Fine Subgrain Structure Research Articles

Strength stability at high temperatures for additively manufactured alumina forming austenitic alloy

Several fast-spectrum nuclear reactors designed to generate high power (∼450 MWe) rely on forced convection of media such as supercritical CO2, sodium, or liquid lead to cool the nuclear core, operating at temperatures up to 600 °C. Cost-effective, high-strength Fe-based alumina forming austenitic (AFA) alloys are a promising candidate for the fabrication of critical nuclear components. This study investigated laser powder bed fusion (LPBF) processing of an AFA alloy composition optimized for improved creep resistance. Electron microscopy revealed an elongated grain structure along the build direction with a fine sub-grain cellular structure decorated with (Cr,Fe,Nb)23C6 carbide precipitates at the intercellular boundaries. At temperatures of 20–900 °C, the LPBF alloy's superior tensile properties compared to its arc-melted counterpart and other advanced steels (e.g., SS316) were attributed to the distribution of nano-sized carbide precipitates, whereas the high ductility was attributed to the LPBF alloy's elongated grain structure.

Microstructure and excellent arc ablation resistance of Ni–8Al coating on copper substrate by high-speed laser cladding

In the case of electrical contact, copper alloys are often unable to withstand arc ablation and premature failure. To solve this issue, Ni-8wt.%Al (Ni–8Al) coatings were successfully prepared on copper substrates utilizing the high-speed laser cladding. In addition, the microstructure evolution, microhardness, electric conductivity and arc ablation resistance of the prepared coatings were investigated in detail. As a result, the achieved Ni–8Al coatings were structurally dense with good metallurgical bonding to the substrate, of which the microstructure showed a bottom-up evolution of fine sub-grain structure. Notably, the microhardness of the coating was 191.7 HV0.2, representing an increase of 82.57% compared to the substrate, and the electrical conductivity was 18.03 %IACS, approaching that of pure nickel. Additionally, the increase in ablation currents led to changes in the ablation behaviors and failure mechanisms of both the substrate and coating. At high currents, a huge arc ablation crater appeared in the center of the substrate due to the reaction force of the anode flame and the difference in surface tension between the liquid metal at the center and the edge of the molten pool. Nevertheless, the coatings showed better arc ablation resistance due to the improvement of molten pool stability during arc ablation by the in situ formed Al2O3 layer on the coating surface.

Investigation of the Effects of Various Severe Plastic Deformation Techniques on the Microstructure of Laser Powder Bed Fusion AlSi10Mg Alloy

In this paper, we present a complete characterization of the microstructural changes that occur in an LPBF AlSi10Mg alloy subjected to various post-processing methods, including equal-channel angular pressing (ECAP), KoBo extrusion, and multi-axial forging. Kikuchi transmission diffraction and transmission electron microscopy were used to examine the microstructures. Our findings revealed that multi-axis forging produced an extremely fine subgrain structure. KoBo extrusion resulted in a practically dislocation-free microstructure. ECAP processing at temperatures between 100 °C and 200 °C generated moderate grain refinement, with subgrain diameters averaging from 300 nm to 700 nm. The obtained data highlighted the potential of severe plastic deformation as a versatile method for tailoring the microstructure of the AlSi10Mg alloy. The ability to precisely control grain size and dislocation density using specific SPD methods allows for the development of novel materials with ultrafine-grained microstructures that offer the potential for enhanced mechanical and functional properties.

