Ultrafine-grained Metals Research Articles

A new severe plastic deformation technique of combined extrusion and torsion to prepare bulk ultrafine grained copper

Reducing grain size is a well-established method for strengthening metals. In this study, a novel severe plastic deformation technique—combined extrusion and torsion (CET) with composite strain—was developed to fabricate bulk ultrafine grained metals. A single pass of CET treatment (with a rotation velocity of 1 r/s and extrusion speed of 3 mm/s) refined coarse-grained copper from 54 μm to 450 nm at room temperature, resulting a significant increase in hardness from 0.55 GPa to 1.3 GPa. The CET technique addresses the limitations of conventional extrusion-processed copper (without torsion), which suffers from gradient microstructure and hardness distribution. It provides enhanced strain accumulation under the same extrusion ratio conditions. The more homogeneous microstructures and properties of CET-processed copper rods are attributed to the reduced strain gradient due to torsion. Additionally, targeted finite element analysis indicated that the CET technology requires 37 % less extrusion load and offers at least 30 % more strain compared to conventional extrusion methods. Compared with other severe plastic deformation methods, such as equal channel angular pressing and high-pressure torsion, which involve simpler deformation processes, the CET technique shows considerable promise for large-scale manufacturing of ultrafine-grained metals.

Friction welding of UFG copper using the W2Mi prototype machine

When welding ultra-fine-grained metals using conventional methods, the microstructure in the joint area is degraded (mainly due to recrystallization) in the heat-affected zone and a significant deterioration of mechanical properties, including joint strength. The aim of the research presented in this article was to identify the possibility of obtaining joints with strength close to initial material using friction welding of metal materials with ultra-fine grain. For this purpose, UFG (ultra-fine-grained) material was produced from technically pure M1Ez4 copper using a hybrid SPD (severe plastic deformation) process. The welding process was carried out on a machine with a prototype design that allows minimizing the welding time, while generating high force. The process parameters used on the prototype machine resulted in an increase in the hardness of the material by 4% in the joint area. The strength of the joint compared to the base material decreased slightly by 2%. The tests carried out proved that, using appropriate process parameters, it is possible to obtain a UFG metal joint without a decrease in its mechanical strength.

Achieving high strength and uniform ductility in high-entropy alloys via dynamic-precipitation accelerated transformation-induced plasticity

This study presents a novel approach to fabricating advanced transformation-induced plasticity (TRIP) high-entropy alloys (HEAs) that exhibit an exceptional combination of high strength and uniform ductility, far surpassing the mechanical performance of previously developed TRIP HEAs and non-TRIP multiphase HEAs. The ultrafine-grained Al5Cr20Fe35Co35Ni5 TRIP HEA sample exhibits a yield strength of gigapascal-level, ranging from 1100 to 1260 MPa, with large uniform elongations that range from 29 % to 39 %. Simultaneously, the alloy displays outstanding toughness, reaching 46,000±5390 MPa·%. The achievement of this significant breakthrough rests upon two key factors: (1) achieving an ultrafine-grained FCC matrix in a fully recrystallized microstructure with the aid of thermally induced B2 phase, and (2) dynamic precipitation of nanometer-sized B2 phase particles on high-density stacking faults and slip bands created within the interiors of ultrafine grains. The dynamic precipitation of the B2 phase, facilitated by the formation of an amorphous phase that serves as a precursor to the B2 crystalline phase, is a rare occurrence in HEAs at room temperature. This dynamic precipitation process greatly enhances TRIP-assisted strain hardening (from FCC to HCP phase) of the current alloy at ultrafine grains, enabling the overcoming of the inherent tensile ductility constraints typically associated with ultrafine-grained metals. The current development of TRIP HEAs with an outstanding combination of high strength and high toughness is believed to mark a remarkable milestone in the application field of TRIP HEAs.

