Concurrent Recrystallization Research Articles

The impact of precipitation on recrystallization in a severely plastically deformed CoCrFeMnNi multi–principal element alloy

The present study investigated the rate-controlling mechanism for recrystallization kinetics in a severely plastically deformed (SPD) multi–principal element alloy. For this purpose, an equiatomic Cantor alloy (CoCrFeMnNi) was subjected to equal channel angular processing (ECAP) at room temperature and annealed at 773–973 K. The major recovery process was found to be concurrent recrystallization with precipitation. The recrystallization kinetics, investigated in the frame of the Johnson–Mehl–Avrami (JMAK) theory, gives a retarded recrystallization with a high recrystallization temperature of above 0.54Tm and an anomalous low JMAK exponent value of 1.1–1.8. The measured apparent activation energy for recrystallization exhibited a steady increase over the duration, with initial values of 220.8 ± 7.5 kJ/mol rising to 313.9 ± 23.8 kJ/mol towards the end. The systematic analysis leads to the conclusion that the rate-controlling mechanism for recrystallization in ECAP processed CoCrFeMnNi MPEA is the grain boundary migration-determined grain growth instead of nucleation. During this process, the Zener pinning effect exerted by precipitates from the concurrent precipitation with recrystallization plays the dominant role.

Effect of initial grain structure on the concurrent precipitation and recrystallization of Co37Cr20Ni37Ti3Al3 high-entropy alloy

High performance precipitation strengthened high-entropy alloys (HEAs) can be prepared by concurrent recrystallization and precipitation in the process of aging at intermediate temperatures after rolling. Generally, the homogenized samples are subjected to rolling before aging, where the effects of initial grain structure before rolling on the concurrent recrystallization and precipitation are overlooked. In this study, we revealed the inherited effect of initial grain on the concurrent recrystallization and precipitation in Co37Cr20Ni37Ti3Al3 HEA. The heredity of grain homogeneity to the initial grain structure was revealed, where the initial grain structure influences the homogeneity of strain distribution during rolling and influences the recrystallization in the subsequent aging. Moreover, the inherited effect of the initial grain structure also influences the precipitation behavior. TEM characterization confirmed that the lamellar precipitation decreases, while spherical precipitation increases with the increase of the initial grain size. By tailoring the initial grain size, the strength and ductility of Co37Cr20Ni37Ti3Al3 alloy were simultaneously improved. The present study provides a good understanding of the concurrent recrystallization and precipitation in HEAs.

Dual effects of pre-strain on continuous and discontinuous precipitation of L12-strengthened high-entropy alloys

The effects of pre-strain on the continuous precipitation (CP) and discontinuous precipitation (DP) as well as mechanical properties of L12-strengthened HEAs were thoroughly studied. It is found that the pre-strain has a dual effect on the L12 precipitation behavior depending on the pre-strain level. At low pre-strains, the plastic deformation increases the dislocation density without significantly affecting the grain structure, which accelerates the CP reaction by promoting the CP nucleation and growth, leading to the CP-dominant microstructure. At high pre-strains, the server plastic deformation induces the formation of a high density of deformation bands and subgrains, the boundaries of which provide preferred nucleation sites for the DP reaction. In addition, the high stored energy induced by the cold deformation enhances the grain boundary migration, which promotes the DP growth and concurrent recrystallization. Mechanical tests further reveal that the pre-strain substantially improves the strength of the alloys without significantly deteriorating the ductility. The contributions of dislocations, grain boundaries, and precipitates to the strengthening of the pre-strained and aged alloys were quantitatively evaluated.

