Zirconium alloying and grain refinement of magnesium alloys

Factors that influence alloying zirconium to magnesium with a Mg-33.3Zr master alloy and the subsequent grain refinement are discussed based on a large number of experiments conducted at the laboratory scale (up to 30 kg of melt). It is shown that the zirconium particles released from the Zirmax(R) master alloy must be brought into thorough contact with the melt by an appropriate stirring process in order to attain a good dissolution of zirconium. The influence of alloying temperature on the recovery of zirconium was found to be negligible in the range from 680 to 780 degreesC. An ideal zirconium alloying process should end up with both high soluble and high total zirconium in the melt in order to achieve the best grain refinement in the final alloy. The distribution of zirconium in the final alloy microstructure is inhomogeneous and almost all of the zirconium in solution is concentrated in zirconium-rich cores in the microstructure.

A new zirconium-rich master alloy for the grain refinement of magnesium alloys

A new zirconium-rich magnesium-zirconium master alloy (designated AM-cast) has been developed by the CRC for Cast Metals Manufacturing in collaboration with Australian Magnesium Corporation for use as a grain refiner for magnesium alloys that do not contain aluminium. This work describes the microstructural characteristics of this new grain refiner and its grain refining ability when added to different magnesium alloys under various conditions (alloying temperature from 680 °C to 750 °C; weight of melt from 1 kg to 150 kg and sample thickness from 7 mm to 62 mm). Owing to its highly alloyable microstructure, AM-cast can be readily introduced into molten magnesium at any temperature when assisted by a few minutes of stirring or puddling. Little sludge has been found at the bottom of the alloying vessel in these trials due to the fine zirconium particles contained in the master alloy. The recovery of zirconium is normally in the range from 40% to 60% with respect to 1% zirconium addition as the master alloy. It is shown that this new master alloy is an excellent grain refiner for aluminium-free magnesium alloys.

Texture evolution and grain refinement of ultrafine-grained copper during micro-extrusion

Samples of oxygen-free high conductivity (OFHC) coarse-grained (CG) and ultrafine-grained (UFG) copper were micro-extruded to an equivalent strain of 2.8 in one pass at room temperature. Samples of the OFHC copper were annealed at 650C for 2 h to produce CG copper. Some samples were subsequently processed by equal channel angular pressing of eight passes, route Bc, at room temperature to produce the UFG material. Crystallographic texture and misorientation distributions were obtained locally from EBSD mappings at different radial positions after micro-extrusion. To model the strain path during micro-extrusion, the analytic flow line model of Altan etal. [J Mater. Process. Tech. 33 (1992) p.263] was used and also validated by finite element calculations. Modelling was carried out using the viscoplastic self-consistent (VPSC) model and a recently developed grain refinement model. The results showed large texture variations along the cross-section of the extruded sample for both UFG and CG copper. These cyclic drawing textures in UFG copper were simulated in good agreement with experiments using the presented modelling framework.

Evolution of Grain-Boundary Microstructure and Texture in Interstitial-Free Steel Processed by Equal-Channel Angular Extrusion

The equal-channel angular extrusion (ECAE) of Ti-bearing interstitial-free (IF) steel was performed following two different routes, up to four passes, at a temperature of 300 degrees C. The ECAE led to a grain refinement to submicron size. After the second pass, the grain size attained saturation thereafter. The microstructural analysis indicated the presence of coincident-site lattice (CSL) boundaries in significant fraction, in addition to a high volume fraction of high-angle random boundaries and some low-angle boundaries after the deformation. Among the special boundaries, Sigma 3 and Sigma 13 were the most prominent ones and their fraction depended on the processing route followed. A deviation in the misorientation angle distribution from the Mackenzie distribution was noticed. The crystallographic texture after the first pass resembled that of simple shear, with the {112}, {110}, and {123} aligned to the macroscopic shear plane.

Solidification Microstructure and Texture in Grain-refined Titanium alloys

In the present study, solidification microstructure and texture evolution in grain-refined Ti-6Al-4V and γ-TiAl alloys via trace boron addition are compared with their baseline counterparts. Boron addition resulted in dramatic grain refinement by almost an order of magnitude. The texture developed in these alloys is also markedly different from the baseline alloys.

Microstructure and Evolution of Mechanically-Induced Ultrafine Grain in Surface Layer of Al-Alloy Subjected to USSP

Experiments were conducted to investigate the ultrafine-grained (UFG) microstructures in the surface layer of an aluminum alloy 7075 heavily worked by ultrasonic shot peening. Conventional and high-resolution electron microscopy was performed at various depths of the deformed layer. Results showed that UFG structures were introdued into the surface layer of 62 μm thick. With increasing strain, the various microstructural features, e.g., the dislocation emission source, elongated microbands, dislocation cells, dislocation cell blocks, equiaxed submicro-, and nano-crystal grains etc., were successively produced. The grain subdivision into the subgrains was found to be the main mechanism responsible for grain refinement. The simultaneous evolution of high boundary misorientations was ascribed to the subgrain boundary rotation for accommodating further strains. Formed microstructures were highly nonequilibratory.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Grain size in Mg alloys: recrystallization and mechanical consequences

The present paper examines the development of grain size during the recrystallization of magnesium alloys and the influence the grain size has on the mechanical response. In magnesium alloys grain refinement improves the strength-ductility balance. This simultaneous increase in both strength and ductility is ascribed to the impact the grain size has on deformation twinning. The mechanisms by which the grain size is established during hot working are shown to be conventional dynamic recrystallization followed by post-dynamic recrystallization. The role of alloying additionon both of these reactions is briefly considered.