Comparison of strengthening in wire-drawn or rolled Cu-20% Nb with a dislocation accumulation model

Strengthening after large deformations by wire-drawing or rolling of Cu, Nb and Cu-20% Nb was compared with the predictions of a proposed modified substructural strengthening model for ductile two-phase alloys. The comparisons indicate that the more extensive and refined model of Funkenbusch and Courtney offers no improvement over the original model of Ashby in predicting the strengthening with increased deformation processing or the dislocation densities necessary to produce the observed strengthening in Cu-20% Nb. Both models can predict the strengthening behaviour of Cu-20% Nb. However, neither model is in accord with the observations that the dislocation density in the Cu matrix is essentially independent of the degree of deformation processing, and that the magnitudes of the dislocation density are much the same in the Cu in Cu-20% Nb and pure Cu identically deformation-processed. In addition, there is no experimental support for the Funkenbusch and Courtney model prediction of an order of magnitude greater dislocation density in the Nb filaments than in the Cu matrix in Cu-20% Nb. It appears that a mechanism that does not require an accumulation of dislocations for strengthening, such as the difficulty in propagating dislocations between closely spaced barriers, is more likely to be responsible for strengthening in Cu-Nb-type deformation-processed composites.     

Effect of temperature on the strength and conductivity of a deformation processed Cu-20%Fe composite

The high temperature (22–600 °C) properties were evaluated for a Cu-20%Fe composite deformation processed from a powder metallurgy compact. The ultimate tensile strengths decreased with increasing temperature but were appreciably better than those of similarly processed Cu at temperatures up to 450 °C. At 600 °C, the strength of Cu-20%Fe was only slightly better than that of Cu as a result of the pronounced coarsening of the Fe filaments. However, at temperatures of 300 and 450 °C, the strength of Cu-20%Fe is about seven and six times greater, respectively, than that of Cu, as compared to about a two fold advantage at room temperature. Therefore, Cu-20%Fe composites made by deformation processing of powder metallurgy compacts have mechanical properties much superior to those of similarly processed Cu at room temperature and at temperatures up to 450 °C. The pronounced decrease in electrical conductivity of deformation processed Cu-20%Fe as compared to Cu is attributed to the appreciable dissolution of Fe into the Cu matrix which occurred during the fabrication of the starting compacts where temperatures up to 675 °C were used. While the powder metallurgy compacts used for the starting material for deformation processing in this study did not lead to a high conductivity composite, the powder metallurgy approach should still be a viable one if processing temperatures can be reduced further to prevent the dissolution of Fe into the Cu matrix.     

DEFORMATION PROCESSED Cu-REFRACTORY METAL COMPOSITES

Recent studies have shown that heavily drawn Cu-Nb alloys can achieve strengths above 2000 MPa and outstanding combinations of strength plus conductivity (both electrical and thermal). The properties result from an aligned composite structure formed by a mechanical reduction which produces ribbon shaped Nb filaments In the Cu matrix. These metal-metal matrix composites are referred to here as deformation processed composites (DPC). A whole series of metal-metal matrix Cu-X alloys may be prepared by deformation processing, where X may be any of the BCC metals: V, Nb, Ta, Cr, Mo, W or Fe. Processing consists of two primary steps: preparation of the Cu-X billet with the X phase uniformly dispersed as small particles, and then a very large mechanical reduction. The billet may be prepared by either solidification or powder processing. Existing experimental results are reviewed, some new data on powder processed Cu-W are presented, and the general applicability of solidification processed and powder processed billets are presented.     

