐carbon steel has been verified for the first time, and the crystal structure determined by electron diffraction analysis as a trigonal CdI2‐type with a P3 m1 space group and lattice parameters of a= 0.341 nm and c= 0.569 nm.

Hybrid structure of white layer in high carbon steel – Formation mechanism and its properties

This study identifies for the first time, the hybrid structure of the white layer in high carbon steel and describes its formation mechanism and properties. The so-called ‘white layer’ in steel forms during high strain rate deformation and appears featureless under optical microscopy. While many researchers have investigated the formation of the white layer, there has been no definitive study, nor is there sufficient evidence to fully explain the formation, structure and properties of the layer. In this study, the formation, morphology and mechanical properties of the white layer was determined following impact testing, using a combination of optical and SE- microscopy, HR-EBSD, TKD and TEM as well as nano-indentation hardness measurements and FE modelling. The phase transformation and recrystallization within and near the white layer was also investigated. The microstructure of the steel in the white layer consisted of nano-sized grains of martensite. A very thin layer of austenite with nano sized grains was identified within the white layer by HR-EBSD techniques, the presence of which is attributed to a thermally-induced reverse phase transformation. Overall, the combination of phase transformations, strain hardening and grain refinement led to a hybrid structure and an increase in hardness of the white layer.

Effect of microstructural morphology on the mechanical properties of titanium alloys

Different morphologies of ?+? microstructures were obtained in a commercial Ti-6Al-4V alloy by cooling at different rates from the single ?-phase region into the two phase region. The effect of such morphologies on mechanical properties was studied using hot compression tests in a Gleeble thermomechanical simulator. A variety of complex morphologies could be obtained since the cooling rate has a significant influence on the ? to ? phase transformation and the resulting morphological development. While most of the ? phase transformed to colonies of ? at high cooling rates, it was possible to obtain a complex mixture of a colonies, grain boundary a and lamellar structure by decreasing the cooling rate. These complex morphologies each exhibited distinctive mechanical properties and characteristic dynamic phase transformation behaviour during deformation as a function of strain rate.

SolTrack: an automatic video processing software for in situ interface tracking

High‐Resolution in situ observation of solidification experiments has become a powerful technique to improve the fundamental understanding of solidification processes of metals and alloys. In the present study, high‐temperature laser‐scanning confocal microscopy (HTLSCM) was utilized to observe and capture in situ solidification and phase transformations of alloys for subsequent post processing and analysis. Until now, this analysis has been very time consuming as frame‐by‐frame manual evaluation of propagating interfaces was used to determine the interface velocities. SolTrack has been developed using the commercial software package MATLAB and is designed to automatically detect, locate and track propagating interfaces during solidification and phase transformations as well as to calculate interfacial velocities. Different solidification phenomena have been recorded to demonstrate a wider spectrum of applications of this software. A validation, through comparison with manual evaluation, is included where the accuracy is shown to be very high.