Microstructure and wear resistance of CoCrFeNiMn coatings prepared by extreme-high-speed laser cladding

In this study, CoCrFeNiMn high-entropy alloy coatings with remarkable formability were deposited on a 316-steel substrate using conventional laser cladding (CLC) and extreme-high-speed laser cladding (EHLA) techniques. The microstructure, phase composition, element distribution, microhardness, and wear properties of the coatings prepared at various deposition speeds were analyzed. The results illustrated that the coatings comprised a stable FCC solid solution phase. The microstructure of the CLC-HEA coating was predominantly composed of columnar grains with an average size of 60.8 μm, whereas the grain size of the EHLA-HEA coating decreased to approximately 32.9 μm and 24.5 μm at deposition speeds of 20 m/min and 40 m/min, respectively. The quick cooling and heating properties of the EHLA-HEA coating finally led to a lowered dilution rate, changed grain growth orientation, and enhanced dislocation density. Furthermore, the surface hardness of the EHLA-HEA coating (251 HV) surpasses that of the CLC-HEA coating (173 HV), particularly at higher deposition speeds. The wear test analysis demonstrates a transition in wear mechanisms from abrasive to adhesive, fatigue, and oxidative wear as the load increases. Moreover, EHLA-HEA coating exhibited superior wear resistance, especially at low loads (5 N); the wear rate of EHLA-HEA coating (1.07 × 10−6 mm3N−1 m−1) deposited at 40 m/min is nearly an order of magnitude lower than for CLC-HEA coating (9.97 × 10−6 mm3N−1 m−1), which is owing to its fine sub-grain structure and its higher dislocation density.

Subgrain Coalescence Simulation by Means of an Advanced Statistical Model of Inelastic Deformation

The development of technological methods for processing and manufacturing of functional (with a priori targeted properties) polycrystalline materials and products made of these materials still remains an acute problem. A multilevel modeling approach offers researchers the opportunity to describe inelastic deformation by applying internal variables that give an effective characterization of the material structure at different structural scale levels. High temperature plastic deformation is accompanied by these processes, which leads to a significant rearrangement of the meso- and microstructure of the material. The most substantial contribution to changing the properties of polycrystals is made by the evolution of grain and defect structures at the expense of dynamic recrystallization, which significantly depends on dynamic recovery. In this paper, we consider the problem of the coalescence of subgrains undergoing rotation during inelastic hot deformation. This process is called subgrain coalescence, and it is one of the dynamic recovery mechanisms responsible for changes in the fine subgrain structure. Under applied thermomechanical loads, the coalescence process promotes the formation of recrystallization nuclei and their subsequent growth, which can greatly change the grain structure of a polycrystal. The problem was solved in terms of the advanced statistical model of inelastic deformation, modified to describe the subgrain coalescence process. The model takes into account the local interactions between contacting structural elements (subgrains). These have to be considered so that the grain coalescence caused by a decrease in subboundary energies during their progressive merging can be adequately analyzed. For this purpose, a subgrain structure quite similar to the real structure was modeled using Laguerre polyhedra. Subgrain rotations were investigated using the developed model, which relies on the consideration of the excess density edge component of the same sign dislocations on incidental subgrain boundaries. The results of modeling of a copper polycrystal are presented, and the effects of temperature and strain rate on the subgrain coalescence process is demonstrated.

The influence of Sc and Zr additions on microstructure and corrosion behavior of AA5182 alloy sheet

Minor additions of Sc and Zr to a 5182 base alloy were used to study the effects of microstructure on the susceptibility to intergranular corrosion (IGC) of 5xxx sheet alloys. The results showed that minor Sc and Zr additions significantly decrease the IGC susceptibility as measured by the depth of IGC for the high-temperature annealed sheet. The improvement in IGC resistance is attributed to a high number density of nano-scaled coherent Al3(Sc,Zr) dispersoids which resist recrystallization, and produce a fine subgrain structure. The character of the subgrain structure is critical to prevent the occurrence of continuous intergranular Al3Mg2 precipitates during sensitization.