Microstructural evolution in Cu–Nb processed via friction consolidation

Immiscible alloys, whether in well-mixed or layered forms, are of increasing interest based on their novel structural and functional properties, such as enhanced thermal stability against grain growth or radiation-induced defect trapping at the interfaces. To address the need for new approaches to tailor microstructures, the microstructural development of an immiscible Cu-4 wt.% Nb alloy processed via friction consolidation of elemental powders is investigated. Friction consolidation is a solid phase processing technique that imparts severe plastic strain into a deforming volume resulting in elevated temperatures below the melting temperature of the alloy. Two distinct processing pathways were chosen to understand the effect of thermomechanical conditions on the final microstructure. The microstructure was characterized using scanning electron microscopy, scanning transmission electron microscopy, and X-ray diffraction techniques. Path 1 exhibited larger strain, strain rate, and temperature as compared with path 2. In path 1, agglomerated Nb particles were present in the recrystallized ultrafine-grained Cu matrix, while in path 2 extremely fine and dispersed Nb particles were present in a highly deformed Cu matrix. In both pathways, supersaturation of Cu in Nb lattices was noted, but not vice versa. The asymmetry in mixing is explained based on deformation-based, thermodynamic and kinetic factors. These findings provide a pathway for creation of novel tailored microstructures and improved properties in any number of binary immiscible alloy systems.

     

INFLUENCE OF AXISYMMETRIC DEFORMATION PROCESSING METHODS ON THE MECHANICAL PROPERTIES OF Cu-Nb COMPOSITES

It has been shown that deformation processed Cu-19% Nb alloys with good strength and electrical conductivity can be developed in sizes that are useful for engineering applications. Mot extrusion of bundled sub-elemental Cu-19% Nb wires followed by cold drawing to make a composite wire of diameter equal to that of the initial sub-elemental wires resulted in a 67% increase in the ultimate tensile stress. However, on subsequent cold drawing of this composite wire the strength increased at a slower rate than that obtained on continuing cold drawing of the sub-elemental wire and the strength differential decreased. In addition, after cold drawing to equivalent diameters the electrical conductivity of the composite wire was less than that of the sub-elemental wire. These results indicate that while high strengths and good electrical conductivities can be produced in larger size deformation processed Cu-Nb composites by a process of bundling, extrusion and cold drawing of sub-elemental wires, there appears to be a limit to the amount of subsequent cold drawing feasible before the benefit in properties ceases.     

The effect of copper granules on interfacial bonding and properties of the copper-graphite composite prepared by flake powder metallurgy

The study evaluates the effect of introducing Cu granules and control milling on the microstructure, interfacial bonding and mechanical properties including sintered density, hardness, compressive strength, flexural strength and electrical conductivity of Copper-Graphite (Gr) composite synthesize by flake powder metallurgy (Flake PM). It develops the flake composite particles by control mechanical alloying (MA) which further laminates over the refine granules surface. This encapsulation facilitates the strong interfacial bonding among the composite constituents during sintering. Results highlight that the 10% Cu granules in Cu-10Gr composite exhibit excellent mechanical properties. It increases the relative density, hardness, compressive strength, and flexural strength by 4.19%, 28.23%, 98.31%, and 11.8% respectively. However, the electrical conductivity increases by 6.73% (%IACS) for 15% of Cu granules in the Cu-10Gr composite. The improvements in the results are the synergistic coordination of dispersion homogeneity, surface integrity, work hardening, and the superior interfacial adhesion between composite powder and Cu granules.     

Creep characteristics of wire-drawn Cu-20%Nb

In this study the creep deformation and rupture characteristics of a wire-drawn Cu-20%Nb composite are investigated at 500°C and compared with the results obtained from pure copper. The results show that the creep rupture strength of Cu-20%Nb is considerably higher than that for pure copper. The rupture strength of Cu-20%Nb increases up to a certain draw ratio, in contrast to the room temperature strengthening. The stress exponents n are in the range 6.6–8.1 for Cu-20%Nb, in comparison with 2.75 in pure copper. The increase in the creep strength and the higher stress exponent values in the Cu-20%Nb are associated with the reduction in the power law creep damage mechanism. This is due to the constraint introduced on the matrix creep flow by the niobium phase rather than the development of high threshold stress values. While the increase in the length of niobium filaments and reduction in interfilament spacing with increasing draw ratio increase the constraint on the creep flow of the matrix, they also enhance the creep damage caused by the diffusion mechanisms because of the easy diffusion paths along the niobium filaments and the reduction in the matrix gain size.     

