Nong Gao

Room temperature precipitation in quenched Al-Cu-Mg alloys : a model for the reaction kinetics and yield strength development

The microstructural evolution during low temperature ageing of two commercial purity alloys (Al–1.2Cu–1.2Mg–0.2Mn and Al–1.9Cu–1.6Mg–0.2Mn at.%) was investigated. The initial stage of hardening in these alloys is very rapid, with the alloys nearly doubling in hardness during 20 h ageing at room temperature. The microstructural evolution during this stage of hardening was investigated using differential scanning calorimetry (DSC), isothermal calorimetry and three–dimensional atom probe analysis (3DAP). It is found that, during the hardening, a substantial exothermic heat evolution occurs and that the only microstructural change involves the formation of Cu–Mg co–clusters. The kinetics of cluster formation is analysed and the magnitude of the hardening is discussed on the basis of a model incorporating solid solution hardening and modulus hardening originating from the difference in modulus between Al and clusters.

Significance of stacking fault energy on microstructural evolution in Cu and Cu-Al alloys processed by high-pressure torsion

Disks of pure Cu and several Cu–Al alloys were processed by high-pressure torsion (HPT) at room temperature through different numbers of turns to systematically investigate the influence of the stacking fault energy (SFE) on the evolution of microstructural homogeneity. The results show there is initially an inhomogeneous microhardness distribution but this inhomogneity decreases with increasing numbers of turns and the saturation microhardness increases with increasing Al concentration. Uniform microstructures are more readily achieved in materials with high or low SFE than in materials with medium SFE, because there are different mechanisms governing the microstructural evolution. Specifically, recovery processes are dominant in high or medium SFE materials, whereas twin fragmentation is dominant in materials having low SFE. The limiting minimum grain size (d min) of metals processed by HPT decreases with decreasing SFE and there is additional evidence suggesting that the dependence of d min on the SFE decreases when the severity of the external loading conditions is increased.

Tribological properties of ultrafine-grained materials processed by severe plastic deformation

Processing by severe plastic deformation (SPD) has been developed extensively over the last two decades in order to produce ultrafine-grained (UFG) materials having submicrometre or nanometre grain sizes. An important material property for UFG materials is good wear resistance so that they may be used in a range of structural applications. An examination of the published data shows that only limited reports are available to date on the wear behaviour of SPD-processed materials and, furthermore, many of these results appear to be conflicting. The correlation of hardness and wear is limited because the wear property is a system property that in practice is influenced by a range of factors. Accordingly, this review is designed to examine recent reports related to the wear resistance of materials processed by SPD with particular emphasis on alloys processed using equal-channel angular pressing (ECAP), high-pressure torsion (HPT) and accumulative roll-bonding (ARB).

Processing of an ultrafine-grained titanium by high-pressure torsion: an evaluation of the wear properties with and without a TiN coating.

A commercial purity (CP) Grade 2 Ti was processed by high-pressure torsion (HPT) using an imposed pressure of 3.0GPa at room temperature. The HPT processing reduced the grain size from ∼8.6 μm in the as-received state to ultra-fine grains (UFG) of ∼130 nm after HPT. Tensile testing showed the HPT-processed Ti exhibited a good combination of high ultimate tensile strength (∼940 MPa) and a reasonable elongation to failure (∼23%). Physical vapour deposition was used to deposit TiN coatings, with a thickness of 2.5 μm, on Ti samples both with and without HPT processing. Scratch tests showed the TiN coating on UFG Ti had a critical failure load of ∼22.5 N whereas the load was only ∼12.7 N for the coarse-grained Ti. The difference is explained using a simple composite hardness model. Wear tests demonstrated an improved wear resistance of TiN coating when using UFG Ti as the substrate. The results suggest that CP Ti processed by HPT and subsequently coated with TiN provides a potentially important material for use in bio-implants.

A model of grain refinement and strengthening of Al alloys due to cold severe plastic deformation

This paper presents a model which quantitatively predicts grain refinement and strength/hardness of Al alloys after very high levels of cold deformation through processes including cold rolling, equal channel angular pressing (ECAP), multiple forging (MF), accumulative roll bonding (ARB) and embossing. The model deals with materials in which plastic deformation is exclusively due to dislocation movement within grains, which is in good approximation the case for many metallic alloys at low temperature, for instance aluminium alloys. In the early stages of deformation, the generated dislocations are stored in grains and contribute to overall strength. With increase in strain, excess dislocations form and/or move to new cell walls/grain boundaries and grains are refined. We examine this model using both our own data as well as the data in the literature. It is shown that grain size and strength/hardness are predicted to a good accuracy.

