Showing papers on "Grain growth published in 2022"

In-situ grain structure control in directed energy deposition of Ti6Al4V

Abstract: In metal additive manufacturing (AM), long columnar grains along the building direction due to highly directional thermal gradients often lead to a strong texture and severe anisotropy in mechanical properties of the deposits, which significantly impair part qualification and targeted applications. Here, a novel depositing strategy by periodically alternating processing parameters is designed to partially preserve the equiaxed grains resulted from the columnar-to-equiaxed transition at the top of each deposited layer and, at the same time, to interrupt the epitaxial growth of columnar grains in AM titanium alloy. With the help of the competitive growth of the new grains and the potential coarsening effect during subsequent thermal-cycles, a microstructure of full equiaxed prior-β grains in Ti6Al4V fabricated by laser directed energy deposition (DED) is finally obtained without either using any auxiliary equipment or adjusting alloy chemistry like previous researches. This further contributes to a superior mechanical property and a remarkable reduction on both the crystallographic textures and the property anisotropy.

Enhanced strength and ductility in Al-Zn-Mg-Cu alloys fabricated by laser powder bed fusion using a synergistic grain-refining strategy

Abstract: Grain refinement is critical to surpassing the bottlenecks of inherent hot tearing of high-strength aluminum alloys fabricated by additive manufacturing (AM). In this study, a synergistic grain-refining strategy including heterogeneous nucleation, solute-driven growth restriction and nanoparticle-induced growth restriction was introduced to control the microstructure of Al-Zn-Mg-Cu alloys during the laser powder bed fusion (LPBF) process. Crack-free Al-Zn-Mg-Cu alloys with significantly refined grains were safely fabricated via LPBF by coincorporation of TiC and TiH2 particles. In-situ L12-Al3Ti particles were produced to promote the heterogeneous nucleation. The grain growth was restricted by adding Ti solute, while introduced TiC nanoparticles (NPs) improved the density of heterogeneous nucleation sites and blocked grain growth physically. The resultant elimination of columnar grains and hot cracks in the (1 wt.%) TiC- and (0.8 wt.%) TiH2-modified Al-Zn-Mg-Cu alloy resulted in excellent ultimate tensile strength (UTS) of 593 ± 24 MPa, yield strength (YS) of 485 ± 41 MPa and elongation (EL) of 10.0% ± 2.5% under the T6 condition. This study provides new insights into improving the grain microstructure and mechanical properties of high-strength aluminum alloys during LPBF.

Development of ultrafine grained cobalt-free AlCrFe2Ni2 high entropy alloy with superior mechanical properties by thermo-mechanical processing

Abstract: The microstructure and properties of cobalt-free cost-effective AlCrFe2Ni2 high entropy alloy (HEA) in the as-cast condition and after thermo-mechanical processing by severe cold-rolling and annealing were investigated in the present work. The as-cast HEA showed a heterogeneous microstructure consisting of relatively coarse lamellar and much finer intertwined regions. The coarse regions consisted of eutectic mixture of FCC and ordered B2 (along with minor BCC) phases. The FCC phase was enriched in Fe and Cr, while the B2 phase was found enriched in Ni and Al but depleted in Cr. The BCC/B2 phase in the as-cast material showed phase separation due to spinodal decomposition to two different B2 phases in the fine regions. The overall FCC phase and BCC/B2 phase fractions were ∼60% and 40%, respectively. Despite the complex microstructure, the presence of a high fraction of the ductile FCC phase rendered remarkable workability, allowing heavy cold-rolling up to 90% reduction in thickness. Heavy deformation resulted in the development of intriguing microstructural features such as folding and bending of the lamellae, local shearing, and finally, deformation-induced nanocrystallization of the FCC phase. However, the B2 phase retained the ordered structure even after 90% cold-rolling. Annealing at 800 °C resulted in the formation of an ultrafine microduplex structure with significant resistance to grain growth even up to an annealing temperature of 1200 °C. A high yield (∼880 MPa) and tensile strength ( 1100 MPa) coupled with appreciable elongation (∼10%) could be achieved after annealing at 800 °C. The tensile properties obtained were superior to the other cobalt-free HEAs, which indicate the promising application of the cobalt-free cost-effective AlCrFe2Ni2 HEA as an advanced structural material.

Significantly enhanced grain boundary Zn and Ca co-segregation of dilute Mg alloy via trace Sm addition

Abstract: Increasing the level of grain boundary (GB) segregation is one of the effective strategies for the preparation of new high-performance Mg alloys. Here based on careful experimental design, we clearly demonstrate that the addition of trace Sm could significantly increase the GB segregation concentration of Zn and Ca in the as-extruded dilute Mg–Zn–Ca–Mn alloy, and confirm that the increased GB segregation could improve the yield strength of the as-extruded alloy and further inhibit the grain growth and the separation of the segregation atmosphere from GBs during annealing. The proposed idea of multi-element promoting efficient GB segregation is expected to contribute to the design strategy of advanced Mg alloys.

