Laser surface alloying of ni film on al-based alloy

Based on SIMS analyses, X-ray diffraction patterns, scanning and transmission electron microscopy observations and chemical analyses at various scales, the mechanisms of LASER surface alloying of Ni coating of Al-based alloy have been investigated. The formation of Al₃Ni primary dendrites in the case of the low thickness of the initial Ni coating is reported and a double dendritic mode is observed in the case of the thicker coating, Al₃Ni₂ followed by Al₃Ni dendrites. A segregation in Si has been observed just below the exposed surface and related to the dendritic solidification mode. The optimized surface treatment parameters have been experimentally determined as a function of the initial thickness of the Ni coating.     

Metallography of fatigue crack initiation in an overaged high-strength aluminum alloy

Crack initiation was observed by optical microscopy using Nomarski interference contrast during fatigue cycling of an overaged 2024 aluminum alloy. The number of cracks more than five microns long at any given fraction of the fatigue life, and the distribution of cracks among various possible initiation sites, both depend on the applied stress amplitudeσa). The crack density at failure falls from approximately 300/mm² when σ^a is 90 pct of the yield strength, to less than I/mm² when σ^a is less than 60 pct of the yield strength. Cracks may begin in the matrix, in grain boundaries, or at constituent particles. At all stress amplitudes, however, the most common initiation sites areβ (Al₇Cu₂Fe) constituent particles. At low stress amplitudes in particular, fatigue cracks develop from the interface between closely-spaced fragments of β particles broken during prior processing (cluster sites). The stress-raising effect of voids which often occur at cluster sites may be responsible for their effectiveness in initiating fatigue cracks. Formerly Graduate Students in the Department of Metallurgical Engineering at Wayne State University During 1982–83 he is on leave as Associate Director of the Metallurgy Program, Division of Materials Research, National Science Foundation, Washington, DC 20550.     

Structure-property relations in a metastable β Ti alloy containing Si

The hardness response, tensile behavior, and phase transformations occurring in a quenched and aged metastable β phase Ti-30 at. pct V-l at. pct Si alloy have been inves-tigated. Upon aging at 570°C, as-quenched samples show a broad hardness peak which is associated with the formation of rod-like, hexagonal (Ti,V)_xSi_y transition phase precipi-tates. The equilibrium silicide is observed upon aging at 570°C in the form of faceted, tetragonal particles. A loss of tensile ductility and a transition to intergranular fracture occurs after extended aging at 570°C and is related to Si segregation to the grain bound-aries. Comparing the behavior of Ti-30V to that of Ti-30V-lSi shows that the presence of Si strongly retards α-phase formation. However, a substantial age hardening re-sponse still occurs upon aging at 450°C, especially after prior cold work (the yield strength increases from 635 to 982 MPa). This hardening response is combined with a retention of a ductile, transgranular fracture even after extended aging at 450°C. Aging first at 570°C followed by aging at 450°C results in an increase in the volume fraction of α-phase formed but a subsequent decrease in ductility and hardness response upon aging at 450°C. These results are discussed in terms of the structure/property relationships which result from the influence of Si on the formation of, a) (Ti.V)_xSi_y precipitates, b) the equilibrium silicide, and c) the α-phase.     

7449高强韧铝合金均匀化热处理及微观组织演变

利用光学显微镜(OM)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、能谱分析(EDS)、差示扫描量热(DSC)、X射线物相分析(XRD)研究了7449高纯高强韧铝合金均匀化热处理制度及其微观组织演变。结果表明:7449合金铸锭组织存在枝晶偏析,Zn,Cu原子沿晶界偏析严重,非平衡共晶相沿晶界呈连续网状分布,共晶相组成为α(Al)+Mg(AlZnCu)2四元相,初始回溶温度为477℃;合金经不同均匀化热处理后,非平衡共晶相发生回溶,且析出弥散分布的Al3Zr粒子。研究了不同的均匀化热处理制度对7449合金微观组织的影响,均匀化热处理温度越高,合金非平衡共晶相回溶效果越好,合金经(440℃/12 h)+(470℃/24 h)均匀化热处理后第二相回溶充分,且析出弥散的Al3Zr粒子,抑制后续加工和固溶处理过程中的再结晶。最终确定合金合理的均匀化热处理制度为(440℃/12 h)+(470℃/24 h)。     

Yield strength of overaged alloys containing coherent ordered precipitates

The existing models for yielding of overaged alloys do not account for the observed strain rate sensitivity of the yield stress and predict a sharper decrease in stress as a function of particle size than the experimental observations. A dynamic model has been presented here to account for the yield strength of overaged alloys containing coherent ordered precipitates which takes into consideration the intrinsic characteristics of the precipitates. Concentric glide loops are supposed to form around precipitate particles. Their sizes as a function of applied stress have been computed. The stress on the innermost loop rises with increasing applied stress and when it exceeds the opposing force due to antiphase boundary, the loops shrink. The rate of shrinkage of loops has been calculated for different values of applied stresses. The yielding occurs when the external stress is high enough to account for the applied strain rate. The model has been applied to calculate the yield strength of an austenitic stainless steel containing γ′ precipitates. The antiphase boundary energy of the precipitates was measured by dislocation pair spacing method.The mode of variation of calculated yield stress values with particle size is predicted more accurately than by earlier models. The model qualitatively accounts for the observed sensitivity of yield stress to strain rate and antiphase boundary energy of the precipitates. Formerly Manager Research & Development, Alloy Steels Plant, Durgapur     

