Equiaxed Grain Research Articles

Effect of Die Design parameters on materials processed by Equal Channel Angular Pressing

Equal Channel Angular Pressing technique is used for grain refinement in bulk materials, where the material is passed through the die in which the two channels intersect at some angle, that result in finer grains. There are number of die design parameters which affects the production of ultra-fine grains. This paper presents the effect of die design parameters such as channel angle, corner angle, several passes and route followed for processing of the material for grain refinement. The results of different parameters are compared, and it was observed that the optimum values for producing maximum shear strain is for channel angle 90° and it reduces with the increase in channel angle. About 20° corner angles is found to suitable for filling the corner gap, around 4 to 6 passes, and route Bc leads to the ultrafine grained structure of homogeneous equiaxed grains separated by high angle grain boundaries in the materials. The sample of Al-TiO2 was prepared and its microstructure and mechanical properties were tested after equal channel angular pressing.

Grain refinement and crack inhibition of hard-to-weld Inconel 738 alloy by altering the scanning strategy during selective laser melting

Additive manufacturing of hard-to-weld nickel-based superalloys leads to the formation of elongated columnar grain structures and hot cracks along the build direction. Changing the scanning strategy during selective laser melting (SLM) without changing alloy composition is a promising method for the inhibition of hot cracking by grain refinement. In this work, the effect of grain structure on the cracking inhibition behaviour of hard-to-weld Inconel 738 alloy fabricated by SLM with different scanning strategies (rotation of 0°, 90° and 67° between layers, respectively) was investigated. Nearly crack-free, local equiaxed microstructures and an excellent combination of strength and ductility were obtained by rotation of 67° for the SLM-built Inconel 738 alloy due to bimodal grain structure with local equiaxed grains and columnar grains. The local equiaxed grains are attributed to dynamic recrystallization and give rise to thermal stress relaxation. Meanwhile, the aspect ratio of columnar grains is related to thermal gradient variation. In addition, the inhibition of initiation and propagation of cracks is attributed to grain refinement and reduced residual stress. This work demonstrates the feasibility of grain refinement by changing the scanning strategy and improved suppression of cracks in a hard-to-weld superalloy without changing the alloy composition.

Thermal Stability of Grain Structure for Ag Nanotwinned Films Sputtered With Substrate Bias

In this study, different grain growth paths after aging at various temperatures for 1 hour were observed in Ag nanotwinned thin films with and without substrate bias, which led to large differences in thermal stability. As-deposited Ag nanotwinned thin films with and without substrate bias were characterized by FIB, EBSD, and TEM, which revealed thinner equiaxial fine-grained regions, higher amounts of (111)-oriented grains and denser nanotwin stacking perpendicular to the growth direction of columnar grains in thin films with -150 V substrate bias voltage. Due to the better lattice arrangement and densification of the crystal structure due to substrate bias, the microstructures remained almost unchanged, except for a little equiaxial grain growth, even after aging at 450 °C for 1 hour. However, in the Ag nanotwinned thin film without substrate bias, severe abnormal grain growth (AGG) occurred after aging at only 250 °C for 1 hour. Better understanding of the differences in thermal stability in nanotwinned thin films is provided in this study, and the findings will be beneficial for experimental design for further applications.

Fabrication and deformation behavior of a novel laminated TiAl matrix composite

Two-phase titanium aluminides are desirable for a wide range of high-temperature structural applications for their excellent thermo-physical properties. In this paper, a novel laminated TiAl matrix composite (LMC) consisting of alternating equiaxed grain layers (EGLs) and fully-lamellar grain layers (FGLs) has been fabricated successfully by hot-pressing and subsequent reaction annealing. After microstructure tailoring, the LMC exhibited a good combination of strength and ductility at 850 °C. Experiments demonstrated that the thermal residual stress induced by the fabrication process had a periodic distribution across the layers. The competition between residual stress and externally applied stress changed the stress state and accordingly the Ti3Al twining was activated, which benefited the accommodation of the incompatibility between alternating constituent layers. The crack tolerance was remarkably improved by layered structure in the LMC. Crack initiation occurred mainly in either the EGLs or the layer interface. However, they were constrained effectively by the laminated structure. Instead, these dense microcracks may relief the stress concentration and improve the deformation stability. This work is expected to enrich the current understanding on the stress-mediated deformation behavior of TiAl alloys, and provide guidance towards design of high-performance intermetallic laminated composites.