Strengthening in deformation-processed Cu-20% Fe composites

Three Cu-20% Fe composites with different iron powder sizes were fabricated using powder metallurgy processes. The strengths of these composites after extensive deformation processing by rod swaging and wire drawing were shown to be anomalously higher than those predicted by rule of mixtures equations. However, the strengths obey a Hall-Petch type relationship with the iron filament spacings. The strengths of the Cu-20% Fe composites after equivalent deformation processing increased with decreasing initial iron powder size. Comparison of a Cu-20% Fe composite with a similar Cu-20% Nb composite showed that Cu-20% Fe was stronger after an identical degree of deformation processing. This increase in strength of a Cu-20% Fe composite over that of a Cu-20% Nb composite correlated with the greater shear modulus of iron compared to niobium using a barrier model for hardening.     

Microstructural and mechanical behavior of Zr-based metallic glasses with the addition of Nb

The effect of Nb content on the microstructure and mechanical properties of Zr-based bulk metallic glass (BMG) were investigated. The addition of Nb led to the formation of the Zr-based metallic glass composites with a ductile dentritic phase by in situ precipitation. The presence of the in situ precipitated phase enhanced significantly the plasticity of the composite under uniaxial compressive test. The interactions between the precipitated phase and the shear band affect the deformation mechanism and fracture mode of the BMG by enhancing the affecting level of the normal stress on the shear surface, and the constant α in the Mohr–Coulomb criterion can reflect the extent of the interactions among particles and the amorphous matrix.     

Novel In Situ Nanostructure-Dendrite Composites in Zr-Base Multicomponent Alloy System

The evolution of a nanostructure-dendrite composite microstructure of two Zr-base alloys solidified through different casting routes is presented. The alloys were designed by adding different amounts of Nb to the Zr-based multicomponent glass-forming alloy system. The refractory metal Nb promotes the formation of a primary phase having dendritic morphology, whereas the residual melt solidifies to a nanostructured/amorphous matrix. The volume fraction and the morphology of the dendritic phase varied with the Nb content and the adopted casting route. A correlation between the alloy composition and adopted casting method with evolved microstructures and mechanical properties is revealed. These composites exhibit a unique combination of high fracture strength up to 1922 Mpa, as well as plastic strain over 15.8% under uniaxial compression testing at room temperature. The high strength of these composites is imparted by the nanostructured matrix, whereas the large plastic strain is a consequence of the retardation of excessive localized shear banding in the matrix by ductile dendrites. The significant work hardening of the composites prior to fracture is attributed to dislocation multiplication in the solid solution-strengthened dendritic phase.     

Effect of High‐Energy Ball Milling on Mechanical Properties of the Mg–Nb Composites Fabricated through Powder Metallurgy Process

New biocompatible and biodegradable Mg–Nb composites used as bone implant materials are fabricated through powder metallurgy process. Mg–Nb mixture powders are prepared through mechanical milling and manual mixing. Then, the Mg–Nb composites are fabricated through cold press and sintering processes. The effect of mechanical milling and Nb particles as reinforcements on the microstructures and mechanical properties of Mg–Nb composites are investigated. The mechanical milling process is found to be effective in reducing the size of Mg and Nb particles, distributing the Nb particles uniformly in the Mg matrix and obtaining Mg–Nb composite particles. The Mg–Nb composite particles can be bound together firmly during the sintering process, result in Mg–Nb composite structures with no intermetallic formation, lower porosity, and higher mechanical properties compared to composites prepared through manual mixing. Interestingly, the mechanical properties of manually mixed Mg–Nb composites appear to be even lower than that of pure Mg.

     

Toughening MoSi2 with niobiúm metal—effects of morphology of ductile reinforcements

Morphology effect of ductile reinforcements was evaluated using a four-point bend test on chevron-notched MoSi₂ composites reinforced with 20 vol. % niobium. The niobium used had three different morphologies, i.e., fibre, foil and particle. The thickness of the foils was 250 m, while the fibres and particles had diameters of 250 and 200 m, respectively. Toughness of MoSi₂ composites was increased from 3.3 MPa m^1/2 for the matrix to 15 MPa m^1/2 with the incorporation of the Nb fibres or foils. The particulate composites also exhibited an increase in toughness (7 MPa m^1/2). The toughening achieved was mainly attributed to ductile phase bridging in all the composites tested. The relatively small toughness improvement in the particulate composites was ascribed to the embrittlement of the Nb particles. The results indicate that toughening by crack bridging depends mainly on the intrinsic properties of the ductile bridging ligaments rather than on their morphology, and that the embrittlement of the bridging ligament is detrimental to the toughening of the composites.Previously known as L. Xiao.     

