John M. Papazian

EFFECTS OF SIC WHISKERS AND PARTICLES ON PRECIPITATION IN ALUMINUM MATRIX COMPOSITES

The age-hardening precipitation reactions in aluminum matrix composites reinforced with discontinuous SiC were studied using a calorimetric technique. Composites fabricated with 2124, 2219, 6061, and 7475 alloy matrices were obtained from commercial sources along with unreinforced control materials fabricated in a similar manner. The 7475 materials were made by a casting process while the others were made by powder metallurgy: the SiC reinforcement was in the form of whiskers or particulate. It was found that the overall age-hardening sequence of the alloy was not changed by the addition of SiC, but that the volume fractions of various phases and the precipitation kinetics were substantially modified. Precipitation and dissolution kinetics were generally accelerated. A substantial portion of this acceleration was found to be due to the powder metallurgy process employed to make the composites, but the formation kinetics of some particular precipitate phases were also strongly affected by the presence of SiC. It was observed that the volume fraction of GP zones able to form in the SiC containing materials was significantly reduced. The presence of SiC particles also caused normally quench insensitive materials such as 6061 to become quench sensitive. The microstructural origins of these effects are discussed.

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.

Calorimetric Studies of Precipitation and Dissolution Kinetics in Aluminum Alloys 2219 and 7075

Differential scanning calorimetry (DSC) was used to study the kinetics of precipitation and dissolution of metastable and stable phases in aluminum alloys 2219 and 7075. A comparison of DSC scans obtained at heating rates of 1, 5, 10, and 20 K per minute showed that, during a DSC scan, the rates of precipitation of θ′ and θ in 2219 and η′ and η in 7075 were limited by their reaction kinetics. Likewise, the rates of dissolution of GP zones, θ′ and η′, were found to be dominated by kinetics. In contrast, the dissolution of θ and η was dominated by the thermodynamic equilibrium between these phases and the matrix. Analysis of the kinetically dominated reaction peaks and their dependence on heating rate and particle size showed that the GP zone dissolution reaction could best be described by a three-dimensional volume diffusion limited rate expression with an activation energy equal to that for diffusion. The rate of formation of θ′ was best described by an Avrami expression withn = 1.1, indicating that nucleation was not the rate controlling step. A pronounced dependence of the θ′ formation rate on prior plastic deformation was observed and ascribed to the influence of the matrix dislocation density on diffusivity.

Lithium depletion during heat treatment of aluminum-lithium alloys

The loss of lithium from the near surface region during heat treatment of two commercial aluminum-lithium alloys was studied using a nuclear reaction analysis technique. A finely collimated 2.5 MeV beam of3He ions was used to stimulate the7Li(3He,p)9Be reaction in samples of BAACo 8090 and 8091 alloys heat treated for 1, 4, and 16 hours at 500 °C. The emitted protons were detected as a cross section of the sample was traversed through the beam, thereby determining the lithium content as a function of distance from the external surface. Suitable calibration and control samples were used to validate the technique. The lithium concentration data were fit with assumed concentration profiles calculated from diffusion equations and modified for the particular experimental configuration employed. Extensive lithium depletion was found in both alloys, and the concentration profiles were found to be accurately predicted by the diffusion calculations. For heat treatment in either wet or dry air, the depth of lithium loss was the same, and can be approximately given as x = 1.5 √Dt. When heat treated in an argon atmosphere, the depth of lithium loss was reduced. The lithium loss appeared to be limited by the diffusive flux of lithium to the surface of the sample in wet and dry air, but was limited by other factors in argon. Porosity was observed in the lithium depleted region; this was ascribed to the accumulation of vacancies generated by the unequal fluxes of aluminum and lithium atoms.

Optimized Tooling Design Algorithm for Sheet Metal Forming over Reconfigurable Compliant Tooling

New forming technologies based on compliant “discrete‐die” reconfigurable tooling are available for production of sheet metal parts, but there is a need for a formalism that can accurately predict the three‐dimensional shape of the required tooling, a priori. An optimized tooling design algorithm has been developed based on a methodology that uncouples the effects due to springback and those due to the compliance of the polymeric layer. A springback compensated die shape is initially predicted for the case of stretch forming over smooth rigid dies based on an iterative approach using the finite element method. The tooling design algorithm is based on an inverse springback approach that uses the elastic‐plastic stress state prior to unloading to elastically deform the sheet in a direction opposite to springback. A procedure has been developed to improve the convergence behavior of the inverse springback approach utilizing an interpolation scheme. The interpolation scheme uses prior iterations to predict th...