Tensile and Fatigue Properties of Twin-Roll-Cast AZ31 Magnesium Alloy Strips with Different Thicknesses

This study investigates the microstructure, tensile properties, and high-cycle fatigue resistance of twin-roll-cast Mg-3Al-1Zn (wt%) alloy strips with thicknesses of 1 mm, 1.5 mm, and 3 mm. The investigation results reveal that the 1-and 1.5-mm-thick strips show a fully dynamically recrystallized (DRXed) microstructure consisting of fine equiaxed DRXed grains, whereas the 3-mm-thick strip shows a partially DRXed microstructure containing very coarse elongated unDRXed grains because of the insufficient strain imposed during twin-roll casting. The inhomogeneous microstructure of the 3-mm-thick strip leads to a large deviation in its tensile elongation. The average grain size of the strips increases with increasing strip thickness, which results in reductions in both their tensile strength and their ductility because of the weakened grain-boundary hardening effect and the promoted formation of undesirable twins, respectively. The high-cycle fatigue resistance in the stress regime with finite fatigue life is similar for all three strips, but the fatigue strength with infinite fatigue life decreases from 175 MPa to 140 MPa as the strip thickness increases from 1 mm to 3 mm. The fatigue strength (FL) increases linearly with increasing yield strength (YS) according to the relationship FL = -199.5 + 2.03·YS.

Effects of hyper-high-temperature solid-solution treatment on microstructure evolution and nanoprecipitation of the Al-Ni-Cu-Fe-Zr-Sc alloy manufactured by selective laser melting

The novel selective laser melting (SLM) Al-Ni-Cu-Fe-Zr-Sc alloy has high temperature stability, but the segregation of the melting pool boundary limits its potential applications. Because conventional heat treatment with a solid solution at 530 °C is inappropriate for the SLM Al-Ni-Cu-Fe-Zr-Sc alloy, this study used hyper-high-temperature solid-solution treatment at three temperatures (560, 570, and 580 °C for 1 h) to investigate the evolution of the melting pool structure. An artificial aging treatment was applied at 175 °C for 6 h to investigate the precipitation mechanism. The results indicated that as-printed presented a bimodal microstructure consisting of a coarse-grained (CG) region and a fine-grained (FG) region. The hyper-high-temperature solid-solution treatment promoted the decomposition of the melting pool boundary and expansion of FG region. The FG region was composed of Ni-, Cu-, and Fe-rich equiaxed grains. After aging treatment, Al9Fe(Cu)Ni and Al3(Sc, Zr) nano-scale strengthening phases could be found in the FG region. The decomposition of the melting pool boundary, bimodal microstructure, and nano-scale phases can strengthen the matrix and increase hardness. In addition, the mechanism of element diffusion in melting pool was also explored to confirm that heat treatment can reduce material deterioration caused by boundary precipitation. These characteristics mitigate the disadvantages of the SLM Al-Ni-Cu-Fe-Zr-Sc alloy in terms of its strength directionality and establish its potential for use in aerospace applications.

Bio-corrosion impacts on mechanical integrity of ZM21 Mg for orthopaedic implant application processed by equal channel angular pressing

The mechanical integrity of rolled ZM21 Mg was improved by equal channel angular pressing (ECAP) to function as a potential biodegradable bone screw implant. Electron backscattered diffraction (EBSD) revealed deformed grains of 45 µm observed in rolled ZM21 Mg. They were transformed to equiaxed fine grains of 5.4 µm after 4th pass ECAP. The yield strength of rolled and ECAPed ZM21 Mg alloys were comparable. In contrast, 4th pass ZM21 Mg exhibited relatively higher elongation when compared to rolled sample. The mechanical properties of rolled and ECAPed ZM21 Mg were dependant on both grain refinement and crystallographic texture. The rolled and 4th pass ECAPed tensile samples exhibited nonlinear deterioration of mechanical properties when tested after 7, 14, 21 and 28 days immersion in Hank’s solution. The evaluation signifies that regardless their processing condition, ZM21 Mg alloys are suitable for surgical areas that requires high mechanical strength. In addition, the 4th pass ECAP samples were viable to MG-63 cells proving themselves to be promising candidates for future in vivo studies.

