Predictive model of superplastic properties of aluminum bronze and of the superplastic extrusion test

The superplastic properties of aluminum bronze were studied by way of artificial neural network. The model was established using Levenberg–Marquardt algorithm. It was improved by studying superplastic tension test data of aluminum bronze such that the superplastic forming parameters were optimized. According to the parameters, the experiment of superplastic extrusion of a solid bearing was performed. It is shown that the model reflected well the relationship between superplastic properties of aluminum bronze and superplastic tension conditions. The relative error between the test values and the predicted values of the network is less than 8.5%, which meets perfectly the demands of superplastic deformation of aluminum bronze. Moreover, the superplastic forming of solid cage of aluminum bronze show that it is feasible to produce solid cage using superplastic extrusion process. This extrusion process has remarkable economic benefits as well.     

The constant stress tensile creep behaviour of a superplastic zirconia-alumina composite

A detailed study was conducted to evaluate the constant stress tensile creep behaviour of a superplastic 3 mol% yttria-stabilized zirconia-20 wt% alumina composite. The comprehensive experimental results indicate that creep deformation may be expressed in the form exp(–585 000/8.3T), where is the steady-state creep rate, is the imposed stress, is the linear intercept grain size andT is the absolute temperature. Microstructural observations revealed that there is very little dislocation activity, or change in grain size or shape. A detailed analysis was conducted to evaluate the possible rate-controlling mechanisms in terms of the experimentally determined mechanical properties and the microstructural observations. Based on the maintenance of an equiaxed microstructure and the strong grain size and stress dependence, it is concluded that creep occurs by a grain-boundary sliding/grain rearrangement process.     

Subzero tensile properties of vapour quenched aluminium alloys

Supersaturated and metastable aluminium alloy solid solutions containing a dispersed phase have been produced by a vapour quenching technique. Binary alloys contained 3.5% Fe and 5.5% Mn; ternary alloys contained 6 to 9% chromium and 0.5 to 1.2% iron. After rolling into sheet the tensile properties were determined in the temperature range 293 to 77 K. At 77 K tensile strengths of 1115 and 1036 MPa were obtained for two Al-Cr-Fe alloys, equivalent to E/82 and E/83, respectively. These are the highest strengths ever reported for an aluminium alloy. The deformation behaviour at subzero temperatures has indicated the potential for further strengthening of metastable rapidly solidified aluminium alloys by dislocations alone.     

The fracture resistance of nickel aluminium bronze tubes

Burst tests on internally pressurised cast nickel aluminium bronze tubes containing artificial defects indicate that the alloy has a toughness of 150 Mn/m^3/2 at a thickness of 9.53 mm. Short axial through cracks only propagated after general yield at a very high effective toughness.

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Résumé Des essais de gonflement de tubes en bronze de Nickel, Aluminium moulé, soumis à une pression interne et comportant des défauts artificiels indiquent que l'alliage a une ténacité de 150 Mn/m^3/2 à une épaisseur de 9,53 mm. Des fissures traversantes axiales et courtes ne se sont propagées qu'après plastification générale à une ténacité effective très élevée.

     

Microstructures and tensile properties of spray-deposited high-strength aluminium alloys

Spray deposition is a new rapid solidification technique which produces bulk preforms directly from the melt metals. A spray deposition process was used to develop several high-strength aluminium alloys based on their commercial chemical compositions. These alloys include 2024 alloy, 7075 alloy and 7075 alloy modified with 1.0% Fe and 1.0% Ni.The deposits possessed rapid solidification microstructure with grain size of about 20 μm and a relative density of over 94%. The hardening phases of the materials in T4 or T6 conditions consisted of supersatured solid solution, stable and unstable ageing precipitates and disperse phases. The formation of the fine distributed disperse phases was due to the addition of iron and nickel to the 7075 alloy. The spray-deposited materials exhibited substantial improvement in tensile strengths and maintained acceptable ductility when compared to the corresponding ingot metallurgy processed materials. This revised version was published online in November 2006 with corrections to the Cover Date.     

Rapid forming of superplastic aluminium alloy 5083

Abstract

Decreasing the cycle time significantly for forming the commercially available superplastic aluminium alloy 5083 has been achieved. Forming results and conditions are compared with previous relevant works which are actually scarce. A circular cup having a depthdiameter ratio of 1:2 can be formed in 70 s. This ratio requires flat sheet to be stretched in area by up to three times, which should be large enough when dealing with actual industrial sheet forming. On average, the thickness is decreased by two-thirds; in fact, the thickness distribution is not uniform and the gradient is concentrated at the wall of the cup. The location of minimum thickness in rapid forming is different from that in conventional forming. Disregarding the traditional approach, the pressure-time profile employed in this work was not restricted to yield the so called optimum strain rate, which is usually low. Following the same processing profile, but proceeding in stages of partial forming, a series of progressive forming configurations was obtained in order to analyse the strain rate path leading to the successful rapid forming. For a specimen processed at 500C, the maximum volume fraction of cavities is 4 existing at the location of minimum thickness.     

