Effect of alloying and extrusion temperature on the microstructure and mechanical properties of Mg-Zn and Mg-Zn-RE alloys

Extruded Mg-Zn-RE alloys have been shown to exhibit excellent combinations of yield strength and ductility, but it is not completely clear how adding rare earth metals to Mg-Zn alters the microstructure and affects the mechanical properties. Microstructural changes and the resulting mechanical properties from changes in composition and extrusion temperature have been investigated for Mg-. x Zn-. y RE (. x=2.5 and 5. wt.%, y=0 and 1. wt. %, and RE=Gd and Y) alloys. Adding RE to Mg-Zn increased the strength and reduced the ductility, while increasing the zinc concentration in the Mg-Zn-RE alloys had the reverse effect.

Low-temperature compaction of Ti-6Al-4V powder using equal channel angular extrusion with back pressure

Equal channel angular extrusion (ECAE), with simultaneous application of back pressure, has been applied to the consolidation of 10 mm diameter billets of pre-alloyed, hydride-dehydride Ti-6Al-4V powder at temperatures ≤400 °C. The upper limit to processing temperature was chosen to minimise the potential for contamination with gaseous constituents potentially harmful to properties of consolidated product. It has been demonstrated that the application of ECAE with imposed hydrostatic pressure permits consolidation to in excess of 96% relative density at temperatures in the range 100-400 °C, and in excess of 98% at 400 °C with applied back pressure ≥175 MPa. ECAE compaction at 20 °C (back pressure = 262 MPa) produced billet with 95.6% relative density, but minimal green strength. At an extrusion temperature of 400 °C, the relative density increased to 98.3%, for similar processing conditions, and the green strength increased to a maximum 750 MPa. The relative density of compacts produced at 400 °C increased from 96.8 to 98.6% with increase in applied back pressure from 20 to 480 MPa, while Vickers hardness increased from 360 to 412 HV. The key to the effective low-temperature compaction achieved is the severe shear deformation experienced during ECAE, combined with the superimposed hydrostatic pressure.

Deformation field variations in equal channel angular extrusion due to back pressure

Flow lines were analysed in aluminium alloy 6061 during equal channel angular extrusion (ECAE) in a 90° die with and without the application of back pressure during pressing. The lines appeared much more rounded when a back pressure was applied compared to the case of conventional ECAE testing. With the help of an analytic flow function, the deformation field was obtained. It is shown that back pressure slightly lowers the total strain, strongly increases the size of the plastic zone and significantly reduces the plastic strain rate.

A kinematics study of variable lead axisymmetric forward spiral extrusion

Variable lead axi-symmetric forward spiral extrusion (VLAFSE) allows far more efficient strain accumulation than constant lead counterpart AFSE. It eliminates the rigid body rotation of the sample in an AFSE die which only causes frictional loss and has no contribution to the deformation. Based on a proposed velocity field, the kinematic formulation for VLAFSE is presented, utilized to predict strain components in the deformation zone and compared with experimental measurements. A simple closed form solution of the VLAFSE problem is presented which describes the flow of material inside the extrusion die. The results confirm that the effective strain of the process accumulates non-linearly in the both radial and extrusion directions.

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.

Comparison of different extrusion methods for compaction of powders

Densification of metallic powders by means of extrusion is regarded as a very attractive processing technique that allows obtaining a high level of relative density of the compact. However, the uniformity of the relative density depends on that of strain distribution and on the processing parameters. Several variants of extrusion can be used for compaction of metal particulates, including the conventional extrusion (CE) and equal channel angular pressing (ECAP), often referred to as equal-channel angular extrusion. Each of these processes has certain advantages and drawbacks with respect to compaction. A comparative study of these two extrusion processes influencing the relative density of compacts has been conducted by numerical simulation using commercial finite element software DEFORM2D. The results have been validated by experiments with titanium and magnesium powders and chips.

Expression of Na+/H+ exchanger mRNA in the gills of the Atlantic hagfish (Myxine glutinosa) in response to metabolic acidosis

Sodium/proton exchangers (NHE) are transmembrane proteins that facilitate the exchange of a Na⁺ ion for a H⁺ ion across cellular membranes. The NHE are present in the gills of fishes and are believed to function in acid-base regulation by driving the extrusion of protons across the branchial epithelium in exchange for Na⁺ in the water. In this study, we have used reverse transcriptase-polymerase chain reaction (RT-PCR) to detect the presence of a branchial NHE in the gills of the Atlantic hagfish, Myxine glutinosa. The subsequent partial cDNA sequence shares homology with other vertebrate and invertebrate NHE isoforms. In addition, using semi-quantitative, multiplex RT-PCR we demonstrate that mRNA expression of hagfish gill NHE is upregulated following an induced metabolic acidosis. Expression was increased to 4.4 times basal levels at 2-h post-infusion and had decreased to 1.6 times basal by 6 h. Expression had returned to basal levels by 24-h post-infusion. The inference from this study is that a gill NHE which is potentially important in acid-base regulation has been present in the vertebrate lineage since before the divergence of the hagfishes from the main vertebrate line.

Influence of grain size on the compressive deformation of wrought Mg-3A1-1Zn

The influence of the grain size on the flow stress of extruded Mg–3Al–1Zn tested in compression is examined. Samples with grain sizes varying between 3 and 23 μm were prepared by altering the extrusion conditions. Compression testing of the extruded bar was carried out at temperatures between ambient and 200 °C. Twinning dominated the deformation at lower temperatures but this gave way to slip dominated flow when the temperature was raised. For tests carried out at intermediate temperatures, a similar transition was observed when the grain size was reduced. The transition was accompanied by a change in flow curve shape and Hall–Petch slope. The peak stresses achieved when twinning dominated the deformation were up to 100 MPa greater than those seen when slip dominated the flow. Critical grain sizes marking the twinning–slip transition were identified and these are described in terms of the deformation conditions.