High strength aluminum-cerium alloy processed by laser powder bed fusion

Readily available high strength Al-alloys for laser power bed fusion (LPBF) typically require post-processing such as mechanical working or heat treatment to evolve strengthening phases. Through the calculation of phase diagrams (CALPHAD) and a method based on Scheil solidification, the Al-8 wt%Ce-10 wt%Mg alloy was evaluated for LPBF to be microstructurally favorable to yield components with full volumetric density, without the need for post-processing. Through an exhaustive LPBF parametric study, documentation of defects was performed for the use of a wide range of laser power and scan speed combinations, i.e., varying the energy density, followed by microstructural characterization with X-ray diffraction, optical and electron microscopy, and mechanical testing with both Vickers hardness and quasi-static uniaxial tension. Overall, defects were observed with use of high and low laser powers and scan speeds, but a processing window for LPBF was observed at both high and low laser powers, where > 99% volumetric density was achieved. Besides defects, decreasing the scan speed, i.e., increasing the energy density, resulted in decreases in the concentration of Mg, down to a maximum of 4.5 wt% due to vaporization. Moreover, a decrease in Mg led to a lower Vickers hardness, attributed to a decrease in the solid solutioning of Mg in the α-Al matrix. Samples selected for tensile mechanical testing were produced with a LPBF laser power of 200 W traversing at 800 mm/s, which yielded a density > 99% and Mg concentration of approximately 7 wt%. A yield strength and ultimate tensile strength of 377 and 468 MPa, respectively, was found for those produced with the power of 200 W. However, the ductility of the alloy was less than 2% elongation at failure. A fine (< 1 µm) sub-grain cellular solidification structure was found distributed throughout the α-Al matrix, with ribbons of the eutectic Al11Ce3 intermetallic distributed along the intercellular boundaries. The high strengths achieved in this alloy were attributed to contribution from Orowan dislocation looping of the Al11Ce3 phase, the high degree of solid solutioning of Mg in the α-Al matrix, and cellular boundary strengthening due to impeding dislocation movement.

High temperature creep behavior of a low alloy Mn-Mo-Ni reactor pressure vessel steel

Creep behavior of a low alloy Mn-Mo-Ni reactor pressure vessel steel received in post weld heat treatment condition has been studied in the temperature range of 600–900 °C. The steel exhibited three stage creep behavior. The steady or secondary creep rates for different stress conditions for a particular temperature were determined using stress change tests. Temperature change tests were conducted to determine the steady state creep rates at different temperatures at a particular stress. The creep rates obtained from different tests were analyzed using the Norton's and Bird-Mukherjee-Dorn creep equations to determine the stress exponent (n) and the creep activation energy (Q). The values of n and Q obtained signify dislocation climb as the underlying creep mechanism in steel. Electron back scatter diffraction (EBSD) study indicated formation of fine subgrain structure in the crept sample.

Effects of hot-forging and subsequent annealing on microstructure and mechanical behaviors of Fe35Ni35Cr20Mn10 high-entropy alloy

Effects of hot-forging and subsequent annealing on the microstructure and mechanical behaviors of Fe35Ni35Cr20Mn10 high-entropy alloy (HEA) were investigated by the microstructure observation and mechanical property test. The hot-forging results in a significant rise in the strength of the prepared HEA but a great reduction in the fracture elongation too. The dominant fine sub-grain structure and much higher density of dislocations formed during forging are responsible for it. The annealing treatment after forging and increasing the annealing temperature make the strength decreased but the elongation improved. With the increase of the annealing temperature, the combined effects of the reduced dislocation density, the coarsened recrystallized grains and the decreased number of annealing twins make the yield strength decreased but the elongation improved significantly. Different microstructure features in the prepared HEAs before the tensile test results in various softening mechanisms and diverse mechanical behaviors during the tensile deformation. In as-annealed HEA at 1000 °C, the relatively low dislocation density and the coarse recrystallization grains become the main features of microstructure. Besides the formation of dislocation cells, another softening mechanism, the formation of a large number of fine deforming twins during the tensile deformation, is found to stand up to further strain hardening in this HEA, and thus its strain hardening exponent is high to 0.48.

