Philip N. Adler

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.

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.

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.

Effect of activity differences on hydrogen migration in dissimilar titanium alloy welds

The effect of alloy composition on hydrogen activity was measured for seven titanium alloys as a means to determine the tendency for hydrogen migration within dissimilar metal welds. The alloys were: Ti-CP, Ti-3A1-2.5V, Ti-3Al-2.5V-3Zr, Ti-3Al-2Nb-lTa, Ti-6A1, Ti-6A1-4V, and Ti-6Al-2Nb-lTa-0.8Mo. Hydrogen pressure-hydrogen concentration relationships were determined for temperatures from 600 ‡C to 800 ‡C and hydrogen concentrations up to approximately 3.5 at. pct (750 wppm). Fusion welds were made between Ti-CP and Ti-CP and between Ti-CP and Ti-6A1-4V to observe directly the hydrogen redistribution in similar and dissimilar metal couples. Hydrogen activity was found to be significantly affected by alloying elements, particularly Al in solid solution. At a constant Al content and temperature, an increase in the volume fraction ofΒ reduced the activity of hydrogen in α-@#@ Β alloys. Activity was also found to be strongly affected by temperature. The effect of temperature differences on hydrogen activity was much greater than the effects resulting from alloy composition differences at a given temperature. Thus, hydrogen redistribution should be expected within similar metal couples subjected to extreme temperature gradients, such as those peculiar to fusion welding. Significant hydrogen redistribution in dissimilar alloy weldments also can be expected for many of the compositions in this study. Hydride formation stemming from these driving forces was observed in the dissimilar couple fusion welds. In addition, a basis for estimating hydrogen migration in titanium welds, based on hydrogen activity data, is described.

Hydrogen related fatigue fracture of Ti-6Al-4V

Hydrogen concentrations were measured in the fracture surface of a fatigued Ti-6A1-4V specimen. Results show that hydrogen migrated to the triaxial stress region of the crack during slow crack growth where it accumulated and remained trapped before fracture. (FS)

Deuterium surface segregation in titanium alloys

Deuterium surface segregation has been investigated in α, α + β, and β-phase titanium that were deuterium charged over the range of 2 to approximately 300 wppm. Surface segregation was observed in samples that were essentially α-phase materials,i.e., high-purity commercial α-Ti, Ti-6A1, and Ti-3A1-2.5V, whereas Ti-6A1-4V had slight enrichment and β-Ti-13Mn had no detectable segregation. Nuclear reaction analysis (NRA) techniques were used to measure the near-surface deuterium concentration, and the segregation has been localized to within 50 nm of the surface. The time-dependent increase of deuterium at the surface is consistent with deuterium diffusion from the bulk to the surface and a room-temperature diffusivity of approximately 3 × 10-9 cm2/s. Surface enrichment in excess of 30 times the bulk concentration was observed in charged samples and in excess of 60 times for samples that had been charged and then vacuum annealed. Polishing was found to be of importance in causing segregation. The presence of deuterides or a surface defect state is suggested to explain the deuterium surface enrichment.