Coarse-grained Counterparts Research Articles

An innovative solid state manufacturing approach for developing high performance Al/Cu bimetals

Al/Cu bimetals are extensively used in various sectors like electronics and electrical, cookware and decorative industries to make parts and components. The bimetallic components developed by most of the existing manufacturing processes often shows both poor interfacial bond strength and inferior tensile strength. The present work explores an alternate microstructural engineering based novel manufacturing approach to improve the product performance of Al/Cu bimetals via a hybrid manufacturing process consisting of: (a) cryorolling + short-term annealing to engineer microstructural variation in parent materials and (b) laboratory plane strain bonding process for joining/bonding. Four different microstructural combinations are engineered in Al and Cu parent materials (such as ultrafine grained, bimodal/heterogeneous grained, fine grained and coarse-grained microstructures) and the influence of such microstructural combinations on joining potential of Al/Cu bimetals at wide range of process parameters are established. Among all microstructural combinations, ultrafine grained Al + bimodal grained Cu combination has shown excellent bond quality due to dual advantage of high ductility from its coarse-grained counterparts and interfacial strain localization from the UFG counterparts. When deformed at high deformation speed, the bimodal material shows signatures of excellent bonding in the form of microscopic waves and swirls. A possible mechanism is cyclic interaction between the back stress and forward stress arising due to the differences in the strain accommodation capabilities between the fine and coarse grains in this microstructure. This cyclic strain provides a ratcheting effect which results in exceptionally high shear strain at interface and facilitates enhanced bond quality due to wave-like material interlocking. Fine element simulations are also in agreement with experimental results when upscaled to industry scale roll bonding process.

Achieving high strength and high ductility in ZK60 alloy with a heterogeneous lamella structure

Heterogeneous structure emerges as a novel design philosophy for metal materials to break through the strength-ductility tradeoff. Here we present a heterogeneous lamella (HL) structure in ZK60 alloy that was fabricated by conventional powder metallurgy techniques, i.e., powder pretreatment and hot extrusion. The distinctive structure consists of soft coarse-grained (CG) lamella embedded in the hard ultrafine-grained (UFG) matrix. Compared to the homogeneous CG and UFG counterparts, the HL ZK60 alloy exhibits an excellent combination of tensile yield strength (404.4 MPa) and uniform elongation (13.5 %). This strength-ductility synergy can be attributed to the hetero-deformation induced (HDI) strengthening and HDI strain hardening as a result of the deformation incompatibility and mutual constraint between the hetero-zones.

Enhancing strength-ductility synergy via hierarchical microstructure in fine-grained Ti-10Mo-8V-1Fe-3.5Al alloy

• Partially-recrystallized β structure with abundant substructures and defects was characterized. • Microstructural hierarchies were obtained in β + α microstructure. • Excellent strength-ductility synergy could be achieved no matter in β or α + β phased condition. • Enhanced strain-hardening rate postponed the occurrence of plastic instability. Microstructural hierarchies of a fine-grained (∼6μm) Ti-10Mo-8 V-1Fe-3.5Al alloy fabricated by hot working process were investigated in β-phased and (α + β)-phased condition respectively. The β-phased microstructure was characterized by a fraction (∼24%) of recrystallized β-grains distributed randomly in the matrix, including subgrains with abundant substructures and defects. After aging, the consequent precipitation behavior of α featured by heterogeneous and insular areas with coarse α(∼0.9 μm) embedded in the ultrafine α(∼0.13 μm). It was clarified that excellent strength-ductility synergy can be achieved by hierarchical microstructure no matter in β or α + β phased condition, in sharp contrast to their coarse-grained counterparts. The microstructural dependence of mechanical properties was discussed in terms of the strain-hardening rate arising from the formation of dislocation pile-up influenced by a dispersion of precipitates, and by grain size (GS).

