Equiaxed Grain Research Articles

The microstructures and mechanical properties of Al-Zn-Mg-Cu-Si-Zr alloy: A comparison between directed energy deposition and laser powder bed fusion

Al–Zn–Mg–Cu-Si-Zr powders were manufactured by directed energy deposition (DED) and laser powder bed fusion (LPBF) routes, and their microstructures as well as mechanical behaviors were comparatively investigated. The sizes of pores and grains in the DED sample (30 and 15.36 μm) are way higher than that in the LPBF sample (3 and 3.12 μm). The microstructure of DED sample is composed of equiaxed grains, while the LPBF sample exhibits a mixed microstructure consisting of large columnar grains in the melt pool center and small equiaxed grains on the melt pool boundary. The mechanical properties of the LPBF sample are: Yield strength 356.9 MPa, tensile strength 451.8 MPa and elongation 5.50 %, which are way higher than the DED sample (Yield strength 238.8 MPa, tensile strength 305.8 MPa and elongation 2.80 %).

Effects of Ti2SnC MAX Phase on Microstructure, Mechanical, Electrical, and Wear Properties of Stir‐Extruded Copper Matrix Composite

Herein, the effect of Ti2SnC MAX phase reinforcement and friction stir back extrusion (FSBE) parameters on the microstructure, mechanical, electrical, and tribological behavior of Cu‐Ti2SnC composite is investigated. The results indicate that, depending on the rotational speed of the extrusion process, an equiaxed grain microstructure with a uniform distribution of reinforcing particles is formed after the extrusion process. Comparing the extruded samples to the unprocessed samples reveals a larger grain size after extrusion. The absence of MAX phase particles causes the formation of finer grain size in the extruded sample. Under the influence of heat and plastic strain, a reactive layer containing a solid solution of Cu(Sn) or Cu3Sn compounds is formed at the interface of the particle and the copper matrix. By performing the extrusion process, the reactive layer at the interface is broken and scattered on the copper matrix. Furthermore, by applying MAX phase reinforcing particles and performing the extrusion process, the copper matrix's hardness, tensile strength, and wear resistance increase by 128, 99, and 84%, respectively. The five vol% Ti2SnC MAX phases in the extruded copper matrix composite wire results in 7.3% reduction in electrical conductivity.

Tailoring microstructure of additively manufactured Ti6Al4V titanium alloy using hybrid additive manufacturing technology

An innovative hybrid additive manufacturing (HAM) technology combining laser shock peening without coating (LSPwC) and laser powder bed fusion (LPBF) has been proposed to tailor the microstructure of additively manufactured Ti6Al4V titanium alloys. Herein, we investigated the characteristics and evolution of interfacial microstructures between adjacent LPBF layers using electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM). The experimental results demonstrated that the martensitic laths in the LSPwC-treated region changed to equiaxed ultrafine grains under alternating mechanical and thermal effects. Moreover, the LSPwC-induced shock wave caused a transformation in the interface bonding behavior at the interface between the adjacent LPBF layers from the transitivity mechanism to the impregnation mechanism. Based on these results, the LSPwC-induced microstructural evolution mechanism at the interface between adjacent LPBF layers is related to LSPwC pre-activated high-density dislocations and nano twins that promote recrystallization during LPBF new layers and the formation of ultrafine equiaxed grain interlayers. Consequently, compared with the LPBF specimen, the HAM specimen indicated a high ultimate tensile strength of ∼1303 MPa and an excellent elongation of ∼12.7 %.