The role of grain size in achieving excellent properties in structural materials

Advanced structural materials are expected to display significantly improved mechanical properties and this may be achieved, at least in part, by refining the grain size to the submicrometer or the nanocrystalline range. This report provides a detailed summary of the role of grain size in the mechanical properties of metals. The effect of grain size on the high temperature behavior and the development of superplasticity is illustrated using deformation mechanism maps and the development of exceptional strength through grain refinement hardening at low temperatures is also discussed. It is shown that the deformation mechanism of grain boundary sliding, as developed theoretically, can be used to effectively predict both the high and low temperature behavior of metals having different grain sizes. This analysis explains the increase in strain rate sensitivity in ultrafine-grained metals with low and moderate melting points and the ability to increase both the strength and ductility of these materials to thereby overcome the strength-ductility paradox. The recent development of hybrid materials is also reviewed and it is demonstrated that, although these hybrids have received only limited attention to date, they provide a potential for making significant advances in the production of new structural materials.

Strain hardening and microstructure evolution in ECAP-processed ultrafine-grained metals: a comparative study of copper, aluminum, and magnesium alloys

Strain hardening and microstructure evolution in ECAP-processed ultrafine-grained metals: a comparative study of copper, aluminum, and magnesium alloys

Refractory high-entropy nanoalloys with exceptional high-temperature stability and enhanced sinterability

Nanocrystalline alloys (nanoalloys) are prone to grain growth. It is known that grain boundary segregation and precipitation can stabilize nanoalloys, but the stabilization becomes less effective at high temperatures and adding grain growth inhibitors often reduces sinterability. Herein, we have simultaneously achieved exceptional high-temperature stability and improved sinterability for a class of TiNbMoTaW-based refractory high-entropy nanoalloys (RHENs). Bulk pellets of RHENs were fabricated through ball milling and spark plasma sintering, achieving 93–96% relative densities with 50–100 nm grain sizes for three compositions. For example, Ti17.8Nb17.8Mo17.8Ta17.8W17.8Ni6Zr5 sintered at 1300 °C attained ~ 96% relative density with ~ 55 nm mean grain size. Moreover, these RHENs exhibited exceptional stability at 1300 °C. Both Ti17.8Nb17.8Mo17.8Ta17.8W17.8Ni6Zr5 and Ti18.8Nb18.8Mo18.8Ta18.8W18.8Ni6 retained < 150 nm grain sizes after five hours annealing at 1300 °C. Notably, the addition of Ni, a well-known sintering aid for activated sintering of refractory metals such as W and Mo, in high-entropy TiNbMoTaW can promote sintering while maintaining high-temperature stability against rapid grain growth. This may be explained by hypothesized high-entropy grain boundary (HEGB) effects, while we recognize the possible (additional) effects of compositional inhomogeneity and secondary phase (Zener) pinning. These RHENs possess some of the highest temperature stability achieved for nanoalloys and ultrafine-grained metals.

Powder Metallurgy Route to Ultrafine-Grained Refractory Metals.

Ultrafine-grained (UFG) refractory metals are promising materials for applications in aerospace, microelectronics, nuclear energy, and many others under extreme environments. Powder metallurgy (PM) allows to produce such materials with well-controlled chemistry and microstructure at multiple length scales and near-net shape manufacturing. However, sintering refractory metals to full density while maintaining a fine microstructure is still challenging due to the high sintering temperature and the difficulty to separate the kinetics of densification versus grain growth. Here an overview of the sintering issues, microstructural design rules, and PM practices towards UFG and nanocrystalline refractory metals are sought to be provided. The previous efforts shall be reviewed to address the processing challenges, including the use of fine/nanopowders, second-phase grain growth inhibitors, and field-assisted sintering techniques. Recently, pressureless two-step sintering has been successfully demonstrated in producing dense UFG refractory metals down to ≈300nm average grain size with a uniform microstructure and this technological breakthrough shall be reviewed. PM progresses in specific materials systems shall be next reviewed, including elementary metals (W and Mo), refractory alloys (W-Re), refractory high-entropy alloys, and their composites. Last, future developments and the endeavor towards UFG and nanocrystalline refractory metals with exceptionally uniform microstructure and improved properties are outlined.