Effect of welding thermal treatment on the microstructure and mechanical properties of nickel-based superalloy fabricated by selective laser melting

Strengthening the heat-affected zone in actual weld joint of Inconel 625 alloys was crucial for engineering applications in aerospace, petroleum, nuclear energy, and marine industries. In this study, Gleeble thermal simulations were employed to investigate the influence of welding heat on grain morphology, carbide evolution, and mechanical properties of Inconel 625 alloys manufactured by selective laser melting (SLM). Typical elongated columnar grains along the building direction and parallel columnar array with obvious layer boundaries along the scanning direction were observed in SLM-processed Inconel 625 alloys with hot isostatic processing treatment. After Gleeble thermal simulations, the prominent equiaxed grains appeared in the entire gauge region that was directly exposed to the simulated welding heat owing to the recrystallization process. Meanwhile, the size of equiaxed grains significantly increased with the increasing peak temperature. Particularly, only MC-type carbides were observed in the γ-Ni matrix before and after the thermal simulation owing to the fairly rapid heating (100 °C/s) and cooling (30 °C/s) process. Simulated welding heat produced a negative effect on the mechanical properties of the Inconel 625 alloys, especially for the peak temperature of 1350 °C. Then, the respective contributions of the four conventional strengthening mechanisms were successfully quantified for understanding the effect of the simulated peak temperature on the yield strength of the Inconel 625 alloys. Finally, the observations of microstructural evolution indicated that the cross-slip of dislocations was the dominant deformation mechanism to account for the large strain-hardening rate of the Inconel 625 alloys.

Study on processing and strengthening mechanisms of mild steel subjected to laser cavitation peening

A method of laser cavitation peening (LCP) was employed to strengthen the Q235 steel. The plastic deformation, residual stress, microhardness, phase, and microstructure of Q235 steel subjected to laser cavitation peening were investigated through a combination of experiments and simulations. The processing and strengthening mechanisms (LCP impact, plastic strain, grain refinement) were analyzed. Laser-induced cavitation bubble experiences three periodic pulsations during the process of LCP. The strengthening of Q235 steel by LCP can be attributed to the impact of the laser shock wave, bubble collapse shock wave, and water-jet. LCP causes plastic deformation in the surface layer of material and thereby introducing compressive residual stress and enhancing the microhardness. Dislocation structures including dislocation tangles, dislocation walls, and dislocation cells generate within the grains and near grain boundaries after LCP impact. Residual stress, microhardness, and dislocation density increase significantly with the increase of impact times. The high-density dislocation tangles and the sharing of dislocation cells refine the original coarse grains into equiaxed fine grains. The process of grain refinement is accompanied by the dissolution of cementite.

Transformation Delay and Texture Memory Effect of Columnar Grained Cast Slab in Low Grades Non-oriented Electrical Steels

The cast slab of low grades non-oriented electrical steels experiences twice diffusional solid phase transformation and demonstrates the features of strong morphological memory referring to columnar structure and texture memory referring to the preferred <100> texture at room temperature. In this paper, electron backscatter diffraction (EBSD), quasi-insitu observation of heating samples, and dilatometry are used to study and analyze the two kinds of memory phenomena. The results show that the cast slab consists of about 70% coarse columnar grains and 30% small equiaxed grains. Many small equiaxed grains show Σ3 misorientations with columnar grains indicating K-S orientation relationship obeyed during phase transformation. Coarse columnar grains show a typical <100>||growing axis orientation which is the same as solidified columnar grains. The quasi-insitu observation shows that transformation of columnar grained ferrite to austenite is very sluggish and columnar grained ferrite can still be seen even at a superheating degree of 176°C for 1 hour. Dilatometry measurement indicates that the starting transformation temperatures for a columnar-grain-dominant sample and a small-equiaxed-grain-dominant sample are similar, whereas their transformation extents are quite different with columnar grained sample showing a low dilatational amount due to insufficient transformation. It is most likely that the coarse columnar grains in cast slab are retained and untransformed high temperature δ-ferrite are not subjected to twice complete transformations. These retaining columnar grains in low grades of electrical steels can be used to improve magnetic properties through optimizing processing parameters.

Al2O3/ZrO2 Materials as an Environmentally Friendly Solution for Linear Infrastructure Applications.

The present work deals with the evaluation of the effect of ZrO2 on the structure and selected properties of shapes obtained using the centrifugal slip casting method. The samples were made of alumina and zirconia. The applied technology made it possible to produce tubes with a high density reaching 99–100% after sintering. Very good bonding was obtained at the Al2O3/ZrO2 interphase boundaries with no discernible delamination or cracks, which was confirmed by STEM observations. In the case of Al2O3/ZrO2 composites containing 5 vol.% and 10 vol.% ZrO2, the presence of equiaxial ZrO2 grains with an average size of 0.25 µm was observed, which are distributed along the grain boundaries of Al2O3. At the same time, the composites exhibited a very high hardness of 22–23 GPa. Moreover, the environmental influences accompanying the sintering process were quantified. The impacts were determined using the life cycle analysis method, in the phase related to the extraction and processing of raw materials and the process of producing Al2O3/ZrO2 composites. The results obtained show that the production of 1 kg of sintered composite results in greenhouse gas emissions of 2.24–2.9 kg CO2 eq. which is comparable to the amount of emissions accompanying the production of 1 kg of Polyvinyl Chloride (PVC), Polypropylene (PP), or hot-rolled steel products.

