Refinement Mechanism Research Articles

Surface morphologies and microstructure of high-strength steel AISI 4820 with different heat treatment during grinding process

High-strength steel AISI 4820 has been widely used in the manufacturing process for functional components (gear parts) at varying service environments. Surface morphologies and microstructure caused by the grinding process have important implications for the service life of components, which can be determined by the machining parameters and heat treatment conditions. In this paper, to further understanding of underlying deformation mechanism of AISI 4820 induced by grinding parameters, the surface morphologies and microstructure of AISI 4820 steel with three different heat treatment conditions, i.e., the carburized and quenched heat treatment (CQHT), the normalized heat treatment (NHT), the quenched heat treatment (QHT), during the grinding process was investigated. Surface morphologies analysis by SEM. The microstructure, plastic deformation, and grain refinement mechanism was revealed by EBSD. The results showed that better machined surface quality is performed in the CQHT condition. In NHT condition, the subsurface layer grains were significantly deformed and refinement was exhibited after the grinding process in NHT condition. Additionally, the internal grains were refined after carburizing in QHT condition.

Preparation and mechanism of copper ultra-fine grain strip by ultrasonic vibration extrusion cutting-model, FEM and experiment

Ultra-fine grain materials are widely used due to their superior material properties. In presented work, ultrasonic vibration extrusion cutting (UVEC) is put forward to prepare ultra-fine grain strip. UVEC induces material to produce strain and strain rate far exceeding that of ordinary severe plastic deformation process. The mathematical model and finite element simulation model of physical field parameters was established. UVEC test platform was set up to verify each other with previous model. Relationship and mechanism among processing parameters, physical field parameters and microstructure changes were analyzed, mechanism of grain refinement in UVEC was revealed. The results show that the grain size can be refined to ultra-fine grain size by UVEC method. With the increase of tool rake angle, the strip equivalent strain/equivalent strain rate decrease gradually, and the grain refinement weakens gradually. Besides UVEC, ordinary turning (OT) and ordinary extrusion cutting (OEC) tests are also carried out. The average grain size of UVEC-strip (0.75 μm) is about 3/4 that of OEC-strip (1.03 μm). The passivation layer depth of OT-strip (38 μm), OEC-strip (58 μm) and UVEC-strip (75 μm) increases gradually.

A multi-level generative framework for community detection in attributed networks

AbstractCommunity detection in attributed networks is one of the most important tasks in complex network analysis. Many existing methods propose to integrate the network topology and node attribute from a generative aspect, which models an attributed network as a probabilistic generation process with the community distribution described by hidden variables. Though they can provide good interpretability to the community structure, it is difficult to infer community membership quickly due to their high computational complexity when inferring. Motivated by the multi-level strategy, in this study, we propose a multi-level generative framework to reduce the time cost of generative models for community detection in attributed networks. We first coarsen an attributed network into smaller ones by node matching. Then, we employ the existing generative model on the coarsest network without any modification for community detection, thus efficiently obtaining community memberships of nodes in this small coarsest network. Last, we project the assignments back to the original network through a local refinement mechanism to get communities. Extensive experiments on several real-world and artificial attributed networks show that our multi-level-based method is significantly faster than original generative models and is able to achieve better or more competitive results.

Influence of Pre-Milling on the Mn Solid Solubility in the Al-Mn-Cu Alloy during Mechanical Alloying

