Effect of point defects on mechanical properties of metals. Progress report, January 1, 1975--December 31, 1975

Results of research activities are presented on electron irradiation softening in pure Fe, effects of impurities on electron irradiation softening, electron microscopic investigation of electron irradiation softening, surface film softening, and dislocation motion. (JRD

Effects of Heavy-Ion Irradiation on Microstructure of V-4Cr-4Ti Alloy at Moderate Temperatures

V-4Cr-4Ti is promising for first-wall and structural applications in magnetic fusion reactors. Fast neutron sources were used to evaluate postirradiation properties of fusion candidate materials, but FFTF and EBR-II have been shutdown. Under these circumstances, heavy-ion irradiation is an attractive alternative. We used 3-MeV V{sup +} and 4.5-MeV Ni{sup 2+} ions to investigate effects of irradiation on microstructure of V-4Cr-4Ti at 200-420 C. This paper report results of ion irradiation experiments and compare with fast-neutron irradiation data. From TEM, the dominant feature of postirradiation microstructure was a high density of dislocation loops and point- defect clusters. Density and defect size depend on irradiation dose and temperature. Precipitates and voids/bubbles were not observed, even in specimens simultaneous injected with he and exposed to heavy ions. Increased transport of point defects to internal interfaces was observed, as manifested by defect denuded zones along grain boundaries. Defect denuded zones along grain boundaries could lead to segregation of impurities and solutes and formation of precipitates on grain boundaries

Threshold energy surface and Frenkel-pair resistivity for Cu

In-situ electrical resistivity damage-rate measurements in the high voltage electron microscope have been used to study electron-irradiation-induced defect production in copper single crystals at T and surrounded by regions of much higher threshold energy. The corresponding damage function exhibits a plateau of 0.6 Frenkel pairs. the present results imply a Frenkel pair resistivity for C of (2.75/sub -0.2//sup +0.6/) x ..cap omega..-cm

A unified description of crystalline-to-amorphous transitions

Amorphous metallic alloys can now be synthesized by a variety of solid-state processes demonstrating the need for a more general approach to crystalline-to-amorphous (c-a) transitions. By focusing on static atomic displacements as a measure of chemical and topological disorder, we show that a unified description of c-a transformations can be based on a generalization of the phenomenological melting criterion proposed by Lindemann. The generalized version assumes that melting of a defective crystal occurs whenever the sum of thermal and static mean-square displacements exceeds a critical value identical to that for melting of the defect-free crystal. This implies that chemical or topological disorder measured by static displacements is thermodynamically equivalent to heating, and therefore that the melting temperature of the defective crystal will decrease with increasing amount of disorder. This in turn implies the existence of a critical state of disorder where the melting temperature becomes equal to a glass-transition temperature below which the metastable crystal melts to a glass. The generalized Lindemann melting criterion leads naturally to an interpretation of c-a transformations as defect-induced, low-temperature melting of critically disordered crystals. Confirmation of this criterion is provided by molecular-dynamics simulations of heat-induced melting and of defect-induced amorphization of intermetallic compounds caused either by the production of Frenkel pairs or anti-site defects. The thermodynamic equivalence between static atomic disorder and heating is reflected in the identical softening effects which they have on elastic properties and also in the diffraction analysis of diffuse scattering from disordered crystals, where the effect of static displacements appears as an artificially-enlarged thermal Debye-Waller factor. Predictions of this new, unified approach to melting and amorphization are compared with available experimental information

Experimental determination of the energy dependence of defect production

The damage function nu(T), i.e., the number of Frenkel pairs as a function of recoil energy is determined for Cu from electron and ion damage-rate measurements. nu(T) shows a plateau at nu = 0.54 which extends up to approx. 7xT/sub d//sup min/. Therefore, simple damage models, such as the modified Kinchin-Pease expression, are inappropriate not only at high recoil energies where stimulated recombination in cascades reduces defect production, but also in the single displacement regime. As a consequence, no simple relation between T/sub d//sup min/ and T/sub d//sup av/ is expected to exist. A procedure is suggested which uses anisotropy measurements in combination with polycrystal electron and ion irradiations to construct absolute damage functions in metals

Correlation between the elastic-shear-instability-mechanism and empirical criteria for irradiation-induced amorphization

In an attempt to correlate the shear instability mechanism with empirical criteria for irradiation-induced amorphization, shear moduli of an A{sub 3}B-type fcc crystal were calculated as a function of the chemical long range order parameter (S) using a Morse potential. The shear moduli were found to decrease with decreasing S. When the depth and the curvature of the A-B potential were changed while keeping the A-A and B-B potentials constant, the magnitude of the decrease in shear moduli is greater for deeper and narrower A-B potentials. The present results indicate that a shear instability should occur more readily in compounds with larger ordering energy and larger elastic moduli. These results agree with the reported empirical criteria for irradiation-induced amorphization, therefore providing further support for the shear instability mechanism for solid-state amorphization. 9 refs., 3 figs

Effect of point defects on mechanical properties of metals. Technical progress report

The major research effort was on iron. The recovery of the electron irradiation effect was studied in pure iron and iron-carbon alloys. This investigation resulted in several new findings in addition to the confirmation of two assumptions used in the interpretation of the mechanical effects of electron irradiation on high-purity iron single crystals. The irradiation softening effect was studied in single crystals oriented for the (011) slip and polycrystalline specimens. Then, a striking orientation effect was discovered and a large irradiation softening was found in single crystal specimens oriented for the hard (121) slip. In this oriertation, more than 60% of the yield stress is reduced by electron irradiation; thereby, the hardest oriertation becomes the softest orientation following electron irradiation. The solution softening and hardening effects of carbon atoms were investigated using highpurity iron single crystills similar to those used in the irradiationsoftening study. The general behavior of the softening and hardening was in good agreement with a proposed theory based on the irradiation- softening experiments. (auth

Effect of point defects on mechanical properties of metals. Technical progress report, December 1, 1979-January 31, 1980

This progress report consists of five sections. The importance of morphological study in conjunction with mechanical tests is emphasized and is explained in niobium, the surface effect on crystal plasticity of niobium, surface film softening in Ni-plated iron, and the hydrogen charging effect in iron