Papers by Stefano Spigarelli
Acta Materialia, 1997
Creep tests were conducted over a range of temperatures from 523 to 603 K on an unreinforced 2024... more Creep tests were conducted over a range of temperatures from 523 to 603 K on an unreinforced 2024 Al alloy fabricated by powder metallurgy processing. The creep curves under all testing conditions exhibit a brief quasi-steady-state condition and then a very extended tertiary stage leading to failure. A logarithmic plot of the minimum creep rate against the applied stress leads to a high and variable stress exponent and a high apparent activation energy. Prior to creep testing, the specimens contained large particles identified as Al2Cu and Al2CuMg. A precipitation of fine particles occurred during testing; this was especially evident in the long-term tests at the highest testing temperature. An analysis of the creep data is developed based on incorporating a threshold stress into the creep equation and using measurements of particle densities in order to include the influence of continuous precipitation during testing. The significance of the threshold stress is examined, and it is shown that the analysis leads to a stress exponent of 5 and an activation energy approximately equal to the value for lattice self-diffusion in pure aluminum.
Formability and Microstructure of AZ31 Magnesium Alloy Sheets
Key Engineering Materials, 2007
An analysis of hot deformation of an Al–Cu–Mg alloy produced by powder metallurgy
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2003
The high-temperature plasticity of a 2014 aluminium alloy produced by powder metallurgy was inves... more The high-temperature plasticity of a 2014 aluminium alloy produced by powder metallurgy was investigated in a wide range of temperatures and strain rates. When the strain rate was plotted as a function of stress (either peak flow stress in torsion, or applied stress in tensile creep), the alloy exhibited the same threshold-like behaviour observed in similar materials. The microstructure of representative torsioned samples was analysed in a transmission electron microscope (TEM) and the characteristics of particles and precipitate distribution were estimated. The dependence on stress and temperature was analysed by means of the conventional constitutive equations used for describing the hot-working behaviour and by means of a modified form of the sinh-equation, where the stress was substituted by an effective stress i.e. by the difference between the actual stress and a threshold stress. This temperature-dependent threshold stress was found to be a constant fraction (15%) of the Orowan stress generated by the dispersion of alumina particles and of precipitated intermetallic phases.
Analysis of the creep behaviour of a thixoformed AZ91 magnesium alloy
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2000
Experiments were conducted to determine the creep response of a thixoformed Mg–Al–Zn AZ91 alloy i... more Experiments were conducted to determine the creep response of a thixoformed Mg–Al–Zn AZ91 alloy in the temperature range between 120 and 180°C. The initial microstructure of the alloy consisted of large globular dendrites of α phase (solid solution of Al in Mg) surrounded by a divorced eutectic of β phase (Mg17Al12) and particles of α phase rich in Al. The minimum creep rate () dependence on applied stress σ and temperature T was analysed in detail. The high stresses applied in this study resulted in power law breakdown, leading to the use of the equation instead of the conventional power law. The stress exponent n was close to 5 with α0=0.024 MPa−1. The activation energy for creep Q was close to 140 kJ mol−1. These results, and the comparison with creep data obtained by testing the same material after solution treatment, led the authors to conclude that in the considered temperature regime, creep of the thixoformed AZ91 is controlled by high temperature climb of dislocations.