Bulk Ultrasonic Treatment of Crystalline Materials

Ultrasound is widely used in the treatment of materials. Its applications in melt processing, surface hardening or finishing, metal forming, welding, etc., are well known and have been reviewed in numerous review articles and books. Among ultrasound-assisted processes, the direct action of ultrasonic waves on the structure and properties of bulk materials is of special interest. Ultrasonic waves induce oscillating shear stresses in materials which exert mechanical forces on crystal lattice defects, primarily on dislocations, which can yield a number of interesting effects on the structure and properties of crystals. The present paper aims to review studies on the effects of ultrasonic treatment (UST) on crystalline materials. First, the methods for the excitation of standing ultrasonic waves in bulk samples of materials are analyzed. Then, early studies on the effect of UST on the dislocation structure and phase composition, hardness and strength of materials with different initial structures are analyzed. An emphasis is then made on the influence of UST on the structure and mechanical properties of advanced ultrafine-grained (UFG) metals and alloys processed by severe plastic deformation (SPD). The results of simulations of ultrasound’s effect on the dislocation and atomic structures of materials by dislocation and molecular dynamics methods are also reviewed.

Equal channel angular extrusion (ECAE): Die design, affecting variables and development of mechanical behavior and properties of bio-metals

There are numerous practices for producing nanocrystalline (NC) and ultrafine-grained (UFG) metals, composites, polymers and alloys; amongst which Equal Channel Angular Extrusion (ECAE) is one of the renowned techniques in metal forming processes under the severe plastic deformation process (SPD) in which a massive plastic strain is enforced on a bulk material. The novelty of this work is the design and fabrication of a split type of die specifically for carrying out ECAE on difficult-to-deform bio-metals. The die is designed for a channel angle of 120°. The study provides a detailed discussion of the pre and post-extrusion tests, process flowchart, and operational variables. This work offers a valuable opportunity to investigate the implications of the ECAE process for other challenging-to-deform metals. Additionally, the ultrafine-grained materials produced through ECAE have potential applications in industries such as automobile, power, and aerospace.

Developing methods of constrained groove pressing technique: A review

Among various severe plastic deformation (SPD) techniques, constrained groove pressing (CGP) has arisen as a famous technique for fabricating ultrafine-grained sheet metals and alloys in recent times. The severe strain is applied to improve the mechanical performance and properties. However, inhomogeneous grain refinement, lower formability, surface unevenness, and microcracks are the common limitations of conventional CGP-processed samples. Therefore, this review inspects the latest progress through modifications of the CGP process and compares the process efficacy with the conventional process. Advanced CGP processing techniques, such as modification of die design, annealing of the processed sample, cross CGP, ring CGP, hybrid SPD, covered sheet casing CGP, and rubber pad CGP, are emerging techniques to up-scale the procedure and improve the grain refinement and thereby produce superior mechanical properties. In this review paper, the historic research of conventional CGP and its effect on grain refinement and mechanical properties is initially briefed and then recent progress through different advanced techniques is highlighted, especially in alloys. Albeit significant progress has been observed in CGP, the future scope and successful usage of CGP-processed samples in actual practice are discussed in the last stage.

Pressureless two-step sintering of ultrafine-grained refractory metals: Tungsten-rhenium and molybdenum

• Pressureless two-step sintering of W-10Re and Mo has been demonstrated. • 98.4% dense W-10Re with an average grain size of 260 nm can be pressurelessly sintered below 1200 °C. • 98.3% dense Mo with an average grain size of 290 nm can be pressurelessly sintered below 1120 °C. • W-10Re and W have similar grain boundary diffusivity at the same homologous temperature and similar activation energy. • Mo has more sluggish grain boundary diffusivity and larger activation energy. • Pressureless two-step sintering to be a general method to produce dense ultrafine-grained refractory metals and alloys. The challenge of sintering ultrafine-grained refractory metals and alloys to full density is hereby addressed by pressureless two-step sintering in tungsten-rhenium alloy and pure molybdenum. Using properly processed nano powders (∼50 nm average particle size), we are able to sinter W-10Re alloy to 98.4% density below 1200 °C while maintaining a fine grain size of 260 nm, and sinter molybdenum to 98.3% density below 1120 °C while maintaining a fine grain size of 290 nm. Compared to normal sintering, two-step sintering offers record-fine grain sizes and better microstructural uniformity, which translates to better mechanical properties with higher hardness (6.3 GPa for tungsten-rhenium and 4.0 GPa for molybdenum, both being the highest in all pressurelessly sintered samples of the respective material system) and larger Weibull modulus. Together with our previous demonstration in tungsten, we believe that two-step sintering is a general effective method to produce high-quality fine-grained refractory metals and alloys, and the lessons learned here are transferable to other materials for powder metallurgy.

