Formation Of Grain Structure Research Articles

Hot Deformation and Dynamic Recrystallization Behavior of CoCrNi and (CoCrNi)94Ti3Al3 Medium Entropy Alloys

The CoCrNi and precipitate-hardened (CoCrNi)94Ti3Al3 medium entropy alloys (MEAs) have attracted much attention, due to their exceptional mechanical properties, whereas the hot deformation characteristics have not been revealed. In the present study, we investigated the dynamic recrystallization behavior and microstructure evolutions of the two MEAs hot-compressed at single-phase temperatures. The constitutive equation was obtained, and microstructures were observed. Discontinuous dynamic recrystallization acted as a key mechanism of grain refinement at a relatively higher temperature and lower strain rate, which leads to the formation of a homogeneous grain structure. The addition of Ti and Al promoted dynamic recrystallization due to the solid solution hardening effect. The results provide valuable guidelines for microstructure refinement via thermomechanical processing.

The effect of vibratory conditioning on tensile strength and microstructure of 1018 mild steel

PurposeThe purpose of this paper is to investigate the prominence of mechanical excitations at the time of welding. In the past years, the process of welding technology has expanded its influence in manufacturing. The crucial drawback of conventional welding is prompted by internal stresses and distortions, which is the focal reason for weld defects. These weld defects can be diminished by the process called post-weld heat treatment (PWHT), which consumes more working hours and needs skilled workers. To replace these PWHT processes, mechanical vibrations are introduced during the process of welding to diminish these weld defects.Design/methodology/approachIn the current research, the mechanical vibrations are transferred to weld-pool through vibro-motor and DC motor connected to the electrode. As per standards, the tensile test specimens were prepared for welding with different voltages of vibro-motor and DC motor respectively. The weld joints were tested for tensile strength and analyzed the microstructure at the fusion zone.FindingsMelt-ability at fusion zone of 1018 mild steel was investigated by the single-stroke intense heat process of fusion welding. It is observed that the mechanical vibrations technique has a profound influence on the enhancement of the fusion zone characteristics and grain structure. The peak value of the tensile strength is observed at 100 s of vibration, 190 V of vibro-motor voltage and 18 V of electrode voltage. The tensile strength of the welded joints with vibrations is increased up to 22.64% when it is compared with conventional welding. The enhancement of the tensile strength of the weld bead was obtained because of the formation of fine grain structure. So, mechanical vibrations are identified as the most convenient method for improving the mild steel alloys weld quality.Originality/valueA novel approach called mechanical vibrations during the process of welding is implemented for fusion zone refinement.

Influence of undissolved second-phase particles on dynamic recrystallization behavior of Mg–7Sn–1Al–1Zn alloy during low- and high-temperature extrusions

This study investigates the effects of fine and coarse undissolved particles in a billet of the Mg–7Sn–1Al–1Zn (TAZ711) alloy on the dynamic recrystallization (DRX) behavior during hot extrusion at low and high temperatures and the resultant microstructure and mechanical properties of the alloy. To this end, partially homogenized (PH) and fully homogenized (FH) billets are extruded at temperatures of 250 and 450 °C. The PH billet contains fine and coarse undissolved Mg2Sn particles in the interdendritic region and along the grain boundaries, respectively. The fine particles (<1 μm in size) retard DRX during extrusion at 250 °C via the Zener pinning effect, and this retardation causes a decrease in the area fraction of dynamically recrystallized (DRXed) grains of the extruded alloy. In addition, the inhomogeneous distribution of fine particles in the PH billet leads to the formation of a bimodal DRXed grain structure with excessively grown grains in particle-scarce regions. In contrast, in the FH billet, numerous nanosized Mg2Sn precipitates are formed throughout the material during extrusion at 250 °C, which, in turn, leads to the formation of small, uniform DRXed grains by the grain-boundary pinning effect of the precipitates. When the PH billet is extruded at the high temperature of 450 °C, the retardation effect of the fine particles on DRX is weakened by their dissolution in the α-Mg matrix and the increased extent of thermally activated grain-boundary migration. In contrast, the coarse Mg2Sn particles in the billet promote DRX during extrusion through the particle-stimulated nucleation phenomenon, which results in an increase in the area fraction of DRXed grains. At both low and high extrusion temperatures, the extruded material fabricated using the PH billet, which contains both fine and coarse undissolved particles, has a lower tensile strength than that fabricated using the FH billet, which is virtually devoid of second-phase particles. This lower strength of the former is attributed mainly to the larger grains and/or absence of nanosized M2Sn precipitates in it.