Concurrent Recrystallization and Precipitation for Combination of Superior Precipitation and Grain Boundary Hardening in Co37Cr20Ni37Ti3Al3 High-Entropy Alloy

Concurrent Recrystallization and Precipitation for Combination of Superior Precipitation and Grain Boundary Hardening in Co37Cr20Ni37Ti3Al3 High-Entropy Alloy

Atomic scale understanding of the defects process in concurrent recrystallization and precipitation of Sm-Co-Fe-Cu-Zr alloys

Identifying the defects process in concurrent recrystallization and precipitation during aging a supersaturated solid solution is essential for understanding their interaction mechanisms and for manipulating the microstructure, but was rarely done at the atomic scale. Herein, the concurrent recrystallization and precipitation in the supersaturated hexagonal Sm(Co, Fe, Cu, Zr)7.5 alloys were studied through detailed transmission electron microscopy investigations, where the recrystallization, growth of recrystallized subgrains (cells) and precipitates stem from the gradual formation and dissociation of defects, including basal stacking faults (SFs), vacancies and excess interstitial atoms. The diffusion-controlled glides of <a>-type partial dislocations associated with the SFs not only transform the matrix from the mixture of hexagonal Sm(Co, Fe, Cu, Zr)7 (1:7H) and Sm2(Co, Fe, Cu, Zr)17 (2:17H) to Sm-depleted rhombohedral Sm2(Co, Fe, Cu, Zr)17 (2:17R) cells but also provide continuous diffusion channels to reduce the point defects to form the Sm-enriched hexagonal Sm(Co, Fe, Cu, Zr)5 (1:5H) cell boundary precipitates and Zr-enriched rhombohedral (Sm, Zr)(Co, Fe, Cu)3 (1:3R) platelets. It indicates a diffusion-controlled displacive phase transformation mechanism, characterized by the composition-dependent 2:17R’ intermediate phase due to incomplete basal slip and incomplete solute partitioning. The growth velocities of both recrystallized cells and precipitates are closely related to the defects density, being faster due to the high defects density at early stage, and being slower due to the reduced defects density at later stage. A basal slip model is proposed to explain the formation and dissociation of defects along with the stacking period change and the simultaneous formation of continuous atomic diffusion channels. These new findings may yield a deep understanding of the interaction between recrystallization and precipitation.

Modifying transformation pathways in high entropy alloys or complex concentrated alloys via thermo-mechanical processing

Often the experimentally-observed, single-phase high entropy alloy (HEA) is the result of second-phase precipitation constrained by thermodynamic and kinetic factors. Using Al0.3CoCrFeNi as a candidate HEA, this paper demonstrates the strong influence of thermo-mechanical processing on the transformation pathway adopted for isothermal second-phase precipitation. A traditional thermo-mechanical processing route comprised of homogenization cold-rolling solution treatment in the single fcc phase region, followed by a precipitation anneal at a lower temperature, results in a homogeneous distribution of nanometer scale-ordered L12 (gamma prime-like) precipitates within the fcc matrix. In contrast, if cold-rolling is followed directly by annealing at the precipitation temperature, then the resulting microstructural evolution pathway changes completely, with concurrent recrystallization of the matrix fcc grains and precipitation of B2 and sigma phases, largely at the grain boundaries. These experimentally observed variations in transformation pathway have been rationalized via the competition between the thermodynamic driving force and activation barrier for second-phase nucleation in this alloy, coupled with the kinetics of the process. The microstructural variations that result from these dramatically different phase transformation pathways can lead to some rather exceptional mechanical properties that can be varied over a large range even for a single Al0.3CoCrFeNi HEA composition.

Effects of non-isothermal annealing on microstructure and mechanical properties of severely deformed 2024 aluminum alloy

Microstructure and mechanical properties of AA2024 after severe plastic deformation (SPD) and non-isothermal annealing were investigated. The non-isothermal treatment was carried out on the severely deformed AA2024, and the interaction between restoration and precipitation phenomena was investigated. Differential scanning calorimetry, hardness and shear punch tests illustrate that static recovery and dissolution of GPB zones/Cu–Mg co-clusters occur concurrently through non-isothermal annealing. Scanning electron microscope and electron backscatter diffraction illustrate that non-isothermal annealing of deformed AA2024 up to 250 °C promotes the particle-free regions and also particle stimulated nucleation. Results show that through heating with the rate of 10 °C/min up to 250 °C, the ultimate shear strength and the hardness are maximum due to the presence of S'/S phases which have been detected during non-isothermal differential scanning calorimetry experiment. Also, recrystallization phenomenon occurs in temperature range which includes the dissolution of S'/S phases. The concurrent recrystallization and dissolution of S'/S phase at 380 °C have been verified by differential scanning calorimetry, mechanical properties, and optical microscope.

