Effect Of Microstructural Refinement Research Articles

The role of different electromagnetic fields in magnesium alloys direct-chill casting: Numerical simulation and experimental investigation

Based on the magnetic-fluid-thermal multiphysics transient coupling numerical simulation results of the magnesium alloy direct-chill (DC) casting, the effects of conventional vibration electromagnetic field (VMF), differential phase vibration electromagnetic field (DP-VMF), conventional low-frequency electromagnetic field (LFMF), and differential phase low-frequency electromagnetic field (DP-LFMF) on melt flow were systematically investigated from the perspective of impulse. Based on thermal behavior and crystal growth theory, the relationships between the velocity field, temperature field, and the morphology of the solidification structure were discussed, and the effect and mechanism of different electromagnetic fields in improving the solidification structure were revealed. Simultaneously, the effects of different electromagnetic fields on AZ31B and AZ80 alloys were investigated. The DC casting experiment verified the theoretical results. Results show that applying low-frequency electromagnetic fields (LFMF and DP-LFMF) can effectively inhibit the formation of columnar grain, but the effect of microstructure refinement is weak; the impact of vibration electromagnetic fields (VMF and DP-VMF) is precisely the opposite. The structure refinement effect of DP-VMF and the inhibition effect of DP-LFMF on columnar grains are better than those of their conventional electromagnetic fields. In the presence of DP-VMF, the average grain size of the center, 1/2 radius, and the edge of the ingot decrease by about 42%, 49%, and 77%, respectively, compared with no electromagnetic field.

Effect of microstructure refinement of pure copper on improving the performance of electrodes in electro discharge machining (EDM)

The paper presents an analysis of the impact of plastic deformation using hydrostatic extrusion (HE) on the structural, mechanical and functional properties of pure copper for use as electrodes in the process of electro discharge machining (EDM). As part of the research, copper was subjected to the HE process with the maximum cumulative true strain equal to ɛcum = 3.89 obtained in 5 stages. The result was, a refinement of the microstructure with the grains elongated in the direction of extrusion, with a cross-sectional size of d2 = 228 nm. As the obtained material can be potentially used in the process of electro discharge machining, the copper specimens after the HE process were subjected to a comprehensive analysis to determine the mechanical, physical and functional properties of the material. A significant increase in strength (UTS) and yield strength (YS) of the HE-processed copper was obtained, reaching respectively UTS = 464 MPa and YS = 456 MPa at the maximum strain of ɛ = 3.89. Despite the clear strain-induced strengthening of the material, a very high electrical conductivity of not less than 97% was obtained. The electrodes made of copper after HE process have reduced erosion wear while maintaining a comparable or better quality of the machined surface. The best results were obtained for finish machining, where the electrical discharge wear was lower by 60% compared to the electrode made of non-processed copper. In addition, an improvement in the surface quality after the EDM process by 25% was observed when using the HE-processed electrodes.

Combined Effect of In and Ce on Microstructure and Properties of Ag10CuZnSn Low-Silver Brazing Filler Metals

In this study, trace amounts of In and Ce elements were composite added into a Ag10CuZnSn low-silver brazing filler metal, and the effects of the composite alloying on the solidus and liquidus temperatures, the spreading performance, the microstructure of the filler metal, and the mechanical properties of the joints prepared with these filler metals were studied. The results reveal that the In element can significantly decrease the solidus and the liquidus temperatures of the Ag10CuZnSn alloy, while the Ce element has little effect on the melting temperature. Trace amounts of In and Ce elements can obviously increase the spreading areas of the filler metals on the pure Cu and 304 stainless steel base metals. The In and Ce elements can refine the microstructure of the filler metals. When the contents of In and Ce are 1.5 wt% and 0.15 wt%, respectively, the microstructure refinement effect is the most obvious, and the shear strength of the 304 stainless steel brazed joint also achieves a maximum value of 375 MPa. Excessive addition of In and Ce can form brittle intermetallic compounds in the filler metal, decreasing the brazed joints' shear strength.

