Particle Simulated Nucleation Research Articles

The hot deformation characteristics and the associated microstructural evolution of an Al–Cu–Li–Zn alloy studied by constitutive equations and processing maps

The hot deformation behavior and the associated microstructural evolution (which were the keys to decide mechanical properties) were studied using different deformation conditions for the quaternary Al–Cu–Li–Zn alloy containing 1.0 wt% Zn. The constitutive equations and processing map were established. Dynamic recrystallization (DRX) was related to the orientation of the initial grains. The DRX rarely occurred in the grains with an orientation near <110>Al and mainly occurred in the grains with an orientation near <100>Al. Particle simulated nucleation (PSN) promoted by pre-existed coarse T1 phase particles was the main mechanism of the DRX when the deformation temperature was below 450 °C. In the temperature range of 450–500 °C with a strain rate of 0.01 s−1, the DRX occurred by grain boundary bulging or the increase in the cumulative misorientation, but only small fraction of the DRX grains appeared. This work provided an important guideline for the optimization of deformation techniques and microstructures.

Effect of Al addition on microstructure and mechanical properties of Mg−Gd−Zn alloys

The microstructure evolution and mechanical properties of Mg−15.3Gd−1Zn alloys with different Al contents (0, 0.4, 0.7 and 1.0 wt.%) were investigated. Microstructural analysis indicates that the addition of 0.4 wt.% Al facilitates the formation of 18R-LPSO phase (Mg12Gd(Al, Zn)) in the Mg−Gd−Zn alloy. The contents of Al11Gd3 and Al2Gd increase with the increase of Al content, while the content of (Mg, Zn)3Gd decreases. After homogenization treatment, (Mg, Zn)3Gd, 18R-LPSO and some Al11Gd3 phases are transformed into the high-temperature stable 14H-LPSO phases. The particulate Al−Gd phases can stimulate the nucleation of dynamic recrystallization by the particle simulated nucleation (PSN) mechanism. The tensile strength of the as-rolled alloys is improved remarkably due to the grain refinement and the fiber-like reinforcement of LPSO phase. The precipitation of the β′ phase in the peak-aged alloys can significantly improve the strength. The peak-aged alloy containing 0.4 wt.% Al achieves excellent mechanical properties and the UTS, YS and elongation are 458 MPa, 375 MPa and 6.2%, respectively.

The dynamic recrystallization and mechanical property responses during hot screw rolling on pre-aged ZM61 magnesium alloys

In order to investigate the microstructure and texture evolution induced by precipitation during severe plastic deformation (SPD), pre-aging was conducted on ZM61 alloys, followed by screw rolling (at 300 °C), a special type of SPD technology. The mechanical properties were measured by uniaxial tensile testing at room temperature. The results showed that a gradient structure was obtained, and the grain size was more refined in the outer region along the radial direction. All samples expressed a basal texture, and the intensity gradually weakened when the pre-aging time increased, with the strength and ductility improving concurrently. The fracture elongation improved from 15.2% on as-received samples to 27.8% after pre-aging for 12 h, an 82.9% increase. This is mainly related to dynamic recrystallization mechanisms, including continuous dynamic recrystallization (CDRX), discontinuous dynamic recrystallization (DDRX), and particle simulated nucleation (PSN).

Comparative study on the high temperature deformation behaviour of an as-cast Ni base superalloy subjected to different cooling rates after homogenization