Bimodal grain structures and tensile properties of a biomedical Co–20Cr–15W–10Ni alloy with different pre-strains

The influence of pre-strain on the formation of bimodal grain structures and tensile properties of a Co–20Cr–15W–10Ni alloy was investigated. The bimodal grain structures consist of fine grains (FGs; 2–3 μm in diameter) and coarse grains (CGs; 8–16 μm in diameter), which can be manipulated by changing the pre-strain (ɛ = 0.3–0.7) and annealing temperatures (1000–1100 °C). High pre-strain applied in the samples can intensify the plasticity heterogeneity through increasing the total dislocation density and the local volumes of high-density dislocations. This can essentially result in finer FGs, a higher FG volume fraction, and overall grain refinement in the samples after annealing. High-temperature essentially increases both the size and volume fraction of CGs, leading to an increase in the average grain size. The tensile test suggests that the bimodal grain structured samples exhibited both high strength and ductility, yield strengths of 621–877 MPa and ultimate tensile strengths of 1187–1367 MPa with uniform elongations of 55.0%–71.4%. The superior strength-ductility combination of the samples arises from the specific deformation mechanisms of the bimodal grain structures. The tensile properties strongly depend on the size ratio and volume fraction of FGs/CGs in addition to the average grain size in the bimodal grain structures. The grain structures can be modified via changing the pre-strain and annealing temperature.

Effects of initial texture on deformation behavior during cold rolling and static recrystallization during subsequent annealing of AZ31 alloy

This study demonstrates that the initial texture of Mg alloy significantly affects the microstructure developed during cold rolling and the recrystallization behavior during subsequent annealing. In a sample with a texture oriented toward the normal direction (ND sample), thick and large-sized shear bands are intensively formed during cold rolling and the deformation is strongly localized along these bands. In contrast, in a sample with a texture oriented toward the transverse direction (TD sample), many {10-12} twins are formed at the early stage of cold rolling, and then, numerous small-sized shear bands are formed in these twins. Results of nanoindentation tests reveal that the cold-rolled ND sample has internal strain energy that is substantially concentrated along the shear bands, whereas the cold-rolled TD sample has a large amount of internal strain energy that is homogeneously distributed throughout the material; this latter behavior is strongly related to the extensive {10-12} twinning and the resultant microstructural variations in the TD sample. During subsequent annealing, recrystallization occurs locally along the shear bands in the ND sample, which leads to the formation of a bimodal grain structure comprising fine recrystallized grains and coarse unrecrystallized grains. In contrast, during subsequent annealing of the TD sample, static recrystallization occurs homogeneously throughout the material, which results in the formation of a uniform grain structure that mostly comprises equiaxed recrystallized grains.

Microstructure and Charpy Impact Toughness of a 2.25Cr-1Mo-0.25V Steel Weld Metal.

The demand for heat-resistant steel has increased owing to its utility in numerous devices that must withstand high steam pressures and high temperatures, such as turbine rotors and blades in ultra-supercritical power plants. It is inevitable to join heat-resistance steel part by welding method, so it is important to maintain the toughness of the weld metals. In this study, the microstructure, low-temperature impact toughness, and fracture surface of as-welded and post-weld heat treatment (PWHT) of 2.25Cr-1Mo-0.25V weld metal were investigated. The microstructures of the as-welded and PWHT specimens are granular bainite and ferrite, respectively. This work revealed the relationship between effective microstructure nearby crack initiation origin and low temperature impact toughness for both the as-welded and PWHT specimens. The evolution of the microstructure and prior austenite was then investigated using confocal laser scanning microscopy (CLSM) to observe the formation of coarse ferrite grain structures. A suggestion for enhancing the low-temperature toughness was provided based on the effect of adjusting Mn content and forming acicular ferrite.