Experiments and Crystal Plasticity Finite Element Simulations of Texture Development during Cold Rolling in a Ti-15V-3Cr-3Sn-3Al Alloy

The effect of deformation on the evolution of crystallographic texture in a Ti-15V-3Cr-3Sn-3Al (Ti-15333) alloy after unidirectional cold rolling was studied experimentally and numerically in the present investigation. An optical microscope (OM) and scanning electron microscope (SEM) were used to study the microstructures, while the crystallographic texture after cold rolling was studied with X-ray diffraction. The rolling process (deformation) was simulated with PRISMS-plasticity, open-source crystal plasticity software. Micro-indentations were performed on the initial solution-annealed sample with an equiaxed grain structure. The experimentally obtained load–displacement curve for a particular grain (orientation-φ1, Φ, φ2 = 325.2°, 18.0°, 66.2° (Bunge notation)) was compared with the crystal plasticity finite element method (FEM)-simulated load–displacement curve to obtain the calibration parameters. The obtained parameters, along with the experimental stress–strain curve, were used to recalibrate the PRISMS-plasticity software for the rolling simulations of the Ti-15333 alloy. It was observed that the γ-(normal direction, ND//<111>) and α-(rolling direction, RD//<110>) fibers strengthened with cold rolling, experimentally as well as numerically. The simulated orientation distribution functions (ODFs) matched reasonably well with those obtained from the experiments. The average values of von Mises stress and von Mises strain increased with an increase in deformation.

Grain structure orientational change in Ti6Al4V alloys induced by sea water quenching and novel stress relief annealing process

We report on the microstructures and properties of Ti6Al4V alloys, which were achieved upon quenching in sea water medium with potential high cooling rate. The Ti6Al4V alloys were quenched at 1000 and 1100 °C, respectively. Moreover, the effect of post-quenching annealing performed at 900 °C was analyzed. As a result, the quenched alloy experienced surface thermal stress, due to rapid cooling and thermal shock, due to exposure to high temperature annealing. The alloy quenched at 1000 °C developed equiaxed grain structure after annealing, while the 1100 °C-quenched alloy generated irregular shaped lamellae structures. TEM analysis for the 1000 °C-quenched samples revealed the α′-martensite decomposed into the equilibrium α + β phases. Moreover, the 1100 °C-quenched Ti6Al4V alloy revealed an α′-acicular martensitic structure.

A 3D characterization of equiaxed grains induced by mediated nucleation in additively manufactured Inconel 718

In additive manufacturing of Ni-based alloy IN718, what causes Icosahedral Short-Range Order (ISRO) or iQC mediated nucleation of equiaxed grains is still not understood. In order to have a better comprehension of the microstructure formation, a more comprehensive characterization both on grain structure and chemical heterogeneities must be achieved. In this work, a scanning electron microscope equipped with a focused ion beam (FIB) allowed to perform a 3D EDS chemical analysis and EBSD orientation mapping of the volume in an equiaxed grain zone. After the 3D reconstruction of the equiaxed grains has been performed, grain-to-grain orientation analysis allowed to analyze multiple twin boundaries orientation relationships in a volume containing hundreds of grains. Chemical mapping using EDS allowed to identify TiC inclusions from which a five-fold symmetry axis seem to emerge, indicating a potential nucleation site. Such areas were analyzed in transmission electron microscopy (TEM) to investigate the different present phases. The nucleation sequence and potential origin of mediated nucleation are further discussed, opening new prospects for grain refinement in additively manufactured nickel-based alloys.

Additive friction stir deposition of SS316: Effect of process parameters on microstructure evolution

Solid state nature of additive friction stir deposition (AFSD) additive manufacturing process is very advantageous in terms of defect formation and microstructural refinement in the material. Current study presents the process optimization, microstructural evolution and kinetics of recrystallization for AFSD deposited low stacking fault energy material - SS316. As deposited microstructure shows equiaxed ultra fine grains with an average grain size of ~5.0 ± 0.5 μm. Shear deformation at high temperature during processing leads to the operation of restoration mechanisms. Observation of necklace type microstructure in the as deposited SS316 is attributed to discontinuous dynamic recrystallization during processing. Recrystallization kinetics of the AFSD SS316 is characterized using Johnson-Mehl-Avarami-Kolmogorov (JMAK) model. Deformation – thermal cycling during AFSD process resulted in inconsistent recrystallization kinetics. Variation in strain, strain rate and temperature during processing with processing parameters result in varying microstructure and tool wear. High strength-ductility combination and sustained work hardening in as deposited SS316 appear to arise from transformation and twinning during deformation, leading to the formation of hierarchical twins and martensitic phase after deformation. • A high strength and work hardening alloy – SS316 is additively manufactured using a novel additive friction stir deposition. • Porosity free builds with fine equiaxed grain structure is formed in as deposited material as a result of discontinues dynamic recrystallization during processing. • Microstructure of the deposited material and the tool wear during processing varied with variation in the process parameters. • Activation of twining and transformation during deformation resulted in excellent tensile properties with yield strength of 415 ± 10 MPa and ductility 75 ± 5% in as deposited material.

