Dislocation mechanisms for the relaxation of thermal stress at metal-ceramic interfaces

Thermal stresses in the vicinity of the interface between metal and ceramic components result from the difference in the thermal expansion coefficients between the two materials when the temperature is changed. The dislocation structure in and near NiOPt interfaces that survived large temperature changes was studied in order to understand how a metal-ceramic couple accomodates thermal stresses that frequently can exceed the yield stress of the metal and the fracture stress of the ceramic. The interface structure for the exact (001) epitaxial orientation relationship between Pt and NiO is a square grid of misfit dislocations. A larger misfit dislocation spacing is expected to be present at room temperature than at the high temperature used during hot pressing to form the metal-ceramic bond. A dislocation mechanism based on climb and glide was suggested to produce this change of spacing and relieve the thermal stress. Experimental observations were obtained which are consistent with this mechanism.     

Cyclic near-tip fields for fatigue cracks along metal-metal and metal-ceramic interfaces

Finite element analyses were conducted with the objective of determining the cyclic near-tip fields ahead of a stationary tensile fatigue crack lying along the interface between two dissimilar solids. The model systems analyzed are (i) a metal-metal bimaterial whose components have the same elastic properties but different plastic deformation characteristics, and (ii) a metal-ceramic bimaterial. In both cases, monotonic loading to the peak tensile load results in a predominantly mode I field ahead of the crack. However, unloading in one or both components of the bimaterial with prior plastic deformation generates mixed-mode conditions at the crack tip. The mode mixity persists during a significant portion of the next loading phase and is gradually removed upon reloading to the peak stress. The cyclic plastic zone size directly ahead of the fatigue crack is approximately one-fifth the size of the monotonic plastic zone. The residual compressive stresses within the cyclic plastic zone are computed. The implications of reversed yielding and mixed-mode near-tip fields to constant-amplitude and variable-amplitude fatigue fracture are discussed.     

Nonequilibrium molecular dynamics simulations of metallic friction at Ta/Al and Cu/Ag interfaces

With the advent of large-scale parallel computers in the past decade, it has become possible to study the microphysics of complex nonlinear many-body systems at scales approaching the mesoscale. One of the areas where nonequilibrium deformation arises is that of dry sliding friction between ductile metals. We discuss the results of atomistic simulation studies of sliding friction in the velocity range 10 to 10³ m/s in the high-pressure regime of 5 to 15 GPa for Ta/Al and Cu/Ag tribopairs. We discuss the velocity dependence and the near interface deformation seen in these simulations. This article is based on a presentation given in the symposium “Dynamic Deformation: Constitutive Modeling, Grain Size, and Other Effects: In Honor of Prof. Ronald W. Armstrong,” March 2–6, 2003, at the 2003 TMS/ASM Annual Meeting, San Diego, California, under the auspices of the TMS/ASM Joint Mechanical Behavior of Materials Committee.     

钛合金超塑成形/扩散连接的数值模拟及工艺研究

利用有限元软件Marc对钛合金TC4板的超塑成形/扩散连接(SPF/DB)进行数值模拟.通过模拟获得优化的压力-时间(p-t)曲线,为后续的超塑成形/扩散连接试验提供参考基础.经过试验得到钛合金TC4板超塑成形/扩散连接的最佳工艺参数为:超塑成形,θ=900 ℃、p=2.5 MPa、t=790 s;扩散连接,θ=900 ℃、p=3.0 MPa、t=80 min. 钛合金TC4;超塑成形/扩散连接(SPF/DB);数值模拟;工艺研究     

AB5型与AB3.5型贮氢合金的电化学性能对比研究

为了对比研究AB5型与AB3.5型贮氢合金的电化学及动力学特性,以AB5型MlNi3.68Mn0.32Co0.73Al0.27和AB3.5型Ml0.80Mg0.20 （NiMnAlCu）3.6、La0.6 Mg0.4 Ni3.5为研究对象.采用XRD分析了合金的相结构,利用电化学方法测试合金的电化学及动力学特性.结果发现,MlNi3.68 Mn0.32Co0.73Al0.27合金由单相LaNi5相组成,而Ml0.80Mg0.20（NiMnAlCu）3.6、La0.6Mg0.4Ni3.5均有LaNi5相和La2Ni7相组成.AB3.5型合金的放电容量、荷电保持率以及动力学特性高于AB5型合金.     

Equipment for injection moulding of thermosetting materials based on ceramic and metal-ceramic powders

Results of design and process investigations of the construction of technological equipment for injection moulding of articles with complex shapes from thermosetting materials based on ceramic and metal-ceramic powders are presented. Numerical simulation of the proces of heat transfer is conducted. The parameters of a commercial plant (GShP-2) and of a newly designed plant for injection moulding are determined. The newly designed plant produces a uniform distribution of temperature in the functional loop and creates a thermosetting material and injectible material that are uniform in terms of properties.     

Equipment for injection moulding of thermosetting materials based on ceramic and metal-ceramic powders

Results of design and process investigations of the construction of technological equipment for injection moulding of articles with complex shapes from thermosetting materials based on ceramic and metal-ceramic powders are presented. Numerical simulation of the proces of heat transfer is conducted. The parameters of a commercial plant (GShP-2) and of a newly designed plant for injection moulding are determined. The newly designed plant produces a uniform distribution of temperature in the functional loop and creates a thermosetting material and injectible material that are uniform in terms of properties.     

Effect of FeO and Al2O3 on the MgO Solubility in CaO–SiO2–FeO–Al2O3–MgO Slag System at 1823 K

The MgO solubility in CaO–SiO₂–FeO–Al₂O₃–MgO quinary slag system at 1823 K was measured to evaluate the effect of FeO and Al₂O₃ on the MgO solubility. It was found that the MgO solubility was decreased with higher optical basicity, FeO concentration, and increased with higher Al₂O₃ concentration. The MgO solubility was affected by activity coefficient of Mg²⁺ ($\gamma _{{\rm Mg}_{{\rm 2 + }} } $ ). Increase of the activity coefficient of Mg²⁺ ($\gamma _{{\rm Mg}_{{\rm 2 + }} } $ ) with higher FeO or lower Al₂O₃ decreased the MgO solubility. The increment in MgO solubility is remarkably reduced beyond a critical $X_{{\rm Al}_{2} {\rm O}_{{\rm 3}} } /(X_{{\rm Al}_{2} {\rm O}_{{\rm 3}} } + X_{{\rm FeO}} )$ ratio. The significant decrease of the increment of MgO solubility is caused by the change of the molten slag structure. The excess stability function of Al₂O₃ and the Fourier transform infrared (FT‐IR) analysis were applied to indirectly verify the existence of the spinel structure in the CaO–SiO₂–FeO–Al₂O₃–MgO slag system.