Materials Science Forum Vol. 753

Abstract: In this study, the constitutive flow behavior and static recrystallization characteristics of a Nb-microalloyed TWIP (Fe-20Mn-1.5Al-0.1Nb) steel under hot deformation conditions have been determined and results compared with those of Fe-25Mn-Al TWIP steels. Investigations using compression testing in a Gleeble simulator, including the double-hit technique, enabled the acquisition of flow stress and recrystallization data. These were analyzed to determine the powers of strain and strain rate as well as the activation energies of deformation and recrystallization (Qdef and Qrex). For given deformation and grain size parameters, the time for 50% recrystallization (t50) of the 0.1% Nb TWIP steel was significantly longer than for the Nb-free TWIP steel: it was comparable to that of Nb-microalloyed carbon steel. Qrex was higher than that of Type 304 stainless steel that has nevertheless much longer t50 times.

Abstract: Less attention has been paid to study the recrystallization and grain growth behavior of severe plastically deformed (SPD) metals specially steels that are deformed to very high strain by conventional rolling method. Present work has been focused on systematic investigation of recrystallization and grain growth behavior of a Aluminium Killed (AK), an Interstitial Free (IF) and an Interstitial Free High Strength (IFHS) steels that were subjected to very high levels of strain (ԑeqv= 4.51) by cold rolling. The cold rolled steels show fine lamellar structure with very strong texture consists of both γ and α fibre. All the steels show formation of ultrafine grains and dramatic rise in the intensity of α fibre component in the early stages of annealing. However, progress of annealing for longer time leads to an increase in the mean grain size as well as γ fibre intensity. The results also indicate that the heavily cold rolled material exhibit selective growth of specific texture components.It appears that microstructure and texture is closely related to the observed phenomenon.

Abstract: The effect of micro-alloying elements of niobium and boron and titanium on recrystallization behavior is researched after the cold rolled IF steels are annealed at high temperature. The results show that there is high density dislocation in the cold rolled steel and the microstructure is fibrous tissue. The recrystallization grains appear when the cold steel annealed at 655 °C and then the grains grow up with the annealing temperature increased to 840 °C. The recrystallization temperature and time of B-Ti-IF steel is lower than that of Nb-Ti-IF and Ti-IF steels. The recrystallization activation energy of Nb-Ti-IF steel is 181.7KJ/mol which is 56.6KJ/mol larger than that of B-Ti-IF steel.

Abstract: Texture evolution during static primary recrystallization of an austenitic Fe-28Mn-0.28C TWIP steel was analyzed. The cold-rolled material, which showed a Brass-type texture at medium (30% and 50%), and additionally a γ-fiber at high (80%) deformation degrees, was subjected to isothermal annealing at 700°C. The influence of rolling degree/starting texture on the development of particular texture components was studied. After recrystallization a weakened, retained rolling texture was observed for the examined reduction levels. In addition to the deformation components, Brass and Goss, further α-fiber components were formed mainly by annealing twinning leading to the development of this fiber.

Abstract: The cold rolled band of the niobium stabilized type ASTM 430 ferritic stainless steel with 85 % thickness reduction was annealed with heating rates of 0.10, 6.8, 23.5 and 41.5 °C/s and a soaking time of 24 s. The changes in microstructure and texture were followed by interruptions in the annealing cycle at temperatures of 780, 830 and 880 °C. Annealing at the lower heating rate was more effective for the development of γ-fiber than the annealing performed with high heating rate. The increased rate of heating provided an increase in the onset recrystallization temperature, a reduction in average grain diameter and a more homogeneous distribution throughout the thickness. The specimens with higher volume fraction of the γ-fiber annealed with low heating rate showed a high average coefficient of anisotropy R =1.99.

