Stefano Spigarelli | Università Politecnica delle Marche, Italy (original) (raw)
Papers by Stefano Spigarelli
Materials Science Forum, 2009
ABSTRACT The aim of the present work was to evaluate the potential for superplastic deformation o... more ABSTRACT The aim of the present work was to evaluate the potential for superplastic deformation of the AZ31 magnesium alloy produced by Twin Roll Casting (TRC), a continuous casting technology able to convert molten metals directly into a coiled strip. In order to develop a superplastic microstructure, the TRC sheets were heated at 400 °C for 2 h, then rolled by multiple passes with re-heating between them, with a total thickness reduction of about 60%. The superplastic behaviour of the alloy was studied by tensile tests, carried out at in the temperature range from 400 °C to 500 °C and with initial strain rates of 1-10-3 s-1 and 5.10-4 s-1. The microstructural and fractographic characterization of the alloy was carried out by means of optical (OM) and scanning electron microscopy (SEM). The tensile tests evidenced a superplastic behaviour of the processed AZ31 Mg alloy, with a maximum elongation to failure of about 500% at 460 °C, with a strain rate of 5-10-4 s" 1. The microstructure of the alloy after superplastic deformation showed fine and equiaxed grains, with a large fraction of high angle boundaries. Analyses of the fracture surfaces evidenced flow localization around the grains, suggesting that grain boundary sliding (GBS) was the main deformation mechanism. Failure occurred by cavitation, mainly at the higher testing temperature, due to the prevailing effect of grain growth.
International Journal of Pressure Vessels and Piping, 2008
The Ni-22Cr-12Co-9Mo alloy (UNS alloy N06617, also known as alloy 617), of relatively common use ... more The Ni-22Cr-12Co-9Mo alloy (UNS alloy N06617, also known as alloy 617), of relatively common use at high temperature, is also one of the candidate materials for the most ambitious EU and US projects for future generations of power plants. Its microstructure in the solution-annealed condition and after long-term creep tests at 700 and 800 1C was characterized by means of light optical, scanning electron and transmission electron microscopy. Differential scanning calorimetry (DSC) analyses were also performed to check the usefulness of this technique to detect microstructural modification in specimens exposed to high temperature. Microstructural features were correlated to the hardness evolution in a chronological range of several thousand hours.
Materials & Design, 2002
The creep response at 600 8C of a P91 (9Cr-1Mo-NbV) steel welded joint was investigated by testin... more The creep response at 600 8C 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 heataffected 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. ᮊ
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2002
ABSTRACT
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2005
The present study aims at investigating the effect of small additions (0.3, 0.5, 0.8 and 1.5%) of... more The present study aims at investigating the effect of small additions (0.3, 0.5, 0.8 and 1.5%) of Si on the creep response of an AM50 alloy. Die-cast samples not previously heat-treated were subjected to tensile tests at room temperature and to constant load creep tests at 423 and 398 K. The effect of chemical composition was investigated by testing five alloys with rising Mg content, from 0 (basic AM50 alloy) to 1.5%. Irrespective of Mg content, the creep curves exhibited a well-defined primary stage followed by a short secondary region and by a short tertiary stage that led to fracture without extensive necking. Addition of 0.3% Si hardly affected creep strain rate, while greater additions induced a substantial increase in creep strength, an effect that was partially offset by a decrease in strain to fracture.
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
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2007
The microstructure and mechanical properties of a friction stir welded 6056-T6 aluminum alloy pla... more The microstructure and mechanical properties of a friction stir welded 6056-T6 aluminum alloy plate were investigated by using polarized optical and transmission electron microscopy techniques. The microstructure revealed different grain morphologies in the thermo-mechanically affected zones, in proximity of the weld nugget; the advancing side had fairly elongated, bent grains, whilst the broader retreating side had more elongated, narrower grains.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 1998
An investigation of the effect of creep exposure on the microstructure of a 9Cr–1Mo alloy for ste... more An investigation of the effect of creep exposure on the microstructure of a 9Cr–1Mo alloy for steam tubing was performed. The samples were machined from a tube, austenised at 1323 K for 15 min and air cooled to room temperature, followed by tempering at 1023 K for 1 h. Creep tests were performed at 848, 873, 898 and 923 K
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, 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.
