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Isothermal and thermomechanical fatigue of 63Sn/37Pb solder is studied under total strain-controlled tests. A standard definition of failure is proposed to allow inter-laboratory comparison. Based on the suggested failure criterion, load drop per cycle, the Young’s modulus and the ratio of the maximum tensile to maximum compressive stresses remain constant, and the fatigue response of the solder is stable before failure, although cyclic softening was observed from the beginning. Experimental results of isothermal fatigue tests for a total strain range from 0.3 to 3 percent show that the log-log plot of the number of cycles to failure versus the plastic strain range has a kink at the point where the elastic strain is approximately equal to the plastic strain. In this paper, it is shown how the isothermal fatigue life of near-eutectic solder at lower strain ranges can be predicted by using the experimental data of fatigue tests at high strain ranges and early stage information of a fatigue test at the strain range in question. A thermomechanical fatigue life prediction is also given based on a dislocation pile-up model. Comparison with experimental results shows a good agreement.

A detailed study was conducted of the microstructure and particle-matrix interfaces in Al/TiCpmetal matrix composites prepared by theXDprocess and subsequent extrusion. A study of the morphology of the TiC particles showed that the surfaces are low index (111) and (200) planes, the former being more common. Direct contact on an atomic scale is established between Al and TiC, allowing chemical bonds to form. Young's modulus is in the range expected for a composite of Al and TiC with good interfacial bonding and load transfer to the particles. No third element has been detected at the interfaces, showing that they are clean. Both incoherent and semicoherent interfaces are seen. The interface character depends on the size of the particles and their orientation with respect to the neighboring Al grains. “Special” interfaces with evidence for nearly periodic dislocations were observed in bothXDAl/TiC and Al/TiB2composites, indicating the general tendency ofin situcomposites to lower their interfacial energy by forming such boundaries.

The effect of solid state heat treatment at 913 K on extruded XD Al/TiC metal matrix composite with 0.7 and 4.0 /nm particle sizes has been investigated. The interfaces between Al and TiC after extrusion were atomically abrupt, as observed by HRTEM. On holding at 913 K, the composite with submicron particle size showed substantial changes in the phases present due to reaction between Al and TiC at 913 K. The stable reaction products are Al 3 Ti and A1 4 C 3 . A substantial increase in Young's modulus occurs. The room and elevated temperature strength and hardness of the composite with submicron particles also increase significantly with time of heat treatment, but at the expense of ductility. The effect of heat treatment over the time range investigated is limited to the interfaces for the 4.0 /xm TiC particle size composite due to longer diffusion paths.

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Articles you may be interested inDielectric and ac conductivity studies on undoped and MgO-doped near-stoichiometric lithium tantalate crystals PHOTOLYSIS OF ACETONE N ate added in proof. In subsequent work by Mr. A. Shilman in this laboratory, an extensive investigation has been made of pressure effects near 260, 280, and 300 mJ.'. Near 260 mJ.' and near 300 mJ.', the CO quantum yield was pressure-independent. Near 280 mJ.', it was pressure-dependent in agreement with Roebber et al. 6 Thus, the difference between Roebber's results at 280 mJ.' and those obtained at low light intensities6-8 (usually at 254 and 313 mJ.') appear to represent wavelength effects.The electrical conductivity of a-Nb20. monocrystalline and sintered specimens, measured under a constant ambient oxygen pressure and over the temperature range 300 to 900°C, exhibits an exponential temperature dependence with an activation energy of 1.65 ev. The isothermal conductivity, IT, of near-stoichiometric a-Nb20. is related to the ambient oxygen partial pressure, P02, by the power law expression IT = const P0 2 ""il·24o!;).0l. Both sets of observations may be rationalized on the basis that slightly-reduced a-Nb.O. becomes a metal-excess, n-type semiconducting oxide containing oxygen vacancies capable of trapping electrons. A detailed analysis of the relation between conductivity and ambient oxygen pressure has indicated that the conductivity depepdency may be ascribed to a mechanism whereby defect levels (due to oxygen ion vacancies with either one or two trapped electrons) are created and electrons are thermally excited from these levels into the conduction band.

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