Materials & Design, 2002
The creep response at 600 °C of a P91 (9Cr–1Mo–NbV) steel welded joint was investigated by testin... more The creep response at 600 °C of a P91 (9Cr–1Mo–NbV) steel welded joint was investigated by testing small samples from different weld locations. The microstructure of the weld was analysed by light and transmission electron microscopy. The heat-affected zone material exhibiting a fine-grained structure showed the highest minimum creep-rate values, i.e. the lowest creep strength. The coarse-grained weld metal generally had the greatest creep strength, whereas the parent metal exhibited an intermediate behaviour. The minimum creep-rate dependence on applied stress for weld metal, fine-grained heat-affected zone, and base material was described by means of the conventional power-law equation with a stress exponent close to 15. The experimental results, both in the form of strain vs. time creep curve and of minimum creep rate, were successfully used to simulate the creep response of cross-weld samples.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2007
High-temperature plastic deformation and dynamic recrystallization of AZ31 extruded (EX) and heat... more High-temperature plastic deformation and dynamic recrystallization of AZ31 extruded (EX) and heat treated (FA) alloy was investigated in the temperature range between 200 and 400 °C. High-temperature straining resulted in partial dynamic recrystallization above 250 °C; in the EX alloy recrystallization was complete at 300 °C, while a moderate grain growth was observed at 400 °C. The peak flow stress dependence on temperature and strain rate are described by means of the conventional sinh equation; the calculation of the activation energy for high temperature in the whole range of temperature deformation gives Q = 155 kJ/mol, i.e. a value that was reasonably close but higher than the activation energy for self diffusion in Mg. The microstructure resulting from high-temperature straining was found to be substantially different in EX and FA alloys; in particular, the EX alloy was characterized by a lower flow stress, a higher ductility and by a finer size of the dynamically recrystallized grains. These results are then discussed on the basis of the “necklace” mechanism of dynamic recrystallization.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2000
Among cast aluminium alloys, 319 ranks as one of the commercially important alloys used in automo... more Among cast aluminium alloys, 319 ranks as one of the commercially important alloys used in automotive applications, on account of its excellent casting characteristics and good mechanical properties. It has become one of the candidates for shaping the aluminium alloys in the semisolid state or thixocasting. In this study, thixocast bars of 319 aluminium alloy were heat treated in T4, T5 and T6 conditions, and the microstructural evolution was followed by optical and scanning electron microscopy. Electrical conductivity and hardness measurements were also performed on aged samples to follow the precipitation process. After aging, samples were prepared for tensile testing at room temperature, to study the effect of heat treatment on the mechanical properties. Longitudinal sections of tensile-tested samples were examined to identify the failure mechanism. The rupture propagates in the eutectic region or where Si particles are present, leading to a fracture of the particles themselves. The mechanical properties of the thixocast samples are, in some cases, higher than those obtained from traditionally cast 319 alloys.
Scripta Materialia, 2003
The high-temperature plasticity of an AA6082 aluminium alloy was investigated in a wide range of ... more The high-temperature plasticity of an AA6082 aluminium alloy was investigated in a wide range of temperatures and strain rates. The peak stress dependence on strain rate and temperature was analysed by means of the conventional constitutive equation and by means of a modified form, where the stress was substituted by the difference between stress and a threshold stress.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2002
The hot workability of a 6061 alloy reinforced with 20% Al2O3 particulate was investigated by mea... more The hot workability of a 6061 alloy reinforced with 20% Al2O3 particulate was investigated by means of torsion and compression tests in the temperature range between 350 and 500 °C. Different stability criteria were used to identify the ‘processing window’. Gegel's approach, based on the use of the Liapunov function, resulted in instability areas larger than those identified with the other criteria. The conventional approach of the maximisation of the strain-rate sensitivity and the criterion based on Ziegler's plastic flow theory, in its more general form, valid for any type of stress vs. strain-rate curve, gave substantially similar indications for the determination of the processing window.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2002
The mechanical behaviour of a particulate-reinforced composite 6061+20% Al2O3 produced by ingot m... more The mechanical behaviour of a particulate-reinforced composite 6061+20% Al2O3 produced by ingot metallurgy was investigated with compression tests at different temperatures and strain rates to evaluate the activation energy and to calculate the constitutive equations. The tests were performed in the temperature range between 350 and 500°C and at strain rates between 10−1 and 10−3 s−1. The compression flow curves exhibited an initial sharp increase with strain, followed by monotonous hardening. The maximum stress decreased with decreasing strain rate and increasing temperature. The activation energy of deformation was 155 kJ mol−1, close to the activation energy for self-diffusion in Al. The processing maps were calculated to evaluate the efficiency of the forging process in the range of temperatures and strain rates investigated and to recognise the instability zones of flow behaviour. Scanning electron microscopy observations on polished samples demonstrated damage development in conditions of plastic flow instability.
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Papers by Stefano Spigarelli