Microstructural evolution and mechanical properties of 6Cr13 martensitic stainless steel subjected to cold rolling and heat treatments

Microstructure evolution and mechanical properties of 6Cr13 martensitic stainless steel subjected to cold rolling and subsequent heat treatments were investigated. The microstructure was checked by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The phases were identified by X-ray diffraction (XRD). The results show that the carbides are dissolved and fragmented during the cold rolling process, and the size of the carbides decreases with increasing the rolling reduction. The refinement of the microstructure, including martensite laths, twins and carbides, occurs due to the dislocation activities induced by the cold rolling. A high density of dislocations persists in the steel even after the heat treatments. Furthermore, reversed austenite forms during tempering of the steel with a rolling reduction of 67% at 200 °C and 300 °C. The steel with a reduction of 67% exhibits an ultra-high strength of ∼1913 MPa, an elongation of ∼7% and a microhardness of ∼58 HRC after the heat treatments (i.e., quenched at 1050 °C and tempering at 200 °C), which is attributed to the synergetic effect of microstructure refinement, the formation of reversed austenite and the high retained dislocation density.

Grain refinement and modification of an Al–Si–Mg–Ti–Be cast alloy: microstructure and tensile property

In this study, the combined use of grain refinement and modification was studied on a hypoeutectic Al–Si–Mg–Ti–Be cast alloy. The primary α-Al grains were transformed from columnar dendrites in the as-received alloy into fine equiaxed dendrites. Meanwhile, the eutectic Si with irregular morphologies were modified to fine fibrous. The room-temperature ultimate tensile strength is increased by ∼14% from 145 MPa to 165 MPa, while the elongation to fracture is increased from 3% to 5%. The comprehensive enhancement of mechanical properties is due to the synergistic effect of microstructural refinement and isotropic improvement by α-Al grain refinement and eutectic Si modification.

Influence of SiC particles and post-heat treatment on the properties of Ti-6Al-4V-based surface nanocomposite fabricated by friction stir processing

Friction stir processing was used to successfully fabricate a Ti-6Al-4V-based surface nanocomposite with a uniform dispersion of SiC reinforcement particles (FSP). Reinforced SiC particles showed good interfacial bonding with the matrix material. Microstructural modification after FSP and post-FSP heat treatment in terms of grain refinement and reinforcement behavior of SiC particles was investigated using an optical microscope (OM), scanning electron microscope (SEM), and electron back-scattered diffraction (EBSD). Microhardness test, Charpy impact test, and pin-on-disc wear test were carried out to measure the hardness, impact toughness, and tribological behavior of fabricated surface composites, respectively. Fully β-transformed microstructure comprising a mixture of fine basket-weave lamellar α/β and ultrafine martensite α′ was evident in the FSPed stir zone. The incorporation of SiC reinforcement resulted in additional microstructure refinement during FSP due to Zener pinning and particle-stimulated nucleation (PSN) caused by SiC particles. Nearly two-fold improvements in microhardness and wear resistance, whereas a slight but noteworthy improvement in impact toughness was observed. Enhancements in mechanical and wear properties were primarily ascribed to the combined effect of microstructural refinement, formation of martensite (α′), and dispersion strengthening (Orowan Mechanism).

Effect of deformation amount distribution in the austenite recrystallization temperature region on the microstructure and mechanical properties of the X70 pipeline steel

ABSTRACT In this study, the effect of deformation amount distribution of X70 pipeline steel on the microstructure and mechanical properties in the austenite recrystallization temperature region are investigated. The results reveal that the austenite grain size is smaller and more uniform by increasing the last-pass rough rolling deformation amount in the austenite recrystallization region. An increase in the last-pass rough-rolling deformation amount can also improve the strength and low-temperature toughness. The deformation mechanism and microstructural evolution based on the last-pass in the austenite recrystallization temperature region are clarified. The final microstructure refinement effect of granular bainite and acicular ferrite attribute to the finer and more uniform original austenite grain size. The main strengthening mechanisms are grain boundary strengthening and dislocation strengthening. The high proportion of the high-angle grain boundary is significantly greater for improving the strength and toughness, which impedes dislocations to slip across grain boundaries.