Simulation of the solidification organization of X15CrNiSi20-12 alloy castings during investment casting based on the CAFE model

In this work, the CAFE (Cellular Automata Finite Element) model was used to simulate the grain growth of X15CrNiSi20-12 austenitic stainless steel during the investment casting process, and the effects of shell thickness, shell temperature, casting temperature, and cooling rate on the solidification organization were investigated by orthogonal tests. The results show that the cooling method has an important effect on the formation of equiaxed crystals. However, the shell temperature, shell thickness, and casting temperature have a smaller effect on the formation of equiaxed crystals. A more equiaxed fine grain structure can be obtained by using a shell thickness of 7 mm, shell temperature of 1143.15 K, casting temperature of 1913.15 K, and water-cooling conditions. In addition, a decrease in the shell thickness, shell temperature, and casting temperature can lead to an increase in the grain size, while an increase in the cooling rate will lead to smaller grain size and increase the number of equiaxed fine crystals, thus improving the mechanical properties of the material.

Effect of rolling temperature on microstructure and mechanical properties of medium manganese steel

The effect of rolling temperature on the incomplete recrystallization behavior and the relationship to mechanical properties in 0.06C–4Mn-1.2Cr (wt.%) steel was investigated. Three plates of experimental steel were produced by hot rolling at different temperatures and intercritical annealing processes in this study. The microstructure of experimental steels was examined by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD), and transmission electron microscope (TEM). The mechanical properties of test steel were examined by impact test and tensile test. Austenite reversion and cementite precipitation-dissolution was numerically simulated under the assumption of local equilibrium. With the decrease of hot rolling temperature from 1000 °C to 800 °C, the uniform equiaxed prior austenite grains microstructures changed to heterogeneous microstructures of laminated and ultrafine prior austenite grains arranged alternatively. The tensile and impact tests revealed that the medium manganese steels rolling at 800 °C had an excellent combination of mechanical properties. Elongation had a great relationship with retained austenite fraction and stability. The fractography of impact specimens was examined to explore the toughening mechanism of the micro-laminated steels. The steel with uniform equiaxed grain structure exhibited a relatively low tensile strength of 750 MPa and low elongation of ∼20% and an extremely poor low-temperature toughness of ∼40 J at −40 °C. An enhanced low-temperature toughness (∼233 J at −40 °C), as well as an improved tensile strength (823 MPa) and higher elongation of ∼40%, was obtained in the steel with heterogeneous microstructures. The content of austenite in heterogeneous microstructure is twice that in uniform equiaxed grain structure. And cementite dissolved more thoroughly in the heterogeneous microstructure. The high austenite content of heterogeneous microstructures enabled the steel to be significantly stronger and ductile, which contributed to the high tensile strength and elongation to some extent. It is concluded that the effect of interface decohesion resulted in the occurrence of delamination. And the high austenite content and delamination greatly improved the low-temperature toughness of experimental steel.

Study of Process Parameters on Solidification Structure and Centre Grain Size of 2311 in 420 mm Extra-Thick Continuously Cast Slabs

Based on the solidification heat transfer model and the CAFE model, the solidification behavior and structure of 2311 die steel, with a cross-section dimension of 415 × 2270 mm at different casting speeds, specific water flow and superheat, is numerically simulated. Nail-shooting and acid-etching experiments are carried out on the slab to verify the model’s macroscopic size. With the increase in casting speed, the slab’s central equiaxed grain ratio (ECR) decreases and the average grain size increases. The increase in superheat promotes the growth of columnar grains and inhibits the growth of central equiaxed grains. When the superheat increases from 23 to 38 K, the ECR decreases from 43.2 to 29.64%, and the average radius of grains increases from 0.89 to 1.01 mm. With the increase in specific water flow, the ECR decreases, and the average grain radius is the smallest when the specific water content is 0.32 L kg−1. Finally, the slab quality is improved by process optimization, and the central segregation index of carbon decreases from mean value of 1.15 to 1.05.