Increasing the strength of Al-based alloys is an important issue of physical metallurgy and industrial processing. Severe plastic deformation and related extension of solid solubility during mechanical alloying provide an opportunity for significant strengthening due to grain refinement, solid solution, and precipitation strengthening mechanisms. During mechanical alloying, an anomalous increase in the solid-state solubility of alloying elements occurs. The present study focuses on the investigation of the pre-milling treatment to the microstructure, phase composition, and solubility in Al-7.7 Mn-3.5 Cu (wt%) alloy processed by a high-energy ball milling of Al-14.3 Mn-6.5 Cu (wt%) master alloy diluted with Al powder. During milling, the mean granular size decreased to ~5 µm, and a strong grain refinement occurred. According to our TEM and XRD data, ball milling provided a mean grain size of 13–14 nm and a microhardness of 490–540 HV. The lattice parameter of the Al-based solid solution decreased with an increase in the milling time to 7.5–10 h, which suggested the dissolution of the alloying elements, and the lattice parameter increased at a higher milling time of 12.5–40 h, which suggested the decomposition of the solid solution. The XRD data revealed the dissolution of the Al6Mn and Al20Cu2Mn3 solidification-originated phases with a further precipitation of the Al6Mn dispersoids. Pre-milling of the master alloy entailed a significant decrease in the minimal lattice parameter value from 0.4029 nm to 0.4023 nm due to an increase in the Mn solute content from 6.2 wt% (3.3 at%) to 7.5 wt % (4.0 at%) in the studied alloy during high-energy ball milling.

Grain Refinement of Aluminum and Aluminum Alloys by Sc and Zr

Rare earth elements and transition elements are widely used in aluminum alloys, magnesium alloys and copper alloys due to their unique microalloying effects. With regard to in-depth research on the grain refinement characteristics of rare earth elements and transition elements, the combination of grain refinement and microalloying in the master alloys has a great impact on the theories and technical research of refinement, and the broadening of the application field of master alloys. This paper first summarizes the grain refinement mechanisms and analyzes the effects of rare earth elements and transition elements on the grain refinement of aluminum and aluminum alloys, and summarizes the elements suitable for the preparation of Al-M master alloys and their refinement mechanism.

Formation of ultrafine grain and mechanical properties in commercial pure titanium subjected to heavy-reduction thermomechanical processing around β transus temperature

The purpose of this study was to investigate the flow stress behavior and formation of ultrafine grains (UFGs) by focusing on the effects of strain-induced dynamic transformation (SIDT) and dynamic recrystallization (DRX) under hot forming near the β transus temperature. Heavy-reduction thermomechanical controlled processing (TMCP) of commercial pure (CP) titanium was performed at deformation temperatures ranging from 700 to 1000 °C, a strain rate of 1 s−1, and a 70% height reduction, followed by water cooling immediately after compression. The flow stress at 0.2 strain decreased significantly from 96 to 25 MPa when the deformation temperature increased from 800 to 900 °C. Equiaxed UFGs with a grain size of 2–4 μm formed in this temperature range. The primary mechanisms of grain refinement in the CP titanium under heavy-reduction TMCP were DRX occurring at 800 °C and a combination of DRX and SIDT (β → α) at 900 °C. Of the 700, 900, and 1000 °C TMCP-treated specimens subjected to a 90° V-bending test, the yield force of the 700 °C TMCP-treated specimen was the highest. When the TMCP temperature was increased from 900 to 1000 °C, the yield force decreased significantly, whereas the springback amount increased. The α phase grain refinement achieved by the TMCP at 700 °C increased the force, and the retained β phases within the 1000 °C TMCP-treated specimen increased the springback amount.

Investigation of the grain refinement mechanism in machining Ti-6Al-4V: Experiments and simulations

The thermal and mechanical loading may induce the grain refinement in the deformation zone during machining, which has great impacts on the mechanical behavior of the workpiece. In order to investigate the grain refinement mechanism in the shear band of Ti-6Al-4V during machining, we quantitatively analyzed the final microstructural characteristics including the grain size, grain boundary misorientation, and geometrical necessary dislocation (GND) density distribution in shear bands by precession electron diffraction (PED). It is found that the grains are highly refined in the shear band, and obvious continuous dynamic recrystallization (CDRX) characteristics have been captured. Besides, discontinuous dynamic recrystallization (DDRX) is verified not happen in shear bands of Ti-6Al-4V during machining theoretically while DDRX is often considered as the main grain refinement mechanism to predict the grain size in shear bands in literature reports. Then, a physically-based CDRX model is applied in machining, which describes the formation and rotation of subgrain, as well as the migration of the grain boundary. Subsequently, the cross-scale simulation method combining finite element (FE) and cellular automata (CA) is used to reproduce the microstructure evolution process of the shear bands. And, the effectiveness of the proposed model is verified by comparing the microstructure morphology, grain size, and dislocation density obtained by simulation with those characteristics observed experimentally. Finally, the model was applied to analyze the microstructure evolution in machined surface.