Microstructure and mechanical properties of thixoformed GH4037 parts before and after heat treatment

In this work, the effect of heat treatment on the microstructure and high temperature mechanical properties of the GH4037 cylindrical parts fabricated via thixoforming was investigated. The microstructure of the thixoformed GH4037 parts before heat treatment was composed of solid grains and eutectic phases, while it mainly consisted of equiaxial grains after heat treatment. After heat treatment, a large number of spherical γ′ precipitates dispersed inside the grains, while primary γ′ particles, streaks of M23C6 carbides and blocks of MC carbides precipitated along the grain boundaries. When the tensile test temperature increased from room temperature to 1000 °C, the tensile strength of GH4037 alloy decreased gradually, while the elongation presented a trend of descending first and then ascending. After heat treatment, the tensile strength realized a remarkable improvement while the elongation was inevitably reduced. The results indicated that the microstructure and mechanical properties of the thixoformed GH4037 parts were significantly improved by heat treatment, and verified the feasibility of thixoforming in the field of nickel-based superalloys.

Synthesis and microstructure evaluation of ODS steel 316L with zirconia dispersion

An Austenitic ODS steel was developed for reactor structural material by dispersed 0.5 wt % of nano powder zirconia (ZrO2) to the AISI 316L steel. The synthesis was carried out by the powder metallurgy process with high energy milling and cooled compacting process. A new apparatus of APS (Arc Plasma Sintering) was used for consolidation the sample in the sintering process. Characterizations of the microstructure and elemental composition distribution were performed using the Scanning Electron Microscope (SEM) with X-ray Diffraction Spectroscopy (EDX) and area mapping. Identification for the change of phasesand hardness were analyzed using the XRD-test and Vickers Hardness measurement. Austenitic phase with relatively equiaxed grain and homogeny distribution of the ZrO2 dispersoid were identified after the sintering process followed by the improvement of hardness due to the pinning effect of the grain boundaries.

Effect of spiral scan strategy on microstructure for additively manufactured stainless steel 17–4

The effect of the scan strategy on microstructure is investigated using a custom-built laser powder bed fusion additive manufacturing testbed. Eight stainless steel cubic parts were built to compare two different scan strategies: raster and spiral. The processes were monitored in-situ by a high-speed camera coaxially aligned to the heating laser, and the parts built were characterized by micro-indentation and optical microscopy. The spiral scan strategy creates a much larger meltpool area and the resulting parts exhibit an equiaxed grain structure with 54% higher hardness value compared to the raster scan strategy.

The corrosion behavior and film properties of Al-containing high-entropy alloys in acidic solutions

Aluminum has shown promising application in reducing the high-entropy alloys’ weight and enhancing their mechanical properties. In this work, the impact of solid-solution aluminum on the corrosion behavior and film properties of FeCoCrNiAlx (x = 0.1, 0.3) high-entropy alloy in 0.5 M H2SO4 and 0.5 M HCl solutions was investigated by electrochemical measurement, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. The results demonstrate that both of the high-entropy alloys exhibit an equiaxed grain structure with numerous annealing twins. FeCoCrNiAl0.3 alloy still remains the stable FCC substrate without any second phase precipitates. The increasing aluminum content can improve the corrosion resistance of FeCoCrNiAlx by simultaneously increasing Cr oxide content in the passive film and making it thicker in the H2SO4 solution. Nevertheless, the effect is negative in the HCl solution because chloride ions are inclined to combine with the Al-containing metastable ion complexes, which accelerates the dissolution of HEA.