Fabrication of ultrafine-grained Ti-15Zr-xCu alloys through martensite decompositions under thermomechanical coupling conditions

• Ultrafine-grained microstructure was successfully achieved in Ti-15Zr- x Cu system by hot rolling. • Cu alloying increased dynamic recrystallization nucleation rate and formed Ti 2 Cu along grain boundaries, which inhibited grain growth. • The comprehensive mechanical properties of the ultrafine grained Ti-15Zr-5Cu alloy are better than that of the Roxolid Ti-Zr alloy currently used for dental implants. Grain refinement is a well-recognized method to simultaneously increase the strength and ductility of metallic materials. Fabrication of ultrafine-grained metals in bulk using a simple low-cost approach is a long-term goal for material scientists. In this work, based on the chemical composition of a biomedical Ti-15Zr alloy, a series of novel Ti-15Zr- x Cu ( x = 0, 3, 5, 7 wt.%) alloys were designed and fabricated. The alloys were quenched in the single β phase region to obtain a martensitic microstructure and deformed in the temperature range of 710–750 °C to obtain an ultrafine-grained microstructure through martensite decomposition under thermomechanical coupling conditions. Experimental results showed that Cu alloying could increase the dynamic recrystallization (DRX) nucleation rate due to its role in both refining martensitic lath width and increasing dislocation density. Cu alloying could also suppress grain growth due to the precipitated Ti 2 Cu particles exerting pinning forces on the grain boundaries. The optimal Cu content in the Ti-15Zr- x Cu alloy was determined to be 5 wt.%. After being subjected to a compression leading to a 70% height reduction at 730 °C and 1 s –1 , the grain size of the Ti-15Zr-5Cu alloy was only 180 ± 70 nm. The tensile strength of the as-prepared alloy reached 975 ± 10 MPa, which was 45% higher than that of the conventional Ti-15Zr alloy (673 ± 16 MPa). This increase in strength was achieved without any reduction in ductility. The comprehensive mechanical properties of the ultrafine-grained Ti-15Zr-5Cu alloy are better than that of the Roxolid Ti–Zr alloy currently used for dental implants.

Investigating the Mechanical Properties of Aluminum Alloy Ultra-Fine Grained Metals by Fluid-Solid Coupling Method

This research examines the mechanical characteristics of metal materials using the FSC (Fluid-solid coupling) technique. It primarily investigates the formation of a regular FSC non-smooth surface by the 7A05 deformed aluminum alloy after laser treatment. The mechanical characteristics of the manufactured aluminum alloy ultrafine grained metal materials are investigated by altering various laser settings, coupling patterns, and cell distribution modes. The findings demonstrate that the tensile strength of aluminum alloy increases by 37.21% and 44.14%, and the fracture strength increases by 38.69% and 46.73%, respectively, when the chip thickness compression ratio is 1.2 and 2.2. According to the study, extruding aluminum alloy improves its Vickers hardness, tensile strength, friction, and wear qualities to varying degrees.

Energy model of transformation of two-time decomposition mode of dislocation structure at triple junction: Ultra-fine grained materials

Ultrafine-grained metal materials exhibit strong mechanical properties, but often have poor plasticity. Since the volume fraction of the triple junctions of the grain boundaries in the material is very high, the dislocation at triple-junction of grain boundaries can play a key role in excellent performance of the material, such as high strength and good plasticity in many cases. In this paper, based on one-time decomposition mode of the dislocation pile-up at the triple junction of grain boundaries proposed by Fedorov et al. [Acta Materialia, 51 (2003)887–898], a new energy model is proposed for the two-time decomposition mode of the dislocation pile-up at the triple junction. With this model, we explore the effect of the critical conditions on the decomposition mode of the second dislocation and their transformation between two modes and get the criterion for the transformation of two modes, such as the magnitude of stress, the angle of triple junction and the distance of the decomposed dislocations moving. The transformation of the different decomposition modes of the second dislocation is related to the displacement distance of the dislocations decomposed from the first dislocation of the dislocation pile-up. The transformation criterion of the different decomposition mode of the second dislocation, the strengthening and weakening of the triple junction of the grain boundaries and the dislocation mechanism of superplasticity in plastic deformation processes under loading can be used to analyze semi-quantitatively.

