On the kinetics of discontinuous precipitation in an Al-14.6 at.% Zn alloy

The influence of the continuous precipitation on the discontinuous precipitation in an Al-14.6 at.% Zn alloy has been studied using optical microscopy, resistivity measurement, high angle X-ray study, small angle X-ray study, and transmission electron microscopy. Results showed that a considerable amount of decomposition (up to 9 ∼ 15 vol.%) takes place in the matrix through the prior continuous precipitation before the onset of the discontinuous precipitation at the grain boundary. The growth rate of the cellular structure due to the discontinuous precipitation appears not to be directly related with the available chemical free energy, nor with the available total free energy, but to be directly related with the strain energy associated with the prior continuous precipitation of coherent G.P. zones.     

Novel faulted structures in rapidly solidified Fe-37 at.% Al-15 at.% Mo alloy

Fe-37 at.% Al-15 at.% Mo has an equilibrium two-phase structure consisting of an ordered B2 (FeAl-based) matrix with Mo₃Al as the second phase. Rapid solidification by chill-block melt-spinning suppresses the formation of the Mo₃Al phase, giving instead single phase B2-FeAl supersaturated with Mo and containing a network of faults revealed by TEM. Imaging with B2 superlattice reflections revealed an anisotropic antiphase domain contrast while imaging with fundamental (b.c.c.) reflections showed typical stacking fault or thin precipitate contrast for the same faults. The faults were identified as a/2〈111〉{100} type B2 antiphase boundaries. It is proposed that the formation of such anisotropic and non-conservative antiphase boundaries is related to the retention of excess Mo within the B2 matrix by rapid solidification.     

Cellular precipitation and discontinuous coarsening of cellular precipitate in an Al-29 at.% Zn alloy

The morphology and growth kinetics of cellular precipitation and discontinuous coarsening of the cellular precipitate in an Al-29 at.% Zn alloy have been investigated at temperatures ranging from 323 to 523 K (50 to 250°C) by light microscopy, electron microscopy and X-ray diffraction. At all aging temperatures the alloy was observed to decompose completely by a cellular precipitation reaction which resulted in a fine lamellar structure of aluminium rich and zinc rich solid solutions. The first cell lamellar structure was then decomposed at all aging temperatures by a second cellular or discontinuous coarsening reaction. The discontinuous coarsening reaction occurred at a much slower rate than the first cellular reaction and resulted in a much coarser lamellar structure. Lattice parameter measurements showed the aluminum rich phase in the cellular precipitate to have a composition far from equilibrium while that in the product of discontinuous coarsening was close to equilibrium. Analysis of the growth kinetics of both the cellular precipitation and the discontinuous coarsening suggested that they were controlled by grain boundary diffusivity.     

Intergranular fracture in an Al-15 Wt Pct Zn alloy

Crack propagation rates for the intergranular fracture of an Al-15 wt pct Zn alloy tested in air, distilled H₂O, and 0.5M NaCl have been studied using a double cantilever beam specimen. This technique has been applied to two different types of specimens: polycrystals with large equiaxed grains and bicrystals. The susceptibility to intergranular fracture in air and 0.5M NaCl increases as the volume fraction of G.P. zones in the matrix increases. The microstructure which is most susceptible to fracture exhibits intense planar slip traces while the more resistant microstructures have a more diffuse surface slip pattern. The dependence of cracking rates on the microstructure is explained in terms of the blocking of inhomogeneous plastic flow in the matrix by grain boundaries, resulting in locally severe stress concentrations at the boundaries. Bicrystal tests substantiate these results and show that orientations favoring partial continuity of slip bands across grain boundaries are less susceptible to stress corrosion cracking than arbitrarily oriented boundaries with no slip continuity. WILLIAM J. KOVACS, formerly Graduate Student, Carnegie-Mellon University, Pittsburgh, Pa. This paper is based upon a thesis submitted by WILLIAM J. KOVACS in partial fulfillment of the requirements of the degree of Doctor of Philosophy in Metallurgy and Materials Science at Carnegie-Mellon University.     

Dislocation structures in zirconium and zircaloy-4 fatigued at different temperatures

The development of characteristic dislocation structures in pure zirconium and zircaloy-4 fatigued under pull-push strain control as the testing temperature and the cyclic strain range varied was examined using a thin-foil transmission electron microscopy (TEM) technique. The slip planes and the twinning planes were determined by a standard stereographic trace analysis technique. The first-order prismatic slip {10 0} is the primary deformation mode in zirconium and zircaloy-4 fatigued from room temperature (RT) to 873 K. The pyramidal sli { 2 } is activated at 673 K and at high cyclic strain ranges, whereas the basal slip {0001} only appears in those specimens fatigued at 873 K. The {10 2}, {11 1}, and {11 2} types of twins were detected in specimens fatigued at RT. Twinning becomes less frequent as the testing temperature increases. The schematic map of the cyclic deformation modes as a function of the plastic strain range and the test temperature is described. The dislocation configurations in fatigued pure zirconium specimens evolve from a planar arrangement to a cell structure as the test temperature and the strain range increase. For zircaloy-4, the fatigued dislocation structure is parallel dislocation lines at RT, cells at 673 K, and two sets of approximate mutually perpendicular dislocation bands at 873 K, respectively. Finally, the fatigued dislocation-structure evolution map with the cyclic strain range and the test temperature are qualitatively established for zirconium and zircaloy-4, respectively. The effect factors on the fatigue mechanism and the thermodynamic and dynamic criteria of the dislocation-pattern evolution are discussed.     