Microstructure Transformation of Undercooled Cu-Ni-Co Alloy and Refinement Mechanism under High Undercooling

Microstructure Transformation of Undercooled Cu-Ni-Co Alloy and Refinement Mechanism under High Undercooling

Effects of hafnium content on microstructures and properties of newly developed (Ti,Hf)(C,N) ceramics

In this study, novel (Ti,Hf)(C,N) ceramics with varying hafnium contents were fabricated via carbothermal reduction–nitridation and subsequent spark plasma sintering. The influence of Hf addition on the mechanical properties, wear properties, and corrosion resistance of the (Ti,Hf)(C,N) ceramics was systematically studied. The introduction of Hf promoted the sintering densification of the ceramics in the sintering process. The prepared (Ti,Hf)(C,N) ceramics exhibited excellent mechanical and wear properties owing to refinement and solution-strengthening mechanisms. The (Ti0.9,Hf0.1)(C0.5,N0.5) ceramic demonstrated higher Vickers hardness and fracture toughness, measuring 1997 HV5 and 4.28 MPa m1/2, respectively, compared to the pure Ti(C0.5,N0.5) ceramic which exhibited values of 1635 HV5 and 3.94 MPa MPa m1/2. The wear scar depth of the (Ti0.9,Hf0.1)(C0.5,N0.5) ceramic sample was 57.36% to that of the Ti(C0.5,N0.5) ceramic. Additionally, the addition of Hf improved the corrosion resistance of (Ti,Hf)(C,N) ceramics in a 0.5 M NaOH solution. The potential applications of (Ti,Hf)(C,N) ceramics include machining tools and wear-resistant parts.

Grain Refinement Mechanism of 5A06 Aluminum Alloy Sheets during Cold Rotary Forging.

This paper studies the grain refinement mechanisms of 5A06 aluminum alloy sheets in cold rotary forging (CRF). The results show that the grains are clearly refined from 25.1 µm to 11.8 µm during the CRF process. The grain refinement mechanism can be divided into two modes: (1) The grains with a small Schmid factor (SF) are activated by multi-slip systems, and dense dislocations are segregated along the boundaries of interior regions with different slip systems, which results in a rapidly increasing strain localization along these boundaries. Since the strain localization restrains the coordinate slip deformation between different interior regions, the grains are directly separated into several finer grains. (2) The grains with a large SF are primarily activated by a single slip system, and the dislocation migrates smoothly along most microband boundaries. Then, a more severe lattice rotation causes a transformation to a hard orientation and multi-slip system activation, which contributes to an increase in the rapid misorientation across microband boundaries and thus promotes significant SF grain refinement.

Graded distribution and refinement of Mg2Si in Al–Mg2Si alloy prepared by traveling magnetic field

To quantitatively analyze the mechanisms of graded distribution and refinement of electromagnetic separation technology, in situ Al–Mg2Si graded materials were prepared using traveling magnetic field. Simulation and experiment were both carried out to analyze the effect of traveling magnetic field on the distribution and size of Mg2Si phase. Simulation results show that 6A excitation current density achieves the best effect of graded distribution. Experiments results show traveling magnetic field changed the distribution of Mg2Si in the alloy from uniform to graded distribution and refined the size of primary Mg2Si particle from 16 μm to 10 μm at the outer region on the cross-section. The mechanism of refinement was quantitatively analyzed, the refinement of Mg2Si particle results from stirring and crushing by magnetic field without promoting nucleation. The graded distribution mechanism is attribute to the resultant force acting on Mg2Si particle, which promote Mg2Si particle distributed near the outer region and graded distribution. Hardness distribution on the cross-section is consistent with the volume fraction distribution of Mg2Si.