Medium alloyed as-cast nickel base superalloy was subjected to two different cooling rates (fast and slow) after homogenization which resulted in mono-modal and bi-modal distribution of gamma prime (γ′) precipitates. Both the materials were subjected to high temperature deformation over a range of temperatures and strain rates. Irrespective of the strain rate used, the material subjected to fast cooling exhibited high compressive yield strength and flow softening characteristics when compared to the slow cooled material in the sub-solvus regime. Detailed microstructural investigation carried out using scanning and transmission electron microscope to ascertain the micro-mechanism revealed varying precipitate – dislocation interaction mechanism in both the materials. Higher flow softening and cracking in the fast cooled material is attributed to glide plane softening due to planar slip and the presence of fine network of carbides along the grain boundaries respectively. In the super-solvus regime, the flow stress of both the materials was comparable and both the materials exhibited discontinuous dynamic recrystallization (DDRX). Further, the slow cooled material exhibited lower critical strain for DDRX initiation and also better recrystallization kinetics irrespective of the processing conditions. Higher recrystallization kinetics in the slow cooled material is attributed to the concurrent particle simulated nucleation (PSN) at the coarse MC type carbide boundaries.

Role of second-phase particles in the microstructural evolution and mechanical properties of extruded Mg-Gd-Y-Ag-Zr alloys

The role of coarse second-phase particles in the microstructural evolution and mechanical properties of as-extruded Mg-Gd-Y-Ag-Zr alloys was investigated. Results show that the volume fraction of coarse β particles was enhanced by the addition of Ag (0.5 and 1.0) (wt.%) in the Mg-10.5 Gd-5 Y-0.5 Zr (wt.%) alloy. These particles have a face-centered cubic structure phase. In addition to Gd and Y, Ag is also partitioned into particles in the Ag-containing alloys. β particles can promote the dynamic recrystallization via particle-simulated nucleation and effectively suppress grain growth by pinning effects, which are responsible for the grain refinement. With the increase of volume fraction of β particles, the texture and dislocation features have not been changed. Since the coarse β particles can greatly lead to the reduction of grain size, the grain boundary strengthening in Ag-containing alloys with high volume fraction of particles contributes to the significantly improvement of the tensile yield strength. The coupled effect of the increase in the coarse β particle fraction and the reduction in grain size causes a slight decrease in elongation for the Ag-containing alloy.

Effects of pre-annealing on microstructure and mechanical properties of as-extruded Mg-Gd-Y-Zn-Zr alloy

An as-extruded Mg-11.5Gd-4.5Y-1.5Zn-0.4Zr (wt %) alloy with an excellent strength-ductility balance has been successfully developed by pre-annealing and hot extrusion. The effects of pre-annealing on microstructure and mechanical properties have been studied. Results show that the Mg5RE (RE: rare earth) particles, which are generated by pre-annealing treatments, are cracked during hot extrusion, and resulting Mg5RE fragments not only enhance the recrystallization of particle simulated nucleation (PSN), but also improve the continuous dynamic recrystallization (C-DRX) by promoting the grain subdivision. After hot extrusion, the studied alloy exhibits a bimodal microstructure consisting of fine DRXed grains with relatively random orientations and coarse un-DRXed grains with strong basal texture. Increasing the pre-annealing duration raises both the quantity of Mg5RE particles and the fraction of DRX, thereby decreasing the strength but increasing the ductility. With pre-annealing for 1 h the studied alloy achieves the best strength-ductility balance with tensile yield strength (TYS) of 377 ± 1.2 MPa and elongation to failure (EL) of 10.8 ± 2.0%. Further pre-annealing degrades the ductility due to the excessive Mg5RE particles. The alloy strengthening is attributed to the bimodal microstructure, Mg5RE and long-period stacking ordered (LPSO) phases, solute segregated SFs, and texture.

Microstructure evolution and mechanical properties of a high strength Mg-11.7Gd-4.9Y-0.3Zr (wt%) alloy prepared by pre-deformation annealing, hot extrusion and ageing

