Materials Science Forum Vols. 558-559

Abstract: Copper, Cu-2%Sn and Cu-4.5%Sn alloys have been deformed in plane strain compression at temperatures up to 700oC and the evolution of the microstructures and textures determined by high resolution EBSD. The effect of the solute is to raise the temperature at which dynamic recrystallization occurs and to significantly reduce the size of the dynamically recrystallized grains. In all the materials, there is a small increase in the cube texture component on dynamic recrystallization. The boundary bulges which precede recrystallization are different in the copper and Cu-Sn alloys, although in both materials there is evidence that local deformation in the boundary regions plays a significant role in dynamic recrystallization.

Abstract: Orientation-controlled copper bicrystals containing symmetrical 70o [0 0 1] tilt boundaries were deformed in tension at 923 K and at three initial strain rates from 4.2 x 10-5s-1 to 4.2 x 10-3s-1. The load was applied parallel to the grain boundary so as to eliminate grain boundary sliding. The nucleation of dynamic recrystallization (DRX) was investigated using optical microscopy and orientation imaging microscopy methods. After grain-boundary migration (GBM) and bulging, nuclei appeared behind the most deeply bulged grain boundary regions. The critical strain for nucleation was less than one-half of the peak strain and largely independent of the strain rate. At a fixed strain, nucleation is more frequent and the grain size finer as the strain rate is increased. All the nuclei were twin-related (Σ3) to the matrices. Furthermore, most of the twinning plane traces were parallel to the inactive slip traces of the bicrystals. This indicates that twin variant selection is essentially unaffected by dislocation motion. The observed mechanism of nucleation of DRX is discussed in relation to the occurrence of GBM and twinning.

Abstract: Intermetallic titanium aluminide alloys are multiphase assemblies with complex microstructure and constitution, involving the phases γ(TiAl), α2(Ti3Al), β, and B2. The earlier stages of phase transformation and dynamic recrystallization occurring upon hot-working of such an alloy were investigated at the atomic scale by high-resolution electron microscopy. Accordingly, the conversion of the microstructure is triggered by heterogeneities in the deformation state and non-equilibrium phase composition. The β/B2 phase is apparently unstable under tetragonal distortion, which gives rise to the formation of the B19 phase via distinct shuffle displacements. These processes lead to a modulated microstructure, which is comprised of several stable and metastable phases. The phase transformations are accomplished by the propagation and coalescence of ledges. Large and broad ledges can apparently easily be rearranged into intermediate metastable structures, which serve as precursor for the nucleation of new grains.

Abstract: Strain induced transformation (SIT) of austenite into ferrite has been frequently used as a powerful ferrite grain refinement mechanism. Ordinarily ferrite grain sizes of the order of 1-3μm are achieved via mechanical testing such as compression and torsion. Nonetheless, most of the work done so far employed continuous deformation in the range of 0.8 for compression experiments and in excess of this for torsion. SIT is a promising technique which may be used during actual hot rolling processing. However, in this case, not only deformations are applied with time interrupts between them but also the amount of total deformation allowable is relatively low, in order to attend to flatness and final gauges requirements. This work explores the consequences on SIT microstructure of deformation given in multiple passes as opposite to the usual continuous deformation presented in the literature. Multiple pass deformation at high temperature led to partial dynamic recrystallization and to a mixture of coarse and fine ferrite grains. Multiple pass deformation at the vicinity of Ar3 produced, on the hand, finer ferrite grains indicating that SIT took place. In this case, ferrite grains in the range of 1-3μm were produced and a much more homogeneous distribution of these grains was present.

Abstract: At the prospect of a lightening of the automobile structures, welded spots have been realized on a stacking of two sheets (a 6008 aluminium alloy on steel) by Friction Stir Welding (FSW). Different process parameters have been tested but only the influence of the dwell time will be described in the present paper. The dwell time corresponds to the time during which the probe stays in rotation at its bottom location before extracting. A study of the microstructures and the crystallographic textures associated to mechanical tests (shear and tensile tests) allowed to determine the best set of welding parameters. The recrystallized area around the welding spot has been characterized by Electron BackScattered Diffraction (EBSD). The aim of the present work is to identify the recrystallization mechanisms which occur during welding, and to understand the influence of the dwell time on the recrystallized area. A mechanism of continuous dynamic recrystallization has been identified since misorientation of sub-boundary increases close to the weld and this for all the dwell times tested. Elsewhere, it has been found that the increase of the dwell time induces a larger recrystallized zone.