The metallography of fracture in cast nickel aluminium bronze

In tube burst tests, and in slow bend tests, crack propagation in nickel aluminium bronze produces crack faces normal to the tension stress even though plastic deformation preceding fracture occurs under conditions of generalized plane stress. Shear lips are observed to be about 1.0 mm deep and, therefore, much less than the critical plane stress plastic zone radius which is around 40 to 50 mm in tube tests. Fractographic and metallurgical examinations reveal that the crack path is dictated by metallurgical structure and that the small shear lips and flatness of the crack faces arise from a cumulative mode of fracture dictated by structural features rather than as a result of constraint stresses.     

High temperature deformation of aluminium bronze

Abstract

Hot compression tests were carried out on commercial Cu–8 wt-%Al alloy to test the effect of the deformation conditions on high temperature deformation characteristics and the final structure of the hot deformed material. Dynamic recrystallisation of the material was found to operate at deformation temperatures above ～900 K. Nucleation and growth of recrystallised grains were observed for specimens deformed at temperatures below ～1000 K. However, the flow stress peaks that usually mark the onset of dynamic recrystallisation were hardly seen on the stress–strain curves. During hot deformation of Cu–8 wt-%Al alloy above ～1000 K the interaction of →β phase transformation and deformation processes affected both the flow stress value and the structure of the material. In particular, post-deformation water quenching of the specimens resulted in martensitic transformation within pre-existing β grains. Moreover, local coherent iron particles were detected within β and neighbouring grains.     

Phase transformations in permanent-mould-cast aluminium bronze

The phases obtained in aluminium bronze (Cu-10Al-4Fe) cast into a permanent mould were investigated. The parameters examined were the pre-heating temperature of the mould and the graphite coating thickness. The phases and ₂ were detected as well as the metastable phases and . The intermetallics of the system Fe-Al were obtained in various stoichiometric compositions. The different cooling rates of the casting resulted in two mechanisms of transformation to grains out of the unstable phase, one being nucleation and growth producing needle-shaped grains, the other exhibiting a massive transformation to spherical grains. These two mechanisms determine the changes in the size of the a grains as result of changes in the cooling rate in its various ranges.     

Pre-holed tensile specimens for superplastic Y-TZP ceramics

A new method was developed to produce tensile specimens with pre-machined holes for superplastic zirconia ceramics. Tensile specimens with effective hole sizes of 450 m were successfully produced using graphite fibers. Although the size of the defect introduced is relatively large compared to the grain size enough deformation is obtained for qualitative and quantitative analysis for Y-TZP. 3Y-TZP and 3Y-TZP co-doped with germanium and titanium oxide revealed cavity growth coefficients that varied with strain rate and temperature, unlike metals, as a direct consequence of grain growth induced strain hardening.     

A metallographic study of superplasticity in a micrograin aluminium bronze

The microstructures of micrograin Cu-9 to 10% Al-0 to 4% Fe alloys, which are superplastic at 800° C, have been determined. Metallographic studies after deformation at 800° C over a range of strain-rates encompassing the three stage strain-rate hardening behaviour common to superplastic materials show that in the low strain-rate range, below that for high values of the strain-rate sensitivity exponent (m), clumps of grains slide together as units with considerable flow in the matrix close to sliding interfaces. After deformation in this low strain-rate range there is no evidence for dislocation motion within the grains. With increasing strain-rate, through and beyond the strain-rate range where peak values ofm are recorded, evidence for dislocation motion steadily increases; the tendency for clumps of grains to slide together diminishes; and there is decreased flow in the matrix about the sliding interfaces. The strain-rate for maximumm shows a strong dependence on the proportion of phase in the microstructure and the presence of iron which acts to refine the grain size. These observations are explained in terms of a flow mechanism whereby the high strain-rate sensitivity range occurs intermediate between a low strain-rate range, where sliding is accommodated by diffusion, and a high strain-rate range, where accommodation is by dislocation movement through the matrix.Formerly Graduate Student, Department of Mechanical Engineering, University of Waterloo, Ontario, Canada.     

Influence of thermomechanical ageing on tensile properties of 2014 aluminium alloy

2014 aluminium alloy was subjected to various thermomechanical ageing (TMA) treatments which included partial peak ageing (25% and 50%), warm rolling (10% and 20%) and further ageing to peak hardness level at 160 ° C. The tensile tests reveal that TMA treatments cause a substantial improvement in tensile properties and thermal stability. The electron microscopic studies reveal that the TMA treatments affect substantially the ageing characteristics. The TMA Ib treatment yields the finest needles having longitudinal dimensions of 40nm. The TMA treatments also lead to precipitate-dislocation networks of different densities. It is observed that TMA IIb treatment results in the densest precipitate-dislocation tangles of all the TMA treatments. As a result, a significant improvement in the tensile properties of 2014 aluminium alloy has been observed.     