Journal of Materials Science, 2003
The high-temperature plasticity of a Zr-stabilised 2014 aluminium alloy was investigated in a wid... more The high-temperature plasticity of a Zr-stabilised 2014 aluminium alloy was investigated in a wide range of temperatures and strain rates. The microstructure of representative torsioned samples was analysed by transmission electron microscopy, and the characteristics of particle and precipitate distribution were quantitatively estimated. The strain-rate dependence on stress and temperature was analysed by means of the conventional constitutive equations used for describing hot-working behaviour as well as a modified form of the sinh equation, where the stress was substituted by an effective stress, i.e., by the difference between the stress and a threshold stress. This temperature-dependent threshold stress was found to be a fraction of the Orowan stress generated by precipitates increasing from 62 to 94% as temperature decreased from 773 to 573 K.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2002
The hot workability of a 6061 alloy reinforced with 20% Al 2 O 3 particulate was investigated by ... more The hot workability of a 6061 alloy reinforced with 20% Al 2 O 3 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% Al 2 O 3 produced by in... more The mechanical behaviour of a particulate-reinforced composite 6061 + 20% Al 2 O 3 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.
Materials & Design, 2003
Two models of strain-time creep curve description were applied to a set of experimental data obta... more Two models of strain-time creep curve description were applied to a set of experimental data obtained recently by testing a newly developed Mg-Al-Si-RE alloy (AS21X) produced by die-casting. Due to the absence of extensive tertiary creep, the simplified equations for constant-stress creep tests of the Li-Akulov and Bagley-Jones-Freed models were used to identify material parameters in the 70-180 8C range. In both models, secondary creep rate data fit well in the whole stress-temperature range considered. The description of primary creep was generally good, except for the high-stress range at 70 8C. In all cases, the predicted strain vs. time creep curves became more accurate as the contribution of secondary creep to strain increased. Above 0.002% strain, the models can reliably be applied in the stress-temperature range investigated. The extrapolation of predicted times at fixed strains were one or more orders of magnitude above the highest experimental times (6=10 s for 0.02 strain) and 6 will require additional testing at lower stress. ᮊ
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 AbCu and AbCuMg.
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2002
... to the activation energy for self-diffusion or to the activation energy for diffusion of solu... more ... to the activation energy for self-diffusion or to the activation energy for diffusion of solute elements in the matrix. The model was applied to an unreinforced alloy (2014PM) and a composite (6061 20 pct Al2O3) tested in ... THE hot workability of aluminum and its alloys[117] has ...
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2004
The mechanical behaviour of the magnesium alloy AS21X (Mg-2% Al-1% Si alloy AS21 modified by addi... more The mechanical behaviour of the magnesium alloy AS21X (Mg-2% Al-1% Si alloy AS21 modified by addition of rare-earth elements) was experimentally investigated in the as die-cast and in the solution treated and artificially aged conditions in the temperature range from 70 to 210 • C. The as die-cast microstructure was characterized by coring effects and by the presence of Mg 2 Si and Mg 17 Al 12 particles. Holding at temperatures below 180 • C caused massive formation of fine-scale precipitates at the grain boundaries. The homogeneous distribution of Al in the Mg-rich ␣-phase of heat treated material caused at these temperatures the formation of further amounts of Mg 17 Al 12 particles, also within the grains. At 210 • C, Mg 17 Al 12 particles dissolved in both material conditions. At low temperatures, longer times to creep rupture and better creep resistance were displayed by the as cast (AC) material. The properties in the heat treated (HT) condition approached those of the as cast material at the highest test temperature. The suggested deformation mechanism of the alloy up to 180 • C is climb-controlled dislocation creep, for which core diffusion could play a significant role at 70 • C in the as cast material.
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.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2002
The warm formability of a solution-treated 2618 aluminium alloy was investigated by means of tors... more The warm formability of a solution-treated 2618 aluminium alloy was investigated by means of torsion tests at temperatures between 423 and 573 K. Dynamic precipitation took place during deformation raising the flow stress and lowering ductility. The flow stresses were substantially higher than those observed by testing the same alloy in the as-extruded state. At the lowest temperatures dynamic precipitation resulted in a continuous increase of flow stress, while above 500 K dynamic precipitation and subsequent coarsening of precipitates resulted in a peak of equivalent-stress versus equivalent-strain curves, followed by softening. These differences in deformational behaviour were attributed to the nature of precipitates, as well as to different kinetics of the ageing processes at each testing temperature.
