Fundamentals of Deformation and Annealing

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Authors: Petra Backx, Roumen H. Petrov, Leo Kestens
Abstract: This research looks at the influence of annealing treatments on twins in magnesium alloy AZ31. Samples containing different twin fractions are created in order to examine the influence of the initial structure on the annealing behavior. It is seen that twins can both expand, taking over an entire grain, or shrink, leaving completely untwinned grains. The grains, containing expanding twins, are found to contain, in general, only one twin variant and limited slip deformation after twin formation. The grains containing shrinking twins, on the contrary, are seen to contain different twin variants and have undergone more slip deformation after twinning.
375
Authors: B. Decreus, Hatem S. Zurob, John Dunlop, Yves Bréchet
Abstract: The effect of low temperature recovery treatments on the recrystallization kinetics during subsequent high temperature annealing was investigated in three Al-2.5%Mg alloys with various Fe additions. Recovery treatments were carried out at 190oC for times ranging from 0.25 to 65 hrs. Recrystallization treatments were carried out at 280oC. The kinetics of recrystallization was followed using the techniques of hardness measurement, optical metallography and calorimetry.
381
Authors: Pavel Lejček
Abstract: Anisotropy of grain boundary motion in a Fe–6at.%Si alloy is represented by a spectrum of values of the activation enthalpy of migration and the pre-exponential factor, depending on the orientation of individual grain boundaries. The general plot of these values exhibits a pronounced linear interdependence called the compensation effect. It is shown that changes of these values, caused by changes of intensive variables, are thermodynamically consistent.
387
Authors: D.T. McDonald, John F. Humphreys, Pete S. Bate
Abstract: The microstructure and texture in dynamically recrystallized copper and two copper – tin alloys (2wt% and 4.5wt% tin) has been investigated. Specimens were deformed in channel-die plane strain compression to true strains from 0.1 to 1.22 within the temperature range 200°C to 700°C, and the resulting microstructures were investigated with the use of high resolution electron backscatter diffraction (EBSD). Dynamic recrystallization was initiated by the bulging of preexisting high angle grain boundaries (HAGB), and occurred primarily by strain induced boundary migration (SIBM) and twinning. The addition of tin led to an increase in the temperature at which dynamic recrystallization initiated, and furthermore to a smaller dynamically recrystallized grain size. This was attributed to the effects of solute drag causing lower HAGB mobility. Dynamic recrystallization was observed to weaken the deformation texture components of brass and Goss, as well as introduce a cube texture component which generally tended to strengthen with temperature but weaken with increasing tin additions.
393
Authors: Jae Hyung Cho, Anthony D. Rollett, Kyu Hwan Oh
Abstract: Copper and gold bonding wires were characterized and compared using electron backscatter diffraction (EBSD). During drawing, <111> and <100> fiber textures are the main components in the wires and shear components are mainly located under the surface. Grain average misorientation (GAM) and scalar orientation spread (SOS) of the <100> component in copper and gold bonding wires are lower than those of the <111> or other orientations. The bonding wires experience three stages of microstructural changes during annealing. The first stage is subgrain growth to keep elongated grain shapes overall and to be varied in aspect ratio with annealing time. The grain sizes of the <111> and <100> components increase during annealing. The volume fraction of the <100> component increases whereas that of the <111> decreases. The second stage is recrystallization, during which equiaxed grains appear and coexist with elongated ones. The third stage is grain growth which eliminates the elongated grains and enlarges equiaxed grains. The <111> and <100> grains compete with each other and the <111> grains grow faster than the <100> grains during the third stage.
399
Authors: J.I. Chaves, S.F. Medina, Manuel Gómez, L. Rancel, Pilar Valles
Abstract: In this work the pinning forces exerted by TiN particles in the austenitic phase in two Ti microalloyed steels have been determined and compared with the driving forces for austenite grain growth and for static recrystallisation between hot rolling passes, respectively. TiN precipitate sizes were measured by transmission electron microscopy (TEM) and the precipitated volumes were calculated. These results were then used to calculate pinning forces. The driving forces for recrystallisation were found to be approximately two orders of magnitude higher than the pinning forces, which explains why the austenite in these steels barely experiences hardening during rolling and why the accumulated stress prior to the austenite→ferrite transformation is insufficient (low dislocation density) to refine the ferritic grain.
405
Authors: Patricia Gobernado, Leo Kestens
Abstract: The (relative) grain boundary energy of random high angle boundaries has been measured in several Fe-based polycrystals. Crystallographic data obtained by orientation contrast microscopy (OIM) are combined with the geometrical configuration of grain boundaries at triple junctions. A two-parameter representation of the relative grain boundary energy in terms of misorientation angle and misorientation axis is presented. In the applied procedure a variation of the energy values assigned to one boundary was observed depending on the triangulation path chosen by the operator to connect the arbitrary initial boundary with the boundary under consideration. Results show no evidence of correspondence between the observed energy cusps and the presence of CSL boundaries.
411
Authors: Manuel Gómez, S.F. Medina, J.I. Chaves
Abstract: The austenite static recrystallization kinetics at several temperatures and the recrystallization-precipitation-time- temperature (RPTT) diagrams of a medium-carbon vanadium microalloyed steel have been determined for a strain ε = 0.35. Unlike many other studies carried out previously on V microalloyed steels, the recrystallized fraction against time curves showed the formation of a double plateau that indicates two stages of inhibition of recrystallization due to the formation of different types of strain induced precipitates. This work makes use of transmission electron microscopy to study the nature and size distribution of these precipitates capable of inhibiting recrystallization. The values of driving and pinning forces for static recrystallization are calculated and an analysis of the relationship between the net balance of these forces, the precipitation state and the progress or inhibition of the recrystallization is accomplished. A value of driving force that decreases as recrystallized fraction grows during isothermal holding time is estimated and helps to interpret the behavior of austenite after deformation.
417
Authors: C. Herrera, R.L. Plaut, Angelo Fernando Padilha
Abstract: The phenomena of strain hardening, strain induced martensite formation, recovery, martensite reversion and recrystallization have been studied in austenitic stainless steels of the AISI 304L and 316L types, after solution annealing, followed by rolling at different temperatures (-196, 25, 100 and 200°C) and subsequent annealing of the worked samples. Strain hardening and the percentage of α’ martensite formed showed strong dependency with the deformation temperature and with the austenite chemical composition. As expected, both strain hardening as well as the amount of the martensite formed was higher in the 304L steel and for lower temperatures. Reversion temperature of the α’ martensite was close to 550°C for both steels, independent of the amount of martensite. The 316L steel presented a higher resistance to recrystallization when compared to the 304L steel. The recrystallization temperature of both steels was about 150°C higher than the α’ phase reversion temperature. Rolling temperature did not influence significantly the recrystallization temperature. Proper thermal and mechanical treatments lead to interesting combinations of mechanical properties in both steels with values such as yield strength YS of about 1000 MPa, with an elongation around 10%.
423
Authors: C. Iparraguirre, Ana Isabel Fernández-Calvo, Beatriz López
Abstract: The influence of initial grain size on the softening-precipitation interaction in a low niobium microalloyed steel has been investigated. The study has revealed that for the largest initial grain size (1000 μm), the recrystallised fraction remains lower than the softening fraction until relatively long times are reached. In contrast, for the smallest initial grain size (166 μm) both magnitudes are similar. As a result, precipitation interacts with recrystallisation in the case of the finest austenite grain size, whereas for the coarsest one, since recrystallisation is significantly retarded, interaction with recovery process is observed. Apparently, the initial austenite grain size does not affect precipitation kinetics.
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