Pulsed laser-assisted additive manufacturing of Ti-6Al-4V for in-situ grain refinement

Metal additive manufacturing (AM) enables rapid customization of complex parts. However, it leads to forming of columnar grain structures which give the AM parts anisotropic properties. In this study, we propose a pulsed laser-assisted AM (PLAAM) technique for in-situ grain refinement of Ti-6Al-4V parts. A nanosecond pulsed laser was focused onto a melt pool to generate a favorable environment for the promotion of fine equiaxed grains. The PLAAM technique provided an average prior-β grain size of 549.6 μm, compared to that of 1297 μm provided by the conventional AM technique. Moreover, the maximum value of multiples of uniform distribution of the β phase decreased from 16 to 7.7 when using the PLAAM technique, which indicates a weakened crystallographic texture. These changes confirm that the proposed PLAAM technique promotes finer and more equiaxed prior-β grains. Furthermore, because the proposed technique is a non-contact technique, it can be applied to existing processes without adjusting tool paths.

MORPHOLOGICAL DETAILS OF STRAIN-INDUCED MARTENSITE IN COLD-ROLLED Cr – Ni STEEL

"The investigation presents the evolution of strain-induced martensite during cold rolling of AISI 304 stainless steel at 10% and 50% degree of deformation. The morphologies and characteristics of ’ martensite have been studied using optical-image analysis, scanning and transmission electron microscope. The influence of strain-induced martensite on mechanical properties and hardness of AISI 304 steel have been reported. Plastic deformation during the cold rolling of AISI 304 steel induces a martensitic transformation (γ→α’) in the whole range of applied deformation degree. The equiaxial grains of austenite containing annealing twins were observed within the microstructure of steel at delivery state. During the deformation process the shape of austenite grains was changed into long slim bands parallel to the rolling direction. Many shear bands and micro-twins were observed in the microstructure at large deformations degrees. The martensite, which was formed as a result of the transformation (γ→α’) induced by deformation, has a distinct influence on properties of the investigated steel grade. It was found that the effect of steel hardening is a result of a significant increase of the volume fraction of the ’-martensite in the structure. "

A 3D Polycrystalline Plasticity Model for Isotropic Linear Evolution of Intragranular Misorientation with Mesoscopic Plastic Strain in Stretched or Cyclically Deformed Metals

Two-dimensional electron back-scattered diffraction (2D-EBSD) mapping has been widely used for indicating the polycrystalline plasticity through intragranular misorientation parameters KAM and GROD, based on the empirically linear relationship between their average values and the mesoscopic plastic strain, in both stretched and cyclically deformed metals. However, whether the intragranular misorientation measured on the 2D-EBSD observational plane objectively reflects the 3D polycrystalline plasticity or not is a rarely reported issue. In this research, we firstly compared the KAM and GROD values measured on 2D-EBSD observational planes with different angles to loading axis of a specimen in both undeformed clamp sections and deformed gauge section, to verify whether their average values increase isotropically or not with mesoscopic plastic strain. Then, we proposed six fundamental assumptions and developed a modified 3D polycrystalline plasticity model based on the 2D polycrystalline plasticity model in our previous work. This 3D polycrystalline plasticity model can explain the isotropic linear evolution of intragranular misorientation in deformed low alloy steel with uniform equiaxial grains.

