Richard DeIasi

Calorimetric studies of 7000 series aluminum alloys: II. Comparison of 7075, 7050 and RX720 alloys

Differential scanning calorimetry (DSC) in conjunction with transmission electron microscopy (TEM) are used to characterize the matrix precipitate structure of high strength and overaged tempers of three 7000 series aluminum alloys. Excellent consistency exists between the DSC results, based on the dissolution behavior of existing precipitates, and TEM observations. Comparison is made between matrix precipitate constituency and mechanical properties. A significantly high GP zone particle density was observed in a high strength 7050 alloy temper, but this temper did not have higher strength than other predominantly GP zone matrix tempers. Maximum strength was observed in a 7050 alloy temper that contained approximately equal amounts of GP zones and ή phase precipitates. Strengthening appears to be based on the contribution of both coherent GP zones and semicoherent ή precipitates. Use of the DSC approach and the free energy of activation for precipitate dissolution are recommended as rapid and quantitative means of precipitate identification.

Calorimetric studies of 7000 series aluminum alloys: I. Matrix precipitate characterization of 7075

Both differential scanning calorimetry (DSC) and hot stage transmission electron microscopy were used to characterize the solid state reactions accompanying heating of the highest strength (T651) and overaged (T7351) tempers of 7075 aluminum alloy. Each of the observed endothermic or exothermic reactions that occurs over the 20 to 500°C temperature range has been ascribed to the dissolution or formation of a specific precipitate. The dissolution of each matrix phase,i.e., GP zones, ή and ή, can be characterized by a distinguishable dissolution temperature, dissolution enthalpy, activation energy, and activation entropy. The values of activation energy and activation entropy for the dissolution of the initial matrix precipitate of these phases indicate that the relative stability of the matrix precipitates is primarily influenced by the entropy rather than the energy term. This investigation provides a basis for the use of DSC analysis in the rapid, quantitative identification of the matrix microstructure of 7075 aluminum alloy as well as in the characterization of the matrix microstructure of other 7000 series aluminum alloys.

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.

Moisture Detection in Composites Using Nuclear Reaction Analysis

An experimental method for evaluation of the localized moisture con tent in graphite epoxy composites or other organic solids is described. The method involves specimen conditioning in D2 O, followed by measurement of the localized deuterium concentration by means of the D ( 3 He, p)4 He nuclear reaction. Equilibrium moisture levels in two epoxy resins and two graphite epoxy composites measured by the nuclear technique are con sistent with those obtained from bulk gravimetric methods. Moisture pro files measured in a unidirectional graphite epoxy composite are compared to predictions from diffusion theory.

Effect of Water on the Properties of a Glass-Polyimide Laminate.

Effects of accelerated moisture aging on glass-polyimide laminates were investigated using flexural strength measurements and scanning electron microscopy. The degradation rate at 60, 80 and 100° C in water and steam followed first order kinetics. The calculated value of the activation energy for aqueous degradation (Ea=16.4 kcal mol−1) indicates that the probable mechanism of attack in water and flowing steam is selective hydrolysis of uncycled “amic acid” groups. This causes a decrease in average molecular weight of the matrix with subsequent breakdown of the polymer-fibre interface. The computed value of the activation energy for a single laminate is used to illustrate the calculation of degradation rates and service lifetime of the same or other laminates under various conditions of temperature and water vapour pressure.