Transformation Of Low-angle Boundaries Research Articles

Substructure and texture evolution of a novel near-α titanium alloy with bimodal microstructure during hot compression in α+β phase region

Ti65 alloy has extensive potential applications in manufacturing high-temperature components in the aerospace industry. During thermomechanical processing in the α+β phase region, complicated microstructure and texture evolution occurs. In this work, isothermal hot compression experiments were conducted to comprehensively investigate the substructure and texture evolution of Ti65 alloy with bimodal microstructure via the electron backscatter diffraction (EBSD) technique, and the influence of processing parameters on the evolution of substructure and texture was systematically analyzed. The experimental results showed that the distribution of grain boundaries and the transformation of low-angle boundaries (LABs) to high-angle boundaries (HABs) exhibited sensitivity to temperature and strain rate. The fraction of sub-grain boundaries decreased, while the fraction of HABs increased with the increasing of strain rate or temperature due to the spheroidization of the lamellar α phase accompanied by the wedging of the β phase during deformation and the relative misorientation angle distribution between different α variants precipitated from β phase. Temperature rising at a high strain rate resulting from adiabatic heat promoted atomic activity, caused dislocations to be absorbed by sub-grain boundaries or HABs, and finally reduced the dislocation density in both αp and αs phases. Dynamic spheroidization was the main mechanism with increasing temperature, and coarsening of the lamellar α phase was the main mechanism with increasing strain rate. With the increase of temperature or decrease of strain rate, the texture of the αp phase became stronger and the basal and pyramidal slip could be activated. For the αs phase, spheroidization and variant selection together influence the orientation distribution. As a result, the texture evolution showed a complicated tendency.

Deformation and fracture behavior of the novel NiAl alloy sheet with bimodal laminated structure by in-situ reaction synthesis

Due to the intrinsic brittleness of NiAl alloy, its sheets are difficult to fabricate with traditional casting and rolling methods. To solve this problem, a novel NiAl alloy sheet with a special bimodal laminated structure was successfully designed and fabricated via the in-situ solid-state reaction synthesis using pure Ni and Al foils. The mechanical properties of the bimodal laminated NiAl alloy sheet were tested by hot tensile tests at temperatures ranging from 850 °C to 1000 °C and strain rates ranging from 0.01/s to 0.0005/s. To clarify the deformation mechanism, the deformation microstructure was characterized by scanning electron microscopy and electron backscatter diffraction technology. The fracture behavior was characterized by a three dimensional X-ray microscope. It was found that the sheet with the bimodal laminated structure composed of overlapped coarse-grained layers (CGLs) and fine-grained layers (FGLs) exhibits a significant deformation incompatibility between CGLs and FGLs, leading to a special deformation and fracture behavior. During hot deformation, dislocations mainly accommodate in FGLs at the beginning, and the coarse grains then begin to play a major role in replacing the privilege of FGLs and vital in storing dislocations during further deformation. The bimodal laminated grains are broken up into many fragments and the average grain sizes are refined after deformation due to the transformation of low angle boundaries into high angle boundaries. The nucleation of dynamic recrystallization grains preferentially takes place in FGLs, and DRX will occur in CGLs when sufficient strain and storage energy are accumulated. The fracture surface demonstrates a jagged fracture pattern parallel to the loading direction, which results from the special laminated grain structure. The fracture morphology along the thickness direction experiences dramatic changes related to the non-uniform strain distribution, which presented as the density and size distribution of voids in 3D space. The study is vital to guiding the high temperature forming and service performance of NiAl alloy.

A continuous dynamic recrystallization model to describe the hot deformation behaviour of a Ti5553 alloy

A physical based model is developed to describe recrystallization phenomena of titanium alloys during hot working of the β phase. Continuous dynamic recrystallization is attributed as the restoration mechanism based on the progressive transformation of low angle boundaries (subgrains) into high angle boundaries (grains). The model describes both, the microstructure, and the flow stress evolutions during hot deformation with large strains. The microstructure is conceived as been formed by three different populations of dislocations, and high as well as low angle grain boundaries. Evolution rates of all microstructural features are determined based on the effects of generation, interaction and annihilation of dislocations during deformation due to dynamic recovery, continuous dynamic recrystallization and static recovery. Continuous dynamic recrystallization is modelled and is considered to occur homogeneously within the microstructure. The model is able to predict the formation of subgrains from a fully annealed microstructure and the progressive formation of high angle grain boundaries. The subgrain and grain sizes are also obtained as output of the model. The model was applied to describe the hot compression behaviour of a Ti5553 alloy deformed between 880°C to 920°C and strain rates from 0.001 s−1 up to 10 s−1. The model is validated with flow curves and microstructural characterisation of hot deformed, and with microstructural information of non-deformed samples. The critical strain rate increases with increasing strain rate and decreasing temperature, similar to the discontinuous dynamic recrystallization phenomenon. The model can be implemented to simulate the microstructure and predict flow stresses of titanium alloys in industrial processes.

Recrystallization of cold rolled oxide dispersion strengthened copper during room temperature annealing

Static discontinuous recrystallization was studied during room temperature annealing of a newly designed 80% room temperature rolled oxide dispersion strengthened copper. At early stages of annealing, fine new oriented nanosized/submicron grains were recrystallized in the unique matrix of single brass-oriented deformed grain. Upon longer annealing time up to 14 months, the size, area fraction and number density of the recrystallized grains increased significantly along with changing the crystallographic textures. The analysis of misorientation angle distribution of boundaries indicated transformation of low angle boundaries to high angle boundaries results in nucleation of recrystallized grains by significant contribution of static recovery. Furthermore, the constant level of mechanical hardness after recrystallization was interpreted by the balance between grain size hardening, oxide particle hardening and strain hardening.