Materials Science Forum, 2009
ABSTRACT The aim of the present work was to evaluate the potential for superplastic deformation o... more ABSTRACT The aim of the present work was to evaluate the potential for superplastic deformation of the AZ31 magnesium alloy produced by Twin Roll Casting (TRC), a continuous casting technology able to convert molten metals directly into a coiled strip. In order to develop a superplastic microstructure, the TRC sheets were heated at 400 °C for 2 h, then rolled by multiple passes with re-heating between them, with a total thickness reduction of about 60%. The superplastic behaviour of the alloy was studied by tensile tests, carried out at in the temperature range from 400 °C to 500 °C and with initial strain rates of 1-10-3 s-1 and 5.10-4 s-1. The microstructural and fractographic characterization of the alloy was carried out by means of optical (OM) and scanning electron microscopy (SEM). The tensile tests evidenced a superplastic behaviour of the processed AZ31 Mg alloy, with a maximum elongation to failure of about 500% at 460 °C, with a strain rate of 5-10-4 s" 1. The microstructure of the alloy after superplastic deformation showed fine and equiaxed grains, with a large fraction of high angle boundaries. Analyses of the fracture surfaces evidenced flow localization around the grains, suggesting that grain boundary sliding (GBS) was the main deformation mechanism. Failure occurred by cavitation, mainly at the higher testing temperature, due to the prevailing effect of grain growth.
International Journal of Pressure Vessels and Piping, 2008
The Ni-22Cr-12Co-9Mo alloy (UNS alloy N06617, also known as alloy 617), of relatively common use ... more The Ni-22Cr-12Co-9Mo alloy (UNS alloy N06617, also known as alloy 617), of relatively common use at high temperature, is also one of the candidate materials for the most ambitious EU and US projects for future generations of power plants. Its microstructure in the solution-annealed condition and after long-term creep tests at 700 and 800 1C was characterized by means of light optical, scanning electron and transmission electron microscopy. Differential scanning calorimetry (DSC) analyses were also performed to check the usefulness of this technique to detect microstructural modification in specimens exposed to high temperature. Microstructural features were correlated to the hardness evolution in a chronological range of several thousand hours.
Materials & Design, 2002
The creep response at 600 8C of a P91 (9Cr-1Mo-NbV) steel welded joint was investigated by testin... more The creep response at 600 8C 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 heataffected 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. ᮊ
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2002
ABSTRACT
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2005
The present study aims at investigating the effect of small additions (0.3, 0.5, 0.8 and 1.5%) of... more The present study aims at investigating the effect of small additions (0.3, 0.5, 0.8 and 1.5%) of Si on the creep response of an AM50 alloy. Die-cast samples not previously heat-treated were subjected to tensile tests at room temperature and to constant load creep tests at 423 and 398 K. The effect of chemical composition was investigated by testing five alloys with rising Mg content, from 0 (basic AM50 alloy) to 1.5%. Irrespective of Mg content, the creep curves exhibited a well-defined primary stage followed by a short secondary region and by a short tertiary stage that led to fracture without extensive necking. Addition of 0.3% Si hardly affected creep strain rate, while greater additions induced a substantial increase in creep strength, an effect that was partially offset by a decrease in strain to fracture.
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
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2007
The microstructure and mechanical properties of a friction stir welded 6056-T6 aluminum alloy pla... more The microstructure and mechanical properties of a friction stir welded 6056-T6 aluminum alloy plate were investigated by using polarized optical and transmission electron microscopy techniques. The microstructure revealed different grain morphologies in the thermo-mechanically affected zones, in proximity of the weld nugget; the advancing side had fairly elongated, bent grains, whilst the broader retreating side had more elongated, narrower grains.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 1998
An investigation of the effect of creep exposure on the microstructure of a 9Cr–1Mo alloy for ste... more An investigation of the effect of creep exposure on the microstructure of a 9Cr–1Mo alloy for steam tubing was performed. The samples were machined from a tube, austenised at 1323 K for 15 min and air cooled to room temperature, followed by tempering at 1023 K for 1 h. Creep tests were performed at 848, 873, 898 and 923 K
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, 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.