Effect of Supergravity Field on the Microstructure and Mechanical Properties of Highly Conductive Cu Alloys

In consideration of the characteristics of supergravity to strengthen solidification structures, the effect of the supergravity field (SGF) on the grain refinement and mechanical properties of Cu-0.5Sn alloys was investigated in this paper. Firstly, it was experimentally verified that the addition of Sn could effectively refine the grain. Subsequently, the variations in grain size, tensile strength, and plasticity of the Cu-0.5Sn alloy were compared in normal and SGF conditions. The results revealed that the tensile strength and plasticity of the alloy increased with the increase in gravity coefficient. The ultimate tensile strength of the Cu-0.5Sn alloy in a normal gravity field was 145.2 MPa, while it was 160.2, 165.3, 167.9, and 182.0 MPa in an SGF with G = 100, 300, 500, and 1000, respectively, and there was almost no effect on conductivity. Finally, it was clarified that the mechanism of grain refinement by SGF was that the intense convection caused the fracture of the dendrites to become new nucleating particles. The increased viscosity under SGF hindered the diffusion of atoms in the melt and slowed down the movement of atoms toward the nucleus, leading to a decrease in grain size.

Revealing the interface energy and electronic properties of NbN/α-Fe in shipbuilding steels

In this work, the low index interfacial configuration, interface energy and electronic properties of the NbN/α-Fe interface in shipbuilding steels were investigated by using the first-principles method and ab initio molecular dynamics simulation calculations. Additionally, the NbN/α-Fe interface with the low index interfacial configuration was calculated by two-dimensional disregistry method. It is shown that the relaxation optimization of the NbN(001)/α-Fe(001) structure is carried out and the interfacial energy of Fe-Nb type, Fe-bridge type and Fe-C type NbC(001)/α-Fe(001) structure is calculated, the Fe-N type has an interfacial energy of −1.659 J/m2, the smallest interfacial energy and thus is the most stable. Thereby, the Fe-N type is the most stable conformation of NbN(001) /α-Fe(001). There are strong orbital resonance phenomena between Fe-3d, Nb-4d and N-2p, producing strong metallic and covalent bonds. The results of disregistry calculation by different interfacial structures of NbN/α-Fe showed that the disregistry of NbN(001)/α-Fe(001) crystalline surface was the smallest at 8.65 %, which matches well with the results of interfacial energy calculation. The results indicated that the α-Fe(001) is the optimal orientation surface of the NbN(001). This study reveals the stable bonding mode and site-oriented relationship of NbN/α-Fe interface in steel, which provides important theoretical guidance to explore the mechanism of grain refinement of NbN particles in shipbuilding steels.

An Overview on the Effect of Severe Plastic Deformation on the Performance of Magnesium for Biomedical Applications.

There has been a great interest in evaluating the potential of severe plastic deformation (SPD) to improve the performance of magnesium for biological applications. However, different properties and trends, including some contradictions, have been reported. The present study critically reviews the structural features, mechanical properties, corrosion behavior and biological response of magnesium and its alloys processed by SPD, with an emphasis on equal-channel angular pressing (ECAP) and high-pressure torsion (HPT). The unique mechanism of grain refinement in magnesium processed via ECAP causes a large scatter in the final structure, and these microstructural differences can affect the properties and produce difficulties in establishing trends. However, the recent advances in ECAP processing and the increased availability of data from samples produced via HPT clarify that grain refinement can indeed improve the mechanical properties and corrosion resistance without compromising the biological response. It is shown that processing via SPD has great potential for improving the performance of magnesium for biological applications.