Enhanced Toughness of High‐Temperature Carburizing Gear Steel via Refining Twin Martensite and Retained Austenite by Nb Microalloying

High‐temperature carburizing technology is replacing traditional carburizing technology due to energy savings and emission reductions. However, further effort is required to understand the mechanical properties for practical high‐temperature carburizing. Herein, the microstructures and mechanical properties of the 20MnCr5 gear steel with different Nb contents after carburizing at the temperature from 900 to 950 °C and the corresponding holding time from 8 to 5 h are investigated. For a given steel, the toughness slightly decreases with the carburizing temperature because of the increase of prior austenite grain size. For high‐temperature carburizing, the impact energy increases from 45.9 to 84 J after Nb microalloying. The toughening mechanism is analyzed through theoretical discussion and various characterizations of fracture morphology and microstructure. It is indicated that the refinement of prior austenite grain caused by the solute drag effect of Nb leads to the refinement of acicular martensite and retained austenite. The combined effect of microstructure refinement results in the significant enhancement of impact toughness and accompanies by coordinated plastic deformation. The fracture spreads along the martensite blocks due to the decrease in the thickness of twin lamellas in acicular martensite, which also contributes to the enhancement of toughness.

The Effects of Microstructure Refinement and Coincidence Site Lattice Boundaries on the Plasticity of FeCrAlY Alloys

FeCrAlY alloys are widely used as thin‐gauge catalyst carriers for automotive exhaust gas purification. However, they exhibit low ductility due to coarse Fe–Y phases in the alloys, which decreases the carrier yield. To improve the plasticity of the alloys, a novel method for fabricating FeCrAlY alloys using twin‐roll strip casting (TRSC) is proposed. It is found that the plasticity of the FeCrAlY alloy is significantly improved by TRSC compared to conventional ingot casting. Then, to understand the mechanism of plasticity improvement, the microstructures of the alloys are investigated. The Fe–Y phases and grains are significantly refined by TRSC due to its characteristics of subrapid solidification. In addition, the TRSC samples produce several low‐Σ coincidence site lattice (CSL) boundaries, and some of these CSL grain boundaries are inherited by the cold‐rolled sheets. The impact characteristic of steel liquid in the TRSC is the major reason for the formation of CSL grain boundaries. The finer microstructure and greater number of low‐Σ CSL boundaries in the TRSC samples result in strong resistance to fracture, which improves the plasticity of the FeCrAl alloys.

Developing sustainable Zn-MWCNTs composite coatings using electrochemical co-deposition method: Tribological and surface wetting behavior

ABSTRACT Carbon nanotube (CNT) nanofillers are a part of carbon allotrope family materials with numerous and effective features. Multiple CNTs nested one another is a low-cost and scalable category of CNTs, known as multi-walled CNTs (MWCNTs), which are well-stablised for their excellent mechanical, electrical and thermal properties. In this present work, MWCNTs nanofiller embedded Zn matrix (Zn-MWCNTs) composite coatings were fabricated by adding the different concentrations of MWCNTs (0.5, 0.1 and 0.2 g/L) in the acidic electrolyte bath using electrochemical co-deposition route with uniform embedment. The prepared composite coatings were characterised using advanced characterisation techniques (XRD, FESEM and EDS) for their obtained surface morphology and microstructures. All Zn-MWCNTs composite coatings exhibited enhanced tribo-mechanical performance compared to pure Zn coatings. Zn-MWCNT (0.2 g/L) composite coating exhibited the highest average microhardness value of 119 HV and minimum wear rate of 12.5 × 10−3 mm3N−1m−1 among all the prepared composite coatings, which attributed to the synergistic effect of microstructural refinement and superior mechanical properties of MWCNTs. Surface wetting studies reveal that MWCNTs play an important role in increasing hydrophobicity in terms of contact angle, which increased by 1.93 times for Zn-MWCNT (0.2 g/L) composite coating. The present work may be helpful for facile fabrication of low-cost biocidal Zn-MWCNTs nanocomposite coating.