A high strength Mg-11.7Gd-4.9Y-0.3Zr (wt%) alloy with a weak tension-compression yield asymmetry has been successfully developed by pre-deformation annealing, hot extrusion and ageing. The effects of pre-deformation annealing on the microstructure evolution and mechanical properties are studied. The results reveal that pre-deformation annealing generates a large number of Mg5RE (RE: rare earth) phase particles and raises the fraction of dynamic recrystallization (DRX). The preformed Mg5RE particles not only enhance the DRX by particle simulated nucleation (PSN), but also facilitate the continuous DRX (C-DRX) by promoting the grain subdivision during hot extrusion. Without pre-deformation annealing, the as-extruded alloy exhibits a high tensile yield strength (TYS) of 376 ± 9.6MPa but a low elongation to failure (EL) of 4.3 ± 0.1% due to the bimodal microstructure consisting of coarse un-DRXed grains with strong basal texture and fine DRXed grains with weak basal texture. After T5 treatment, the TYS further increases to 500 ± 5.5MPa, whereas the EL reduces to 2.7 ± 0.4%. An excellent balance of strength and ductility (TYS of 343 ± 0.2MPa and EL of 9.3 ± 0.9%) can be realized by pre-deformation annealing for 1h due to the raised fraction of DRX and the weakened basal texture. T5 treatment further increases the TYS to 446 ± 3.8MPa but reduces the EL to 3.0 ± 0.2%. The studied alloy exhibits good compressive performance, resulting in a weak tension-compression yield asymmetry. The grain refinement, Mg5RE and β′ phases, and solute-segregated SFs contribute to the alloy strengthening.

Thermal stability of Cu-Cr-Zr alloy processed by equal-channel angular pressing

Thermal stability of a Cu-Cr-Zr alloy processed by equal-channel angular pressing up to16 passes was investigated using isochronal annealing ranging from 250 to 850°C for 1h. The microstructure, crystallographic texture and micro hardness of samples were characterized through electron back scatter diffraction and Vickers micro hardness measurements. The recrystallized grain size was stable between 250°C and 500°C then increased quickly. The achieved mean grain size, after 1, 4 and 16 ECAP passes, was around 5.5μm. A discontinuous mode of recrystallization was found to occur and a Particle Simulated Nucleation mechanism was evidenced. The evolution of the high angle grain boundary fraction increased notably after annealing above 550°C. The crystallographic texture after isochronal annealing was similar to that of ECAP simple shear, no change of the texture during annealing was observed but only slight intensity variations. Micro hardness of all Cu–Cr–Zr samples showed a hardening with two peaks at 400 and 500°C associated with precipitation of Cu cluster and Cu5Zr phase respectively, followed by a subsequent softening upon increasing the annealing temperature due to recrystallization.

Stretching the engineering strain of high strength LPSO quaternary Mg-Y-Zn-Al alloy via integration of nano-Al2O3

In the present study, an attempt is made for the first time to reinforce long-period stacking ordered (LPSO) MgY1.06Zn0.76Al0.42 (at.%) alloy with 0.5, 1.0, and 1.5 vol% of nano-Al2O3 particles to form nanocomposites. Microstructure characterization revealed the ability of nano-Al2O3 in inhibiting the formation of 14H LPSO phases in the nanocomposites during solidification. Homogenization at 723 K (450 °C) for 2 h led to the subsequent precipitation of fine Mg-Y-Zn-Al precipitates (≤1 µm) in the nanocomposites. The fine Mg-Y-Zn-Al precipitates and nano-Al2O3 particles were established to be active in promoting dynamic recrystallization (DRX) of α-Mg via particle-simulated nucleation during extrusion, which was responsible for weakening the basal texture in the nanocomposites and improving failure strain. As a result, failure strain was significantly increased from 10.8 % in the monolithic alloy to beyond 15 % in the nanocomposites with the highest strength among nanocomposites achieved in NC5 (nanocomposite reinforced with 0.5 vol% of nano-Al2O3 particles).