Bulk nanocrystalline W-Ti alloys with exceptional mechanical properties and thermal stability

Nanocrystalline (NC) W metals and alloys often exhibit higher radiation tolerance and strength than their coarse-grained counterparts. However, their thermal stability is low, making it difficult to achieve bulk NC W metals and alloys by consolidation using conventional techniques such as pressure-less sintering, hot-explosive-compaction sintering, and spark plasma sintering. Here we report the synthesis and mechanical properties of bulk NC W100-xTix (x = 10 at.%–30 at.%) alloys prepared by consolidating mechanically alloyed NC powders under a high-temperature/high-pressure condition. Adding 20 at.%–30 at.% Ti largely improves the sinterability of NC W-Ti alloy powders. The room-temperature microhardness and compressive yield strength of consolidated bulk NC W80Ti20 alloy are ∼ 16.9 and 6.0 GPa, respectively, which are mainly caused by grain boundary strengthening and significantly higher than those of previously reported W and W alloys. The ultimate compressive strength of bulk NC W80Ti20 measured between 900 and 1100 °C deceases with increasing temperature. This behavior can be explained by the activation of Rachinger grain boundary sliding. No grain growth is observed in bulk NC W80Ti20 after compression at 1000 °C. Theoretical calculation suggests that it is the segregation of Ti at grain boundaries that decreases the specific grain boundary free energy and makes the NC W80Ti20 alloy thermodynamically stable.

Enhanced bonding property of ion-plated TiN coating on stainless steel by mechanically pre-forming a gradient nanostructure

An enhanced coating/substrate bonding property is crucial for the application of hard coatings (e.g., TiN) on soft substrates (e.g., stainless steel). In the present work, a gradient nanostructure, in which the grain size is ~50 nm in the top surface layer and increases gradually with depth, was pre-formed in 304 stainless steel by means of plate surface mechanical rolling treatment (PSMRT). Subsequently, a TiN coating was deposited on it by using a commercial arc ion plating approach. Scratch tests demonstrated that the critical load of coating detachment was significantly enhanced in the coated PSMRT sample, ~47 % higher than that in the coated coarse-grained counterpart sample. Analyses of the underlying bonding mechanisms revealed that a higher interfacial strength was achieved in the coated PSMRT sample, mostly due to the depressed formation of compounds and the accelerated diffusion of Ti in the interfacial region. In addition, such factors as a higher substrate hardness, a lower friction coefficient between the scratch tip and the coating, and a lower internal stress in the coating might also contribute to the enhanced bonding property in the coated PSMRT sample. This work provided insights on applying a simple mechanical approach to enhance the bonding properties of hard coatings on soft substrates.

Recent Progress on Nanocrystalline Metallic Materials for Biomedical Applications.

Nanocrystalline (NC) metallic materials have better mechanical properties, corrosion behavior and biocompatibility compared with their coarse-grained (CG) counterparts. Recently, nanocrystalline metallic materials are receiving increasing attention for biomedical applications. In this review, we have summarized the mechanical properties, corrosion behavior, biocompatibility, and clinical applications of different types of NC metallic materials. Nanocrystalline materials, such as Ti and Ti alloys, shape memory alloys (SMAs), stainless steels (SS), and biodegradable Fe and Mg alloys prepared by high-pressure torsion, equiangular extrusion techniques, etc., have better mechanical properties, superior corrosion resistance and biocompatibility properties due to their special nanostructures. Moreover, future research directions of NC metallic materials are elaborated. This review can provide guidance and reference for future research on nanocrystalline metallic materials for biomedical applications.

Influence of processing and microstructure on the corrosion behavior of ultrafine grained Al 5083 alloy

Ultrafine grained (UFG) material has received significant research interest due to its combination of strength and toughness when compared with its coarse-grained counterpart. In the present investigation, a distinct UFG microstructure of Al 5083 alloy fabricated through cryorolling (CR), CR followed by warm rolling (WR), and CR followed by short annealing (SA) have been subjected to salt immersion, polarization, and intergranular corrosion tests in 3.5% NaCl solution. A salt immersion test revealed that UFG structures have resistance to pitting corrosion for all conditions when compared to their coarse-grained counterpart. The presence of coarse intermetallic second phase particles leads to the formation of micro galvanic cells with matrix resulting in highly localized pitting corrosion in the solution treated (ST) material, whereas crystallographic pits along the lateral direction have been observed in deformed material. The lower values of corrosion current density ( icorr = 3.640 and 3.910 μA/cm2) were observed for CR and WR conditions, respectively, than ST samples ( icorr = 8.140 μA/cm2) were attributed to the presence of a higher volume fraction of low angle grain boundaries along with a high density of dislocations. SA after CR has slightly increased the corrosion current ( icorr = 3.8 μA/cm2) values thereby shifting the corrosion potentials to a noble direction.

