X. Wu

Ultrafine-grained titanium of high interstitial contents with a good combination of strength and ductility

A dehydrided Ti powder of very high oxygen content was successfully consolidated using back pressure equal channel angular processing into a fully dense bulk ultrafine-grained Ti showing apparent compressive ductility as well as high true yield and ultimate strengths of 1350 and 1780MPa, respectively. Interstitial solid solution strengthening contributed to the majority of the increase in strength with additional contribution from ultrafine grains. Significantly, the material also exhibited much improved ductility for such a high interstitial content, thanks probably to the nonequilibrium grain boundaries and bimodal grain structure introduced during severe plastic deformation.

High strength ultrafine/nanostructured aluminum produced by back pressure equal channel angular processing

High strength ultrafine/nanograined aluminum materials with ultimate strength up to 740MPa and Vickers microhardness up to 2285MPa were produced using back pressure equal channel angular processing of ultrafine-sized aluminum powder at 400°C. Microstructure analyses revealed that the attained high strength and microhardness were derived from the presence of nanosized aluminum and γ-alumina grains (5–10nm) as well as residual amorphous alumina. The interaction between the severe shear deformation and the preexisting amorphous alumina, concurrent oxidation, and amorphous to γ-alumina transition was considered to be responsible for the formation of such a refined and complex nanostructure.

Creep behaviour at elevated temperatures of an Al-Cu-Mg-Ag alloy

Abstract Tensile creep tests were conducted on an extruded and age-hardened Al-Cu-Mg-Ag alloy using constant load at 150, 180 and 210 °C. The normal three-stage creep was observed with a well defined steady state. The values for the stress exponent were between 11 and 15 for the temperatures and the activation energy for creep was about 152 kJ mol−1. The results obtained in this investigation were compared with those on some other Al-Cu-Mg-Ag alloys.

Synthesis of Bulk Materials by Equal Channel Angular Consolidation of Particles

Pure aluminium and titanium powders were successfully synthesised into bulk materials using equal channel angular (ECA) consolidation. Powders were used directly without the need to cold-compact them into green bodies. The processing temperatures were significantly lower than the usual sintering temperatures for aluminium and titanium. Fully dense bulk samples were achieved after one pass of ECA deformation through a 90 degree die. Mechanical properties of the as-ECA processed materials were comparable to those of wrought aluminium and titanium through ingot metallurgy. Multiple passes of ECA deformation resulted in refined microstructure and improved mechanical properties. The new process has many advantages over conventional powder sintering and is capable of producing bulk nanomaterials of high integrity.

Effects of oxidation and boron addition on tensile creep properties of cast Ti–46Al–2V–1Cr based alloys

Abstract Creep behaviours of Ti–46.5Al–2.5V–1Cr (at.%) and Ti–46.2Al–2V–1Cr–0.5B–0.4Ni alloys were investigated at 750xa0°C under constant stresses of 200 and 120 MPa, respectively, both in air and in vacuum. A well-defined steady state was achieved for Ti–46.5Al–2.5V–1Cr tested in vacuum. However, the specimens tested in air and those of Ti–46.2Al–2V–1Cr–0.5B–0.4Ni generally showed a lack of steady state deformation. Although the secondary creep rates were comparable for Ti–46.5Al–2.5V–1Cr, the total creep life was much shorter for the specimens tested in air compared to those tested in vacuum. For Ti–46.2Al–2V–1Cr–0.5B–0.4Ni, the secondary creep rate was increased and creep life much shortened when tested in air. Compared to Ti–46.5Al–2.5V–1Cr, the boron containing alloy exhibited much reduced creep resistance in terms of secondary creep rate and the total creep life irrespective of the testing environment. It is concluded that oxidation and addition of B both have considerably detrimental effects on creep strength of the material.