Age hardening of EN AW 2014 alloy extruded in the semi-solid state

Response to T6 heat treatment of thixoextruded EN AW 2014 aluminium alloy was investigated in the present work. Extrusion of a 2014 slug heated to a liquid fraction of 15%, takes place in the semi-solid state until the liquid fraction in the final part of the slug is reduced via segregation to a level where semi-solid forming is no longer possible. Hence, the final part of the slug is extruded in the solid-state with a concurrent recrystallization process. This process has produced two distinctly different structures at the front and rear ends and an unexpected hardness profile in T6 temper along the length of the thixoextruded rod. The response to T6 heat treatment of the globular front has been age hardening as usual. The inferior age hardening potential with respect to the hot extruded counterpart is attributed to the grain boundary Al 2Cu phase which has grown too coarse via liquid segregation to be readily solutionized at typical solutionizing temperatures. The rear end of the extrudate on the other hand, has softened upon T6 heat treatment owing to Cu depletion and a fully recrystallized structure.

Evolution of Microstructure and Precipitation in Heat-Treatable Aluminium Alloys during ECA Pressing and Subsequent Heat Treatment

The precipitation and evolution of microstructure in a spray-cast Al-7034 alloy and a commercial wrought Al-2024 alloy were studied after equal-channel angular pressing (ECAP) using transmission electron microscopy and differential scanning calorimetry (DSC). Microstructural examination showed the grain sizes of both alloys were reduced to the range of ~0.3–0.5 μm through ECAP. The DSC analysis identified the occurrence of thermal effects involving the formation, coarsening, dissolution and melting of the precipitate phases and concurrent recrystallization. The heating and ageing response of the alloys processed by ECAP was identified by micro-hardness testing of the samples after interrupted heating and ageing treatments.

Hot rolling simulations of austenitic stainless steel

The dynamic, static and metadynamic recrystallization behavior of austenitic stainless steel during hot rolling was analyzed. In this approach, each of those recrystallization behaviors is described by appropriate kinetics equations. The critical strain for dynamic recrystallization was determined so that a distinction could be made between static and metadynamic recrystallization; then the amounts of strain accumulation compared with the critical strain each pass. The effects of grain size on the fraction recrystallized and of the latter on the flow stress were evaluated for each type recrystallization behavior. In this way, the dependence of the mean flow stress (MFS) on temperature could be analyzed in terms of the extent and nature of the prior or concurrent recrystallization mechanisms. Finally, an example is given of an industrial process in which DRX/MDRX can play an important role. More grain refinement can be achieved by increasing the strain rate, decreasing the interruption time and lowering the temperature of deformation.

Chemical and mechanical processes during burial diagenesis of chalk: an interpretation based on specific surface data of deep‐sea sediments

Burial diagenesis of chalk is a combination of mechanical compaction and chemical recrystallization as well as cementation. We have predicted the characteristic trends in specific surface resulting from these processes. The specific surface is normally measured by nitrogen adsorption but is here measured by image analysis of scanning electron micrographs. This method concentrates on the micritic matrix alone. Deep‐sea sediments are ideally suited to the study of burial diagenesis because they accumulate in a relatively conservative tectonic setting. We used material from the Ontong Java Plateau in the Pacific, where a &gt; 1 km thick package of chalk facies sediments accumulated from the Cretaceous to the present. In the upper 200–300 m the sediment is unconsolidated carbonate ooze, throughout this depth interval compaction is the principal porosity reducing agent, but recrystallization has an equal or larger influence on the textural development. In the chalk interval below, compaction is not the only porosity reducing agent but it has a larger influence on texture than concurrent recrystallization. Below 850 m grain‐bridging cementation becomes important resulting in a lithified limestone below 1100 m. This interpretation is based on specific surface data alone, and modifies current diagenetic models.