Heterostructure alleviates Lüders deformation of ultrafine-grained stainless steels

Ultrafine-grained (UFG) 304 stainless steel is much stronger than its coarse-grained counterpart. However, it develops undesired Lüders bands during deformation. Here we report a bi-modal heterostructure that can effectively suppress Lüders deformation in the UFG 304 stainless steel, which produces a strong strain hardening to improve the ductility and toughness, without sacrificing much strength. These superior mechanical properties are attributed to strain delocalization capability of the bi-modal heterostructure, which results in lower stress concentration and consequently postpones the martensitic transformation.

An engineering route to synthesize stable bulk nanocrystalline magnesium with an average grain size of 20 nm

A simple and effective route to preparing bulk nanocrystalline (NC) pure Mg with an average grain size of 20 nm is first proposed by hydrogenation-disproportionation-desorption-recombination (HDDR) followed by spark plasma sintering (SPS) and hot extrusion (HE) techniques. NC Mg is thermodynamically super-stable against annealing at 550 °C and straining under compression and tension. Moreover, the as-extruded NC Mg samples show high tensile yield stress (TYS) of 259 MPa and compressive yield strength (CYS) of 157 MPa, which are about 2 times and about 7–10 times higher than those of their coarse-grain counterparts and the as-cast samples, respectively.

Effect of concurrent grain growth on radiation-induced segregation in nanocrystalline Fe–Cr–Ni alloys

Irradiation of crystalline materials modifies their microchemistry and microstructure. This includes solute segregation toward defect sinks such as grain boundaries (GBs), a phenomenon commonly known as radiation-induced segregation (RIS). Unlike in coarse-grained alloys where GBs are nearly static, RIS is usually accompanied and affected by either thermal or irradiation-induced grain growth in nanocrystalline materials. This work presents a modeling study of concurrent grain growth and RIS in austenitic Fe–Cr–Ni adopting realistic 2D grain structures. RIS can be significantly affected by concurrent grain growth due to (i) increasing grain size, (ii) motion of GBs as defect sinks, and (iii) their combined effect. Consequently, RIS is enhanced by grain growth due to increased grain size and sink motion. More notably, RIS in nanocrystalline materials were found to induce grain-level compositional redistribution in addition to RIS at defect sinks, resulting in grain-size-dependent compositions in individual grains. Without concurrent grain growth, elements depleted at the sinks due to RIS, such as Cr in austenitic steels, will have lower concentrations in smaller grains than in larger grains. The opposite trend becomes true when concurrent grain growth takes place. The compositional difference in individual grains can be significant enough to affect local phase stability. These findings are not discernible with the classical 1D bicrystal model, and they highlight the different effects of RIS in nanocrystalline alloys compared to their coarse-grained counterparts.

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.

Grain boundary engineering in roll-bonded copper to overcome the strength-ductility dilemma

Copper sheets with graded and fine-grained (FG) microstructures were processed by accumulative roll bonding (ARB) and subsequent annealing to evaluate the effect of structural heterogeneities on the strength–ductility relationship. By an appropriate design of the individual ARB-process-steps, two types of graded structures (GSs) have been achieved where the grain size gradually changes from a more refined grain structure at the surface to a coarser one in the interior (CGC) or a coarser one at the surface to a finer one in the interior (FGC). For both types, the grains size gradually changes between 4 and 19 microns across the thickness of samples. In terms of the uniform tensile work, GSs exhibited a superior combination of strength and elongation owing to the hetero-deformation induced strengthening. In addition to a good strain delocalization capability compared to their coarse-grained (CG) counterpart, the graded microstructures affect the crack paths in the non-uniform deformation regime.

Low-temperature plasma nitriding of AISI 304 stainless steel with nano-structured surface layer

Abstract A nanostructured surface layer was produced on an AISI 304 stainless steel plate by means of a surface mechanical attrition treatment (SMAT). Low-temperature plasma nitriding of the SMAT stainless steel sample was investigated in comparison with the coarse-grained sample by using structural analysis (X-ray diffraction, scanning electron microscopy, and transmission electron microscopy) as well as mechanical property measurements. It shows a much thicker nitrided layer is formed on the SMAT sample relative to that in the coarse-grained counterpart nitrided under the same condition. The nitrided layer in the SMAT sample is composed of nanostructured austenite expansion and martensite expansion with high supersaturations of nitrogen. The surface hardness and the hardened surface layer thickness, as well as the abrasive and the adhesive wear resistances of the nitrided SMAT sample are much enhanced relative to the nitrided coarse-grained sample.

