Al-Zn合金GP区的价电子结构分析

运用固体经验电子理论(EET)计算了Al-Zn合金GP区的价电子结构。计算结果表明，由于GP区晶胞的最强键上的共价电子数远比纯Al晶胞的最强键共价电子数多，M原子与Zn原子形成较强的共价键，因此，在Al-Zn合金中即使以较快的速度淬火，也容易在淬火过程中形成GP区。由于GP区的共价键络较强，使得合金硬度在GP区一开始形成时就稳步地提升，合金的硬度到中间相形成时达到最大值.     

Porosity and tensile ductility in Al-Zn alloys

The tensile ductility of Al-Zn alloys processed in two different ways was examined over a broad range of temperature and strain rates. The different processing conditions produced materials with differing initial void andJor inclusion contents. If all the defects present are assumed to be in the form of voids, their respective volume fractions in the two alloys are 0.5 and 1.5 pct. Testing at temperatures from 77 to 480 K encompasses a range of mechanical behavior, ranging from brittle to superplastic. The poorly processed material has markedly less ductility than the one containing fewer defects both at high temperatures and at near cryogenic ones (196 K), where the alloys are marginally ductile. Increasing strain rate embrittles alloys in the temperature range 196 K to 300 K. At 196 K the strain rate needed to induce brittleness in the low defect alloy is over two orders of magnitude higher than in the alloy with a greater voidJinclusion content. Critical embrittling strain rates at room temperature are in the range common to conventional metal-forming operations for the poorly processed alloy, and thus cold formability devolves importantly on initial processing. In the vicinity of room temperature and at low strain rates, tensile ductility is greater for alloys with a higher defect concentration; this behavior is related to the development of multiple necks in the poorly processed materials. At the highest temperatures investigated, tensile ductility correlates directly with strain rate sensitivity and material defect concentration. Materials with low defect concentrations resist neck development during quasi-uniform flow to a much greater degree than do those with a high defect concentration. B. HARRIPRASHAD, formerly Graduate Student at Michigan Technological University T. H. COURTNEY, formerly Professor, Department of Metallurgical Engineering, and Dean of the Graduate School at Michigan Technological University.     

Discontinuous precipitation and coarsening in Al-Zn alloys

The morphology and kinetics of discontinuous precipitation (DP) and discontinuous coarsening (DC) in solution treated and isothermally aged Al-Zn alloys containing 39.3 and 59.3 at.% Zn have been investigated at temperatures ranging from 323 to 523 K by light microscopy, electron microscopy and X-ray diffraction. At all aging temperatures the supersaturated α solid solution was observed to decompose rapidly by DP into a lamellar mixture of solute depleted α phase and β phase precipitate. DP occurred so rapidly in the 59.5 at.% Zn alloy that the heat of transformation raised the temperature of the alloy significantly. With further aging a slower DC reaction transformed the lamellar DP into a coarser lamellar structure of the same two phases; however, the composition of the α phase of the DC was closer to the equilibrium solvus composition than that of the DP. With still further aging a second, much slower DC reaction was observed to decompose the lamellar product of the first DC reaction in the 59.5 at.% Zn alloy into a still coarser lamellar structure. Analysis of the kinetics of both the DP and DC reactions showed them to be controlled by boundary diffusion in the advancing reaction interface. Reaction front migration rates for both DP and DC increased markedly with increasing Zn content. This increase seems to be associated partially with an increase in boundary diffusivity with increasing Zn content.     

Defect structures and interactions in Al-Zn eutectoid alloys

Defect structures in superplastic and nonsuperplastic Al-Zn eutectoid alloys were studied by transmission electron microscopy. Slowly cooled alloys develop a fine lamellar microstructure and are not superplastic. The equiaxed quenched and quench-aged alloys, however, are super-plastic under the proper conditions of temperature and strain rate. Quench-aging produces sub-boundaries in the aluminum-rich α phase and dislocation loops in the zinc-rich β phase. Subsequent room temperature deformation creates a typical cold-worked dislocation structure in the α phase. At higher deformation temperatures, the cold-worked structure is increasingly replaced by a recovered structure. At the same time, the dislocation loop density in the β phase decreases to lower values. During superplastic deformation at 250βC, the dislocation loops in the β phase are annihilated by interactions with glide dislocations. In the α phase, a continuous, or steady-state, dynamic recovery process appears to operate. A completely recovered structure is maintained with dislocation-free subgrains.     