Microstructural Characteristic of the Al-Fe-Cu Alloy During High-Speed Repetitive Continuous Extrusion Forming

High-speed repetitive continuous extrusion forming process (R-Conform process) was performed on the Al-Fe-Cu alloy. The microstructural evolution and mechanical properties were studied by x-ray diffraction, electron backscatter diffraction, transmission electron microscopy and tensile testing. The results show that a significant improvement of tensile ductility concurs with a considerable loss of tensile strength before four passes, after that the process on mechanical properties variation tends to be steady, indicating an accelerated mechanical softening occurs when comparing to low-speed R-Conform process. Microstructure characterization indicates that the accumulated strain promotes the transformation of low angle boundaries to high angle boundaries, thus leading to the acceleration of continuous dynamic recrystallization process, and the precipitates are broken, spheroidized and homogeneously distribute in Al matrix as increasing R-Conform passes. Massive microshear bands are observed after initial passes of R-Conform process, which may promote continuous dynamic recrystallization and further grain refinement during high-speed R-Conform process.

Evolution of misorientation spectrum of grain boundaries of submicrocrystalline molybdenum upon deformation under conditions of grain-boundary diffusion of nickel

Comparative investigations of the concurrent evolution of the structure and the misorientation spectrum of grain boundaries of submicrocrystalline molybdenum have been performed upon free annealing and under the creep conditions at a temperature of 1023 K. It has been established that changes in the misorientation spectrum of grain boundaries of submicrocrystalline molybdenum upon annealing and creep are observed concurrently with boundary migration. The grain-boundary diffusion of nickel upon annealing leads to an increase in the fraction of grain boundaries of the special type with Σ17a in the grain-boundary ensemble of submicrocrystalline molybdenum. Under the creep conditions grain-boundary diffusion fluxes of nickel atoms favor transformation of low-angle boundaries into high-angle boundaries and growth in the angle of misorientation of the high-angle boundaries of submicrocrystalline molybdenum to 45°–60°.

Substructure evolution of Ti-6Al-2Zr-1Mo-1V alloy isothermally hot compressed in α+β two-phase region

Substructure evolution significantly influences the flow behavior of titanium alloys in isothermal hot compression. This paper presents a physical experiment (isothermal hot compression and electron backscatter diffraction, EBSD) and a cellular automaton (CA) method to investigate the substructure evolution of a near-α titanium alloy Ti-6Al-2Zr-1Mo-1V (TA15) isothermally compressed in the α + β two-phase region. In the CA model, the subgrain growth, the transformation of low angle boundaries (LABs) to high angle boundaries (HABs) and the dislocation density evolution were considered. The dislocation density accumulating around the subgrain boundaries provided a driving force and made the transformation of the LABs to HABs. The CA model was employed to predict the substructure evolution, dislocation density evolution and flow stress. In addition, the effects of strain, strain rate and temperature on the relative frequency of the HABs were analyzed and discussed. To verify the CA model, the predicted results including the relative frequency of the HABs and the flow stress were compared with the experimental values.

Substructure and texture evolution and flow behavior of TA15 titanium alloy compressed in the alpha + beta two-phase field

This paper investigates the substructure and texture evolution of a near-α titanium alloy Ti–6Al–2Zr–1Mo–1V (TA15) during isothermal hot compression in an α+β two-phase field. The microstructures of deformed samples were analyzed using an electron backscatter diffraction (EBSD) technique to study the effects of process parameters on the evolution of substructure and texture. The activated energy and the stress exponent were calculated to identify the deformation behaviors and the softening mechanisms. The experimental results showed that the transformation of low angle boundaries (LABs) to high angle boundaries (HABs) was sensitive to strain rate, strain and temperature. The volume fraction of the HABs increased with the increasing strain rate or strain but decreased with increasing temperature. During the transformation process, the dislocations inside the subgrains and around the subgrain boundaries were annihilated by dislocation reaction and absorption. The transformation of the LABs into HABs significantly influenced the flow stress during the thermomechanical processing. With the increase of strain or temperature, the texture in the alpha phase became weaker and the pyramidal slip systems began to be activated, while the texture in the beta phase tended to be stronger with more plastic deformation.

未再結晶７０７５アルミニウム合金における微細粒の動的形成と超塑性発現

Dynamic evolution of fine grains and superplasticity of an unrecrystallized 7075 aluminum alloy were studied by means of two-step tensile testing as well as optical, SEM and TEM metallographic observation. Typical superplasticity takes place accompanied by the evolution of new fine grains in the medium region of strain rate at 798 K. Newly evolved grain size decreases with increase in the reduction of prior cold-rolling, i.e. with decrease in the thickness of layered initial grains (DST). Strain rate dependence of flow stress and total elongation to fracture is sensitively affected by cold-rolling reduction because of changes in DST. Grain boundary sliding (GBS) frequently takes place just after yielding even in the layered pre-existing grain boundaries parallel to tensile axis. With further straining, GBS brings about subgrain rotation, followed by transformation of low angle boundaries to high angle ones. Repitition of this process can result in the evolution of new fine grains with high angle boundaries at high strains. It is concluded from the mechanical and metallographic results that GBS can play a key role in the dynamic evolution of fine grains as well as the appearance of superplasticity.