Grain Refinement of Alloys in Fusion-Based Additive Manufacturing Processes

One of the less desirable aspects of fusion-based additive manufacturing is the propensity for coarse columnar grain structures crossing build layers to form. This paper initially attempts to explain the reason for the formation of columnar grain structures in terms of the high thermal gradients typically observed during solidification and the alloy compositions that are typically used which promote epitaxial growth. Successful approaches to the grain refinement of titanium alloys using alloying elements that produce constitutional supercooling are discussed along with the difficulty with nucleant additions. Much of the grain-refining technology already used in aluminium casting is shown to also be applicable to additive manufacturing, although the novelty of the effective use of nanoparticles as nucleants is highlighted. It is also shown that for other alloy systems for which there is a lack of grain-refining technology using chemical means, mechanical means, such as ultrasonic treatment, can be effective across a wide range of alloys. Finally, consideration is given to the difficulties and the possible solutions of producing parts layer by layer. In particular, the importance of understanding nucleation in solidification conditions characterized by high cooling rates and thermal gradients; the importance of melt dynamics; and how previous layers could provide possibilities for refinement of the subsequent layer are highlighted.

Effect of Multidirectional Forging and Subsequent Annealing to the Microstructure of Al-Mg-Mn Type Alloy

The grain refinement is important to improve both service properties at room temperature and superplasticity at elevated temperatures. This study focuses on the effect of multidirectional forging in isothermal conditions on the microstructure of Al-Mg-Mn-type alloy. The evolution of dislocation and grain structure, and precipitates of Mn-rich phase during multidirectional forging in a temperature range of 200 to 500 °C was studied. Multidirectional forging at temperatures of 200 and 300 °C leads to the formation of shear bands in the deformed grains. The multidirectional forging at 400 and 500 °C leads to the formation of a bimodal grain structure with fine- and coarse-grained areas. Subsequent recrystallization annealing at 500 °C increases the grain size and decreases the fine grains fraction in the samples pre-deformed at 400-500°C, and, on the contrary, annealing leads to formation homogeneous and fine grain structure with size up to 6.5 μm in samples pre-deformed at 200 and 300 °C.

Effects of CoAl2O4 inoculants on microstructure and mechanical properties of IN718 processed by selective laser melting

This work investigated the effects of CoAl2O4 inoculants on the microstructure and mechanical properties of a superalloy – Inconel 718 (IN718), processed by selective laser melting (SLM). IN718 powder was blended with 0.2 wt. % of CoAl2O4 particles. After the SLM process, the CoAl2O4 particles reacted with IN718 and formed a dispersion of Al-rich nano-oxide particles in the matrix. These inoculants were observed to facilitate the formation of fine, equiaxed grains and reduce the degree of crystallographic texture in the as-built microstructure. The presence of the nano-oxide particles in the microstructure served to restrict the mobility of grain boundaries during heat-treatment and promoted the formation of a bimodal grain structure with comparatively finer average size. Due to the reduction of crystallographic texture, the elastic anisotropy present in as-built specimens was greatly minimized with the addition of CoAl2O4 inoculants. Specimens containing inoculants were also found to possess improved tensile properties following heat-treatment at both room temperature and 650 °C. Furthermore, creep testing at 650 °C/650 MPa revealed that the steady strain rate of IN718 was lowered from 8.8×10−9 s−1 to 4.9×10−9 s−1, and the creep rupture life was extended by 52 hours in samples that were fabricated with CoAl2O4. These results strongly suggest that the decomposition of CoAl2O4 into Al-rich oxides during SLM processing not only contributes to grain refinement and a reduction in the degree of crystallographic texture but also forms an oxide dispersion that restricts the mobility of dislocations and grain boundaries in IN718.

