Coarse-grained Alloy Research Articles

Superplastic deformation mechanisms of coarse-grained rolled Mg-4Y-3RE magnesium alloy

The Mg-4Y-3RE (WE43) magnesium alloy possesses significant advantages such as high specific strength, excellent shock absorption, strong electromagnetic shielding capabilities and recyclability. However, its close-packed hexagonal structure leads to poor plasticity at room temperature, which limits its broader engineering applications. Therefore, superplastic forming at high temperatures is used to manufacture the components from this alloy. This study conducted tensile tests on hot-rolled WE43 rare-earth magnesium alloy with coarse grains at various temperatures and strain rates. The high-temperature superplastic properties were characterized, revealing the intrinsic mechanisms of thermal deformation behavior. The results indicate that the best superplasticity is achieved at 460 °C. This is attributed to the smallest grain size, the weakest texture, and the relatively uniform distribution of the second phase at this temperature. The influence of strain rate on elongation at temperatures among 440 °C∼500 °C is not significant as the impact of strain rate is multifaceted. Meanwhile, the elongation can reach up to 367.7 ± 3.7 % at a strain rate of 0.0 1s−1, which exhibits the high strain rate superplasticity (HSRS). Under these conditions, the deformation of coarse-grained WE43 rare-earth magnesium alloy is controlled by grain boundary sliding (GBS) and solute drag dislocation creep. Furthermore, the GBS involves deformation coordination mechanisms such as grain boundary diffusion, lattice diffusion, dislocation climbing, and dynamic recrystallization accommodation mechanisms.

High-Temperature Creep Resistance of FeAlOY ODS Ferritic Alloy.

A significant effort in optimizing the chemical composition and powder metallurgical processing led to preparing new-generation ferritic coarse-grained ODS alloys with a high nano-oxide content. The optimization was aimed at high-temperature creep and oxidation resistance at temperatures in the range of 1100-1300 °C. An FeAlOY alloy, with the chemical composition Fe-10Al-4Cr-4Y2O3 (wt. %), seems as the most promising one. The consolidation of the alloy is preferably conducted by hot rolling in several steps, followed by static recrystallization for 1 h at 1200 °C, which provides a stable coarse-grain microstructure with homogeneous dispersion of nano-oxides. This represents the most cost-effective way of production. Another method of consolidation tested was hot rotary swaging, which also gave promising results. The compression creep testing of the alloy at 1100, 1200, and 1300 °C shows excellent creep performance, which is confirmed by the tensile creep tests at 1100 °C as well. The potential in such a temperature range is the target for possible applications of the FeAlOY for the pull rods of high-temperature testing machines, gas turbine blades, or furnace fan vanes. The key effort now focuses on expanding the production from laboratory samples to larger industrial pieces.

Achieving low-temperature superplasticity of a coarse-grained BAZ531 magnesium alloy

An as-extruded Mg-5Bi-3Al-1Zn (wt%) alloy containing a coarse-grained structure and a large amount of nano-scale Mg3Bi2 precipitates showed 330 % tensile elongation-to-failure at 200 °C and 10-4 s-1. Superplasticity was mainly due to grain boundary sliding (GBS) and grain matrix slip deformation (GMD) at low and moderate strains, along with grain refinement at high strain making more GBS be activated.

Ultrafine-grained refractory high-entropy alloy with oxygen control and high mechanical performance