Hot tensile behavior of a TiAl alloy with a (β0 + γ) microduplex microstructure prepared simply by heat treatments

A (β0 + γ) microduplex microstructure was prepared in a Ti-42Al-8.5V (at%) alloy by means of quenching-tempering treatments, which was characterized by ultra-fine γ platelets embedded in millimeter-sized β0 grains according to a definite K-S orientation relationship. Hot tensile tests were performed at 850–1000 °C with strain rates of 5 × 10−4–5 × 10−3 s−1 and a near-superplastic creep behavior was yielded. As a consequence, high elongations (>100%) were obtained with a stress exponent of ~3. Deformation mechanism was further analyzed by examining the microstructure and orientation evolution in individual β0 grains. The results showed that significant crystal rotation and fragmentation occurred in β0 matrix due to plastic flow and recrystallization, leading to an equiaxed grain structure with a weak preferential orientation resembling ND fiber where {111}<112>was the dominant component. In contrast, evident globularization/coarsening of the γ platelets was observed during deformation. Meanwhile, the γ platelets were slightly deformed but drifted with the β0 matrix. The spontaneous rotation due to grain boundary/interface sliding resulted in dramatic orientation divergence of γ phase. A weak and unusual<110>fiber was produced due to the preservation of K-S orientation relationship.

Analytical Description of the Criterion for the Columnar-To-Equiaxed Transition During Laser Beam Welding of Aluminum Alloys

An equiaxed grain structure in a laser welded seam is beneficial. In current literature the material and process perspective are addressed separately. This paper combined the material and process perspective in one analytical expression in order to access a deeper understanding of the key parameters for an equiaxed solidification during laser beam welding. For this, the steady state model of Hunt for growth of equiaxed dendritic grains was combined with heat conduction calculations and nucleation theory. The derived analytical expression identified the minimum values of the welding parameters required for the growth of equiaxed dendritic grains and describe their interaction with the metallurgically given nucleation density. The theoretically derived analytical description agrees well with experimental results obtained with two different batches of EN AW-6016 over a wide-range of laser welding parameters. The analytical description summarizes the effects of nucleation processes, local solidification conditions and the influence of process parameters on the growth of equiaxed dendritic grains. Furthermore, the equation identifies the key alloy properties, which influence the required process parameter for an equiaxed solidification. For the first time, the full range of phenomena, including metallurgical and thermal effects, was merged in one analytical expression.

Enhanced dielectric, ferroelectric and magnetic properties of Ba4Sm2Fe2Nb8O30 RT multiferroics prepared by microwave sintering

High density Ba4Sm2Fe2Nb8O30 (BSFN) multiferroics ceramics with tetragonal tungsten bronze structure had been prepared by microwave sintering (MS) for 30min and conventional sintering (CS) methods for 4 h at 1275 °C. Single tungsten bronze phase and equiaxial grains are obtained for the MS BSFN ceramics, while a small amount secondary phase of SmNbO4 is observed in the CS BSFN ceramics with columnar grains. Compared to Ba4Sm2Fe2Nb8O30 ceramics prepared by CS method, enhanced dielectric, ferroelectric and magnetic properties are achieved for the MS BSFN ceramics. The values of electric polarization Pr and coercive electric field Ec are 2.11 μC/cm2 and 7.14 kV/cm for the MS BSFN ceramics, respectively. Meantime, the magnetic polarization Mr of 0.410emu/g and coercive magnetic field Hc of 2930Oe are also obtained for the MS BSFN ceramics. Based on the density, crystal structure, point defect and grain, the reasons of enhanced dielectric, ferroelectric and magnetic properties are discussed for the MS BSFN ceramics It is indicated that Ba4Sm2Fe2Nb8O30 is an intrinsic room temperature multiferroic materials.

The Feasibility and Process Control of Uniform Equiaxed Grains by Hot Deformation in GH4720Li Alloy with Millimeter-Level Coarse Grains

The Feasibility and Process Control of Uniform Equiaxed Grains by Hot Deformation in GH4720Li Alloy with Millimeter-Level Coarse Grains

Comparative investigation between fiber laser and disk laser: Microstructure feature of 2219 aluminum alloy welded joint using different laser power and welding speed

In this investigation, the 2219 aluminum alloy sheets with a thickness of 1.5 mm are welded by different types of lasers. By analyzing the macro-morphology and microstructure of the weld obtained by two different laser welding methods, the characteristics of welding aluminum alloy by using these different lasers are studied. The results indicate that under the gravity of molten pool and different laser power densities, the weld seams (WS) welded by fiber laser and disc laser are in X-shape and Y-shape respectively. Besides, under the influence of Marangoni convection, the equiaxed fine grains of the weld are brought into the middle part, forming a small and regular equiaxed crystal zone (EQZ). It is found that the widths of the WS and the heat-affected zone (HAZ) obtained by fiber laser are both wider than those welded by disc laser. The grain size of columnar dendrites and equiaxed dendrites in the WS welded by fiber laser is coarser than that welded by disc laser. The tensile strengths of disc-laser-welded joints are also higher. In short, disc lasers are more suitable for high-power laser welding aluminum alloys.