Using differential scanning calorimetry as an analytical tool for ultrafine grained metals processed by severe plastic deformation

Abstract Differential scanning calorimetry (DSC) is a thermal analysis technique that measures the energy absorbed or released by a sample as a function of temperature or time. Analysis by DSC has wide applications for examining solid state reactions and solid–liquid reactions in many different materials. Analysis of the kinetics of reactions may be assessed by activation energy analysis methods. In recent years, DSC has been applied in the examination and analysis of bulk ultrafine grained materials processed through the application of severe plastic deformation. This overview examines these recent reports with reference to materials processed using the procedures of equal channel angular pressing, high pressure torsion and accumulative roll bonding. In addition, some critical issues related to DSC analysis are also discussed.

Precipitation in Stretched Al-Cu-Mg Alloys with Reduced Alloying Content Studied by DSC, TEM and Atom Probe

The hardening and microstructural evolution during ageing of a Al-1.2Cu-0.5Mg and a Al-1.2Cu-1.2Mg (at.%) alloy has been investigated. Artificial ageing at 150°C of stretched and naturally aged samples initially (up to about 48 h) leads to very limited further strengthening, but ageing at 190°C results a quick increase in strength. Detailed microstructural investigation using differential scanning calorimetry, transmission electron microscopy and three-dimensional atom probe demonstrated that hardening at 150oC is mostly dominated by the formation of solute clusters with varying compositions and plate-like zones rich in copper at early stages of ageing (t<24h) and by the formation of S phase at the later stages of ageing. At higher 190oC no zones or clusters form and the ageing is dominated by the formation of S precipitates.

Determination of kinetic parameters from calorimetric study of solid state reactions in 7150 Al­Zn­Mg alloy

Differential scanning calorimetric (DSC) study was carried out at different heating rates to examine the solid state reactions in a 7150 Al-Zn-Mg alloy in water-quenched (WQ) state, naturally and artificially aged tempers. The exothermic and endothermic peaks of the thermograms indicating the solid state reaction sequence were identified. The shift of peak temperatures to higher temperatures with increasing heating rates suggests that the solid state reactions are thermally activated and kinetically controlled. The artificial aging behaviour of the alloy was assessed by measuring the variations of hardness with aging time. The fraction of transformation (Y), the rate of transformation (dY/dt), the transformation function f(Y), and the kinetic parameters such as activation energy (Q) and frequency factor (k0) of all the solid state reactions in the alloy were determined by analyzing the DSC data, i.e. heat flow involved with the corresponding DSC peaks. It was found that the kinetic parameters of the solid state reactions are in good agreement with the published data.

Fabrication of MEMS components using ultrafine-grained aluminium alloys

A novel process for the fabrication of a microelectromechanical systems (MEMS) metallic component with features smaller than 10 µm and high thermal conductivity was investigated. This may be applied to new or improved microscale components, such as (micro-) heat exchangers. In the first stage of processing, equal channel angular pressing (ECAP) was employed to refine the grain size of commercial purity aluminium (Al-1050) to the ultrafine-grained (UFG) material. Embossing was conducted using a micro silicon mould fabricated by deep reactive ion etching (DRIE). Both cold embossing and hot embossing were performed on the coarse-grained and UFG Al-1050. Cold embossing on UFG Al-1050 led to a partially transferred pattern from the micro silicon mould and high failure rate of the mould. Hot embossing on UFG Al-1050 provided a smooth embossed surface with a fully transferred pattern and a low failure rate of the mould, while hot embossing on the coarse-grained Al-1050 resulted in a rougher surface with shear bands

Microstructure and precipitation in Al-Li-Cu-Mg-(Mn, Zr) alloys

Abstract Hot rolled Al–6Li–1Cu–1Mg–0·2Mn (at.-%) (Al–1·6Li–2·2Cu–0·9Mg–0·4Mn, wt-%) and Al–6Li–1Cu–1Mg–0·03Zr (at.-%) (Al–1·6Li–2·3Cu–1Mg–0·1Zr, wt-%) alloys developed for age forming were studied by tensile testing, electron backscatter diffraction (EBSD), three-dimensional atom probe (3DAP), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). For both alloys, DSC analysis shows that ageing at 150°C leads initially to formation of zones/clusters, which are later gradually replaced by S phase. On ageing at 190°C, S phase formation is completed within 12 h. The precipitates identified by 3DAP and TEM can be classified into (a) Li rich clusters containing Cu and Mg, (b) a plate shaped metastable precipitate (similar to GPB2 zones/S″), (c) S phase and (d) δ′ spherical particles rich in Li. The Zr containing alloy also contains β′ (Al3Zr) precipitates and composite β′/δ′ particles. The β′ precipitates reduce recrystallisation and grain growth leading to fine grains and subgrains.