The effects of warm working on aluminum alloy 7075-T651

Abstract Aluminum alloy 7075 in the high strength T651 condition was preheated for 10 min at temperatures between 155 and 245 °C and then quenched, air cooled or rolled to 10% reduction in area. The effects of these treatments on the hardness, conductivity and precipitate microstructure were evaluated. Differential scanning calorimetry was used for microstructural characterization. For preheating temperatures below approximately 195 °C, the hardness and conductivity of the material were within the allowable limits for T651, but significant microstructural changes were observed. These changes consisted of precipitate dissolution and overaging, with the former predominating. Above 195 °C, overaging became more severe. Deformation caused significant precipitate dissolution in addition to that caused solely by the thermal treatment. Repeated thermal cycling showed that the precipitate microstructure reached a metastable equilibrium typical of the preheating temperature within the first 10 min at temperature; an additional 50 min had little further effect. Deformation significantly increased the rate of overaging during subsequent preheating cycles.

The effect of reduced gravity on solidification microstructures of NH4Cl-H2O alloys

The effect of gravity on the columnar-to-equiaxed microstructural transition was studied in small samples of NH4Cl-H2O. The behavior of the samples during laboratory (one gravity) experiments was contrasted with their behavior during a (low gravity) sounding rocket flight. In one gravity, the columnar zone accounted for 25 to 100 pct of the structure, depending on the superheat and orientation of the chill. Grain multiplica-tion occurred by showering and by convection induced dendrite arm remelting. Convec-tion was caused by both thermal gradients and solutal gradients. In low gravity, however, completely columnar structures were obtained; all grain multiplication mechanisms were entirely suppressed. Reduced gravity also modified the thermal conditions and caused the liquid to cool more slowly. This resulted in a steeper temperature gradient in the liquid ahead of the solidification interface. “Big bang” type nucleation occurred in two of the samples, distributing nuclei throughout the liquid. Despite this, an equiaxed zone did not form, indicating that the most significant effect of low gravity on this experiment was modification of the thermal conditions.

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.

Microstructural effects of the cyclic and monotonic hardening of Al5Mg

Abstract The cyclic and monotonic hardening mechanisms in Al5Mg (where the composition is in approximate weight per cent) have been characterized by transmission electron microscopy and by differential scanning calorimetry (DSC). After constant-plastic-strain-amplitude tests to cyclic saturation, a high density of fine dislocation loops is generated. The corresponding DSC plots reveal a characteristic exotherm at 150°C (ΔHx in the range from −0.2 to −0.5 J g−1) which is shown to result from the annihilation of the fine dislocation loops. This behavior is not found after monotonic deformation by cold rolling or drawing to equivalent total plastic strains, for which dense dislocation tangles rather than loops develop. The 150°C exotherm occurs at saturation after fatigue of Al5Mg in both the annealed and the cold-worked states and also of Al5Mg0.5Ag aged to form homogeneous T precipitates. At lower temperatures in the range 50–110°C, an endotherm (probably due to Guinier-Preston (GP) zone dissolution) is also detected (ΔHx ≈ 0.5 Jg−1). The variation in the endothermic peak temperature is consistent with a GP zone size which depends on the aging or fatigue treatment.

Science-Based Multiscale Modeling of Fatigue Damage for Structural Prognosis

Probabilistic microstructurally-based models for fatigue are being developed as part of a DARPA/Northrop Grumman Structural Integrity Prognosis System (SIPS). An integrated framework has been developed to provide probabilistic fatigue life prognosis of individual air vehicles by leveraging research across a large multi-disciplinary team. Development of the framework was made possible by advancements made by the SIPS Modeling and Simulation multidisciplinary research team in the statistical modeling of polycrystalline metal microstructures, finite element meshing, multiscale analysis, modeling of constitutive behavior of polycrystals, and materials testing technology for property measurement and validation procedures. The fatigue models are based on a fundamental understanding of the fatigue process, and trace the structural degradation caused by fatigue back to its physical origins in the microstructure of the metallic component. The objectives are to discover and link all the important damage mechanisms leading to a macroscopically observable crack and to allow defects to emerge naturally from statistically meaningful ensembles of material representations, subject to more accurate, scale-specific, damage inducing fields. An example is modeling the interplay between the grain crystallography and crack incubation at a constituent particle. A microstructurally-based model allows estimation of the total fatigue life from incubation and nucleation at a constituent second phase particle to propagation of micro-cracks to emergence of macro-cracks all in a statistical sense to permit accurate estimation of reliability indices.