Influence of structural factors on the macro- and micromechanisms of fracture of austenitic dispersion-hardening alloys with an interrupted type of decomposition. Report 1

The commercial alloy 36NKhTYu in various structural conditions differing in the type of decomposition, grain size, and degree of dispersion of Y'- and -phase hardening particles was investigated by high-voltage electron microscopy and determination of the mechanical properties in tension in fatigue tests. It was established that in the alloy with the original coarse grain size regardless of the type of structure being formed and the form of loading, fracture is primarily ductile, occurring by a dislocation-shear mechanism. The comparison of structures of different type that was made showed that the structure with interrupted precipitation of -phase has the highest fracture energy requirement as the result of the presence of plastic interlamellar layers, providing effective dissipation of the energy supplied in loading. Deformation and fracture of the fragmented and ultradispersed structures is controlled by collective rotational processes.Translated from Problemy Prochnosti, No. 10, pp. 23–28, October, 1992.     

Fracture toughness of structural alloys under cyclic loading. Communication 2

Strength of Materials -     

Effect of the type of stress state on the micromechanisms of failure of maraging steels during equilibrium deformation

Fractographic and microstructural results are presented on the equilibrium failure of samples made of maraging steel, including those carrying stress concentrators of various form. It has been shown that it is possible to trace the actual micromechanism of failure of ductile materials during the growth and development of microcracks in terms of the stress state.Translated from Problemy Prochnosti, No. 3, pp. 68–73, March, 1991.     

Effect of scandium on the decomposition of aluminium-zinc alloys

The effect of scandium on the hardness, structure and heat evolution of an aluminium-40% zinc alloy was investigated. It was found that contrary to its action in other aluminium-based alloys, scandium does not raise the maximum hardness during an ageing treatment. This may be explained by insufficient scandium redissolution at the rather low temperature necessary for achieving a solid solution of zinc in aluminium. Differential scanning calorimetry studies show a similar heat evolution in both alloys upon linear heating, starting with the reversion of modulated structures. An exothermal effect appears only in the scandium-bearing alloy due to increased heterogeneous precipitation of _r, and on Al₃Sc particles. Above 200 °C, the dissolution of the _r and phases starts, followed by the monotectoid reaction.     

Effect of lithium-lead eutectic and hydrogen on the fatigue behavior of some austenitic alloys

We investigate low-cycle and high-cycle fatigue of 04Kh16N11M3T and 03Kh20N45M4BCh austenitic alloys under rigid loading by pure bending in air, hydrogen, and lithium-lead eutectic at 80, 250, and 350°C. The amplitude of deformation did not exceed ±3%. We discovered a decrease in durability at 250 and 350°C in the presence of lithium-lead eutectic under low-cycle loading and under high-cycle loading in the stage of limited endurance. The greater the amplitude of loading, the sharper the decrease in durability, and this effect is more pronounced at 250°C than at 350°C. Under the action of gaseous hydrogen, the durability of 04Kh16N11M3T alloy increases under low-cycle loading as compared to that in air. In the same situation, the durability of 03Kh20N45M4BCh alloy decreases. Under high-cycle loading, no influence of hydrogen on the limited endurance of these alloys was detected. Both in hydrogen and in lithium-lead eutectic, we observed a decrease in the amplitude of deformation at the fatigue limit on a base of 10⁷ cycles.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 101–106, January – February, 1995.     

Studies on structural transformation and magnetic properties in Sm2Co17 type alloys

Structural transformations and microstructural characterisation of Sm₂Co₁₇ alloys containing Fe, Cu and Zr at different stages of thermal processing have been investigated by X-ray diffraction, optical, scanning electron and transmission electron microscopes. Solution treated samples consist of a mixture of hexagonal TbCu₇ (1:7 H) and rhombohedral Th₂Zn₁₇ (2:17 R) structure types of 2:17 phase. After isothermal aging, TbCu₇ + Th₂Zn₁₇ structures transform into Th₂Zn₁₇ type structure with precipitation of Cu-rich hexagonal SmCo₅ (1:5 H) and Zr-rich platelet phases. In addition to the main phases, a soft magnetic phase of composition Zr₆(FeCo)₂₃ is formed in alloys containing higher Zr composition. Isothermal aging studies reveal that magnetic properties show a peak value when aged at 1108–1123 K for 10 h. TEM studies show cellular precipitate structure with cell interiors having 2:17 R structure, while the fully coherent cell boundaries have the 1:5 H structure. Zr-rich platelets which run across many cells and cell boundaries were found to have 1:7 H structure.     

Effect of structural dispersity on the high-temperature strength of chromium-nickel alloys

The effect of structural dispersity on the high-temperature strength of a KhN45Yu alloy and 12Kh18N10T and 15Kh25T steels was studied in 1000-h tests at 600, 700, and 800°C in air. The material with the coarse-grained structure was shown to posses higher mechanical properties than the material with the fine-grained structure. T. V. Karpenko Physicomechanical Institute, National Academy of Sciences of Ukraine, Lvov, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 68–72, July–August, 1998.     

Effects of strain rate on mechanical properties and failure mechanism of structural Al–Mg alloys

The effects of strain rate on the mechanical properties and failure mechanism of AA5754 and AA5182 sheets were investigated. Tensile tests were conducted at quasi-static (less than 10⁻¹ s⁻¹) and dynamic (600, 1100 and 1500 s⁻¹) strain rates at room and elevated temperature using an INSTRON machine and Tensile Split Hopkinson Bar (TSHB) apparatus, respectively. Shear band decoration, interrupted tensile tests, electron microscopy, and image analysis techniques were also utilized. The results obtained show that the studied alloys exhibit negative strain rate sensitivity at quasi-static rates, but mild positive sensitivity at dynamic rates. Different failure mechanisms were also observed. Strain localization and shear band formation was found to be a necessary pre-requisite for the development of damage and final failure under quasi-static conditions. In the dynamic strain rate regime however, less shear banding was observed. Void nucleation, growth and coalescence that is characteristic of dynamic tensile fracture appears to be the dominant mode for failure under dynamic conditions.