J.D. Boyd

Near-interface microstructure in a SiC/Al composite

Abstract The microstructure of a SiC particulate reinforced Al 2080 alloy has been characterized by focussed ion beam microscopy and transmission electron microscopy. A 40 nm thick amorphous interface layer develops during processing of the composite material by diffusion of Al and Mg into the pre-existing SiO 2 layer on the 3 μm SiC particles. During tensile deformation, damage occurs in the near-interface zone of the matrix, at the particle–matrix interface and within the SiC particles.

Strengthening of low-interstitial steels by strain-ageing treatments

Formable low-carbon automotive sheet steels can be designed to achieve a 30--50 MPa strength increment during paint baking. This so-called bake hardening process is a form of static strain ageing, whereby dissolved carbon or nitrogen atoms segregate to the dislocations generated during temper rolling and press forming. In ultra-low carbon interstitial-free steels, the concentration of C and N in solution is determined by the equilibrium partitioning between ferrite and precipitates of Ti- or Nb carbonitrides. Thus, during continuous annealing the concentration of interstitial atoms could increase due to precipitate dissolution. The reverse process could occur during cooling due to NbC precipitation at 800--700 C or Fe{sub 3}C precipitation at 400--300 C. The purpose of the present study was to determine the effects of the annealing process variables (temperature, time and cooling rate) on the solute carbon concentration in a series of interstitial free (IF) steels.

On the nature of “raised ferrite” in a low-carbon microalloyed steel

In thermomechanically processed (TMP) low-carbon steels, the predominant microstructural constituent is ferrite. There is an interesting feature associated with polygonal ferrite (PF) microstructures when observed in Nital-etched samples by SEM secondary electron imaging. Under these conditions, many PF grains appear in relief, whereas there is no distinguishing contrast in optical microscopy. The present study was carried out to elucidate the nature of raised ferrite by directly measuring the orientation of RF grains in metallographic samples of a microalloyed steel.

Effects of martensite particle size and spatial distribution on work hardening behaviour of fine-grained dual-phase steel

ABSTRACT Model dual-phase steel microstructural variants having fine ferrite grain size and a range of martensite particle sizes and spatial distributions were produced by varying the starting microstructure prior to the intercritical annealing treatment. Superior tensile properties were obtained for the microstructural variant having the smallest (∼1 µm) uniform ferrite grain structure and a corresponding uniform distribution of small (∼0.5 µm) martensite particles. This microstructural variant also exhibited superior work hardening properties, as determined from a Crussard–Jaoul analysis and plots of instantaneous work hardening exponent vs. strain. The true work hardening rate had a positive dependence on at low strain ( < 2%) for all three microstructural variants, consistent with the geometrically necessary dislocation mechanism. At higher strains, Stage III work hardening is operative and the dislocation annihilation factor exhibited a positive dependence on .

Microstructural evolution of a hot-rolled microalloyed complex phase steel

ABSTRACT The current study examines a grade of hot-rolled and continuously cooled complex phase sheet steel comprised of polygonal ferrite (PF), granular bainite (GB) and lath bainite (B). The quantity of each constituent phase depends on the thermomechanical processing conditions, which vary between commercially produced sheets. In this study, the effects of cooling rate and austenite grain morphology on microstructure are determined through a series of dilatometry experiments. The resulting CCT diagrams show a progression in the order PF → GB → B with increasing cooling rate, and that a Pancaked (unrecrystallised) austenite condition promotes the formation of PF to higher cooling rates and the formation of GB to higher temperatures. Application of the CCT results to industrially produced sheet provides a useful approach for interpreting the evolution of microstructure during controlled-cooling and coiling. However, direct comparison is limited by the moderate level of austenite pancaking that can be achieved through laboratory dilatometry experiments in comparison to an industrial hot mill. Notable differences in microstructure are observed between the leading and trailing edges of industrially produced sheets due to relatively small variations in cooling schedules.