Materials Science Forum Vols. 558-559

Abstract: 3N nickel has been deformed by equal channel angular pressing (ECAP) at 400°C up to 3 passes using route A. The texture with respect to position in the deformed billet, i.e. from top to bottom, has been measured with high-energy synchrotron radiation. It is characterized by texture components typical for simple shear in the intersection plane of the square-shaped 90° bent channel. Besides, an oblique cube component is observed. Orientation imaging microscopy clearly shows that this component is due to partial recrystallization. Intensities of the texture components as well as deviations from their ideal shear positions vary from the top to the bottom of the billet and with the number of passes. The change of the intensity of texture components and the texture gradient investigated is discussed. Special emphasis is put on the influence of dynamic recrystallization on texture and microstructure formation during ECAP of fcc metals.

Abstract: Methods of complex X-ray and metallographic analyses have been used to investigate the influence of texture, thin structure and grain boundaries of coiled low-alloyed steel for gas-and-oil pipeline tubes after hot rolling on its mechanical properties. Finish rolling temperature changes in a wide range (ТFR = 8900С-5300С). Specifics of steel structure formation under conditions of dynamic (controlled rolling) and static (high-and-low temperature rolling) recrystallization have been found.

Abstract: An assessment of the inhomogeneity of microstructure generated within plane strain compression test specimens has been performed using the nickel based superalloy, Waspaloy. Two variables were investigated: the effect of strain rate and the effect of friction at the tool/specimen interface. Tests were performed at 1040°C at nominal strain rates of 0.01 and 1 s-1 with and without a glass based lubricant. At the low strain rate the microstructure was relatively homogeneous regardless of the friction conditions. At the high strain rate there was significant microstructure variation from surface to mid plane which was further exaggerated by increased friction. Quantification of the inhomogeneity, however, is non-trivial in this alloy due to the complicated recrystallisation behaviour it exhibits and difficulty in differentiating between recrystallised and non-recrystallised grains.

Abstract: During large strain deformation of materials, the high angle grain boundary spacing approaches the order of mean thermal diffusion distances for given deformation conditions. Based on the results of microstructural and grain size analysis in low carbon steel subjected to large strain-high Z deformation, the evolved ferrite grain size was found to be controlled by the Zener-Hollomon parameter and grain boundary diffusion was found to be the controlling mechanism.

Abstract: The warm-compression microstructures of two nickel-containing tempered martensite steels with and without carbon were investigated. The EBSD measurement and TEM observation/Kikuchi pattern analysis confirm that the ultrafine microstructures contain ferriteaustenite dual phases with more than 18% austenite. The difference in the austenite amount before and after warm compression proves that the dynamic austenite precipitation occurs during warm deformation and the carbon addition is beneficial to the austenite precipitation. It is also found that the ultrafine austenite grains as a hard phase promote the dynamic recrystallization of ferrite during warm deformation.

Abstract: Effect of austenite as a harder second phase on ferrite substructure evolution by intercritical rolling has been investigated using 0.12C-0.3Si-1.35Mn steel. Slab was reheated at 780°C (ferrite; α + austenite; γ phase region) and rolled with 90% reduction down to 12mm in thickness using laboratory mill. 700°C (ferrite; α + cementite; θ phase region) rolled plate was also prepared to compare with 780°C rolled plate. Microstructure distribution along the plate thickness has been observed in detail by SEM and EBSD. Microstructure showed mixed structure of fine-grained ferrite and elongated ferrite for both of the plates. The ratio of fine-grained ferrite region was around 50% at the plate surface, however, fine-grained ferrite formation hardly occurred at mid thickness for 700°C rolled plate. FEM analysis revealed that fine-grained ferrite distribution along the thickness can be well explained by equivalent plastic strain distribution along the plate thickness for 700°C rolled plate. Equivalent plastic strain showed maximum value near the plate surface due to shear strain component, and it could help substructure evolution and fine grained ferrite formation. On the other hand, the ratio of fine-grained ferrite region reached to 50% all through the thickness for 780°C rolled plate. Austenite as a harder second phase seems to promote ferrite substructure evolution even at mid thickness of the plate.

Abstract: Commercial low carbon steel AISI 1010 was subjected to Equal Angular Channel Pressing (ECAP) at different temperatures. The paper describes the refinement of the coarse grained ferrite microstructure to submicrocrystalline range by large plastic strain. The steel was deformed in an ECAP tool with a channel angle φ = 90°, at different temperature in the ranging between 150 – 300°C. The number of passes at each temperature was N = 3. Optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to study the formation of substructure and ultrafine grains in the deformed specimens. The TEM study reveals that at the lowest ECAP temperature of 150°C extensively elongated ferrite grains with dense dislocation network dominate in the structure. The randomly scattered polygonized subgrains have been observed. The activation of dynamic recovery process, even at the lowest temperature of equal channel pressing, contributed to the formation of individual polygonized grains. As the temperature of ECAP processing was increased the process of dynamic polygonization and recrystallization occurred more effectively and the submicrocrystalline structure was formed by sectioning of elongated ferrite grains. The formation of such predominant submicrocrystalline structure resulted in strength increase of the low carbon steel.

Abstract: Dynamic recrystallization(DRX) of ferrite in low carbon steels was investigated by hot compression tests at temperatures of 550 to 700oC at strain rates of 0.001 to 10s-1. The results indicate that DRX of ferrite can occur in low carbon steels and lead to grain refinement. With increasing Zener-Hollomon parameter Z, its mechanism changes from discontinuous dynamic recrystallization to continuous dynamic recrystallization, the turning point is approximately at Z=1×1016s-1 for a low carbon steel with 0.171wt% C. The results also indicate that changing the minor constituents of the low carbon steel from pearlite colonies to fine cementite particles has an effect on promoting DRX of ferrite, and the increase of Mn content and the presence of tiny Nb precipitates have opposite effects respectively. However, all these changes are of benefit to the refinement of recrystallized grains.

Abstract: The multiplicity and variety of grain shapes in three-dimensional polycrystalline metals makes their energetic and kinetic analyses difficult. To help simplify the analysis of isotropic polycrystals, average N-hedra (ANHs) (N=3,4,5,…∞) were created as a set of regular polyhedra, consisting of N identical faces, which act as topological “proxies” for analyzing the corresponding class of irregular grains containing mixed faces of the same number. This paper outlines a further generalization of the ANH concept that extends three-dimensional analysis to include the growth or shrinkage of a small population of grains embedded in a textured matrix.