Electron microscopical investigations on the precipitation of various h.c.p. phases in an Al-6.8 at.% Zn alloy

In an Al-6.8 at.% Zn alloy aged at T = 120°C the precipitation of h.c.p. phases was studied by high-resolution electron microscopy and high voltage electron microscopy. The shapes of the respective precipitates, the hexagonal lattice parameters as well as the orientation relationship between the h.c.p. precipitates and the f.c.c. matrix were determined. Information was mainly gained from the spacings and orientations of lattice fringes and, occasionally, from the Moiré patterns. Three h.c.p. phases were found, two of which are characterized by the same lattice parameters but have different orientation relationships to the f.c.c. matrix crystal.     

Dislocation structures in fatigued polycrystalline copper

The dislocation structures in fatigued polycrystalline copper with small average grain size were investigated over a plastic strain range from 1.5 x 10⁻⁵ to 10⁻². It was found that the dislocation structures are arranged into three types of configurations, which correspond to the three regions in the cyclic stress-strain curve. Cylidirical loop patch structure are present in region A for low strain amplitudes, similar to those observed previously in coarse grained polycrystals. Moreover, irregular loop patches are also formed in this region for small grains polycrystals rather thanin region B at intermediate strain amplitudes for coarse grained polycrystals. In region B, persistent slip band (PSB) structures are formed but with a low volume content compared with the coarse grained polycrystals. In region C, at high plastic strains, the dislocation structures are dominated by dipolar walls. In addition, labyrinth structures are developed in region C instead of region B for coarse grained polycrystals. All the dislocation structures observed are viewed as forms of dipolized structures. A dipolized dislocation arrangement model is proposed to describe the formation process of dislocation structures. It is shown that all the dislocation configurations formed in cycled polycrystalline copper are low energy structures.     

滴定法连续测定银铜铝锌四元合金中Cu,Al,Zn

采用连续滴定的方法同时测定Cu,Al,Zn.加入一定量的EDTA标准溶液,使Cu2+,Al3+,Zn2+全部与EDTA络合,用Zn2+盐滴定过量EDTA,然后加氟盐置换出EDTA-Al中的EDTA,用硫脲抗坏血酸和邻菲啰啉置换出EDTA-Cu中的EDTA,并分别用Zn2+盐滴定,从总量中减去Al量和Cu量即得Zn量.滴定Cu,Al,Zn回收率均在98%以上,本法分析快速准确.     

The formation of cell structures in fatigued iron crystals

Single crystals of α-Fe have been fatigued in constant stress amplitude reversed plane bending. TheS-N curve, the plastic strain amplitude variation, and the dislocation substructures developed in the crystals during fatigue have been studied. The formation of a cell structure was found to occur once a critical dislocation density had accumulated, the nature of which was influenced by the crystallography of the specimen and underwent changes with continued testing. The evolution of a continuous cell structure with testing coincided with a large decline in the level of plastic strain amplitude, and it appears that it is these substructures which account for fatigue hardening. From the observations of crack formation and the mode of dislocation movement during saturation, a mechanism for crack initiation is suggested.     

Electron-microscopic study of the slip systems in an Mg-4.5% Al-1% Zn alloy

Samples of a hot-rolled Mg-4.5% Al-1% Zn alloy plate cut normal to the rolling direction are studied to determine the density of dislocations with different Burgers vectors after warm rolling at low total strains. In the experiment, 22 different grains and their orientations are studied in a JEM-1000 high-voltage transmission electron microscope at an accelerated voltage of 750 kV using the WB DF technique (g, ng, where n = 2, 3, …) for the observation of dislocations. The main method used for the analysis of the Burgers vector of the dislocations is the invisibility criterion gb = 0. In the samples, dislocations with Burgers vectors 〈a〉, [c], and 〈a + c〉 are found. The dislocation dissociation reaction of the 〈a + c〉 dislocation into the 〈a〉 and [c] dislocations has been found for the first time, and an assumption is made that all of the found [c] dislocations result from this reaction. The density of the basal and nonbasal 〈a〉 dislocations does not depend on the orientation of the grains. Rarely found [c] and 〈a + c〉 dislocations are in grains whose orientations are near the texture maximum in the {0002} pole figure.     