Grain boundary engineering plays a significant role in the improvement of strength and plasticity of alloys. However, in refractory high-entropy alloys, the susceptibility of grain boundaries to oxygen presents a bottleneck in achieving high mechanical performance. Creating a large number of clean grain boundaries in refractory high-entropy alloys is a challenge. In this study, an ultrafine-grained (UFG) NbMoTaW alloy with high grain-boundary cohesion was prepared by powder metallurgy, taking advantages of rapid hot-pressing sintering and full-process inert atmosphere protection from powder synthesis to sintering. By oxygen control and an increase in the proportion of grain boundaries, the segregation of oxygen and formation of oxides at grain boundaries were strongly mitigated, thus the intrinsic high cohesion of the interfaces was preserved. Compared to the coarse-grained alloys prepared by arc-melting and those sintered by traditional powder metallurgy methods, the UFG NbMoTaW alloy demonstrated simultaneously increased strength and plasticity at ambient temperature. The highly cohesive grain boundaries not only reduce brittle fractures effectively but also promote intragranular deformation. Consequently, the UFG NbMoTaW alloy achieved a high yield strength even at elevated temperatures, with a remarkable performance of 1117 MPa at 1200 °C. This work provides a feasible solution for producing refractory high-entropy alloys with low impurity content, refined microstructure, and excellent mechanical performance.

Influences of deformation defects on etching behaviors of high-strength and high-conductivity Cu alloy for lead frame

With increasing integration of the semiconductor devices, the requirements on size accuracy and surface quality of the lead frame are higher. Cold-rolling and low-temperature aging are usually used in processing of the Cu alloy for lead frame, which generate/eliminate the deformation defects that affect the etched surface morphology and roughness, while the influence mechanisms have not been well revealed. In this study, the CuCrSn alloy was used as the experimental material, which was cold-rolled to different degree and then aged at 450 °C for different time, and the microstructures and etched surfaces of the specimens were characterized. The results show that etched surface morphologies of grains with different orientations in the coarse-grained Cu alloy are different. With increasing cold-rolling deformation, the influences of grain orientation become less obvious, and the dislocation groups on the slip planes result in grooves approximately parallel to the rolling direction, meanwhile the surface uniformity are improved and the surface roughness decreases. For the 80 % cold-rolled Cu alloy, the density of grooves decreases with increasing aging time and elimination of the dislocations, and the etched surface roughness decreases firstly and then tends to be stable. The etched surface changes significantly and the surface roughness increase obviously after recrystallization of the specimen.

Corrosion behavior of an ultrafine-grained Ti6Al4V-5Cu alloy in artificial saliva containing fluoride ions

Ti6Al4V alloy has been widely used in dental applications, such as orthodontic mini-implants. However, it has been reported that fluoride ions could obviously accelerate the corrosion of implant materials and affect their performance. This work aimed to improve the F− erosion resistance of Ti6Al4V alloy through the strategy of both Cu addition and grain refinement. As contrasted with Ti6Al4V alloy, both the coarse- and ultrafine-grained Ti6Al4V-5Cu alloys effectively mitigated the acceleration of the fluoride ions to the anode process, because Cu substituents blocked the continuous damage of FO· doped in the passive film. Furthermore, grain refinement enhanced the protective ability of the passive film, more oxides and less adsorption amount of fluorides presented in the passive film of ultrafine-grained Ti6Al4V-5Cu alloy than those of coarse-grained Ti6Al4V-5Cu alloy. Under the combination of Cu alloying and grain refinement, the ultrafine-grained Ti6Al4V-5Cu alloy is greatly appropriate for the fabrication of orthodontic devices.

Effect of initial grain size on hot deformation behavior and recrystallization mechanism of Al-Zn-Mg-Cu alloy

Grain size is industrially important for improving mechanical properties and formability. In order to investigate the effect of grain size on the hot workability and deformation mechanisms, Al-Zn-Mg-Cu alloys with different initial grain sizes, 241.2, 100.4 and 48.4 μm, were subjected to hot compression tests. Using the established constitutive equation, the rheological behavior is described and the influence of the initial grain on the thermal deformation behavior is analyzed. Electron back-scatter diffraction (EBSD) analysis was used to study the dynamic recrystallization (DRX) mechanisms of Al-Zn-Mg-Cu alloys with different initial grain sizes. The microstructures obtained under various deformation conditions were analyzed. The results show that decreasing the initial grain size enhances both the peak stress and the activation energy for thermal deformation. Meanwhile, the dynamic recrystallization mechanism varies depending on the initial grain size. Continuous dynamic recrystallization (CDRX) is the primary DRX nucleation mode for coarse-grained alloys. Nonetheless, discontinuous dynamic recrystallization (DDRX) is predominant for alloys with fine grains.