The Effects of Prior Austenite Grain Refinement on Strength and Toughness of High-Strength Low-Alloy Steel

The effects of prior austenite grain (PAG) refinement on the mechanical properties of bainitic/martensitic steels not only come from itself, but also have more complex effects by affecting the substructure formed by coherent transformation. In this study, the samples of a low-alloy steel were water quenched from different austenitizing temperatures and the bainitic/martensitic microstructures with different PAG sizes were obtained. Electron back-scattered diffraction was used to characterize the microstructure and different types of boundaries were identified and quantitatively analyzed. The tensile tests and series temperature Charpy impact tests of different heat treatment were also carried out and comprehensively analyzed with microstructure characterization works. The results show that the uniform refinement of prior austenite grain can increases the density of packet boundary and block boundary, which leads to microstructure refinement with higher density of high-angle grain boundaries with misorientation >45°. The contribution of this microstructure refinement to toughness is significant, but its contributions to strength and elongation are relatively limited. Compared to uniform refined PAG, if the PAGs are mixed crystal, the density of block boundary will be reduced, which leads to a lower density of the high-angle boundary with misorientation >45° and the positive effects of microstructure refinement on toughness improvement are weakened. The observation of fracture surface of impact specimens indicates that refining the PAG can delay the tendency of brittle fracture with the decrease in test temperature, and even in the case of brittle fracture, the cleavage facet of the fracture surface is relatively smaller. This result also verifies that PAG refinement can effectively improve toughness by inhibiting cleavage fracture.

Effect of Microstructure Refinement on Surface Morphology and Dynamic Mechanical Properties of W-Cu Alloys.

Two ultrafine-grained W-Cu alloys and two coarse-grained W-Cu alloys were prepared to study the effect of tungsten grain refinement on the interface characteristics between coating and W-Cu alloys. The experimental results show that in the coarse-grained W-Cu alloys, the tungsten phase near the surface is easy to form pits and cracks during impact loading, while the fine-grained tungsten alloy is not prone to this phenomenon. Simulations show that refining tungsten grains can not only reduce the impact absorption energy, but also increase the contact area of tungsten and copper phases, thereby reducing the fracture probability of tungsten phases and improving the dynamic mechanical properties of the W-Cu alloys under impact loading. The tested results show that the shear strength of gold studs on the coating is increased by about 33%, after grain refinement for the W-Cu alloys.

Formation of Structure and Properties of Two-Phase Ti-6Al-4V Alloy during Cold Metal Transfer Additive Deposition with Interpass Forging.

The paper deals with the main formation patterns of structure and properties of a titanium alloy of the Ti-6Al-4V system during additive manufacturing using cold metal transfer (CMT) wire deposition. The work aims to find the optimal conditions for layer-by-layer deposition, which provides the high physical and mechanical properties of the titanium alloy of the Ti-6Al-4V system hybrid, additively manufactured using CMT deposition. Particular attention is paid to interpass forging during the layered printing of the product. Additionally, we investigate how the heat treatment affects the structure and properties of the Ti-6Al-4V alloy that has been CMT-deposited, both with and without forging. These studies have shown that the hybrid multilayer arc deposition technology, with interpass strain hardening, allows the use of high temperature and high technology titanium alloys to obtain products of a required geometric shape. It has been proven that the interpass deformation effect during CMT deposition contributes to a significant decrease in the sizes of the primary β-grains. In addition, forging enhances the effect of microstructure refinement, which is associated with phase recrystallization in deformed areas. It is shown that the heat treatment leads not only to a change in the morphology of the phases but also to additional phase formations in the structure of the Ti-6Al-4V-deposited metal while the mechanism is realized and consists of the gradual decomposition of the martensitic α′-phase and the formation of a dispersive α2-phase. This structure formation process is accompanied by the dispersion hardening of the α-phase. The strength characteristics of the Ti-6Al-4V alloy obtained using layer-by-layer CMT with forging are given; they exceed the strength level of materials obtained with the traditional technologies of pressure treatment, and there is no decrease in plasticity characteristics. The use of the subsequent heat treatment makes it possible to increase the ductility characteristics of the deposited and forged Ti-6Al-4V material by 1.5–2 times without strength loss.