Modeling Dynamic Recrystallization Behavior in a Novel HIPed P/M Superalloy during High-Temperature Deformation

Abstract: The dynamic recrystallization (DRX) features and the evolution of the microstructure of a new hot isostatic pressed (HIPed) powder metallurgy (P/M) superalloy are investigated by hot-compression tests. The sensitivity of grain dimension and DRX behavior to deformation parameters is analyzed. The results reveal that the DRX features and grain-growth behavior are significantly affected by deformation conditions. The DRX process is promoted with a raised temperature/true strain or a reduced strain rate. However, the grains grow up rapidly at relatively high temperatures. At strain rates of o.1 s−1 and 1 s−1, a uniform microstructure and small grains are obtained. Due to the obvious differences in the DRX rate at various temperatures, the piecewise DRX kinetics equations are proposed to predict the DRX behavior. At the same time, a mathematical model for predicting the grain dimension and the grain growth behavior is established. To further analyze the DRX behavior and the changes in grain dimension, the hot deformation process is simulated. The developed grain-growth equation as well as the piecewise DRX kinetics equations are integrated into DEFORM software. The simulated DRX features are consistent with the test results, indicating that the proposed DRX kinetics equations and the established grain-growth model can be well used for describing the microstructure evolution. So, they are very useful for the practical hot forming of P/M superalloy parts.

Effect of grain size on the mechanical properties and bio-corrosion resistance of pure magnesium

Abstract: In addition to grain size, the mechanical properties and corrosion resistance of magnesium alloys depend on other factors such as texture, distribution of alloying elements, and homogeneity of the microstructure. These attributes might change during grain refining processes, making the situation more complex and masking the real effect of grain size. Accordingly, in the present work, the grain size of commercially pure (CP) Mg was adjusted by grain growth annealing after casting and deformation by hot rolling (for grain refinement via dynamic recrystallization during thermomechanical processing) to investigate the effect of grain size on tensile properties, hardness, and bio-corrosion resistance in simulated body fluid (SBF) for biomedical implant applications. The grain growth kinetics of commercially pure Mg was studied, where the grain growth activation energy (Q) was found to be consistent with that of the grain boundary diffusion in Mg. Based on the Hall–Petch plots, it was revealed that the yield stress (YS), ultimate tensile strength (UTS), and hardness of pure Mg are highly sensitive to average grain size in comparison to common Mg alloys. Moreover, based on the polarization curves and Nyquist plots obtained from electrochemical impedance spectroscopy (EIS) analysis, grain coarsening led to the enhanced corrosion resistance due to the decreased grain boundary regions, where a simple linear formula was obtained for correlating corrosion current density to average grain size.

Synergistic effects of hybrid (HA+Ag) particles and friction stir processing in the design of a high-strength magnesium matrix bio-nano composite with an appropriate texture for biomedical applications.

Abstract: In this work, the effects of hybrid (HA + Ag) particles and triple-pass friction stir processing on the microstructure, texture, hardness, and tensile behavior of magnesium matrix bio-nano composite were investigated. The results showed that the mean grain size of samples was in the range of 1–5 μm owing to the occurrence of dynamic recrystallization and suppression of grain growth by second phase particles. All samples exhibited uniform dispersion of particles in the magnesium matrix caused by triple-pass FSP. However, some agglomerations were visible in the microstructure of AZ91/nHA nanocomposite. The average grain size of the AZ91/nHA/smAg sample (1.4 μm) was smaller than that of the AZ91/nHA/mAg sample (2.1 μm), which was attributed to the formation of higher content of MgxAgy precipitates in the AZ91/nHA/smAg composite. By performing the FSP, the content of Mg17Al12 was significantly decreased due to the dissolution of beta into the alpha caused by the breakup effect of mechanical stirring and temperature increase of samples. The AZ91/nHA/smAg sample had the highest texture parameter for the { 10 1 ‾ 1 } orientation as the high corrosion resistance texture. This was due to the promoting the non-basal slip caused by the dissolution of smAg particles in the magnesium matrix. After the FSP, the microhardness distribution of AZ91, AZ91/nHA, AZ91/nHA/mAg, and AZ91/nHA/smAg samples tended to be uniform and the average hardness was improved owing to the fragmentation of beta particles, grain refinement, and homogeneous dispersion of second phase particles. Compared with the AZ91/nHA/mAg sample, an increase in ultimate tensile strength (291.7 MPa), and a decrease in total elongation (5.6%) and energy absorption (12.3 J/cm3) were observed in the AZ91/nHA/smAg sample due to the formation of a higher content of the silver-rich precipitates in the AZ91/nHA/smAg sample during cooling caused by the higher solubility of silver submicron particles. The fracture surfaces of all processed samples consisted of a large number of fine equiaxed dimples (ductile fracture) owing to the grain refinement and the presence of fine second phase particles.