Formation of the microstructure in Cu-Nb alloys

In order to increase the mechanical strength of copper while retaining its high electrical conductivity the copper matrix is strengthened by Nb particles. For thermodynamical reasons the mutual solubility of Nb in copper is negligible. Therefore, a Cu-Nb alloy is prepared by mechanically alloying and subsequent hot pressing. The formation of a solid solution in a first step and of precipitates in a Cu-10 at.%Nb alloy in the second one as well as the effect of these precipitates on the mentioned properties is discussed. The influence of different temperatures during hot pressing on density, mechanical and electrical properties, etc. is investigated.     

Evaluation of water embrittlement on ‘dual phase’ ADI

Austempered ductile iron (ADI) suffers an embrittlement phenomenon when loaded in contact with water and other liquids. This phenomenon causes noticeable drops in elongation, ultimate tensile strength and fracture toughness of ADI of different strength grades. This paper studies the susceptibility to embrittlement of a new kind of austempered ductile iron called dual phase ADI (DPADI), which shows a matrix composed by the typical ausferrite present in ADI mixed with allotriomorphic ferrite. The new material is obtained by means of specific heat treatments that involve a partial austenitising stage. The susceptibility of DPADI to this type of embrittlement was evaluated by carrying out tensile tests in dry and wet conditions. Fracture surfaces were observed using scanning electron microscopy. The results showed a gradual decrease of the degree of embrittlement caused by contact with water as the ferrite content in the matrix increases.     

Enhancing the mechanical–electrical property simultaneously in pure copper composites by using carbonized polymer dots

In the development of copper-based composite materials, the dilemma of improving the mechanical properties without affecting the electrical properties is an important issue that must be solved. Here, carbonized polymer dot (CPD), as a novel reinforcement, was employed to fabricate CPD/Cu (pure copper) composite via powder metallurgy technique for the first time. The microstructure analysis revealed that the CPD was uniformly dispersed in the copper matrix in the form of nanoclusters, and the nanoclusters of CPD are composed of a three-dimensional amorphous carbon (AC) network structure and inserted carbon dots (some of them have a typical graphene structure, while others not). More importantly, excellent interface combination between the CPD and copper matrix is observed due to the existing of plenty of chemical functional groups. Based on this special microstructure, our prepared CPD/Cu composite achieves excellent mechanical and electrical conductivity simultaneously. Compared to pure Cu, the ultra-tensile strength of 0.2CPD/Cu composite is increased by about 17.0%, while the elongation is only?~?2% lower. The electrical conductivity of the composite is?~?98% IACS, which is much higher than that of pure copper prepared under the same condition (only?~?92% IACS). New insights into how to prepare advanced copper matrix composites with simultaneously improved overall performance will be found from our research.

     

FABRICATION AND CHARACTERIZATION OF Cu-SiCp COMPOSITES FOR ELECTRICAL DISCHARGE MACHINING APPLICATIONS

Cu-SiC_p composites made by the powder metallurgy method were investigated. To avoid the adverse effect of Cu-SiC_p reaction, sintering was controlled at a reaction temperature less than 1032 K. Electroless plating was employed to deposit a copper film on SiC_p powder before mixing with Cu powder in order to improve the bonding status between Cu and SiC particles during sintering. It was found that a continuous copper film could be deposited on SiC_p by electroless copper plating, and a uniform distribution of SiC_p in Cu matrix could be achieved after the sintering and extrusion process. The mechanical properties of Cu-SiC_p composites with SiC_p contents from 0.6 to 10 wt% were improved evidently, whereas electrical properties remained almost unchanged as compared with that of the pure copper counterpart. In the electrical discharge machining (EDM) test, the as-formed composite electrodes exhibited a character of lower electrode wear ratio, justifying its usage. The optimum conditions for EDM were Cu-2 wt% SiC_p composite electrode operating with a pulse time of 150 μsec.     