Microstructural characterization of cast nickel aluminium bronze

The morphology and chemical analysis of the complex phases present in cast nickel aluminium bronze, of nominal composition 10% aluminium, 5% nickel and 5% iron, have been investigated using optical and electron microscopy techniques and energy dispersive analysis. It has been shown that , and four forms of can exist in the ascast microstructure of this alloy. Heat treatment can lead to the precipitation of a further phase which differs in morphology and chemical composition to those present in ascast structures.     

Microstructure of as-cast aluminium bronze containing iron

Abstract

The morphology, crystallography, and chemistry of phases present in an as-cast commercial aluminium bronze containing iron, of nominal composition (wt-%) Cu–10Al–2·5Fe (BS 1400: AB1), and the development of microstructure on cooling from elevated temperatures, have been studied using optical and electron microscopic techniques. The as-cast microstructure consists of α-phase, martensite, and iron-rich intermetallic precipitates. The α-phase is fcc copper-rich solid solution and exhibits a Widmanstätten morphology. The martensitic phase, which is derived from the high-temperature β-phase, has the 9R crystal structure. The intermetallic particles are based on Fe₃Al and have the Do₃ structure. These iron-rich particles are precipitated in the β-phase and cause a refinement of the microstructure by providing sites for the nucleation of the β-phase, to some extent, by impeding the growth of the α-phase.

MST/150     

Microstructure,tensile properties and fracture behaviour of aluminium alloy 7150

A study has been made to understand the microstructure, tensile properties and fracture characteristics of aluminium alloy 7150. Detailed optical and transmission electron microscopical observations were used to analyse the intrinsic microstructural features of the alloy in the T77 condition. The alloy was deformed to failure over a range of strain rates in environments of 3.5% sodium chloride solution and laboratory air. The environment was found to have little influence on strength of the alloy. The strength only marginally increased with an increase in strain rate. However, for all strain rates, the ductility of the alloy degraded in the aggressive environment. The ratio of strain to failure in sodium chloride solution to that in laboratory air indicates that the alloy is only mildly susceptible to stress corrosion cracking. The fracture behaviour was different in the two environments. However, in a given environment the fracture behaviour was essentially the same. In the aggressive environment fracture was predominantly intergranular while fracture revealed a ductile transgranular failure in laboratory air. An attempt is made to discuss the kinetics of the fracture process in terms of competing mechanistic effects involving intrinsic microstructural features, matrix deformation characteristics, environment and strain rate.     

Grain refinement and superplastic behaviour of a modified 6061 aluminium alloy

Abstract

The superplastic properties and microstructure evolution of a 0.15%Zr and 0.7%Cu modified 6061 aluminium alloy were examined in tension at temperatures ranging from 475 to 600°C and strain rates ranging from 7 × 10^-6 to 2.8 × 10^-2 s^-1. The refined microstructure with an average grain size of about 11 μm was produced in thin sheets by a commercially viable thermomechanical process. It was shown that the modified 6061 alloy exhibits a moderate superplastic elongation of 580% in the entirely solid state at 570°C and ? = 2.8 × 10^-4 s^-1. Superior superplastic properties (elongation to failure of 1300% with a corresponding strain rate sensitivity coefficient m of about 0.65) were found at the same strain rate and a temperature of 590°C, which is higher than the incipient melting point of the 6061 alloy (~575°C). The microstructural evolution during superplastic deformation of the 6061 alloy has been studied quantitatively. The presence of a slight amount of liquid phase greatly promotes the superplastic properties of the 6061 alloy, reducing the cavitation level.     

Significantly enhanced tensile strength and ductility in nickel aluminium bronze by equal channel angular pressing and subsequent heat treatment

Nickel aluminium bronze (NAB) was subjected to equal channel angular pressing (ECAP) at 400 °C for up to 4 passes in routes B_A and C, respectively, followed by isothermal heat treatment with a view to improving the κ phase structures and tensile properties. The lamellar κ_III structure was completely broken after 4 passes in route B_A although route C was less efficient. Spheroidisation and coarsening of the highly deformed κ_III continued during heat treatment especially at ≥600 °C. At 800 °C, both the lamellar structure and the fine κ_IV particles transformed completely into a coarse globular morphology with no distinction between the primary and eutectoid α. Significant increases in strength were achieved by ECAP, reaching a maximum yield strength of 960 MPa with a good ductility of ~14 %. Heat treatment after ECAP was shown to considerably improve tensile ductility to >30 % while keeping the strength high at ~700 MPa, a significant enhancement compared to the as-received NAB.