Investigation on the tribocorrosion behavior of casting and spark plasma sintering Ni20Cr alloys in 3.5 wt% NaCl solution: Grain size effects

Tribocorrosion tests were performed on the Cast and SPS Ni20Cr alloys sliding against Si3N4 in 3.5 wt% NaCl solution to investigate the grain size effect on the synergy of corrosion and wear. An appealing result was that the SPS alloy with finer grains exhibited less intense corrosion-induced wear, but more severe wear-induced corrosion, compared to the coarse-grained Cast alloy. The mechanism responsible for the significant difference was attributed to the composition and microstructure of the tribo-layer influenced by grain refinement. The finer grain structure facilitated the rapid replenishment of thicker Cr-rich tribo-layer, while meanwhile the dense layer exhibited high hardness and greatly reduced the wear rate of SPS alloy.

Effect of Al–5Ti–1B Addition on Solidification Microstructure and Hot Deformation Behavior of DC-Cast Al–Zn–Mg–Cu Alloy

This study investigated the effect of adding Al–5Ti–1B grain refiner on the solidification microstructure and hot deformation behavior of direct-chill (DC) cast Al–Zn–Mg–Cu alloys. The grain refiner significantly decreased the grain size and modified the morphology. Fine-grained (FG) alloys with grain refiners exhibit coarse secondary phases with a reduced number density compared to coarse-grained (CG) alloys without grain refiners. Dynamic recrystallization (DRX) was enhanced at higher compression temperatures and lower strain rates in the CG and FG alloys. Both particle stimulated nucleation (PSN) and continuous dynamic recrystallization (CDRX) are enhanced in the FG alloys, resulting in decreased peak stress values (indicating DRX onset) at 450°C. The peak stress of the FG alloys was higher at 300-400°C than that of the CG alloys because of grain refinement hardening over softening by enhanced DRX.

Influence of grain size on α′ Cr precipitation in an isothermally aged Fe-21Cr-5Al alloy

Cr-rich α′ precipitation during aging typically leads to hardening and accordingly embrittlement of FeCrAl alloys, which needs to be suppressed. The influence of grain size on α′ precipitation was studied by aging coarse-grained (CG), ultra-fine grained (UFG), and nanocrystalline (NC) ferritic Kanthal-D [KD; Fe-21Cr-5Al (wt.%) alloy] at 450, 500 and 550 °C for 500 h. After aging at 450 and 500 °C, less hardening was observed in the UFG KD than in CG KD. Atom probe tomography indicated a lower number density and larger sized intragranular α′ in the UFG versus the CG alloy. The smaller grain size and higher defect (vacancy and dislocation) density in the UFG KD facilitated diffusion and accordingly enhanced precipitation kinetics, leading to coarsening of precipitates, as well as saturation of precipitation at lower temperatures, as compared to those in CG KD. No hardening occurred in UFG and CG KD after aging at 550 °C, indicating that the miscibility gap is between 500 and 550 °C. NC KD exhibited softening after aging owing to grain growth. α′ precipitation occurred in NC KD aged at 450 °C but not at 500 °C, indicating that miscibility gap is between 450 and 500 °C. Thus, the significantly smaller grain size in NC KD decreased the miscibility gap, as compared to that in CG and UFG KD. This is attributed to the absorption of vacancies by migrating grain boundaries during aging, suppressing α′ nucleation and enhancing Cr solubility.