Microstructural understanding of the oxidation and inter-diffusion behavior of Cr-coated Alloy 800H in supercritical water

In this study, corrosion tests were performed on Cr-coated Alloy 800 H in 500 °C supercritical water, containing 6000 mg∙L−1 oxygen, for 500 h. After exposure, the structure of the sample can be divided into a Cr2O3 layer with a nanocrystal structure, a residual Cr coating with columnar or equiaxed Cr grains, an inter-diffusion layer including Fe-Ni-rich and Cr-O-rich zones, and an alloy substrate. Transmission electron microscopy was employed for the high-resolution analysis of atom diffusion in the sample. The outward diffusion of Fe and Ni was restrained by the coating, and no oxide exfoliation was observed.

Excellent mechanical properties of a Ti-based alloy via optimizing grain boundary α phase

In this paper, the continuous grain boundary α phase of a Ti-based alloy is optimized by the combined process of deformation and heat treatment, the equiaxial discontinuous grain boundary α phase is obtained, and the ductility is greatly improved while the strength still keep high (strength of 1280 MPa and elongation of 10 %). Firstly, continuous grain boundary α phase is obtained by solid solution-short time aging, then the continuous grain boundary α phase is sheared and broken by 30 % hot rolling deformation, and finally recrystallized by recrystallization annealing, the near duplex microstructure with equiaxial grain boundary α phase and lamellar secondary α phase is obtained, and the comprehensive mechanical properties of this near duplex microstructure are obviously better than that of the traditional lamellar, duplex or equiaxial microstructure. This paper provides a new idea for strengthening and toughening design of metal materials and thus should be of interest.

AISI 2205 DSS for body-in-white automotive structure: effect of welding current and time on the performance of spot welded joint

Duplex stainless steel is an ideal candidate in the automotive sector leading to a significant weight reduction and enhancement in product safety. In the present work, two major influencing process parameters i.e. welding current and welding time have been considered to investigate the mechanical and metallurgical performance of resistance spot welded duplex stainless steel. Other important process parameters, such as electrode force, squeeze time and hold time have been kept constant throughout the process. Results show that tensile shear strength (TSS) increases with an increase in welding current as well as welding time due to more amount of heat generation at interface. However, excessive value of both results in the expulsion of weld material which significantly alters the nugget diameter and cause decrease in TSS. Moreover, the failure mode is changed from interfacial failure (IF) mode to pull-out failure (PF) mode at the higher value of welding current and time. Microstructural observation reveals that the fusion zone (FZ) is consisted of columnar grain structure, whereas heat affected zone (HAZ) contains equiaxed grain structure. Furthermore, the phase balance of austenite and ferrite is completely disturbed in the FZ and HAZ.

Microstructure and texture evolution in (VCoNi)99.9C0.1 medium entropy alloy during deformed and annealing treatments

The microstructure and texture evolution of (VCoNi)99.9C0.1 medium entropy alloy under homogenized, rolled and annealed conditions are systematically investigated. The equiaxed coarse grains of homogenized samples are elongated along rolling deformation direction, and then transformed to equiaxed fine grains after annealing. The distribution of coarse particles develops along grain boundary for as-homogenized and as-rolled samples; while disappears after annealing. Random texture of homogenized sample is changed to Bs-type texture after rolling, which is ascribed to local chemical ordering induced by planar slip and shear band. Bs-type texture is retained in as-annealed sample due to the absence of preferential orientation nucleation and sluggish diffusion.

Hall–Petch relation and grain boundary slipping in Al Mg Sc alloys with fine equiaxed grain structure

Al–Mg–Sc alloys with fine equiaxed grain structure and free of Mg segregation and dislocations were fabricated by friction stir processing and post-solution treatment to study the deformation behavior of Al Mg base alloys at room and elevated temperatures. The k -value in the Hall–Petch equation of the alloys increased with increasing Mg content, and the equation of k -values and Mg content was displayed. Grain refinement and high Mg content not only could reduce the grain boundary (GB) sliding temperature of Al–Mg–Sc alloys during elevated-temperature deformation but also enhance their high-temperature GB sliding capacity. • k -value in H P equation of Al–Mg–Sc alloys increased with increasing Mg content. • k -value of Al–Mg–Sc alloys was developed in the form of k = − 80.56 × exp − c 1.94 + 148.2 . • Grain refinement and high Mg content reduced the GB sliding temperature. • Grain refinement and high Mg content promoted the high-temperature GBs sliding.

