A calorimetric study of precipitation in aluminum alloy 2219

Precipitate microstructures in aluminum alloy 2219 were characterized using transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The DSC signatures of individual precipitate phases were established by comparing the DSC and TEM results from samples that had been aged such that only one precipitate phase was present. These signatures were then used to analyze the commercial tempers. It was found that DSC could readily distinguish between the T3, T4, T6, T8 and O tempers but could not distinguish amongst T81, T851 and T87. Small amounts of plastic deformation between solution treatment and aging had a significant effect on the thermograms. Aging experiments at 130 and 190 °C showed that the aging sequence and DSC response of this alloy were similar to those of pure Al-Cu when the increased copper content is taken into account. Further aging experiments at temperatures between room temperature and 130 °C showed pronounced changes of the GP zone dissolution peak as a function of aging conditions. These changes were found to be related to the effect of GP zone size on the metastable phase boundary and on the GP zone dissolution kinetics.     

The cracking mechanism of silicon particles in an A357 aluminum alloy

The cracking of Si particles in an A357 Al alloy has been investigated over a spectrum of stress and strain by varying aging strength and applying different tensile strains. The variation of the fraction of broken Si particles with stress, strain, and cleavage plane orientation has been obtained. The features of cracking reveal that cracking of Si particles is a very localized event. A dislocation pileup mechanism is the most probable one among all crack-initiation theories for explaining the behavior. Based on this mechanism, further deduction has been made to obtain the relationship between the fraction of broken particles and metallurgical factors. The present data, along with Gurlandrss and that of Lowet al., have been found to verify this relationship for the effect of stress, strain, and cleavage plane orientation.     

On the effect of plastic deformation on the coarsening of ϑ-phase precipitation in an Al-Cu alloy

In the present investigation, the focus is on dynamic coarsening of the equilibrium ϑ phase in an Al-4wt pct Cu alloy. For this purpose, specimens containing a uniform ϑ particle distribution have been produced and deformed in compression at two different temperatures (200 °C and 250 °C) and strain rates in the ranges of 200 °C to 250 °C and 10⁻⁵ to 10⁻² s⁻¹, respectively. The particle size distribution measurements performed in a scanning microscope in back-scattered mode demonstrated a double peak behavior depending on temperature: at the lowest test temperature, the dynamic coarsening is enhanced at the highest strain rate, while at 250 °C, the coarsening seems to be affected by crushing of small and medium size particles during straining.     

Creep and ductility in an Al-Cu solid-solution alloy

High-temperature creep was investigated in an Al-3 wt pct Cu alloy at temperatures in the range of 773 to 853 K and at a normalized shear stress range extending from 10^-5 to 7 × 10^-4. The results show the presence of three distinct regions. In region I (low stresses), the stress exponent is 4.5 and the activation energy is 155 kJ/mole. In region II (intermediate stresses), the stress exponent is 3.2 and the activation energy is 151 kJ/mole. In region III (high stresses), the stress exponent is 4.5 and the activation energy is 205 kJ/mole. Creep curves obtained in the three regions exhibit a normal primary stage, but the extent of the stage is less pronounced in region II than in regions I and III. The creep characteristics in regions I and II, along with the values of the transition stresses between the two regions, are in conformity with the prediction of the deformation criterion for solid-solution alloys. While the advent of region III (high stresses) correlates well with dislocation breakaway from a solute-atom atmosphere, the creep characteristics in this region are not entirely consistent with any of the existing high-stress creep mechanisms. The plot of elongation to fracturevs initial strain rate at 853 K exhibits two peaks at strain rates of 1 × 10^-4 and 6 × 10^-4 s^-1. The first peak (1 × 10^-4 s^-1) is attributed to the variation of the stress exponent for creep in the alloy with strain rate, and the second peak (6 × 10^-4 s^-1) appears to reflect the effect of solute drag on dislocation velocity.     

On the measurements of superplasticity in an Al-Cu alloy

The usual method of measuring the strain rate sensitive ‘m’ values of superplastic materials through differential cross-head speed is found to result in improperly definedm values;m is found to depend strongly on the strain to which the material is subjected, especially at low strains. In this connection, the shape of the log stress-log strain rate curve is examined for the Al-33 wt pct Cu eutectic alloy. The inherent grain growth of the very fine grains which occurs during deformation, and the strain dependence ofm at low strains, are shown to be the causes of the familiarS shape of the log stress-log strain rate curves for the Al-Cu alloy. At high strains (15 to 20 pct and higher) where the stress is no longer importantly strain sensitive, the log stress-log strain rate curve is a straight line of slope near 0.5. The elongation at fracture also does not go through a maximum but continues to increase slowly to the lowest strain rate examined: 10^-7 per s. Formerly Research Assistant, Department of Metallurgy and Materials Science, MIT.     