Enhancement of the three-point bending behavior of a coarse-grained NiTi shape memory alloy with surface heterogeneous gradient microstructure and compressive residual stress

NiTi shape memory alloys offen subject to bending deformation as common functional materials. Laser shock peening (LSP) was utilized to fabricate a gradient layer to improve the bending mechanical behavior of a coarse-grained NiTi plate. A surface gradient layer with a thickness of approximately 100 µm was created in a 1.38 mm thick coarse-grained NiTi plate. The LSPed surface contained nanocrystals, dislocations, and amorphous, while the interior layer contained a substantial number of deformation twins, dislocations, and precipitates. The nanohardness of the surface treated with LSP can reach 10 GPa. The results of a three-point bending experiment indicate that LSP treatment increases the bending resistance of the NiTi plate. This study demonstrated that laser shock peening is an effective method for increasing the deformation resistance of a coarse-grained NiTi by means of surface gradient structures, residual stress, and precipitates.

Interaction and superposition of creep damage and high-cycle fatigue of coarse-grained nickel-base alloy 247

ABSTRACT The cast coarse-grained polycrystalline Alloy 247 is investigated, which is typically used in the rear turbine blades of land-based gas turbines. High rotational speeds up to 10,000 rpm in combination with high temperatures induce creep deformation. Additionally, gas turbine blades are subjected to high-frequency cyclic stresses. This complex loading state requires the investigation of the interaction of creep damage and high-cycle fatigue (HCF). Therefore, HCF tests with a stress ratio of Rσ = −1 were carried out on as-received and pre-crept specimens. In comparison, an HCF test series with high mean stresses (Rσ = 0.5) was conducted to consider the superposition of creep and fatigue. Since nickel-base alloys exhibit a pronounced elastic anisotropy, the grain orientations were considered in the damage analysis. The results show that creep-induced grain boundary damage is a dominant structural parameter for the HCF behaviour of Alloy 247 in both, sequential and superpositioned creep-fatigue loading.

Strength and ductility improvement in a heterostructured Mg-Gd-Y alloy with inversely-gradient hardness distribution

The Mg-8.75Gd-2.85Y (wt%) alloy processed by surface mechanical attrition treatment (SMAT) was annealed at 350 °C followed by peak-ageing. Recrystallization occurred in the surface layer of the specimen induced by the annealing treatment. The obtained heterostructure contained a fine-grained (FG) layer with thickness of about 350 μm on the top and a coarse-grained (CG) layer at the bottom. The sample exhibited a reversely-gradient distribution in hardness, in which the FG layer showed lower values than the CG layer. It was related to the different behaviors of recrystallization and precipitation. Compared with the homogeneous CG alloy, the heterostructured sample exhibited enhancement in both strength and ductility. The increase of yield strength was mainly ascribed to grain boundary strengthening and dislocation strengthening. The random texture of the FG layer and the synergetic effect of the heterostructure were beneficial to the improvement of ductility. This work provides valuable insights for developing gradient-structured Mg-RE alloys for high performance in engineering applications such as automotive, aerospace and defense industries.

Achieving excellent strength-toughness combination in hexagonal closed-packed multi-principle element alloys via 〈c + a〉 slip promotion

To effectively bolster 〈c + a〉 slip is pivotal to improving the mechanical property of hexagonal close-packed (HCP) metallic materials. It has been a longstanding challenge facing the structural material community. Here we introduce a strategy to crack this hard nut. By employing two coarse-grained Ti-Zr-Hf model alloys as examples, we demonstrate for the first time that the two intrinsic properties of multi-principle element alloys (MPEAs)—high alloy concentration and local fluctuation—can work synergistically to narrow the gap in nucleation/gliding resistance between non-prismatic and prismatic slips, leading to enhanced activities of 〈c + a〉 and first order pyramidal 〈a〉 dislocations. This renders the material an exceptional strength-toughness combination, outperforming other coarse-grained HCP alloys and even rivalling most of those with delicately designed microstructrues including nanostructures and heterostructures. Given that high concentration and local fluctuation are intrinsic to all MPEAs, we anticipate that this strategy could be extended to other MPEAs featuring an HCP structure.