Journal of Materials Science, 2003
The high-temperature plasticity of a Zr-stabilised 2014 aluminium alloy was investigated in a wid... more The high-temperature plasticity of a Zr-stabilised 2014 aluminium alloy was investigated in a wide range of temperatures and strain rates. The microstructure of representative torsioned samples was analysed by transmission electron microscopy, and the characteristics of particle and precipitate distribution were quantitatively estimated. The strain-rate dependence on stress and temperature was analysed by means of the conventional constitutive equations used for describing hot-working behaviour as well as a modified form of the sinh equation, where the stress was substituted by an effective stress, i.e., by the difference between the stress and a threshold stress. This temperature-dependent threshold stress was found to be a fraction of the Orowan stress generated by precipitates increasing from 62 to 94% as temperature decreased from 773 to 573 K.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2002
The hot workability of a 6061 alloy reinforced with 20% Al 2 O 3 particulate was investigated by ... more The hot workability of a 6061 alloy reinforced with 20% Al 2 O 3 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% Al 2 O 3 produced by in... more The mechanical behaviour of a particulate-reinforced composite 6061 + 20% Al 2 O 3 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.
Materials & Design, 2003
Two models of strain-time creep curve description were applied to a set of experimental data obta... more Two models of strain-time creep curve description were applied to a set of experimental data obtained recently by testing a newly developed Mg-Al-Si-RE alloy (AS21X) produced by die-casting. Due to the absence of extensive tertiary creep, the simplified equations for constant-stress creep tests of the Li-Akulov and Bagley-Jones-Freed models were used to identify material parameters in the 70-180 8C range. In both models, secondary creep rate data fit well in the whole stress-temperature range considered. The description of primary creep was generally good, except for the high-stress range at 70 8C. In all cases, the predicted strain vs. time creep curves became more accurate as the contribution of secondary creep to strain increased. Above 0.002% strain, the models can reliably be applied in the stress-temperature range investigated. The extrapolation of predicted times at fixed strains were one or more orders of magnitude above the highest experimental times (6=10 s for 0.02 strain) and 6 will require additional testing at lower stress. ᮊ
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 AbCu and AbCuMg.
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2002
... to the activation energy for self-diffusion or to the activation energy for diffusion of solu... more ... to the activation energy for self-diffusion or to the activation energy for diffusion of solute elements in the matrix. The model was applied to an unreinforced alloy (2014PM) and a composite (6061 20 pct Al2O3) tested in ... THE hot workability of aluminum and its alloys[117] has ...
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2004
The mechanical behaviour of the magnesium alloy AS21X (Mg-2% Al-1% Si alloy AS21 modified by addi... more The mechanical behaviour of the magnesium alloy AS21X (Mg-2% Al-1% Si alloy AS21 modified by addition of rare-earth elements) was experimentally investigated in the as die-cast and in the solution treated and artificially aged conditions in the temperature range from 70 to 210 • C. The as die-cast microstructure was characterized by coring effects and by the presence of Mg 2 Si and Mg 17 Al 12 particles. Holding at temperatures below 180 • C caused massive formation of fine-scale precipitates at the grain boundaries. The homogeneous distribution of Al in the Mg-rich ␣-phase of heat treated material caused at these temperatures the formation of further amounts of Mg 17 Al 12 particles, also within the grains. At 210 • C, Mg 17 Al 12 particles dissolved in both material conditions. At low temperatures, longer times to creep rupture and better creep resistance were displayed by the as cast (AC) material. The properties in the heat treated (HT) condition approached those of the as cast material at the highest test temperature. The suggested deformation mechanism of the alloy up to 180 • C is climb-controlled dislocation creep, for which core diffusion could play a significant role at 70 • C in the as cast material.
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.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2002
The warm formability of a solution-treated 2618 aluminium alloy was investigated by means of tors... more The warm formability of a solution-treated 2618 aluminium alloy was investigated by means of torsion tests at temperatures between 423 and 573 K. Dynamic precipitation took place during deformation raising the flow stress and lowering ductility. The flow stresses were substantially higher than those observed by testing the same alloy in the as-extruded state. At the lowest temperatures dynamic precipitation resulted in a continuous increase of flow stress, while above 500 K dynamic precipitation and subsequent coarsening of precipitates resulted in a peak of equivalent-stress versus equivalent-strain curves, followed by softening. These differences in deformational behaviour were attributed to the nature of precipitates, as well as to different kinetics of the ageing processes at each testing temperature.