Bulk Al Materials from Back Pressure Equal Channel Angular Consolidation of Mechanically Milled Particles

Mechanically milled pure aluminium powders were fabricated into bulk materials using back pressure equal channel angular consolidation (BP-ECAC) for four or eight passes at 373K. The bulk materials consolidated from 0 h and 4 h mechanically milled powders were characterised by Vickers hardness tests and density measurements. Thermal stability of the consolidated bulk materials was evaluated by isothermal heat treatments at 673K. The as-consolidated bulk material from the 0 h milled (i.e. unmilled) powder showed nearly full density. However, full density was not obtained with the 4 h milled powder even after eight passes. The HV values for the as-consolidated materials fabricated from the 0 h and 4 h milled powders after four passes and from the 4 h milled powder after eight passes were 57, 121 and 136, respectively. Softening was observed in the bulk material consolidated from the 0 h milled powder during the isothermal heat treatment. However, the hardness of the bulk materials consolidated from the 4 h milled powders after four and eight passes increased to maximum values of 137 and 141 after heat treatment for 28 h and 8 h at 673K, respectively. The maximum hardness was maintained for up to 100 h at 673K in both materials. The hardening and thermal stability in the bulk materials from the milled powders are attributable to dispersion strengthening of Al4C3 particles formed by solid-state reaction during the isothermal heat treatment.

Bulk Mg Produced by Back Pressure Equal Channel Angular Consolidation (BP- ECAC)

Bulk magnesium was consolidated from pure Mg particles with an average size of ~59 µm by back pressure equal channel angular pressing. The Mg powder was processed at 200°C for 4 and 8 passes, respectively, using route C. The consolidated materials displayed density of 1.78 g/cm3, compared to the theoretical value of 1.74 g/cm3 for pure Mg. Vickers microhardness (HV) values were measured to be about 54. Compressive tests at room temperature revealed yield strengths of 100-110 MPa and ultimate strengths of up to 142 MPa with strains to fracture of ~9%, comparable to those for extruded pure Mg. Microstructures were examined using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

Synthesis of aluminium based bulk materials from micro and nano particles using back pressure equal channel angular consolidation

An innovative process for synthesising bulk materials using particles has been developed. The process is termed back pressure equal channel angular consolidation (BP-ECAC). Aluminium based materials were successfully consolidated into bulk materials using particles from nano to micro scales. BP-ECAC allowed the particles to be used directly without pre-compacting and casing and the processing temperatures to be significantly lower than those used in conventional sintering. Fully dense bulk samples were obtained instantaneously as the particles were forced to pass the shearing zone under pressure. Nanostructured materials were obtained from the nanometre-sized Al particles. Significant strengthening of the consolidated materials were observed. The new process is promising in producing porosity free, large volume materials with special compositions and structures.

Different tensile and compressive creep behaviours in a fully lamellar Ti–44Al–1Mn–2.5Nb–0.15Gd alloy

Abstract Creep behaviour of a Ti–44Al–1Mn–2.5Nb–0.15Gd alloy was investigated in both tension and compression. The alloy was cast and heat treated to have a fully lamellar microstructure with an average grain size of ∼265 μm. Creep tests were conducted in vacuum at 750xa0°C and under constant stresses between 100 and 400 MPa. The normal three stage creep was observed in both tension and compression with a well-defined steady state. However, the steady state creep rate in tension was significantly higher than that in compression. The microstructure after creep deformation exhibited lattice dislocations forming well-defined network in tension and dislocation bow-out and straight lattice dislocations in compression in the ‘soft’ grains and individual dislocations and twins in the ‘hard’ grains.

Bulk Ultrafine and Nanostructured Materials from Consolidation of Particles by Back Pressure Equal Channel Angular Pressing

Severe plastic deformation (SPD) has received considerable attention for its capability to produce ultrafine and nano structured materials. On the one hand, SPD, especially in the forms of equal channel angular pressing (ECAP) and high pressure torsion (HPT) is able to refine bulk materials with coarse grain structures. On the other hand, SPD has been used to synthesise bulk materials from particles. It enables particles from nano to micro scales to be consolidated into fully dense materials at much lower temperatures and shorter times, compared to the conventional sintering processing. It is particularly relevant to consolidating particles with non-equilibrium microstructures and to producing complex multiphase alloys. In this summary, ECAP as an effective process to synthesise a range of light metal based materials from particles with various sizes and structures, including aluminium and aluminium composites, titanium and magnesium, will be demonstrated. Full density and good bonding are achieved easily with the application of a back pressure. Microstructures from nano to ultrafine scales have been produced, resulting in significantly enhanced strength. Simultaneous increase in ductility has also been achieved in some alloys by virtue of multi-scale structures.