Effect of prior austenite grain size on the abrasive wear resistance of ultra-high strength martensitic steels

Prior austenite grain size has a marked effect on the hardenability, strength, and impact toughness properties of steels. This study was conducted in order to understand the effect of prior austenite grain size and morphology on the mechanical properties and abrasive wear performance of an ultra-high strength steel. A commercial quenched 500 HB grade wear-resistant steel was selected for the study: the steel was austenitized at two different temperatures and compared to the original, as-received quenched condition. The resulting mean prior austenite grain size was ranging from 14 μm to 34 μm. The decrease in grain size improved the low-temperature impact toughness properties. A high stress abrasive wear testing method with natural granite abrasives was utilized for the evaluation of abrasive wear resistance. The results suggest that decreasing the prior austenite grain size improves the abrasive wear resistance with similar hardness level martensitic steels. In addition, high-resolution electron backscatter diffraction measurements revealed formation of ultra-fine grain structures in the severely deformed regions of the wear surfaces.

Effects of pin diameter and number of cycles on microstructure and tensile properties of friction stir fabricated AA1050-Al2O3 nanocomposite

Nanocomposites of Al-Al2O3 are fabricated by means of friction stir processing (FSP). Effects of pin diameter and number of FSP cycles on tensile properties of the nanocomposites are investigated and the variations are correlated to the evolution of microstructure and distribution of nanoparticles in addition to agglomeration of Al2O3 nanoparticles. Investigation of agglomeration has been considered as an indirect indication for efficiency of the process for distribution of nanoparticles. The ductility of the nanocomposite is found to improve due to grain refinement during FSP. However, the ductility is likely to degrade if Al2O3 particles agglomerate and form coarse particles. The composite fabricated using the 6-mm pin indicates the maximum ductility which is attributed to formation of fine grain structure and efficient distribution of nanoparticles in the composite. Although, the grain structure in the composite fabricated using 8-mm pin is well refined, this sample shows significantly lower ductility with respect to the other samples. This was attributed to formation of coarse agglomerated particles. The second pass of FSP is found to slightly improve ductility and strength in composites fabricated using 4- and 6-mm pins but enforce significant improvements in the one with 8-mm pin. This is indeed because the second pass results in significant change in the distribution of nanoparticles or agglomerated particles in the latter case and negligible in the former ones. Grain structure and nanoparticle distribution and agglomeration can all affect the fracture surface of the tensile specimens of the fabricated nanocomposite to be ductile or brittle.

Novel High-Strength Casting Al−Zn−Mg−Ca−Fe Aluminum Alloy without Heat Treatment

Based on the studies of its structure, as well as technological and mechanical properties, the composition and application prospects are substantiated for a high-strength aluminum Al–Zn–Mg–Ca–Fe casting alloy with no heat treatment. As the subjects of study, we have obtained alloys based on a composition of Al–5.5% Zn–1.5% Mg (wt %) jointly and separately alloyed with 0.5–1.0% Ca, and 0.5% Fe. As the reference, we used standard AK12M2, AMg6lch, and AM4.5Kd casting alloys corresponding to GOST (State Standard) 1583–93: by means of pencil sample casting, the alloy has been tested for the tendency to form hot cracks owing to hindered shrinkage. It is shown that separate alloying with calcium and iron does not promote any improvement of crack resistance and adversely affects mechanical properties. The combined alloying with 1% Ca and 0.5% Fe makes it possible to improve the parameter of hot tearing susceptibility to the level inherent in the AMg6lch alloy. This effect is caused by the formation of eutectic calcium-containing phases, and the formation of a favorable grain structure in which there are no columnar grains. Iron in the structure of the alloy is bound in compact particles, representing a Al10CaFe2 phase, which is a consequence of a nonequilibrium crystallization process occurring in the course of chill-mold casting. The formation of this phase makes it possible to reduce the amount of zinc in the (Al, Zn)4Ca phase and mostly to conserve the composition of the (Al) solid solution, as is indicated by similar hardness values of the base Al–5.5% Zn–1.5% Mg alloy and Al–5.5% Zn–1.5% Mg–1% Ca–0.5%–Fe alloy, as well as the superiority of these values over the hardness values of alloys separately alloyed with calcium and iron. In addition, the hardness of the promising alloy in the as-cast state is more than 20 HV higher than the hardness of branded cast alloys in the same condition. The novel alloy in the as-cast state has shown the following competitive mechanical tensile properties: σUTS ~ 310 MPa, σ0.2 ~ 210 MPa, and δ ~ 4%.