Electro-codeposition and properties of Cu–Ni-MWCNTs composite coatings

ABSTRACT In this work, multi-walled carbon nanotubes reinforced Cu–Ni matrix (Cu–Ni-MWCNTs) composite coatings were produced using the electro-codeposition method by adding different concentrations of MWCNTs (0.05, 0.1 and 0.15 g L−1) to the Cu–Ni plating bath. To achieve effective dispersion of the MWCNTs, the plating solution was stirred using an ultrasonicator (20 kHz and 500 W) for 120 min. Cu–Ni-MWCNTs composite coatings were characterised by scanning electron microscopy and X-ray diffraction to study the surface morphology and microstructure of the coatings. In addition, the effects of MWCNTs on the microhardness, wear resistance and electrical conductivity of the coating were investigated by microhardness tester, pin on disk wear tester and four-point probe system respectively. All Cu–Ni-MWCNTs composite coatings showed enhanced microhardness and wear performance as compared to the Cu–Ni coating. The Cu–Ni-MWCNT (0.15 g L−1) composite coating exhibited maximum microhardness of 438 HV and minimum wear loss of 0.9 mg among all the coatings, in the same pin-on-disk test conditions, which is attributed to the combined effect of microstructural refinement, higher content of MWCNTs and superior properties of MWCNTs. Electrical studies reveal that MWCNTs play a vital role in increasing electrical conductivity by 1.21 times of magnitude.

Evaluating the effect of microstructural refinement on the ignition of Al-Mg-Zr ball-milled powders

Next-generation composite Al-Mg-Zr fuels for bio/chem-agent defeat applications rely on intermetallic formation reactions to reduce ignition temperatures below melting point of Al. Scaling-up the ball-milling synthesis of these powders will require a detailed understanding of how milling parameters and powder composition impact the evolution of powder microstructure. In the present work, we explore the effect of milling duration on the size, microstructure, and properties of (Al-8Mg):3Zr, (Al-8Mg):Zr, 3(Al-8Mg):Zr and 4(Al-8Mg):Zr composite powders. We report compositionally dependent refinement of Zr inclusions in the Al-8Mg matrix that impacts ignition temperatures (Tig). For (Al-8Mg):3Zr and (Al-8Mg):Zr composite powders, Tig initially drops as milling time increases to 60 min and then either plateaus or rises at longer milling times. In contrast, Tig generally decreases over the full 90 min milling duration for the two Al-8Mg-rich compositions. Reductions in heterogeneity and average Zr inclusion size dominate the ignition behavior and lead to steady decreases in Tig.

Effect of microstructure refinement on hydrogen-induced damage behavior of low alloy high strength steel for flexible riser

The effect of different microstructures on hydrogen-induced damage behavior of low alloy high strength steel for flexible riser was studied by hydrogen permeation test and electrochemical hydrogen charging test. The mechanism of hydrogen-induced damage behavior in tested steel was elucidated in terms of microstructure characterization, calculation of hydrogen permeation parameters and sensitivity analysis of hydrogen-induced cracking of tested steel. The results indicated that the microstructure of the tested steel was tempered martensite with precipitated phase, and the microstructure was finer and contained more proportions high angle grain boundaries after multi-stage heat treatment. After multi-stage heat treatment, the irreversible hydrogen traps and total number of hydrogen traps in the microstructure increased significantly, resulting in a significant decrease in hydrogen effective diffusion coefficient (Deff). Compared with quenched and tempered heat treatment, the crack sensitivity of hydrogen-induced cracking in microstructure obtained by multi-stage heat treatment decreased greatly, and the crack mainly presented transgranular propagation and more tortuous.

Effect of Microstructure Refinement on Hardness Homogeneity of Aluminum Alloy 1100 Processed by Accumulative Roll Bonding