Effect of microstructure on hot tensile deformation behavior of 7075 alloy sheet fabricated by twin roll casting

7075 aluminum alloy sheet with a thickness of 1mm was successfully fabricated by twin roll casting and subsequent rolling process. The hot tensile deformation behavior of 7075 alloy was evaluated and large elongation over 200% was obtained at 450°C under the high strain rate of 1×10−1s−1. In order to clarify the reason for the large elongation obtained at high strain rate, the effects of microstructure on hot tensile deformation behavior were investigated. The results show that high solidification rate during TRC casting process induced small particles (~1μm) in homogeneous distribution. The relatively high fraction of particles over 1μm in size attributed the homogeneous recrystallized microstructure with fine grains induced by particle simulated nucleation (PSN). This fully recrystallized fine-grained microstructure of TRC alloy sheet contributed to high ductility and formability of 7075 alloy sheet under high strain rate.

Effect of homogenization on enhancing the failure strain of high strength quaternary LPSO Mg–Y–Zn–Al alloy

The current work investigates the effect of homogenization on enhancing the failure strain of a high strength MgY1.06Zn0.76Al0.42 (at.%) alloy. Bulk 14H long-period stacking ordered (LPSO) phases (≥10µm) were progressively broken down into acicular fine platelets (≤2µm) with increase in homogenization time (1, 2, 4 and 6hours) at 450°C. The high density of fine LPSO platelets (≤2µm inter-particle spacing) were experimentally observed to be more effective than the bulk LPSO phases (≥10µm inter-phase spacing) in promoting dynamic recyrstallization (DRX) via particle simulated nucleation (PSN) during extrusion which was critical for basal texture weakening to enhance the failure strain of the as-extruded alloy. The alloy homogenized for 2hours prior to hot extrusion (as-extruded alloy B) displayed the highest tensile yield (376MPa) and ultimate tensile strength (416MPa) with significant improvement in failure strain (+80%) compared to as-extruded alloy A (homogenized for 1hour prior to hot extrusion).

Surface recrystallization of a Ni3Al based single crystal superalloy at different annealing temperature and blasting pressure

The effects of annealing temperature and grit blasting pressure on the recrystallization behavior of a Ni3Al based single crystal superalloy were studied in this work. The results show that the precipitation of the Y-NiMo phase occurs at 900 and 1000 °C, which precedes recrystallization. The initial recrystallization temperature was between 1000 and 1100 °C. Cellular recrystallization was formed at 1100 and 1200 °C, which consisted of large columnar γ′ and fine γ + γ′. The dendrite arm closed to the interdendritic region may act as nucleation sites during initial recrystallization by a particle simulated nucleation mechanism at 1280 °C. The size of the grains first turned large and then became small upon the pressure while the recrystallization depth increased all the time.

EFFECTS OF THERMAL MECHANICAL TREATMENT ON THE MICROSTRUCTURES AND HARDNESS OF AN Al-Zn-Mg-Cu ALLOY PLATE

The effects of thermal mechanical treatment (TMT: solution-quenching-precipitation- rolling) on the morphology and the amount of the precipitates in an Al-7.81Zn-1.81Mg-1.62Cu alloy plate and the effects of those particles on the microstructures, textures and hardness have been studied. During TMT, MgZn2 particles with nano/submicron scales significantly inhibit the migration of sub- grain/grain boundaries but would not induce particle simulated nucleation (PSN) recrystallization. An appropriate TMT process results in the remarkable diminish of recrystallization and increases Vickers hardness by 15%.

Microstructure evolution of 7050 aluminum alloy during hot deformation

Hot compression of 7050 aluminum alloy was performed on Gleeble 1500D thermo-mechanical simulator at 350°C and 450°C with a constant strain rate of 0.1 s −1 to different nominal strains of 0.1, 0.3 and 0.7. Microstructures of 7050 alloy under various compression conditions were observed by TEM to investigate the microstructure evolution process of the alloy deformed at various temperatures. The microstructure evolves from dislocation tangles to cell structure and subgrain structure when being deformed at 350°C, of which dynamic recovery is the softening mechanism. However, continuous dynamic recrystallization (DRX) occurs during hot deformation at 450°C, in which the main nucleation mechanisms of DRX are subgrain growth and subgrain coalescence rather than particle-simulated nucleation (PSN).