Refinement mechanism and optimization for compact strip production process

To avoid the fracture of cold-rolled sheets, this study investigated the refinement mechanism for the compact strip production process and proposed an optimised method. The primary austenite grain size was 50–475 µm, and the grain size of the hot-rolled strips was 3.6–4.8 µm. The Gleeble-3500 simulator was applied to simulate the hot rolling process. Grain refinement was realised through dynamic recrystallization and the supply of more nucleation sites for transformation by sub-structures. Increasing the coiling temperature from 620 to 730 °C increased the ferrite fraction from 3.95 to 24.16%. The adjacent cementite distance changed from 0.33 to 0.84 µm, resulting in decreased yield and tensile strengths and increased elongation. Thus, the fracture of cold-rolled sheets was avoided.

Atomic insights into heterogeneous nucleation and growth kinetics of Al on TiB2 particles in undercooled Al-5Ti-1B melt

Titanium diboride (TiB2) is an effective grain refiner of Al alloys in the industry that facilitates casting processes by forming uniformly refined microstructures. Although our understanding of the underlying refinement mechanisms has advanced, the atomic kinetics of heterogeneous nucleation of Al on TiB2 remains unknown. Here, we report atomic-scale observations of the heterogeneous nucleation and growth kinetics of Al on self-formed TiB2 particles by in situ heating of undercooled Al-5Ti-1B films. We demonstrate that an ordered Al monolayer forms on the Ti-terminated {0001}TiB2 surface; then, the surrounding Al atoms are initiated to form an island-shaped Al nucleus with face-centered cubic {111} stacking without the assistance of a Ti-rich buffer layer. The interfacial lattice mismatch between {111}Al and {0001}TiB2 causes remarkable out-of-plane strain that decreases gradually with Al nucleus layers increasing to 6 atomic layers. The elastic strain energy originating from this interfacial strain increases the free energy of the Al/TiB2 heterostructure, hence impeding the rapid growth of the Al nucleus. We found that TiB2 particles stabilize the Al nuclei rather than activating their free growth into grains when the experimental undercooling ΔT is lower than the onset undercooling ΔTfgin Greer's free growth model. Our findings provide an atomic-scale physical image of the heterogeneous nucleation and growth mechanisms of Al with inoculator participation and elucidate the strain-dependent growth kinetics of Al nuclei.

Microstructure refinement mechanisms in undercooled solidification of binary and ternary nickel based alloys

Molten glass purification and cycle superheating technologies were used to make Ni65Cu35, Ni65Cu33Co2 and Ni65Cu31Co4 alloys obtain maximum undercoolings of 320 K, 292 K and 300 K respectively. In order to analyze the relationship between morphological characteristics of solidification front and undercooling change during migration of solid–liquid interface, a high-speed camera was used to capture pictures of the recalescence process. Observing the microstructure of the undercooled alloys using metallographic microscope, the characteristics and evolution of microstructure during rapid solidification process of undercooled liquids were analyzed. It was found that grain refinement mechanisms of highly undercooled Ni–Cu–Co alloys was the same as those of the Ni–Cu alloys. Dendrite remelting leads to the grain refinement at low undercooling, while the dominant factor of grain refinement at high undercooling is recrystallization process induced by stress. The internal driving force can be divided into two parts: one is the thermal stress generated by the releasing of solidification latent heat during recalescence process, and the other is the stress and strain accumulated by interaction of liquid flow and primary dendrite during rapid solidification. We also found that addition of third element Co not only played an important role in solidification rate and recalescence effect, but also significantly improved the average hardness of grain refined microstructure, which was about 80% higher than that of as cast alloy. The addition of trace Co was also conducive to the formation of non-segregation solidification microstructure.