Low-angle boundary engineering for improving high-cycle fatigue property of 430 ferritic stainless steel

Grain boundary engineering (GBE) based on the control of a low-angle grain boundary (LAGB) network was investigated to improve the fatigue property in 430 ferritic stainless steel. The development of a subboundary structure during annealing in a specimen that was cold rolled at a high reduction ratio was evaluated quantitatively by electron backscatter diffraction measurements. The fine subgrain structure with a high fraction of LAGBs (FL = 54%) was produced with the formation of a sharp {100} and {111} texture by cold rolling to 95% with subsequent annealing at 973 K for 600 s. The LAGBs interconnected to form networks in the GBEed specimen. The kernel average misorientation maps suggest that the dislocation density within the LAGB network was higher than that in the recrystallized grains, particularly near the LAGBs. The high-cycle fatigue property in specimens with different magnitudes of LAGB networks was compared with that in the annealed commercial 430 stainless steel. The kernel average misorientation maps for the post-fatigued GBEed specimen reveal that the dislocation density decreased during cyclic deformation, whereas the fine-grained structure and high fraction of LAGBs were maintained. It was concluded that the grain refinement and introduction of a high fraction of LAGBs can achieve a higher fatigue strength and longer fatigue life for 430 stainless steel.

Effect of heat treatment on microstructure and mechanical properties of 316L steel synthesized by selective laser melting

The influence of annealing at different temperatures (573, 873, 1273, 1373 and 1673 K) on the stability of phases, composition and microstructure of 316L stainless steel fabricated by SLM has been investigated and the changes induced by the heat treatment have been used to understand the corresponding variations of the mechanical properties of the specimens under tensile loading. Annealing has no effect on phase formation: a single-phase austenite is observed in all specimens investigated here. In addition, annealing does not change the random crystallographic orientation observed in the as-synthesized material. The complex cellular microstructure with fine subgrain structures characteristic of the as-SLM specimens is stable up to 873 K. The cell size increases with increasing annealing temperature until the cellular microstructure can no longer be observed at high temperatures (T ≥ 1273 K). The strength of the specimens decreases with increasing annealing temperature as a result of the microstructural coarsening. The excellent combination of strength and ductility exhibited by the as-synthesized material can be ascribed to the complex cellular microstructure and subgrains along with the misorientation between grains, cells, cell walls and subgrains.

Effect of REE on the Phase Composition and Properties of a New Refractory Magnesium Alloy of the Mg – Zn – Zr – REE System

A novel high-strength refractory alloy VMD16 of the Mg – Zn – Zr – REE system is described. The advantages of the alloy with respect to the counterparts are considered. The processes of formation of phase composition and of structural evolution are studied. A fine dispersed subgrain structure is shown to exist in the deformable magnesium alloy. The combination of the specific morphology and topology of the intermetallic phases with their thermal stability provides a high level of strength properties at room and elevated (300 – 350°C) temperatures at a low anisotropy.

Effect of stamping deformation on microstructure and properties evolution of an Al–Mg–Si–Cu alloy for automotive panels

The effect of 5 % tensile deformation, which simulates the stamping process of Al–Mg–Si–Cu automotive outer panels, on the microstructural evolution during age strengthening, has been investigated. In addition, its benefit on key mechanical properties including hardness, yield strength, ductility, and corrosion resistance has been linked to the microstructural features. It was found that the aging precipitation sequence, SSSS → clusters and G.P. zones → β″ → β′ + Q′ → Q, was not influenced by the dislocations introduced through the stamping deformation prior to aging. On the other hand, stamping deformation could promote the formation of precipitates and refine the precipitates because of the enhanced heterogeneous nucleation and the accelerated precipitation kinetics, leading to superior strength of the alloy at the early stage. Meanwhile, the larger amount of Cu incorporated into nanoprecipitates leads to better intergranular corrosion resistance of the stamped alloy compared with the unstamped one. Due to the reduction in free Si amount at grain boundaries, the formation of fine subgrain structures and the increase of dislocation accumulation, the ductility of the stamped alloy was increased.