The microstructural development of Mg-9 pct Al-1 pct Zn alloy during injection molding

Ex-situ microstructural analysis was used in order to assess the high-temperature transformations of the Mg-9 pct Al-1 pct Zn alloy within the injection-molding system. In addition to as-cast ingot, chipped feedstock, and final products, the alloy samples at different stages of the process were investigated. It was revealed that the cold deformation during ingot chipping and recrystallization during initial stages of residency inside the barrel led to nucleation and growth of the equiaxed grain structure. Subsequent melting of the Al-rich phase followed by liquid-alloy wetting of α-Mg grain boundaries initiated the formation of the slurry with globular solid particles. The structural evolution of the semisolid alloy under the simultaneous influence of temperature gradient and injection-screw shear is considered, and the mechanisms involved are discussed. It is concluded that during processing of partially remelted alloy the temperature profile along the injection-molding barrel in combination with the feedstock structure are key factors that control both the transformation of the thixotropic slurry and the final morphology of the primary solid phase.     

Growth and coarsening of g. p. Zones in Al- Zn alloys

The structural changes during the precipitation of G. P. zones in Al-Zn binary alloys have been investigated by means of anin situ small-angle scattering technique using synchrotron radiation. Defining a specific time, normalized by the half-completion time, the time-dependent evolution of the precipitation process can be divided into three periods, independent of alloy composition and quench-ing conditions. The structural and kinetic features of the first two periods have been analyzed in detail. The first stage represents a growing process of clusters with diffuse interface into the well-defined G. P. zones. The average size of these clusters increases, and the density decreases. The second stage corresponds to the Ostwald ripening process. This mechanism is described by utilizing a modified Lifshitz-Slyozov-Wagner theory.     

Decomposition in Al-Zn alloys: Part I. Isothermal decomposition in an Al-22 at. pct Zn-0.1 at. pct Mg alloy

A small-angle X-ray scattering (SAS) study has been made on solution treated and isothermally annealed specimens of an Al-22 at. pct Zn-0.1 at. pct Mg alloy. The changes in peak position and integrated area of the SAS spectra with time and temperature indicate that de composition is nearly complete immediately after quenching, in agreement with the earlier interpretation that Gerold and Merz placed on the results of Rundman and Hilliard in the binary Al-22 at. pct Zn alloy. Furthermore, structural changes occurring during annealing are consistent with a coarsening or maturation of the fluctuations in the solution. The domi nant wavelength varies ast ^1/3 over a large time span and the temperature dependence of the coarsening process yields an activation energy of 94.2 kJ/mole. The effect of Mg is to re tard the formation of the equilibrium phases while having a small effect on the growth of the composition fluctuations during the coarsening process. Now on sabbatical leave to Caterpillar Tractor Co., Mapleton Plant, Mapleton, Ill. 61554.     

Decomposition in Al-Zn alloys: Part I. Isothermal decomposition in an Al-22 at. pct Zn-0.1 at. pct Mg alloy

A small-angle X-ray scattering (SAS) study has been made on solution treated and isothermally annealed specimens of an Al-22 at. pct Zn-0.1 at. pct Mg alloy. The changes in peak position and integrated area of the SAS spectra with time and temperature indicate that de composition is nearly complete immediately after quenching, in agreement with the earlier interpretation that Gerold and Merz placed on the results of Rundman and Hilliard in the binary Al-22 at. pct Zn alloy. Furthermore, structural changes occurring during annealing are consistent with a coarsening or maturation of the fluctuations in the solution. The domi nant wavelength varies ast ^1/3 over a large time span and the temperature dependence of the coarsening process yields an activation energy of 94.2 kJ/mole. The effect of Mg is to re tard the formation of the equilibrium phases while having a small effect on the growth of the composition fluctuations during the coarsening process.     

Atom probe study of early stage phase decomposition in an Al-7.8 at.% Li alloy

The early stage of phase decomposition of an Al-7.8at.% Li alloy was studied by atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM). The atom probe concentration depht profile clearly that the δ′ with the stoichiometric Li concentration were already precipitated in the as-quenched stage. The high resolution TEM image also showed that small isolated ordered particles were present in the disordered matrix. These two results unambiguously show that the first product which we can observe experimentally is the δ′ precipitates. Therefore, it does not seem possible to obtain convincing evidence to prove that the congruent ordering precedes spinodal decomposition, because such a transformation path would have already been completed during quenching.     

形变热处理对Cu-6.5Ni-1Al-1Si-0.15Mg-0.15Ce合金微观组织及性能的影响

设计并制备 Cu-6.5Ni-1Al-1Si-0.15Mg-0.15Ce(wt.%)合金。采用光学显微镜(OM)、扫描电镜(SEM)、能谱分析和透射电镜(TEM)等测试手段研究合金形变热处理过程中微观组织及性能的变化。合金铸态组织为典型的枝晶组织,铸锭经过920℃热轧后,枝晶组织显著消除。合金的适宜固溶处理制度为960℃/4 h。该合金固溶处理后的冷变形对合金最终性能有很大影响。冷变形程度越大,合金达到硬度峰值的时间越短,硬度峰值和电导率越高。时效温度越高,时效析出过程越快。960℃固溶4 h后冷轧50%,450℃时效2 h硬度峰值可达300.8 HV,电导率20.6%IACS,抗拉强度963.9 MPa,屈服强度950.1 MPa。合金在时效过程中析出纳米级粒子为δ-Ni2Si,其与基体的位相关系为：Cuδ[001][001], Cuδ(110)(010), Cuδ(110)(100)。