Mechanisms of remarkable wear reduction and evolutions of subsurface microstructure and nano-mechanical properties during dry sliding of nano-grained Ti6Al4V alloy: A comparative study

A comprehensive investigation into dry sliding wear and associated evolutions of subsurface microstructure and nano-mechanical properties of a bulk nano-grained Ti6Al4V alloy (NG-Ti6Al4V) was conducted. Comparing with its coarse-grained counterpart (CG-Ti6Al4V), NG-Ti6Al4V always exhibited lower friction coefficient and greater wear resistance in a wide range of tribological conditions. It was also found that the sliding wear induced nano-mechanical properties and microstructure evolutions in Ti6Al4V alloy were largely dependent on its initial microstructural features. For the first time, TEM observations provided a direct experimental evidence that the originally saturated nanostructure in NG-Ti6Al4V could be further refined by sliding wear. This study proposed an effective strategy to improve the load-bearing ability of Ti6Al4V alloy by producing bulk homogeneous nanostructure.

Microstructural Transitions during Powder Metallurgical Processing of Solute Stabilized Nanostructured Tungsten Alloys

Exploiting grain boundary engineering in the design of alloys for extreme environments provides a promising pathway for enhancing performance relative to coarse-grained counterparts. Due to its attractive properties as a plasma facing material for fusion devices, tungsten presents an opportunity to exploit this approach in addressing the significant materials challenges imposed by the fusion environment. Here, we employ a ternary alloy design approach for stabilizing W against recrystallization and grain growth while simultaneously enhancing its manufacturability through powder metallurgical processing. Mechanical alloying and grain refinement in W-10 at.% Ti-(10,20) at.% Cr alloys are accomplished through high-energy ball milling with transitions in the microstructure mapped as a function of milling time. We demonstrate the multi-modal nature of the resulting nanocrystalline grain structure and its stability up to 1300 °C with the coarser grain size population correlated to transitions in crystallographic texture that result from the preferred slip systems in BCC W. Field-assisted sintering is employed to consolidate the alloy powders into bulk samples, which, due to the deliberately designed compositional features, are shown to retain ultrafine grain structures despite the presence of minor carbides formed during sintering due to carbon impurities in the ball-milled powders.

Significant Bauschinger effect and back stress strengthening in an ultrafine grained pure aluminum fabricated by severe plastic deformation process

Bauschinger test in uniaxial tension-compression mode was carried out for the first time on the pure Al specimens having homogeneous ultra-fine grained (UFG) microstructures fabricated by equal-channel angular pressing (ECAP) and subsequent annealing processes. Significant Bauschinger stress (transient softening), Bauschinger energy parameter and their strong dependences on the tensile plastic pre-strain at the very early stage of the tensile deformation were measured in the UFG specimens, in sharp contrast to their coarse-grained (CG) counterpart. The grain size dependence of the Bauschinger effect in pure Al was qualitatively discussed in terms of the back stress arising from the formation of dislocation pile-up against the grain boundary during plastic deformation.

Twinning-Induced Abnormal Strain Rate Sensitivity and Indentation Creep Behavior in Nanocrystalline Mg Alloy.

Nanocrystalline materials exhibit many unique physical and chemical properties with respect to their coarse-grained counterparts due to the high volume fraction of grain boundaries. Research interests on nanocrystalline materials around the world have been lasting over the past decades. In this study, we explored the room temperature strain rate sensitivity and creep behavior of the nanocrystalline Mg–Gd–Y–Zr alloy by using a nanoindentation technique. Results showed that the hardness and creep displacements of the nanocrystalline Mg–Gd–Y–Zr alloy decreased with increasing loading strain rate. That is, the nanocrystalline Mg–Gd–Y–Zr alloy showed negative strain rate sensitivity and its creep behavior also exhibited negative rate dependence. It was revealed that the enhanced twinning activities at higher loading strain rates resulted in reduced hardness and creep displacements. The dominant creep mechanism of the nanocrystalline Mg–Gd–Y–Zr alloy is discussed based on a work-of-indentation theory in this paper.