This work examines the effect of grain refinement via accumulative roll bonding (ARB) on the homogeneity of microstructure and hardness through the thickness of commercially pure aluminum AA1100 sheets. The use of 7 cycles of ARB was shown to produce homogeneous ultrafine-grained microstructure, reducing the grain size to 0.46 and 1.03 µm in normal direction and rolling direction, respectively. Examination by electron backscattered diffraction indicates that continuous dynamic recrystallization was the main mechanism for the formation of submicron equiaxed grains bounded with high-angle grain boundaries (HAGBs). The fraction of HAGBs increased gradually reaching a maximum of 71.6% after 7 cycles. Through-thickness hardness measurements using Vickers and nanoindentation tests show an increase from 43.5 Hv (0.85 GPa) for as-received sample to 63 Hv (1.1 GPa) after 7 cycles. The heterogeneity in hardness through the thickness of the as-received material was shown to restrain the evolution of uniform hardness across the sheet thickness with increasing ARB cycles. Tensile tests showed that the tensile strength is increased to 250 MPa, which is about 2 times its initial value. Owing to the high stacking fault energy of AA1100, strong dynamic recovery occurred with increasing ARB strain which was balanced with the strain hardening property of the material. This resulted in plastic instability at small strains and thus early necking during the tensile test.

Enhancement of impact toughness of β-type Ti–Mo alloy by {332}<113> twinning

The Charpy impact behavior was examined at room temperature on the basis of the metastable β-type Ti-15Mo alloy with {332} twinning-induced plasticity (TWIP) effect. This TWIP alloy exhibited an excellent impact toughness of approximately 250 J/cm2, which was approximately four times higher than those of common titanium alloys. The elongated dimples, fine dimples, and serpentine glide morphology were observed on the fracture surface. The large amounts of twins were formed under impact loading, which induced the adequate plastic deformation to absorb the impact energy. The formation of these dense and fine twins effectively released the stress concentration due to the dynamic microstructure refinement effect, and the abundant cracks along the twin boundaries remarkably increased the crack propagation path. The delayed crack initiation and enhanced crack propagation resistance further consumed the impact energy.

Effects of cerium substitution on phase components, microstructures and magnetic properties of Nd-Fe-Ti-B alloy

(Nd1–xCex)12Fe77Ti5B6 (x = 0, 0.2, 0.3, 0.4, 0.6, 0.8) alloys were prepared by melt-spinning and annealing techniques. The phase constitutions, microstructures and magnetic properties were investigated by powder X-ray diffraction (XRD), a differential scanning calorimeter (DSC), a vibrating sample magnetometer (VSM) and a transmission electron microscope (TEM). It is found that with the increase of Ce content, the coercivity of the (Nd1–xCex)12Fe77Ti5B6 alloys reaches maximum first and then decreases. The maximum coercivity reaches 18.5 kOe obtained in the sample of 20% Ce substituted which is 34% higher than the Ce-free sample. TEM results reveal that the microstructure refinement effect is responsible for the coercivity improvement. This phenomenon implies that in some cases, Ce and Ti co-doping is more beneficial to improving of the coercivity than Ti single doping in Nd-Fe-B alloys. With further Ce addition, magnetic properties deteriorate due to the formation of CeFe2 and TbCu7-type phases.

The Effect of Microstructural Refinement on the Localized Corrosion of Model Zn-Al-Mg Alloy Coatings on Steel

A combination of in-situ Scanning Vibrating Electrode Technique (SVET) and time-lapse immersion optical microscopy (TLM) is used to investigate the effect of microstructural refinement on patterns of localized corrosion affecting zinc-aluminum-magnesium (ZAM) galvanized coatings on steel. Model ZAM coatings comprising Zn-2.7 wt% Al-1.5 wt% Mg are produced on 0.7 mm mild steel sheet by hot dipping, and the resulting coating microstructure is systematically refined by increasing the cooling (solidification) rate from 5°C.sec−1 to 1000°C.sec−1. The intact ZAM coated surface is immersed in 0.17 M aqueous NaCl, and SVET and TLM are used to follow the resulting localized corrosive attack. TLM shows that corrosion initiates preferentially within MgZn2 and spreads laterally over the ZAM surface by preferentially following MgZn2 rich phases. In coarse microstructures, large primary zinc grains tend to deflect and constrain lateral spreading whereas in fine microstructures the smaller primary zinc grains do not. Consequently, lateral spreading rate increases with microstructural refinement. SVET shows that global corrosion rates are similar for all the ZAM coatings but that increased lateral spreading results in lower rates of through-coating penetration for the refined microstructures. These findings are explained in terms of the lateral diffusion of aggressive anolyte species.