Grain size effects and structure origin in high-performance BaTiO3-based piezoceramics with large grains

Abstract: Grain size shows significant influence on electrical properties of piezoceramics. However, there are few works to investigate the grain size effects in high-performance and large-grain piezoceramics and uncover the structure origin. In this work, large grain size from ~53 to ~92 µm was achieved in a high-performance BaTiO3 (BT)-based ceramic via tuning sintering conditions. With grain size increasing, the ceramics exhibit same multiphase coexistence state, similar phase transition temperature, upward TC dielectric peaks and reduced diffuseness degree. Because of the larger and more complex non-180° domains within bigger grains, the improvement in remnant polarization (Pr), coercive field and negative strain were observed in bigger-grain ceramics. The elevated Pr finally leads to the piezoelectric coefficient d33 increasing from 500 to 650 pC/N. However, too large grains may cause the reduced strain due to the high remnant strain in first cycle. Therefore, big grain size is conducive to achieve high piezoelectricity while moderate grain size can facilitate strain response in high-performance ceramics with large grains, which is quite different with pure BT ceramic. This work presents insights into the grain size effects and affords guides to further optimize electrical properties in high-performance piezoceramics with large grains.

Excellent thermal stability and mechanical properties of bulk nanostructured FeCoNiCu high entropy alloy

Abstract: In this work, we report bulk nanostructured equiatomic FeCoNiCu high-entropy alloy (HEA) with superior thermal stability and decent mechanical properties. High pressure torsion (HPT) was employed to refine the grains in the HEA into nanoscale (80 nm), and then Cu-rich particles were in-situ introduced at grain boundaries by subsequent annealing (≥700 °C), forming a multi-phase nanostructure. The Cu-rich particles are highly stable at high temperatures and impose a pinning effect on the grain boundaries, which significantly restricts grain growth (grain size <500 nm up to 900 °C), thus leading to outstanding thermal stability. On the other hand, the nanostructures also possess high yield strengths of 643∼1165 MPa, mainly attributing to grain boundary hardening. Meanwhile, the in-situ formed Cu-rich particles can concurrently deform with the matrix during straining, resulting in large total elongation (15∼20%). This work provides a novel strategy for designing bulk nanostructured HEA with good thermal stability and mechanical properties.

WC coarsening in cemented carbides during sintering. Part I: The influence of WC grain size and grain size distribution

Abstract: Kinetics of WC coarsening during sintering of WC-10 wt% Co cemented carbide grades was examined by use of submicron WC powders with a broad grain size distribution containing much nanograin fraction, medium-fine-grain powders with a narrow grain size distribution containing little nanograin fraction and medium-grain powders with a very narrow grain size distribution not containing nanograin fraction. Based on the kinetic curves re-constructed in the Arrhenius coordinates, values of the apparent activation energy for each carbide grade were obtained, which allowed the limiting stage of the WC coarsening process to be evaluated. The limiting stage of the WC coarsening process for the medium-grain grade is related to the diffusion of W and C atoms in the liquid binder during sintering. For the submicron grade the limiting stage of the WC coarsening process is related to interfacial reactions at WC/liquid interfaces. In this case, the diffusion of W and C atoms dissolved in the liquid binder is fast due to a very significant difference between sizes of the fine/nano WC grains and coarser WC grains. When increasing the sintering temperature and time above a certain level, the significant acceleration of WC coarsening takes place indicating the transition of the process to the stage of anomalous grain growth, at which the formation of abnormally large WC grains in the microstructure determines the whole WC coarsening process. The limiting stage of the WC coarsening process for the medium-fine-grain grade is mainly related to diffusion of W and C atoms dissolved in the liquid binder.

WC coarsening in cemented carbides during sintering. Part I: The influence of WC grain size and grain size distribution

Abstract: Kinetics of WC coarsening during sintering of WC-10 wt% Co cemented carbide grades was examined by use of submicron WC powders with a broad grain size distribution containing much nanograin fraction, medium-fine-grain powders with a narrow grain size distribution containing little nanograin fraction and medium-grain powders with a very narrow grain size distribution not containing nanograin fraction. Based on the kinetic curves re-constructed in the Arrhenius coordinates, values of the apparent activation energy for each carbide grade were obtained, which allowed the limiting stage of the WC coarsening process to be evaluated. The limiting stage of the WC coarsening process for the medium-grain grade is related to the diffusion of W and C atoms in the liquid binder during sintering. For the submicron grade the limiting stage of the WC coarsening process is related to interfacial reactions at WC/liquid interfaces. In this case, the diffusion of W and C atoms dissolved in the liquid binder is fast due to a very significant difference between sizes of the fine/nano WC grains and coarser WC grains. When increasing the sintering temperature and time above a certain level, the significant acceleration of WC coarsening takes place indicating the transition of the process to the stage of anomalous grain growth, at which the formation of abnormally large WC grains in the microstructure determines the whole WC coarsening process. The limiting stage of the WC coarsening process for the medium-fine-grain grade is mainly related to diffusion of W and C atoms dissolved in the liquid binder.