Abstract: In situ orientation mapping using TEM and calorimetric measurements were carried out to investigate the annealing behavior of cold-rolled 6013 aluminum alloy. The recrystallization of the material can be considered to be a number of processes that correspond to two separate stored energy release peaks. In the temperature range of the peak 1, the deformation zones around the large second-phase particles acted as sites for particle-stimulated nucleation. In the matrix, at the same time, some elongation of grains occurred. The elongated matrix grains appeared because of the reduction of the dislocation density and the annihilation of some low-angle grain boundaries between chains of subgrains lying in layers parallel to the sheet plane. The matrix processes in this temperatures range can be considered forms of continuous recrystallization. The matrix high-angle grain boundaries started to migrate at the temperature range of the peak 2. They moved mostly in the direction normal to the sheet plane. Heating of the sample for an appropriate time at those temperatures resulted in the complete discontinuous recrystallization of the material. The recrystallized microstructure was dominated now by elongated grains, which were a few times thicker than those obtained by annealing at the temperatures of the peak 1.

Abstract: Single crystals of Al-0.1%Mn have been channel-die compressed to a true strain of 2.3 and their recovery behaviour at 240-320°C investigated by microhardness measurements, EBSD microtexture mapping and X-ray line broadening analysis. The crystal orientations are the nominally stable Goss {110}, brass {110} and S {123}. For all three orientations the microhardness decreases with a logarithmic time dependency but the instantaneous recovery rates of the Brass oriented crystals are systematically lower than those of the other two orientations by a factor of about 2. The dislocation densities decrease rapidly in the first stages of recovery (<1 min) by dislocation dipole annihilation and more slowly thereafter. In the Goss and S orientations the later stage of recovery is due to sub-grain growth. The orientation dependency is ascribed to the relatively low misorientations developed by plastic straining in the Brass crystals (average about 4°) compared with the Goss and S orientations (about 7-8°).

Abstract: In the present work an Al-Mn-(Fe-Si) model alloy has been subjected to different homogenization treatments, to achieve materials with different microchemistry states in terms of constituents, levels of Mn in solid solution (potential for concurrent precipitation) and dispersoid densities, followed by cold rolling and back-annealing. Characterization of the microchemistry state after homogenization and the evolution in dispersoid precipitation and its effects on the softening behavior after deformation has been performed. It is demonstrated that variations in microchemistry may have dramatic effects on the softening kinetics and the final grain structures, where both pre-existing fine and dense dispersoids before back annealing as well as precipitation concurrent with recovery and recrystallization strongly retard kinetics and generally lead to a coarse grain structure, while conditions with no or limited concurrent precipitation softens much faster and generally results in an even, fine and equi-axed grain structure. The different softening behaviors have been discussed in terms of Zener drag effects derived from the dispersoid evolutions.

Abstract: Motivated by improving current softening models for recycle friendly alloys, softening was investigated in high purity and commercial purity aluminium alloys. Utilizing the electron backscatter diffraction (EBSD) technique, orientation dependent sub-grain growth was characterized with respect to grain size and average boundary misorientation. In the high purity alloys, small additions of Mn in solid solution slowed down the recovery kinetics. The recovery mechanisms were however not altered, but recovery kinetics were found to be orientation dependent. The presence of high angle grain boundaries or transition bands, i.e. large and sharp orientation gradients, seemed to change the growth from slow and continuous to a faster and discontinuous process. This was typical for Cube and Goss, while weak, short and long range orientations gradients observed in Copper, S and Brass, did not alter growth which was slow and continuous. Before detailed studies of recovery of the commercial purity alloy were initiated, a rather slow recovery was observed and further investigated. Preliminary results indicate that iron in solid solution is dramatically slowing down the kinetics but can form clusters by an intermediate annealing in order to speed up recovery.

Abstract: The objective of this study was to determine the effect of ECAP-processing on the texture and the microstructure development of the deformed and recrystallized material. The commercial purity AA4343 aluminium alloy, containing very large second phase particles (SPP) of pure silicon, was deformed via route C up to 10 passes and then annealed. The samples were characterized by TEM and SEM equipped with high resolution EBSD. It was found that the microstructure of the deformed state in areas far from coarse SPP was very similar to that observed in pure aluminium. The orientations of the new grains were scattered but not random and only specific groups of orientations were observed.