Improving Mechanical and Corrosion Behavior of 5052 Aluminum Alloy Processed by Cyclic Extrusion Compression

Background The severe plastic deformation approach and its well-known cyclic extrusion compression (CEC) method have been established as a powerful tool for fabricating bulk ultrafine-grained metals and alloys with improved properties. Objective This study focused on the microstructure evolution, hardness behavior, and corrosion properties of the CEC-processed Al5052 up to four passes compared to the initial annealed state. Methods The initial and CEC-processed Al5052 samples at different pass numbers were examined experimentally by EBSD analyses, hardness measurements, and corrosion resistance. Results Substantial grain refinement was attained from ~23 μm for the annealed sample to ~0.8 μm in the four passes sample. In addition, the hardness values considerably increased up to 75.7% after four passes from the initial value of 80 HV. In addition, the increment of pass numbers led to a more uniform dispersion of hardness values. Furthermore, the production of more stable protective oxide layers on the UFG structure of the CEC-processed sample led to the improvement in electrochemical response with a corrosion rate reduction from 1.49 to 1.02 mpy, respectively, in the annealed and final pass CEC-processed samples. In fact, the annealed sample manifested more large-sized and deeper pits than the CECed samples due to the increment of potential values and electrochemical attack of chlorine ions that finally deteriorates the corrosion performance. Conclusions CEC is an efficient method to improve the mechanical properties of materials due to substantial microstructural changes along with enhancement of electrochemical behavior because of the presence of small-sized and shallow pits.

Atomistic modeling of surface and grain boundary dislocation nucleation in FCC metals

Dislocation nucleation plays a critical role in the plastic deformation of crystalline materials. However, it is challenging to predict the active mode and associated rate of dislocation nucleation under typical experimental loading conditions through molecular dynamics simulation due to timescale limitations. Here we use the free-end nudged elastic band method to determine the activation energies and activation volumes of dislocation nucleation in four typical face-centered cubic metals of Au, Al, Cu and Ni. We focus on the representative processes of surface and grain boundary dislocation nucleation. The atomistically determined activation volumes of these dislocation nucleation processes are larger than 10b3 (with b being the Burgers vector length) under typical experimental loading conditions. These results are compared with experimentally measured activation volumes in ultrafine-grained and nanocrystalline metals, thereby providing mechanistic insight into their rate-controlling deformation mechanisms.

Adiabatic Shear Failure of Ultrafine Grained Titanium Under Impact Loading: An in-situ Experimental Study

Background: Ultrafine-grained (UFG) titanium, which is with high yield strength, biocompatibility and corrosion resistance, is widely used in biomedical and industrial applications. However, adiabatic shear localization (ASL) is often observed in UFG materials due to their worse deformation stability under impact loading. This instability will easily result in formation of adiabatic shear bands (ASB), a narrow band located in the ASL zoom, and finally cause the fracture of material. The main objective of this work is to study the adiabatic shear behavior of UFG titanium under impact loading, including macro- and micro-properties, temperature rise, ASB failure, etc. Methods: A synchronization apparatus, which consisted of Kolsky bar system, high-speed camera system and high-speed infrared temperature measuring system, was set up to carry out the in-situ study of the mechanical properties, temperature rise, and adiabatic shear failure process of UFG pure titanium. Microstructure of the material was also analyzed in this work. Results: The critical strain of UFG pure titanium for adiabatic shear localization is about 0.37 and 0.69 for UFG Ti and CG Ti, respectively. The peak shear stress of UFG Ti is 500MPa. The propagation velocity of ASB in UFG titanium is 533~800m/s, and 160~320m/s for CG Ti. The temperature rsie within ASB of UFG titanium is 307~732℃, and 212-556℃ for CG Ti. The intense temperature rise is after the peak stress and the initiation of ASB most of the time. Conclusion: UFG Ti has good mechanical properties, however, it is easier to form ASB and cause adiabatic shear failure under impact loading when compared with CG Ti. Temperature rise may not play a major role in the formation of ASB in UFG Ti, but may be the consequence of ASB. Results of this work will help researchers better understand the failure of UFG metals under impact loading.