Grain structure control during metal 3D printing by high-intensity ultrasound

Additive manufacturing (AM) of metals, also known as metal 3D printing, typically leads to the formation of columnar grain structures along the build direction in most as-built metals and alloys. These long columnar grains can cause property anisotropy, which is usually detrimental to component qualification or targeted applications. Here, without changing alloy chemistry, we demonstrate an AM solidification-control solution to printing metallic alloys with an equiaxed grain structure and improved mechanical properties. Using the titanium alloy Ti-6Al-4V as a model alloy, we employ high-intensity ultrasound to achieve full transition from columnar grains to fine (~100 µm) equiaxed grains in AM Ti-6Al-4V samples by laser powder deposition. This results in a 12% improvement in both the yield stress and tensile strength compared with the conventional AM columnar Ti-6Al-4V. We further demonstrate the generality of our technique by achieving similar grain structure control results in the nickel-based superalloy Inconel 625, and expect that this method may be applicable to other metallic materials that exhibit columnar grain structures during AM.

Формирование зеренной структуры интерметаллида Ni3Al при высокотемпературном синтезе под давлением

Формирование зеренной структуры интерметаллида Ni3Al при высокотемпературном синтезе под давлением

Texture tailoring and bendability improvement of rolled AZ31 alloy using {10–12} twinning: The effect of precompression levels

Abstract In this study, the texture of a rolled Mg alloy is effectively modified through the application of precompression and subsequent annealing treatment, leading to a remarkable improvement in the bending formability of the alloy at room temperature. Precompression induces lattice reorientation through {10–12} twinning, and annealing treatment reduces the stored strain energy of the precompressed material, which results in the formation of a stable grain structure with two dominant texture components. With an increase in precompression, the tensile strain in the outer region of the bending samples is accommodated to a greater extent due to more pronounced {10–12} twinning and basal slip. As a result, the bending formability of the material at room temperature improves with greater precompression. The variation in microstructure, texture, and bending behavior in relation to the degree of precompression is discussed in detail.

In-situ formed heterogeneous grain structure in spark-plasma-sintered CoCrFeMnNi high-entropy alloy overcomes the strength-ductility trade-off

The conflict between strength and ductility has been a longstanding dilemma for metallic alloys. Herein, we present a facile strategy to overcome the strength-ductility trade-off of face-center-cubic high-entropy alloys (HEAs) via the in-situ formation of heterogeneous grain structure, using the high local temperature gradients generated during spark plasma sintering coupled with the ‘sluggish diffusion’ of HEAs. The fabricated CoCrFeMnNi HEA has a heterogeneous structure over a large range of grain size from 140 nm to 5 μm, which enables a gigapascal yield strength and a uniform elongation of 9.5%. The mechanism responsible for the associated mechanical properties was analyzed.