Pseudopartial wetting of grain boundaries in severely deformed Al-Zn alloys

After severe plastic deformation by the high-pressure torsion, Al-Zn alloys have three various classes of Al/Al grain boundaries (GBs) wetted with a second zinc-rich phase. Completely wetted Al/Al GBs are coated with the layer of a zinc-rich phase more than 30 nm thick. Partially (incompletely) wetted Al/Al GBs contact particles of the zinc-rich phase with a contact angle >60°, but contain no measurable zinc concentration. Pseudopartially wetted Al/Al GBs also contact Zn particles with a contact angle >60°. However, they have a thin interlayer of the zinc-rich phase with a uniform thickness of 2–4 nm, the presence of which explains the unusually high ductility of Al-Zn alloys after high-pressure torsion.     

Decomposition in Al-Zn alloys: Part II. Decomposition during continuous cooling

A small angle X-ray scattering study (SAS) has been made of decomposition during contin uous cooling in four binary Al-Zn alloys with compositions spanning the miscibility gap and in two ternary alloys, each containing 22 at. pct Zn plus small amounts of Sn and Mg. Plots of logλ _m (wavelength receiving maximum amplification during the quench)vs logQ (quench rate) yield slopes of approximately -1/3 for all alloys, indicating that coarsening plays an important role during the quench. In addition, measurements of integrated area under the SAS spectra indicate that decomposition is essentially complete in the quenched condition for all of the alloys studied.     

The influence of thermal and surface treatments on the anelastic creep of Al-Zn and Al-Cu alloys

In many industrial applications, like high precision force measurements or nanopositioning, the elastic and dimensional stabilities of materials are required at a nanometric scale. Therefore, some aspects of room temperature creep of commercial Al-Zn and Al-Cu alloys have been studied. The anelastic creep was measured at room temperature by means of a high resolution laser interferometer. The irreversible component of the deformation was quantified by measuring the viscoelastic after-effect. The anelastic relaxation of Al-Zn has been studied as a function of annealing time and temperature. The results are correlated with the size and the density of precipitates and the extension of the precipitate free zone. The influence of a surface treatment (sandblasting, electropolishing, Cu-galvanizing, chroming, Zn-plating) on anclastic relaxation has also been considered and explained in terms of the anelastic and elastic properties of both the surface layer and the bulk.     

Decomposition in Al-Zn alloys: Part II. Decomposition during continuous cooling

A small angle X-ray scattering study (SAS) has been made of decomposition during contin uous cooling in four binary Al-Zn alloys with compositions spanning the miscibility gap and in two ternary alloys, each containing 22 at. pct Zn plus small amounts of Sn and Mg. Plots of logλ _m (wavelength receiving maximum amplification during the quench)vs logQ (quench rate) yield slopes of approximately -1/3 for all alloys, indicating that coarsening plays an important role during the quench. In addition, measurements of integrated area under the SAS spectra indicate that decomposition is essentially complete in the quenched condition for all of the alloys studied. DENNIS T. LEWANDOWSKI, formerly a graduate student at Michigan Technological University. Now on sabbatical leave to Caterpillar Tractor Co., Mapleton Plant, Mapleton, Ill. 61554.     

FIM observation of GP zones in an Al-4%Cu alloy

GP zones in an aged Al-4%Cu alloy have been examined by a field ion microscope. When the spacing of Cu atoms is large on the observing plane, as, for example, on {024}, {113} or {135} planes, which a GP[1] zone intersects, the images of individual Cu atoms constituting the zone are clearly resolved. A change in image contrast upon successive field evaporation has shown that Cu atoms forming a GP[1] zone are brightly imaged by preferentially ionizing image gas atoms when those Cu atoms protrude from an otherwise smooth plane. When they do not protrude, a dark contrast appears because of the larger electronegativity of Cu. All the GP[1] zones, observed and analyzed with respect to their structures, are identified to consist of a single Cu {200} plane. GP[2] zones with two Cu planes separated by three or four {200} matrix planes are observed.     