On the measurements of superplasticity in an Al-Cu alloy

The usual method of measuring the strain rate sensitive ‘m’ values of superplastic materials through differential cross-head speed is found to result in improperly definedm values;m is found to depend strongly on the strain to which the material is subjected, especially at low strains. In this connection, the shape of the log stress-log strain rate curve is examined for the Al-33 wt pct Cu eutectic alloy. The inherent grain growth of the very fine grains which occurs during deformation, and the strain dependence ofm at low strains, are shown to be the causes of the familiarS shape of the log stress-log strain rate curves for the Al-Cu alloy. At high strains (15 to 20 pct and higher) where the stress is no longer importantly strain sensitive, the log stress-log strain rate curve is a straight line of slope near 0.5. The elongation at fracture also does not go through a maximum but continues to increase slowly to the lowest strain rate examined: 10^-7 per s.     

A calorimetric study of fatigue induced microstructural changes in aluminum alloy 7050

Differential scanning calorimetry (DSC) was used to detect microstructural changes resulting from strain-controlled fatigue of aluminum alloy 7050. Two starting conditions were investigated: a GP zone T6X temper and an overaged T73651. The calorimetric signature of the microstructure was determined for samples that had been cycled either to failure or to preselected percentages of their expected lifetime at various strain amplitudes. Thermodynamic and kinetic analyses of the calorimetric results revealed a pronounced effect of plastic strain during fatigue on the reaction enthalpy and reaction kinetics of the GP zone dissolution peak of T6X, and a lesser effect on theη′ dissolution peak of T73651. No microstructural changes after fatigue to failure in the nominally elastic strain regime were detected by DSC. The calorimetric results were uniform throughout the cross-section of the fatigue specimens. Based upon these results, it is concluded that approximately 75 pct of the GP zones initially present can be affected during low cycle fatigue, and that overaging of the GP zone microstructure does not occur. The results from the T73651 temper show that low cycle fatigue affects this overaged microstructure in a different manner. Reversion or disordering ofη′ does not occur, but some overaging was detected. It is suggested that theη′ precipitate in this alloy is not shearable.     

Columnar-equiaxed transition in Al-Cu alloy ingots

Columnar fractions have been measured in Al-Cu ingots of identical dimensions solidified with various superheats in moulds of different materials. By modelling the solid-liquid front growth of the dendrite tip undercooling and the liquid pool temperature variations it was possible to calculate undercooling in the thermal layer. From the comparison of experimental and theoretical results maximum undercooling in the boundary layer on columnar-equiaxed transition can be evaluated at 3.2 K.     

Growth of silicon particles in an aluminum matrix

The growth of silicon particles in cast aluminum-silicon alloys, during isothermal heat treatment, has been studied three-dimensionally with the aid of the global parameters of quantitative microscopy and with the Coulter Counter. The measured particle volume distribution can be represented as being log-normal, its geometric standard deviation of the distribution maintaining a constant value throughout isothermal growth. Increase in the average particle volume is in direct proportion to time. Its rate is an Arrhenius function of temperature, with an activation energy of about 80 kilo-calories per mol. The growth rate is the same in all alloys from 4 to 12 pct of silicon, showing that it is independent of particle spacing and, therefore, not controlled by long-range diffusion. This is indicated also by the high activation energy. In its mechanism, the growth of silicon particles is analogous, in broad outline, to steady state grain growth in a polycrystalline aggregate. Interfacial tension provides the energy, which is expended in decrease of surface area as growth proceeds. The rate depends jointly upon the mobility of the interface and upon a microstructural rate constant. The latter is related to the geometric standard deviation of the volume distribution and may be expressed in terms of the global parameter ratioM _vS_v/N_v. The growth process has been analyzed as a system.     

Effect of loading condition and stress state on damage evolution of silicon particles in an Al-Si-Mg-Base cast alloy

Damage evolution of Si particles in a Sr modified cast A356(T6) Al alloy is quantitatively characterized as a function of strain under tension, compression, and torsion. The fraction of damaged Si particles, their size distributions, and orientation distribution of particle cracks are measured by image analysis and stereological techniques. Silicon particle cracking and debonding are the predominant damage modes. Particle debonding is observed only under externally applied tensile loads, whereas particle cracking is observed under all loading conditions. The relative contributions of Si particle debonding and fracture to the total damage strongly depend on stress state and temperature. For all loading conditions and stress states studied, the average size of damaged Si particles is considerably larger than the bulk average size. The rate of damage accumulation is different for different loading conditions. At a given strain level, Si particle damage is lowest under compression and highest under torsion. The anisotropy of the damage is highly dependent on the deformation path and stress state. Under uniaxial tension, the cracks in the broken Si particles are mostly perpendicular to the loading direction, whereas in the compression test specimens they are parallel to the loading direction. The Si particle cracks in the torsion and notch-tension test specimens do not exhibit preferred orientations. The quantitative microstructural data are used to test damage evolution models.     

Calorimetric study of pre-precipitation and precipitation in Al-Mg alloy

Precipitation sequence and changes in microhardness measurements of Al-8wt%Mg and Al-12wt%Mg alloys have been investigated by differential scanning calorimetry (DSC). It has been confirmed that the most important hardening is caused by the formation of the intermediate particles β′(Al₃ Mg₂) which precipitate at temperatures above the reversion temperature of the GP zones. The β′ particles nucleation occurs certainly on structural heterogeneities introduced by quenching (dislocations formed by the collapse and shear of discs of vacancies). When the amount of the heterogeneities facilitating the nucleation of β′ (the intermediate phase) decreases a direct formation of β (the equilibrium phase) is observed.     