A model for the electrical conductivity of peak-aged and overaged Al-Zn-Mg-Cu alloys

A physically based model for the electrical conductivity of peak-aged and overaged Al-Zn-Mg-Cu (7xxx series) alloys is presented. The model includes calculations of the η- and the S-phase solvus (using a regular-solution model), taking account of the capillary effect and η coarsening. It takes account of the conductivity of grains (incorporating dissolved alloying elements, undissolved particles, and precipitates) and solute-depleted areas at the grain boundaries. Data from optical microscopy, differential scanning calorimetry (DSC), scanning electron microscopy (SEM) with energy-dispersive X-ray spectrometry (EDS), and transmission electron microscopy (TEM) are consistent with the model and its predictions. The model has been successfully used to fit and predict the conductivity data of a set of 7xxx alloys including both Zr-containing alloys and Cr-containing alloys under various aging conditions, achieving an accuracy of about 1 pct in predicting unseen conductivity data from this set of alloys.     

Understanding texture domains and relations between pre-eutectic and eutectic phase during Al-Si alloy solidification

Phase morphology and crystallographic texture are important components of a solidification microstructure. Probability of nucleation of a nucleus is a strong function of local chemical & thermal conditions and its atomic orientation. Similarly, the growth kinetics for various crystal planes is dependent on its crystal orientation apart from local environmental conditions. The combined effects of nucleation & growth kinetics along with solidification conditions dictate the resulting microstructure of an alloy. In this study, various phases of a near eutectic Al-Si alloy are examined by serial sectioning and 3D reconstruction. A peculiar microstructure is observed in which primary silicon is present in the immediate proximity of primary aluminum. Observations through electron back scatter diffraction show that the microtexture of primary and eutectic Si is similar (with in a twin relation) and thus it is concluded that the formation of eutectic starts with eutectic Si forming on the primary Si.     

Phase relations in the system Cu-Gd-O

The phase relations in the system Cu-Gd-O have been determined at 1273 K by X-ray diffrac- tion, optical microscopy, and electron microprobe analysis of samples equilibrated in quartz ampules and in pure oxygen. Only one ternary compound, CuGd₂O₄, was found to be stable. The Gibbs free energy of formation of this compound has been measured using the solid-state cell Pt, Cu₂O + CuGd₂O₄ + Gd₂O₃ // (Y₂O₃) ZrO₂ // CuO + Cu₂O, Pt in the temperature range of 900 to 1350 K. For the formation of CuGd₂O₄ from its binary component oxides, CuO (s) + Gd₂O₃ (s) → CuGd₂O₄ (s) ΔG° = 8230 - 11.2T (±50) J mol^-1 Since the formation is endothermic, CuGd₂O₄ becomes thermodynamically unstable with respect to CuO and Gd₂O₃ below 735 K. When the oxygen partial pressure over CuGd₂O₄ is lowered, it decomposes according to the reaction 4CuGd₂O₄ (s) → 4Gd₂O₃ (s) + 2Cu₂O (s) + O₂ (g) for which the equilibrium oxygen potential is given by Δμ_{o 2} = −227,970 + 143.2T (±500) J mol⁻¹ An oxygen potential diagram for the system Cu-Gd-O at 1273 K is presented.     

Phase relations in the system Pb-Sn-Se

Phase equilibria have been determined in the portion of the Pb-Sn-Se system bounded by the compositions Pb-Sn-SnSe-PbSe. The PbSe-SnSe join is quasibinary with only small deviations from stoichiometry. The ternary phase boundaries lie close to the Pb-Sn side of the diagram. Subsolidus relations are shown by a series of isoplethal sections.     

Reactions and phase relations in the TiAlO system

Reactions between titanium and alumina were studied experimentally for Al₂O₃ substrates with a titanium-based coating and for planar TiAl₂O₃ diffusion couples in the temperature range between 800 and 1100°C. Isothermal sections through the phase diagram were determined by using these results as well as by investigating equilibrated alloys. These experimental sections agree with those calculated from thermodynamic data. The morphology and layer thickness of the observed reactions zones have been explained on the basis of these phase relations, making use of diffusion data from binary systems TiO and TiAl. The important role of the initial Ti thickness on the type of reaction products has been demonstrated.     

Equilibrium relations in the system nickel oxide-copper oxide

Phase relations in the system Cu-Ni-0 were studied atPo ₂ = 0.21 atm using DTA and quenching techniques. Results were expressed in the form of a NiO-CuO “binary” in which a eutectic occurs at 1090°C and 6 mol pct NiO. The activities of CuO in (Ni,Cu)O solid solutions were determined at 1024°C by establishing the oxygen partial pressures at which Cu₂O precipitates from members of this solution series.     

Sexual relations in therapeutic situations

Patients who feel sexually violated by their therapists may complain to the health authorities. During recent years there has been an increasing number of such complaints. 15 cases were under investigation at the Directorate of Health in July 1992. Most of the doctors and psychologists involved had long therapeutic experience. The sexual contacts were of highly divergent types. In the majority of cases the therapists lacked the capacity to draw clear borderlines between professional and private life. Some of the therapists had a tendency to belittle the contacts. Sexual contact between patient and therapist has a detrimental effect upon the relationship and the mental functioning of the patients. Therapists acquainted with colleagues' sexual violations should react swiftly. Too lenient and permissive ethical and social attitudes towards sexual relationships between therapist and patient may be conducive to further abuse. It is important to incorporate the study and discussion of these problems into both the basic and postgraduate education of therapists. Therapists who have violated the norms for decent behaviour need professional support.