Al-Zn(Cu)合金的相变和Al-Zn-Cu系相图

Al-Zn-Cusystemistheimportantbasisfor practicalAlalloysandZnalloys[1].Amiscibility gapofthefccphaseexistsabove277℃inthe Al Znsystem[2].Thealloywhosecompositionis atthetopofthemiscibilitygapiscalledsym metricalalloyAlZn.Thisalloyhasmanytypical properties.Theimportantoneisthatthespi nodaldecompositioneasilytakesplaceinit.In ordertoavoidthedecomposition,Cuhasbeen added,whichiseffective.Additionof2%Cu molefractioncoulddelaythespinodaldecompo sitionfortwogrades[3,4].AlZn 2Cuisanalloy which…     

Measurement of liquid permeability in the mushy zones of aluminum-copper alloys

Measurements of liquid permeability in the mushy zones of Al-15.42 pct Cu and Al-8.68 pct Cu alloy samples were performed isothermally just above the eutectic temperature, using eutectic liquid as the fluid. A modified method was developed to determine the specific permeability as a function of time (K _s) during the test from the data collected on these alloys. Factors affecting permeability measurements are discussed. The permeabilities are observed to vary throughout the experiment. This is attributed to microstructural coarsening and channeling that occur in the sample during the experiment. Coarsening rates are determined for the isothermal coarsening tests without fluid flow, and the results are observed to be less than the rates indicated from permeability tests where fluid flow is present. Careful measurement of the volume fraction of liquid (g _L) shows that g _L decreases during the test. The permeability is then related to the microstructure of the sample using the Kozeny-Carman equation. The correlation between the measured K _S, g _L, and specific solid surface area (S _V) improves markedly when compared to previous studies, when microstructural parameters at the initial stage of the test are used.     

Grain boundary precipitate free zones in Al-Li alloys

The growth kinetics of δ′ (Al₃Li) precipitate free zones (PFZ) at the grain boundaries has been investigated in several Al-Li alloys at selected aging temperatures ranging from 168 to 225°C and aging times. The PFZs form by a solute depletion mechanism and the PFZ growth can be described as a diffusion controlled process. The activation energy for PFZ growth has been evaluated as 144 kj/mol, which agrees with the activation energy evaluated for the diffusion of Li in α-Al. The PFZ growth has been analyzed on the basis of a diffusion model with due consideration of δ (AlLi) formation at the grain boundaries. With increase in Li content in the alloys, the growth rate of PFZ increases. This observation is explained by a possible increase in the interdiffusion coefficient with the alloy composition. The effect of grain morphology on the PFZs has been studied by comparing the PFZs in recrystallized and unrecrystallized Al-Li alloys.     

Laser shock hardening of weld zones in aluminum alloys

The feasibility of using a high energy, pulsed laser beam to shock-harden weld zones in 5086-H32 and 6061-T6 aluminum sheet was investigated. The tensile strength, hardness, and microstructure of samples 0.3 cm thick were studied before and after laser shocking. After laser shocking, the tensile yield strength of 5086-H32 was raised to the bulk level and the yield strength of 6061-T6 was raised midway between the welded and bulk levels. The increases in ultimate tensile strength and hardness were smaller than the increases in the yield strength. The microstructures after shocking showed heavy dislocation tangles typical of cold working. Formerly with Battelle, Columbus Laboratories     

Application of heterogeneous nucleation theory to precipitate nucleation at GP zones

The accelerated nucleation of precipitates at GP zones is explained using heterogeneous nucleation theory. Nucleation at zone : matrix boundaries is encouraged by several factors: 1) the chemical interfacial energy of zone : matrix boundaries can significantly decrease the interfacial energy barrier to nucleation; 2) destruction of quenched-in excess vacancies at incoherent portions of the nucleus surface may make the change in the volume free energy significantly more negative; 3) the crystal structures of the zone and matrix are identical and parallel which permits the nucleus to be faceted in both phases. Some additional assistance to nucleation at GP zones is provided by: 4) the accelerated diffusivity resulting from the presence of excess vacancies and 5) the large area of zone : matrix boundary per unit volume of matrix. These factors can more than compensate for the decreased solute supersaturation due to the formation of GP zones and provide an explanation for the enhanced nucleation of precipitates in the presence of GP zones.     

Application of heterogeneous nucleation theory to precipitate nucleation at GP zones

The accelerated nucleation of precipitates at GP zones is explained using heterogeneous nucleation theory. Nucleation at zone : matrix boundaries is encouraged by several factors: 1) the chemical interfacial energy of zone : matrix boundaries can significantly decrease the interfacial energy barrier to nucleation; 2) destruction of quenched-in excess vacancies at incoherent portions of the nucleus surface may make the change in the volume free energy significantly more negative; 3) the crystal structures of the zone and matrix are identical and parallel which permits the nucleus to be faceted in both phases. Some additional assistance to nucleation at GP zones is provided by: 4) the accelerated diffusivity resulting from the presence of excess vacancies and 5) the large area of zone : matrix boundary per unit volume of matrix. These factors can more than compensate for the decreased solute supersaturation due to the formation of GP zones and provide an explanation for the enhanced nucleation of precipitates in the presence of GP zones.