Transformation plasticity in high strength, ductile ultrafine-grained FeMn alloy processed by heavy ausforming

The mechanisms contributing to the excellent mechanical properties of the ultrafine-grained (UFG) Fe-23 wt.%Mn alloy processed by heavy ausforming are unraveled based on detailed characterization analysis and modelling. The UFG microstructure is fully austenitic after the heavy ausforming step involving a 90% rolling reduction; while the material quenched from the coarse-grained (CG) austenite consists of epsilon (ε)-martensite and austenite. The UFG Fe23Mn alloy shows a high strain-hardening capacity which leads to a much higher true uniform elongation (0.33) and true tensile strength (1330 MPa) than the CG counterpart (0.17 and 950 MPa, respectively). The high ductility of the heavily-ausformed microstructure with no subsequent annealing step contradicts the general trend of UFG alloys produced by severe plastic deformation. In addition, a ductile fracture mode with improved resistance to damage initiation in the UFG microstructure contrasts with the brittleness of the CG counterpart. Therefore, the UFG Fe23Mn alloy exhibits a high combination of strength, resistance to plastic localization and resistance to cracking. This superior mechanical performance is attributed to the gradual deformation-induced ε-martensitic transformation and to the large plastic co-deformation of the UFG ε-martensite, in which twinning is suppressed at the expense of the activation of non-basal <c+a> slip. A mean-field micromechanical model is used to analyze the contribution of the phase transformation to plasticity, and to further rationalize the mechanical response of the UFG ε-martensite. This finding provides new insight into the effects of microstructural scale on the mechanical behavior of this class of transformation-induced plasticity (TRIP)-assisted alloys.

Nanograin size effects on deformation mechanisms and mechanical properties of nickel: A molecular dynamics study

The grain size refinement of metallic materials to the nanometer scale produces interesting properties compared to the coarse-grained counterparts. Their mechanical behavior, however, cannot be explained by the classical deformation mechanisms. Using molecular dynamics simulations, the present work examines the influence of grain size on the deformation mechanisms and mechanical properties of nanocrystalline nickel. Samples with grain sizes from 3.2 to 24.1 nm were created using the Voronoi tessellation method and simulated in tensile and relaxation tests. The yield and ultimate tensile stresses follow an inverse Hall-Petch relationship for grain sizes below ca. 20 nm. For samples within the conventional Hall-Petch regime, no perfect dislocations were observed. Nonetheless, a few extended dislocations were nucleated from triple junctions, suggesting that the suppression of conventional slip mechanism is not uniquely responsible for the inverse Hall-Petch behavior. For samples respecting the inverse Hall-Petch regime, the high number of triple junctions and grain boundaries allowed grain rotation, grain boundary sliding, and diffusion-like behavior that act as competitive deformation mechanisms. For all samples, the atomic configuration analysis showed that Shockley partial dislocations are nucleated at grain boundaries, crossing the grain before being absorbed in opposite grain boundaries, leaving behind stacking faults. Interestingly, the stress relaxation tests showed that the strain rate sensitivity decreases with grain size for a specific grain size range, whereas for grains below approximately 10 nm, the strain rate sensitivity increases as observed experimentally. Repeated stress relaxation tests were also performed to obtain the effective activation volume parameter. However, the expected linear trend in pertinent plots required to obtain this parameter was not found.

Combination of enhanced strength and sufficient tensile ductility in a sintered ultrafine-grained CoFeMnNi medium-entropy alloy

A CoFeMnNi medium-entropy alloy (MEA) was prepared by mechanical alloying (MA) and spark plasma sintering (SPS). The mechanically alloyed powders were sintered to near-full density and the as-sintered material consists of FCC matrix comprising a minor distribution of oxide particles. A high twin density is observed in MEA bulk samples. The twins are not generated during high-energy ball-milling but formed in the recrystallized grains during the sintering process. Compressive and tensile tests were performed on the sintered specimen at room temperature. The tension-compression yield asymmetry is weakened with increasing sintering temperature. An ultrafine-grained microstructure with an average grain size of 535 nm is obtained at a sintering temperature of 1000 °C. The sintered MEA exhibits an enhanced yield strength of 740 MPa, which is by 244% higher than that of its coarse-grained counterpart, meanwhile maintaining a sufficient tensile ductility of 14.4%. These excellent mechanical properties mainly stem from the high relative density, fully recrystallized ultrafine grains, and precipitation of fine particles that significantly strengthen the material while simultaneously providing high strain-hardening capability.