High-performance co-continuous Al-Ce-Mg alloy with in-situ nano-network structure fabricated by laser powder bed fusion

A novel Al-Ce-Mg alloy with an in-situ nano-network structure and improved tensile properties was fabricated using laser powder bed fusion (LPBF). The microstructure of the as-fabricated alloy exhibited an extremely fine eutectic Al/Al 11 Ce 3 co-continuous structure, i.e., Al 11 Ce 3 intermetallics with an in-situ nano-network surrounding the interconnected Al matrix. It forms an equiaxed grain in the scanning direction (SD) and columnar grain in the building direction (BD). The yield strength (YS) and ultimate tensile strength (UTS) in SD and BD are 300 ± 11.5 MPa, 438 ± 6.9 MPa, and 303 ± 5.0 MP, 442 ± 3.3 MPa, about three times higher than those of as-cast alloy. The elongations (El) are 9.3 ± 0.7% and 6.2 ± 1.2% in SD and BD, respectively, exhibiting anisotropy. The perfect interfacial bonding between the Al 11 Ce 3 phase and Al matrix combined with nano-scale network structures provided a strong ability to hinder the propagation of microcracks formed within Al 11 Ce 3 particles. Besides, the alloy retained outstanding strength at elevated temperatures with 200 MPa at 300 °C and 100 MPa at 400 °C, which may be attributed to the heat resistance of nano-network Al 11 Ce 3 intermetallics and the extremely low diffusion coefficient of Ce in the Al matrix. This work provides a new idea for developing heat-resistant aluminum alloys with a good combination of strength and ductility via AM processing technique. • Microstructure featured extremely fine eutectic Al/Al 11 Ce 3 co-continuous structure. • Al 11 Ce 3 network is connected by nanocrystalline grains with different orientations. • The as-fabricated Al-Ce-Mg alloy displays superior strength-ductility combination. • The as-fabricated Al-Ce-Mg alloy maintains high strength at elevated temperature.

Deformation based additive manufacturing of a metastable high entropy alloy via Additive friction stir deposition

Additive friction stir deposition (AFSD) is a novel solid-state additive manufacturing process which offers unique capability of producing 3-D parts with no solidification related defects and having homogeneous, equiaxed microstructure. However, to fulfill the full potential of the process for structural applications, AFSD needs intense process optimization for a wide range of alloys with excellent mechanical properties. The present study explored AFSD of a novel metastable high entropy alloy (HEA), CS-HEA (Fe 40 Mn 20 Co 20 Cr 15 Si 5 (at.%)), enabled with transformation and twinning induced plasticity during deformation. Intense shear deformation at elevated temperature and strain rate during AFSD led to the operation of restoration mechanisms such as recovery, recrystallization, and grain growth which resulted in refined grains with excellent strength and work hardenability. The average grain size for as-deposited CS-HEA is 3.0 ± 0.5 µm, and the average tensile yield strength of as-deposited CS-HEA is 450 ± 20 MPa. The microstructural variation and mechanical response of the alloy as a function of process parameters were correlated to the AFSD process variables, phase transformation, and recrystallization kinetics. Further, the interaction between recrystallization kinetics and transformation kinetics on microstructural evolution of the material was explored. Additionally, the microstructure and stacking fault energy of the alloy were used to predict the mechanical response of the deposited material using a five parameter work hardening model. • A Metastable high entropy alloy, CS-HEA is additively manufactured using a deformation based additive manufacturing process – additive friction stir deposition. • Operation of discontinuous dynamic recrystallization during AFSD deposition of CS-HEA led to formation of refined equiaxed grain structure in as deposited state. • Variation in AFSD processing parameters affected the microstructure of the alloy, higher heat input resulted in larger grain size with higher fraction of high temperature γ phase. • γ→ε transformation and twinning in ε phase provided excellent strength-ductility combination in as deposited state.