Grain boundary self- and Mn impurity diffusion in equiatomic CoCrFeNi multi-principal element alloy

Grain boundary self-diffusion of Co, Cr and Fe and impurity diffusion of Mn are measured in a coarse-grained equiatomic CoCrFeNi multi-principal alloy. The tracer diffusivities are determined in a wide temperature range of 643 K to 1273 K, which encompasses both the C- and B-type kinetic regimes of grain boundary diffusion in polycrystalline materials after Harrison’s classification. At higher temperatures (T>800 K), only one short-circuit (grain boundary) contribution is observed, while the existence of two distinct contributions is elucidated by thorough analysis of the penetration profiles corresponding to the C-type kinetic regime (643–703 K). The latter observations are explained in terms of a grain boundary phase decomposition after prolonged annealing below 700 K. The product of the segregation factor and the grain boundary width is found to be about 0.5 nm for all constituting elements. The grain boundary diffusion data indicate that Mn does not reveal a strong (if any) segregation in the equiatomic CoCrFeNi alloy.

Variational mode decomposition based self-adaptive denoising imaging method for ultrasonic array testing of coarse-grained titanium alloys processed by additive manufacturing

The coarse columnar grain structure in titanium alloys processed by additive manufacturing leads to strong grain noise in ultrasonic testing, which affects the accurate detection and quantification of its defects. In this paper, a self-adaptive denoising imaging method for ultrasonic array testing is proposed based on variational mode decomposition (VMD). The proposed method uses VMD to decompose complex ultrasonic signals in full matrix capture (FMC) data collected by a linear ultrasonic array transducer. According to the particle swarm optimization (PSO) method, VMD parameters for FMC methods of the total focusing method (TFM), the plane wave imaging (PWI), and the Hadamard spatial encoding are self-adaptively determined. The evaluation parameters are set to select the intrinsic mode function components by VMD to complete the denoising reconstruction and imaging of FMC data. An additive-manufactured titanium alloy specimen was used to verify the improvement of defect detection capability by PSO-VMD denoising method. For a defect at large thickness in the specimen, compared with the conventional TFM and PWI methods, VMD denoising method based on the Hadamard spatial encoding can improve the signal-to-noise ratio by 7.7 dB and 9.0 dB, and reduce the quantitative error by 74 % and 68 %.

Phase transformation kinetics in a coarse-grain Ti17 alloy determined by laser ultrasonics and dilatometry

Different methods are used to determine the phase transformation titanium alloys. While ex-situ methods require a large number of samples and accurate quantification of the phases, in-situ methods can provide the kinetics of transformation continuously and with less samples. However, the results are based on the changes of physical properties, and the interpretation of the results in terms of phase transformation are not direct. This study compares two in-situ methods to measure the kinetics of phase transformation for a Ti17 alloy, namely: laser ultrasonics for metallurgy (LUMet) and dilatometry. While LUMet is coupled to a Gleeble® 3500 thermo-mechanical simulator with ohmic resistance heating, the dilatometer device uses induction heating. We performed two types of experiments by heating the specimens above the β-transus temperature (∼ 865 °C) and 1) and then fast cooled below the β-transus temperature and held at different temperatures, or 2) continuously cooled at different rates. The β→α + β phase transformation is inferred by the evolution of the longitudinal wave speed with the LUMet method and by the change in the length of the specimen by dilatometry. We obtained the information about the isothermal phase transformation using the normalized changes in length for the dilatometric data and in longitudinal ultrasonic velocity. In the investigated isothermal temperature range (580 – 700 °C), the β →α + β transformation is completed within one hour of holding with the fastest transformation rates at 580 °C. For the continuous cooling treatments, the small change in length measured by dilatometry makes it impractical to use the lever rule to accurately determine the relative transformation fraction. However, we could successfully apply the lever rule to the LUMet data. Furthermore, the derivatives of the change in length and ultrasonic velocity are used to estimate the transformation temperatures as a function of the cooling rates. Further, the in-situ measurements are supplemented with ex-situ characterization of the microconstituents after heat treatments using scanning electron microscopy.