Abstract: Double-hit compression tests were carried out at different temperatures and strain rates for a nickel based alloy and a stainless steel. Using microhardness measurements the retained strains after the first and second pass were investigated as a function of the amount of deformation, temperatures as well as strain rates and dwell durations. In general, the retained strain decreases with increasing dwell durations. It is shown that at a given total amount of deformation, the retained strain is reduced for the as deformed grains that have not been recrystallized yet, but increased for the recrystallized grains, when comparing double hit with single hit compression tests.

Abstract: Effect of deformation temperature on subsequent recrystallization characteristics of commercial purity titanium (CP-Ti) was investigated using EBSD and transmission electron microscopy. CP-Ti plates were rolled to at various temperatures between 25°C and 600°C to achieve 60% reduction, and were subsequently annealed at 600°C for various time periods. Increase in rolling temperature resulted in more refined microstructures, which could be attributed to coactivation of secondary slip systems such as basal slip or pyramidal slip in addition to prism slip. During subsequent annealing, a large number of recrystallization nuclei formed in the warm-worked (below 450°C) CP-Ti while nucleation of recrystallization was slower for coldrolled one. This led to the faster recrystallization kinetics and a finer microstructure in asrecrystallized state for warm-worked CP-Ti. Meanwhile, the rate of recrystallization in CP-Ti rolled at 600°C was significantly retarded because a good portion of strain energy stored in deformed matrix was released by dynamic recovery. In the present work, we show that the widely accepted explanation for the influence of the deformation temperature on the recrystallization characteristics, i.e., increase of the deformation temperature retards the recrystallization process, is not always true for the hcp metals since deformation at elevated temperature activates secondary slip systems, providing more sites for nucleation of recrystallization.

Abstract: Warm deformation is one of the promising hot rolling strategies for producing thin hot rolled steel strips. A better understanding of the microstructure evolution during warm deformation is important for a successful introduction of such processing into the industrial production. In the present research, the effect of deformation strain on the ferrite microstructure development in a low carbon Ti-microalloyed steel was investigated through warm torsion testing. Microstructural analysis with optical microscope and electron back-scattering diffraction was carried out on the warm deformed ferrite microstructures. The results show that at the early stage of deformation an unstable subboundaries network forms and low angle boundaries are introduced in the original grains. Then, with further straining, low angle boundaries transform into high angle boundaries and stable fine equiaxed ferrite grains form. It was considered that dynamic softening and dynamically formation of new fine ferrite grains, with high angle boundaries, were caused by continuous dynamic recrystallization of ferrite.

Abstract: The microstructure evolution and mechanical behavior during large strain of a 0.16%CMn steel has been investigated by warm torsion tests. These experiments were carried out at 685 °C at equivalent strain rate of 0.1 s-1. The initial microstructure composed of a martensite matrix with uniformly dispersed fine cementite particles was attained by quenching and tempering. The microstructure evolution during tempering and straining was performed through interrupted tests. As the material was reheated to testing temperature, well-defined cell structure was created and subgrains within lath martensite were observed by TEM; strong recovery took place, decreasing the dislocation density. After 1 hour at the test temperature and without straining, EBSD technique showed the formation of new grains. The flow stress curves measured had a peculiar shape: rapid work hardening to a hump, followed by an extensive flow-softening region. 65% of the boundaries observed in the sample strained to ε = 1.0 were high angle grain boundaries. After straining to ε = 5.0, average ferrite grain size close to 1.5 1m was found, suggesting that dynamic recrystallization took place. Also, two sets of cementite particles were observed: large particles aligned with straining direction and smaller particles more uniformly dispersed. The fragmentation or grain subdivision that occurred during reheating and tempering time was essential for the formation of ultrafine grained microstructure.

Abstract: Previous research works assert that the observed increase in hot flow stress of commercially pure copper is attributed to the interactions between solute atoms and dislocations, specifically by interstitial oxygen. This work shows TEM images of the formation of Cu2O precipitates after warm working temperatures that in part help explain the increase of stress during hot compression of 99.9% pure copper. Three commercially pure large-grained coppers with 26, 46 and 62ppm of oxygen were tested at different temperatures (600°C-950°C) and strain rates (0.3s-1- 0.001s-1). At temperatures below 850°C, the stress differences between coppers, tested at same the strain rate, became increasingly higher. A correlation between stress increase and oxygen content was found. Precipitation of nanometric Cu2O did not show any difference in dynamically recrystallized grain size; however hardness tests showed that the final properties were modified. This work discusses the effect precipitation of Cu2O has on the hot flow curve and the final microstructure of hot formed 99.9% pure copper with different oxygen levels.