Materials Science Forum Vols. 715-716

Abstract: A 0.02%C plain carbon and a 0.22%C TRIP steel were tested in compression in the temperature range 900°C to 1150°C and strain rate range 0.05s^-1 to 1s^-1. Thirty-two experimental flow curves were obtained in this way. The critical conditions for the initiation of dynamic recrystallization were determined by the double differentiation method. Using a dislocation density model to describe the austenite flow stress, the work hardening parameters r and h were derived and are used to model the flow curve in the absence of dynamic recrystallization. The latter was employed to calculate the fractional softening attributable to dynamic recrystallization. The kinetics of dynamic recrystallization are then described using Avrami kinetics. Finally, the dependences of the Avrami and work hardening parameters on Z, the Zener-Hollomon parameter, are used to model compression flow curves at strain rates an order of magnitude greater than the ones employed in the tests.

Abstract: The sheet manufacturing process, which involves various solid-state transformations such as phase transformations, plastic deformation and thermally activated recovery processes, determines the texture of steel and aluminium sheet. The conventional process of flat rolling and annealing only offers limited degrees of freedom to modify the texture of the final product. After annealing a {111} recrystallization fibre in BCC alloys and a cube dominated recrystallization texture in FCC metals is commonly obtained. Many applications, however, require other texture components than the ones achievable by conventional processing. In the present paper it is shown that by asymmetric rolling of a Si-alloyed ultra-low carbon steel a texture can be obtained with increased intensity on the {001} fibre, which is of interest for magnetic applications. Also in aluminium alloys the strong cube annealing texture can be drastically modified by the process of asymmetric rolling. It is argued that by observing the proper rolling and annealing conditions a recrystallization texture with improved normal and planar anisotropy of the mechanical properties may be produced.

Abstract: Recrystallization nucleates at heterogeneities. The impact of this on local texture and stress-strain response in hot worked magnesium is considered in the present paper. Two aspects of bulge nucleation during dynamic recrystallization are considered.

Abstract: Well collimated, high energy X-rays of 90 keV from synchrotron sources have been used to study metals undergoing plastic deformation in-situ, in real time and in the bulk of the materials. The spottiness of poorly illuminated Debye-Scherrer rings showing reflections from individual crystallites is analyzed to obtain grain statistics, mosaic spread and orientation. Upon cold deformation, coarse grained materials show fingerprints of sub-grain formation, grain rotation, grain refinement and the evolution from a single grain into the asymptotic texture. Heating of metals under continuous load drives the observation through the regimes of phase transformation and grain relationships therein, grain coarsening, dynamic recovery and dynamic recrystallization. The paper points out these different phenomena which were observed without precedence.

Abstract: Grain boundary engineering (GBE) to promote degradation-resistant interfaces in the microstructure usually requires that the grain size remains small so that strength is not compromised. Aspects of grain size measurement and control will be reviewed and discussed for a variety of GBE materials such as copper, nickel, nickel-based alloys and austenitic stainless steels, particularly in the light of the high proportion of annealing twins that constitute the GBE microstructure.

Abstract: According to a sub-boundary enhanced solid-state wetting mechanism, abnormally-growing Goss grains in Fe-3%Si steel are expected to have exclusively sub-boundaries which would be formed during primary recrystallization. This microstructural feature is related with the stored energy of Goss grains after cold rolling. To investigate the possibility of existence of sub-boundary exclusively in Goss grains after primary recrystallization, the deformation feature focused on stored energy of Goss and other specific orientations was analyzed by crystal plasticity finite element method calculations. The calculations indicated that Goss grains stored the lowest energy among the orientations formed after plane strain deformation.

Abstract: Dynamic recrystallization was studied for the stainless steels with nitrogen contents of 0.56% to 1.08% during hot deformation at temperatures of 900~1200 with strain rates ranging from 0.003 to 42 s^-1. It was found that flow stress could increase remarkably with increasing nitrogen content. Flow curves during the deformation by 0.1~42/s at temperatures of 900~1200°C show a single peak, indicating the occurrence of dynamic recrystallization during deformation. The peak strain seems to decrease with increasing N content, suggesting that higher content of N facilitates dynamic recrystallization. The quenched microstructures were analyzed by optical microscopy, EBSD and TEM. The recrystallized grain sizes on the quenched specimens were measured and its dependence on temperature and strain rate was analyzed. At high temperature, continuously dynamically recrystallized microstructures were observed; whilst at low temperature, necklace-like partially recrystallized microstructures were found. Key words: High nitrogen stainless steel; dynamic recrystallization; stress-strain curves

Abstract: On the basis of Hillerts model of grain growth, a new model of Goss secondary recrystallization in silicon steel has been developed in which inhibitor and grain boundary energy are taken into account. An analysis shows that these two parameters synergistically affect secondary recrystallization and Goss grain evolves to a coarse grain as inhibitor intensity increases and statistical grain boundary energy decreases. This model successfully explains Goss secondary recrystallization.

Abstract: The solid phase transformation of a metastable phase into a stable phase needs the activation energy. The energy is usually supplied in the form of thermal energy. When the nucleation takes place, the strain energy may develop in the metastable matrix and the stable nucleus. The strain energy can result from differences in density of the nucleus and matrix and the lattice mismatch between the nucleus and matrix. The stable-metastable interface region has the highest strain-energy density in the maximum Youngs modulus direction of the stable phase. Accordingly, the growth rate of the stable phase is the highest in its highest Youngs modulus directions. As the transformation temperature decreases, the strain energy contribution increases and the growth rate anisotropy is likely to increase. When austenite transforms into ferrite at low temperatures, the directed growth of ferrite is observed as forms of Widmanstätten ferrite plates and acicular ferrite plates. The maximum growth direction of ferrite is along the maximum Youngs modulus direction of ferrite, <111>_α, and the broad interfaces are parallel to the maximum growth direction and formed so that they minimizes the shear strain energy in the interface layer. The directed growth results in the Kurdjumov-Sachs orientation relationship between austenite and ferrite, <111>_α//<110>_γ and {110} _α //{111}_γ.

Abstract: In the present work, the oriented nucleation origin of the recrystallized {h11}<1/h,1,2> fibre is characterized. Aiming to investigate the substructural evolution of <110>//RD fibre grains and {001}<110> grains in particular, a detailed microstructure and texture analysis is performed by high resolution orientation scanning microscopy on a cross-rolled sample. The reason to work with cross-rolled material is the increased incidence of rotated cube orientations after cross rolling. The present data have revealed the presence in the deformed substructure of a crystallite volume that has rotated towards the {311}<136> component in the interior of <110>//RD fibre grains as a result of a grain fragmentation process. Preliminary simulations of the deformation texture suggested that the observed orientation fragmentation might be produced by strain localizations of a shear band nature.