Thermoelectric behavior of Bi2Te2.55Se0.45 with a tunable seebeck coefficient: A comparison between coarse needle-like structure and bulk nanostructured alloys

In this study, we compare the thermoelectric properties of coarse-grained n-type Bi2Te2.55Se0.45 alloy prepared by induction melting with the bulk-nanostructured prepared by ball milling and hot-pressing techniques. The corresponding thermoelectric properties showed different behavior for each material processed using different routes. The most striking result of the study is observing a p-type behavior and charge carrier transition from p-to n-type for the coarse-grained alloy. These observed phenomena are related mainly to the unique needle-like microstructure accompanied by high lattice strain. Lastly, the obtained figure-of-merit values for the coarse needle-like structure and bulk nanostructured Bi2Te2.55Se0.45 alloys are 0.20 and 0.80 at their optimum temperatures of 60 and 120 °C, respectively.

Machinability Features of Ti-6Al-4V Alloy with Ultrafine-Grained Structure

Titanium alloys are widely used in various industries. The most common and well-known titanium alloy is titanium alloy with aluminum and vanadium (Ti-6Al-4V). This alloy is used, for example, in the manufacture of aircraft engines. As part of the development of technologies and the emergence of the evolving requirements for materials, Ti-6Al-4V alloys with ultrafine grains less than 1 μm may become promising. This modification of the alloy has excellent strength characteristics, such as increased fatigue resistance. However, manufacturers are aware of the machinability problem of titanium alloys. To date, a sufficiently high level of understanding of this problem has already been achieved. But, there is practically no information about the machinability of ultrafine-grained alloys and their comparison, in this regard, with the usual coarse-grained version. This study presents the results of experimental studies on the influence of cutting parameters (cutting speed, V, m/min; feed rate, Fz, mm/rev) on the roughness and microstructure of the surface of Ti-6Al-4V samples with coarse-grained and ultrafine-grained structures produced via equal-channel angular pressing. It is shown that turning at a low cutting speed (V = 48 m/min) results in a better surface roughness, Ra, for the coarse-grained sample compared to its ultrafine-grained alloy counterpart. When the cutting speed is increased by 1.5 times (up to V = 72 m/min), on the contrary, the ultrafine-grained sample has a lower surface roughness, Ra, compared to the coarse-grained sample. The differences in the morphology and microstructure of the chips, depending on the microstructure type of the processed alloy, are discussed: the presence of plastic flow lines in the chip microstructure of the turned ultrafine-grained sample and the formation of shear bands, cleavages, and microcracks in the chips of the turned coarse-grained alloy.

Grain boundary sliding with diffusional accommodation enable dynamic recrystallization during hot deformation of coarse-grained body-centered cubic HfNbTaTiZr high-entropy alloys

The origin of ultrafine grain refinement durig hot deformation of equiatomic HfNbTaTiZr body-centered cubic (BCC) high-entropy alloy (HEA), having an initial grain size, d ∼ 140 μm is systematically investigated. Microstructural observation at very low strain (ε ∼ 0.04) interval showed unique saw-toothed and stair case-like ledges along the initial grain boundaries (GB). Further analyses showed triple junction folds, and shear offsets at the scratch line and GB intersections suggesting a grain boundary sliding (GBS) mechanism. This study shows direct evidence for the possibility of GBS mechanism in a coarse grained HfNbTaTiZr BCC-HEA, for the first time. The possibility of ultrafine grain refinement is discussed using GBS mechanism with diffusional accommodation. The present study featuring coarse grained GBS mechanism in HfNbTaTiZr alloy provided insights into the high energy GB structure of refractory BCC-HEAs.