The formation mechanism, mechanical properties and thermal stability of the pure copper sheet with gradient structure processed by plastic flow machining

Plastic flow machining (PFM) is a novel severe plastic deformation (SPD) process. Ultrafine-grained metal sheets or strips with gradient structures are processed by PFM in a single step under the combined effects of cutting and pressing. In this paper, the formation mechanism, mechanical properties, and thermal stability of pure copper sheets with gradient structures for different extrusion thicknesses were studied. The results implied that there was a pronounced gradient in terms of strain, grain size, and hardness across the sheet thickness. As the extrusion thickness was varied from 1.0 to 1.6 mm, the increasing thickness of the deformed coarse-grained layer contributed to decreases in the yield strength (YS) and ultimate tensile strength (UTS) and an increase in the elongation after fracture. As a result, the distribution of the gradient structure across the sheet thickness was controllable. There were considerable improvements in hardness, YS, and UTS and only a small reduction in elongation after fracture. In addition, after annealing at 150 °C, the hardness and tensile properties of the sheets were slightly different than those of the untreated sheets, indicating that the sheets had good thermal stability below this annealing temperature. Thus, PFM is an efficient SPD process for preparing the gradient structure sheets with high strength and reasonable ductility. • The formation mechanism of the pure copper sheets with gradient structure processed by plastic flow machining was analyzed. • The variation of the gradient structure of the pure copper sheets with different processing conditions was analyzed. • Thermal stability of the pure copper sheets was analyzed.

Machinability of pure copper before and after processing by severe plastic deformation method

For using ultrafine-grained metals and alloys in industry, further machining sequences and studying their possible effects are necessary to attain these products in their ultimate dimensions. In this work, the machinability of ultrafine-grained pure copper and the corresponding coarse-grained counterpart investigated systematically. The results showed that the processed copper could be machined as efficiently as its initial one. Also, the machining of the Cu sample with the polycrystalline diamond tools requires much less force than tungsten carbide tools. The produced cutting forces were smaller for the processed copper as compared to the initial copper. Also, similar tool wear mechanisms were detectable for the copper specimen before and after the process. Chip morphology altered less through the machining, and the surface quality enhanced during machining the ultrafine-grained copper. Eventually, the polycrystalline diamond tool increased the surface roughness of initial and processed specimens since it caused less than 50% roughness compared to the tungsten carbide tool.

Annealing treatments to enhance thermal and mechanical stability of ultrafine-grained metals produced by severe plastic deformation

Abstract Ultrafine-grained (UFG) metals produced by techniques of severe plastic deformation, such as equal channel angular pressing (ECAP), exhibit extraordinary strength properties. However, in the as-ECAP-processed state, the heavily deformed microstructure of such UFG metals is rather unstable and is prone to undergo grain coarsening (recrystallization) at moderate temperatures. This microstructural instability is enhanced in the presence of modest mechanical stressing as, for example, in cyclic deformation. Thus, all measures to enhance the thermal stability are also considered as beneficial for the improvement of the mechanical stability. One main objective of the present work is to analyse the thermal and mechanical stability of ECAP-processed metals during specific annealing and cyclic deformation tests. As a by-product, some conclusions relating to the separate effects of dislocation density, grain size (in the UFG regime) and internal stresses on the (micro)yielding behaviour will be drawn. Another goal is to explore the potential of different annealing treatments with respect to the stabilization of the microstructure and the optimization of the mechanical properties of ECAP-processed UFG metals in terms of an optimal combination of strength and ductility. In order to demonstrate the potential and the limitations of this approach, experimental work performed on UFG copper, aluminium and α-brass produced by ECAP will be reported and discussed. The results presented indicate strongly that a heat treatment leading to a bimodal grain size distribution provides the best compromise between strength and ductility.