Effect of Nanoscale Mesoscopic Structural States Associated with Lattice Curvature on the Mechanical Behavior of Fe-Cr-Mn Austenitic Steel

The paper explores the effect of high-temperature cross rolling followed by cold rolling on the internal structure of metastable Fe-Cr-Mn austenitic stainless steel, formation of nonequilibrium martensite e and α′ phases in it, dynamic rotations on fracture surfaces, fatigue life under alternating bending, and wear resistance. Scratch testing revealed a strong increase in the damping effect in the formed hierarchical mesoscopic substructure that promotes the formation of a nanocrystalline grain structure, formation of hcp e martensite and bcc α′ martensite in grains, formation of a vortical filamentary substructure on the fracture surface, and an increase in the high-cycle fatigue properties and wear resistance. These processes are associated with a high density of nanoscale mesoscopic structural states that arise in lattice curvature zones during high-temperature cross rolling followed by cold rolling with smooth rolls. The described effects are explained by the self-consistent mechanical behavior of hcp e martensite laths in fcc austenite grains and bcc α′ martensite laths formed in cold rolling of the steel after high-temperature cross rolling.

Improvement in Mechanical and Metallurgical Properties of Friction Stir Welded 6061-T6 Aluminum Alloys through Cryogenic Treatment

An investigation has been made to improve the properties of the friction stir welded (FSW) 6061-T6 aluminum alloys. A cryogenic thermal treatment is developed for the joints during welding and its effects on mechanical and metallurgical properties, and precipitates are evaluated at various welding parameters. The friction stir welded joints with cryogenic treatment attained the better properties than the without cryogenic treatment. The improvement of properties was attributed to the refinement of grain size and to the introduction of a reduction in the softening region of the welded joints. Under cryogenic cooling rates, joints were experienced to the low temperature environments and faster cooling rates, which are contributed to enhance the hardness of the stir zone and heat affected zone regions and the formation of fine grain structure in the stir zone. The results indicated that the formation of finer grains of less than 5 µm in the stir zone, which is smaller than the joints of without cryogenic treatment. Subsequently, mechanical properties drastically improved and the joints achieved a maximum joint efficiency of 74% of the base material

Cooling Behavior and Microstructure of Semisolid A201 Aluminum Alloy Prepared by the SEED Process

The biggest challenge in semisolid processing of high-performance aluminum alloys is the narrow temperature processing windows of these alloys, and as a result, the preparation of qualified semisolid slurries is very important. High solid fraction slurries of high-strength A201 alloy were prepared by the Swirled Enthalpy Equilibration Device (SEED) process. The cooling behavior and microstructures of the A201 slurries produced by the standard, as well as a modified, SEED process were investigated. The results show that qualified A201 slurry can be produced by decreasing the pouring temperature and controlling the processing time in the SEED process. The modified SEED process significantly reduced the radial temperature gradient of the melt, due to the slow cooling rates involved, with the resulting slurries being more uniform, with more spherical microstructures, as compared to those produced by the standard SEED process. The formation of the nondendritic grain structure in the SEED process is attributed to the uniformly distributed large number of nuclei within the melt and the slow cooling of the melt in the containing crucible.

Thin Ag Precursor Layer-Assisted Co-Evaporation Process for Low-Temperature Growth of Cu(In,Ga)Se2 Thin Film.

Achieving favorable band profile in low-temperature-grown Cu(In,Ga)Se2 thin films has been challenging due to the lack of thermal diffusion. Here, by employing a thin Ag precursor layer, we demonstrate a simple co-evaporation process that can effectively control the Ga depth profile in CIGS films at low temperature. By tuning the Ag precursor thickness (∼20 nm), typical V-shaped Ga gradient in the copper indium gallium diselenide (CIGS) film could be substantially mitigated along with increased grain sizes, which improved the overall solar cell performance. Structural and compositional analysis suggests that formation of liquid Ag-Se channels along the grain boundaries facilitates Ga diffusion and CIGS recrystallization at low temperatures. Formation of a fine columnar grain structure in the first evaporation stage was beneficial for subsequent Ga diffusion and grain coarsening. Compared to the modified co-evaporation process where the Ga evaporation profile has been directly tuned, the Ag precursor approach offers a convenient route for absorber engineering and is potentially more applicable for roll-to-roll fabrication system.