The effect of semicoherent precipitation on void swelling in Al-Cu alloys

Aluminum-4 wt pct copper alloys previously heat treated (50 h at 473 K) to contain semi-coherent θ′ precipitate were neutron irradiated to a fast fluence of 2.8 × 10²⁶ n/m² (E > 0.1 MeV) at 328 K to determine the effect of the precipitate on void swelling relative to swelling observed in elemental aluminum under the same irradiation conditions. The ob-served void swelling values were ∼1 and ∼12 pct for the alloy and elemental aluminum, respectively. The broad faces of the θ′ precipitate underwent at least partial coherency loss, the matrix dislocation density increased somewhat, and some of the transmutation-formed silicon segregated to θ′ interfaces and some voids during the irradiation. The θ′ particle size distribution did not change significantly during the irradiation. The swell-ing behavior of the alloy under irradiation was analyzed by extending the rate theory of swelling to include reduction of irradiation-caused point defect concentration by indirect recombination, a result of trapping free point defects at θ′ interfaces. Calculated swell-ing in the alloy using the model agreed well with the experimental observation. Accord-ing to the model the suppression of swelling by θ′ precipitates under the irradiation con-ditions used occurs by extension of the knee of the swelling curve, not by changing the dose exponent for swelling. This paper is based on a presentation made at a symposium on “Radiation In-duced Atomic Rearrangements in Ordering and Clustering Alloys” held at the annual meeting of the AIME, Atlanta, Georgia, March 7 to 8,1977, under the sponsorship of the Physical Metallurgy and Nuclear Metallurgy Committees of The Metallurgical Society of AIME.     

Mechanical properties of a metallic composite material based on an aluminum alloy reinforced by dispersed silicon carbide particles

The mechanical properties of a composite material with a matrix of aluminum alloy D16 reinforced with dispersed silicon carbide particles have been studied. The physicomechanical properties (density, elastic modulus, ultimate tensile strength, and limiting strains) of the composite material with various filler contents are determined experimentally. The experimental results are compared to the results of a theoretical simulation obtained using elastic and elastoplastic models of the composite material. The experimental and the calculated mechanical properties of the composite material with the volume content of the filler up to 30% agree well with each other.     

A transmission electron microscopy investigation of the early stages of precipitation in an Al-Zn-Mg alloy

The growth of spherical precipitates in an Al-5 wt pct Zn-2 wt pct Mg alloy was monitored by hardness measurements and transmission electron microscopy. The growth of precipitates from 15 to 80Å in diameter was followed for various aging times at temperatures of 30, 70, 100, and 150°C. For short times at low aging temperatures precipitate growth followed at _1/9 law, whereas growth at longer times, or higher temperatures, followed at _1’3 law. Various contrast experiments lead to the conclusion that the precipitates produced at all but the shortest aging times at the lowest aging temperatures have a hexagonal structure and are spherical precursors of the η′ phase. The advantages and disadvantages associated with the various imaging techniques for small precipitates utilized in this study are discussed.     

A transmission electron microscopy investigation of the early stages of precipitation in an Al-Zn-Mg alloy

The growth of spherical precipitates in an Al-5 wt pct Zn-2 wt pct Mg alloy was monitored by hardness measurements and transmission electron microscopy. The growth of precipitates from 15 to 80Å in diameter was followed for various aging times at temperatures of 30, 70, 100, and 150°C. For short times at low aging temperatures precipitate growth followed at _1/9 law, whereas growth at longer times, or higher temperatures, followed at _1’3 law. Various contrast experiments lead to the conclusion that the precipitates produced at all but the shortest aging times at the lowest aging temperatures have a hexagonal structure and are spherical precursors of the η′ phase. The advantages and disadvantages associated with the various imaging techniques for small precipitates utilized in this study are discussed. Formerly Research Assistant, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA. 02139     

A high-resolution transmission electron microscopy study of the precipitation process in a dilute Ti-N alloy

The precipitation processes in dilute nitrogen alloys of titanium have been examined in detail by conventional transmission electron microscopy (CTEM) and high-resolution electron microscopy (HREM). The alloy Ti-2 at. pct N on quenching from its high-temperatureβ phase field has been found to undergo early stages of decomposition. The supersaturated solid solution (α″-hcp) on decomposition gives rise to an intimately mixed, irresolvable product microstructure. The associated strong tweed contrast presents difficulties in understanding the characteristic features of the process. Therefore, HREM has been carried out with a view to getting a clear picture of the decomposition process. Studies on the quenched samples of the alloy suggest the formation of solute-rich zones of a few atom layers thick, randomly distributed throughout the matrix. On aging, these zones grow to a size beyond which the precipitate/matrix interfaces appear to become incoherent and theα′ (tetragonal) product phase is seen distinctly. The structural details, the crystallography of the precipitation process, and the sequence of precipitation reaction in the system are illustrated.