Heterogeneous bimodal microstructure and its formation mechanism of Mg-Gd-Y-Zn-Zr alloy during isothermal compression

Heterogeneous bimodal microstructure and its formation mechanism of Mg-Gd-Y-Zn-Zr alloy during isothermal compression at 450 °C and 0.001 s−1 was investigated in detail. Based on microstructure analysis, Mg-Gd-Y-Zn-Zr alloys with bimodal grain size distribution and bimodal texture distribution were obtained by uniaxial isothermal compression. There were more heavily bimodal levels in grain size and texture at true strain greater than 0.8. There was a significant inhomogeneous plastic deformation between and within the grains in Mg-Gd-Y-Zn-Zr alloy during the deformation, which is attributed to the difference the grain orientation and a large number of intergranular block-shaped LPSO phases. Further studies revealed that particle-stimulated nucleation (PSN) caused by a large intergranular LPSO phases and continuous dynamic recrystallization (CDRX) developed layer by layer toward the grain interior were the main grain refinement mechanisms. This PSN and CDRX mechanism contributed to the fine grains in bimodal grain size distribution and the random texture in bimodal texture distribution. However, the original coarse grains with basal orientation were preserved during deformation due to smaller plastic deformation and energy storage. Meanwhile, grain boundary segregation and intragranular lamellar LPSO precipitate promoted CDRX by retarding DDRX. In addition, the grain boundary pinning effect dominated by grain boundary segregation and intragranular lamellar LPSO precipitate hindered the coarsening of fine DRXed grains, which was the ultimate guarantee for the formation of bimodal microstructure. Ultimately, inhomogeneous plastic deformation, DRX, and grain boundary pinning effect were demonstrated as the necessary conditions for the formation of heterogeneous bimodal microstructure during thermal deformation of Mg-Gd-Y-Zn-Zr alloy.

New insights into the colony refinement mechanism by solute boron atoms in Ti-6Al-4V alloy

The coarse lamellar microstructure of as-cast α+β titanium alloys can be effectively refined by boron additions, which has mainly been attributed to the precipitation of TiB whiskers that pin the growth of prior β grains. Here, we report an alternative mechanism of colony refinement in Ti-6Al-4V alloy containing trace amounts of boron solutes (0.02 wt.%). Using atom probe tomography, it is discovered for the first time that boron solutes segregate at the α/β phase boundaries, due to its negligible solubilities in both α and β phases. These segregated boron solutes at the α/β phase boundaries restrict an easy growth of grain boundary α, and in turn promote the nucleation of more grain boundary α variants that serve as nucleation sites of α colonies. Hence, the colony size can be substantially refined without the formation of TiB whiskers that could be harmful to the ductility of the material.

Effect of deformation parameters on microstructure evolution, flow softening behavior and nanoindentation properties of Nb–24Ti–16Si–2Al–4Fe–5V alloy

The microstructural evolution and the flow softening behavior of Nb–24Ti–16Si–2Al–4Fe–5V alloy during hot deformation under different processing parameters were investigated. Results show that the as-cast Nbss/β-(Nb, X)5Si3 eutectic structure disappears and becomes the Nbss matrix and β-(Nb, X)5Si3 blocks after hot deformation, wherein the continuous Nbss matrix consists of fine recrystallized grains. With the increase of deformation temperature, the β-(Nb, X)5Si3 particles evolve from irregular to quadrilateral shape. The refinement mechanism of the β-(Nb, X)5Si3 phase can be divided into silicide splitting and continuous dynamic recrystallization (CDRX). Moreover, the decrease of strain rate makes the DRX degree of Nbss increase, while the influence on that of β-(Nb, X)5Si3 is relatively less. The flow softening behavior of the alloy is mainly controlled by the CDRX behavior of the Nbss phase, and the recrystallization refinement can improve the hardness (H) of the Nbss phase. After deformation, the H values of Nbss will increase with the increase of recrystallization degree, while the H value of β-(Nb, X)5Si3 phase will increase with the decrease of strain rate.