Effects of subgrain size and static recrystallization on the mechanical performance of polycrystalline material: A microstructure-based crystal plasticity finite element analysis

In this paper, the effects of subgrain size and static recrystallization on the mechanical performance of polycrystalline material were investigated using a microstructure-based crystal plasticity finite element (CPFE) model. Firstly, polycrystalline microstructures with different mean subgrain sizes were prepared using simple assumption based on experimental observations, and intermediate microstructures during static recrystallization (SRX) were simulated by a cellular automata model adopting curvature driven grain/subgrain growth mechanism. Then, CPFE method was applied to perform stress analysis of plane strain tension on these virtual microstructures. The results show that the subgrains inside pre-existing grains have an effect on the heterogeneity of the stress distributions. The average stress decreases with increasing the mean subgrain radius. As grain/subgrain grows during SRX, the average stress also decreases. It can be deduced that well-defined and finer subgrain structure may strengthen the polycrystalline material, while grain/subgrain growth during SRX process will degrade the strength.

Mechanical properties of 7475 aluminum alloy sheets with fine subgrain structure by warm rolling

The effect of transition elements on grain refinement of 7475 aluminum alloy sheets produced by warm rolling was investigated. The alloy which contains zirconium instead of chromium showed ultra fine structures with stable subgrains after warm rolling at 350 °C, followed by solution heat treatment at 480 °C. The average subgrain diameter was approximately 3 μm. It became clear that zirconium in solution has the effect of stabilizing subgrains due to precipitation of fine Al3Zr compounds during warm rolling. On the other hand, chromium-bearing compounds precipitate before warm rolling and they grow up to relatively large size during warm rolling. The warm rolled sheets with fine subgrains have unique properties compared with conventional 7475 aluminum alloy sheets produced by cold rolling. The warm rolled sheets solution heat treated had subgrain structures through the thickness with a high proportion of low-angle boundary less than 15°. The strength of the warm rolled sheets in T6 condition was about 10% higher than that of conventional 7475 aluminum alloy sheets. As the most remarkable point in the warm rolled sheets, the high Lankford (r) value of 3.5 was measured in the orientation of 45° to rolling direction, with the average r-value of 2.2. The high r-value would be derived from well developed β-fiber textures, especially with the strong {011}«211» brass component. The warm rolled sheets also had high resistance to SCC. From Kikuchi lines analysis and TEM images, it was found that PFZs were hardly formed along the low-angle boundaries of the warm rolled sheets in T6 condition. This would be a factor to lead to the improvement of resistance to SCC because of reducing the difference in electrochemical property between the grain boundary area and the grain interior.

Microstructure and Properties of TIG/FSW Welded Joints of a New Al-Zn-Mg-Sc-Zr Alloy

A new Al-Zn-Mg-Sc-Zr alloy with low Sc content was welded by tungsten inert gas (TIG) and friction stir welding (FSW) techniques. The microstructure and properties of those two welded joints were investigated by property tests and microstructural observations. The results show that the new Al-Zn-Mg-Sc-Zr alloy has desirable welding property. The ultimate tensile strength and welding coefficient of the TIG joint reach 405 MPa and 76.7%, respectively, and in FSW joint those property values reach 490 MPa and 92.6%, respectively. The studied base metal has a deformed fibrous subgrains structure, many nano-scaled Al3(Sc,Zr) particles, and very fine aging precipitates. In the TIG joint, the fusion zone consists of coarsened dendritic grains and the heat-affected zone (HAZ) has fibrous micro-scaled subgrains. The FSW welded joint is characterized by a weld nugget zone, thermo-mechanically affected zone (TMAZ), and HAZ. Due to plastic deformation around the rotating pin and anti-recrystallized effectiveness of Al3(Sc,Zr) particles, the weld nugget zone has a very fine subgrain structure. The TMAZ experiences some dissolution of aging precipitates. Coarsening of aging precipitates was observed in the HAZ. The better mechanical properties of the FSW joint are derived from a fine subgrain structure and homogeneous chemical compositions.

Mechanical Properties of 5083 Aluminum Alloy Sheets Produced by Isothermal Rolling

The microstructure and mechanical properties of AA5083 aluminum alloy sheets consisting of well developed /?-fiber texture were investigated. In order to maintain rolling textures after final annealing, the materials were rolled isothermally at 623 K by making use of heated rolls and reheating process every pass up to final thickness of 1 mm. The isothermal rolled sheets consisted of fine subgrain structures through the thickness with a high proportion of low angle boundary less than 15°. Tensile properties showed anisotropy clearly regarding elongation and Lankford value. In the isothermal rolled sheets, the elongation of 0° to rolling direction was below 20% and Lankford value of 45° to rolling direction was over 1.5. Therefore, the average Lankford value showed 1.0. The yield strength of the isothermal rolled sheets was about 40% higher than that of the cold rolled sheets because of subgrain structures. The low ductility of 0° to rolling direction on the isothermal rolled sheets seemed to reduce drawability at room temperature. The warm drawability of the isothermal rolled sheets improved and was superior to the cold rolled sheets because of increasing ductility and keeping higher strength than the cold rolled sheets.

Martensitic transformation, shape memory effect and superelasticity of Ti–Nb binary alloys

Shape memory and superelastic properties associated with the martensitic transformation from β to α″ martensite were investigated in Ti–(15–35) at.% Nb alloys. The transformation strain and transformation temperature linearly decreased with increasing Nb content. The low critical stress for slip deformation resulted in only a small superelastic strain in solution-treated Ti–Nb binary alloys. Fine and dense ω precipitates formed during aging in the temperature range between 573 and 673 K were effective in increasing the critical stress for slip deformation in a Ti–26 at.% Nb alloy. An intermediate-temperature annealing at 873 K for 600 s without solution treatment was also effective in increasing the critical stress for slip deformation due to the fine subgrain structure. The higher critical stress for slip deformation resulted in a larger recovery strain and stable superelasticity. Excellent superelasticity was achieved by annealing at 873 K for 600 s followed by aging at 573 K due to the combined effect of work hardening and age hardening.

Effect of Annealing Temperature on Microstructure and Shape Memory Characteristics of Ti&amp;ndash;22Nb&amp;ndash;6Zr(at%) Biomedical Alloy

Effect of annealing temperature on microstructure and shape memory characteristics of Ti–22Nb–6Zr(at%) biomedical alloys wasinvestigatedbyusingtensiletests,XRDmeasurement,scanningelectronmicroscopy(SEM)andtransmissionelectronmicroscopy(TEM).Afterseverecold-rolling,theplatewasannealedattemperaturesbetween773and1173K.The= transustemperatureinthisalloywasdeterminedtobe between 823 and 873K. The specimen annealed at 823K for 3.6ks exhibited a fine subgrain structure. A fully recrystallized structure wasobserved in the specimens annealed above 873K. The annealing temperature less affected the transformation temperature and recovery strain.However, the critical stress for slip decreased considerably with increasing annealing temperature, because the grain size increased. Allspecimens annealed above 823K exhibited stable superelastic behavior at room temperature.(Received November 4, 2005; Accepted December 21, 2005; Published March 15, 2006)Keywords: shape memory effect, superelasticity, biomaterials, titanium based alloy, titanium–niobium–zirconium, Hall–Petch relationship

A Study on Fatigue Properties of Sc Added Al 2519 Alloy

The high cycle fatigue properties of two kinds of wrought Al 2519 alloys without and with scandium of 0.10% were investigated. The fatigue strength was determined at R = 0.1 under constant amplitude loading conditions in air. The alloy with scandium of 0.10% showed a little lower tensile yield strength and higher fatigue strength values. The fine grained Al-0.10Sc alloy exhibited a higher resistance against fatigue crack nucleation despite the lower yield stress in comparison to the coarse grained Al 2519 alloy. The results can be explained mainly with the microstructural differences between both alloys. This results are due to the presence of coherent Al3 (Sc, Zr) precipitates and a very fine subgrain structure.