Cu-10Cr-0.4Zr原位复合材料的微观组织与性能

制备了Cu-10Cr和Cu-10Cr-0.4Zr合金,并经冷变形形成了原位复合材料,观察了Zr的添加对合金铸态组织、复合材料的纤维形貌,研究了Zr的添加和冷变形率对拉伸强度以及导电率的影响.研究表明,在Cu-10Cr合金中添加的0.4%Zr,Cr析出相的直径由15～80μm细化到10～20μm;在相同的冷拔应变下,Cu-10Cr-0.4Zr复合材料较Cu-10Cr材料具有了更高的基体晶格阻力、更加细小均匀的纤维相以及纤维间距,使得Cu-10Cr-0.4Zr复合材料的强度更高.当冷拔应变达到6.2时,Cu-10Cr-0.4Zr原位复合材料抗拉强度高达1089MPa,而Cu-10Cr材料的抗拉强度仅为887MPa.在相同冷拔应变下,Cu-10Cr材料的导电率比Cu-10Cr-0.4Zr材料中的导电率略高.随着材料冷拔应变的增加,决定复合材料电阻率的基体材料内位错散射电阻转变成界面散射电阻,复合材料的电导率逐渐下降.     

In situ strengthening of titanium with yttrium: texture analysis

In situ processing consists of heavily deforming a two-phase alloy of mutually immiscible elements to produce composite sheet or wire. In the well-studied Cu(fcc)-Nb(bcc) system, severe deformation by swaging and drawing reduces as-cast Nb filament phase thicknesses several hundred-fold after deformation. Cu-20 vol % Nb ultimate tensile strengths exceed 2000 MPa for material deformed to a true strain of η = 12, where η = ln (area_original/area_final). In an earlier study of in situ strengthening in immiscible hexagonal close-packed metals, Ti-50 vol % Y and Ti-20 vol % Y alloys were deformed by hot extrusion, hot and cold swaging. The composites were deformation processed to true strains as high as η = 7.6 to form a filamentary microstructure with filament thicknesses on the order of 0.1 μm. The deformation processing of these composites increased their ultimate tensile strengths from 318 to 945 MPa, but the specimens' original diameters were too small to allow deformation processing to the very high true strains achieved with the Cu-Nb composites. In this study, a larger casting of Ti-20 vol % Y was deformation processed to η = 12.8 in an attempt to achieve further refinement of the filament thickness. This composite formed the same filamentary microstructure up to η=7.27 observed in the earlier study of Ti-Y composites; however, at higher η values the filaments recrystallized into equiaxed grains, decreasing the ultimate tensile strength. X-ray texture analysis of the composite specimens showed a strong 〈10ˉ10〉 fibre texture in both the Ti and Y phases in the deformation processing range 2.25 ≤ η ≤ 7.27. This texturing is thought to constrain both the Ti and Y phases to deform by plane strain, which produces severe geometric restrictions on the ability of the plane straining filaments to achieve high η values without fracturing or recrystallizing.     

Nb90W5M5三元固溶体合金的结构和氢渗透性能

合金化是改善Nb金属氢脆问题的有效途径。本文制备了Nb90W5M5三元(M=Co,Ni,Mo,Ti)合金,利用XRD、SEM、PCT、电化学测试和三点弯曲试验研究其相结构、氢化物形成焓、氢扩散系数和机械力学性能。研究证实,Nb90W5M5(M=Co,Ni,Mo,Ti)均为Nb基固溶体结构(Nb-bcc),受原子半径的影响,Nb90W5M5三元合金均有不同程度的晶格畸变现象。Nb90W5Co5合金的晶胞体积畸变收缩明显、晶格点阵常数最小,在Nb基固溶体的晶界和晶内缺陷处有富Co的NbCo化合物固溶组织析出。Nb90W5Co5合金具有低的氢化物形成焓绝对值(-22.3 kJ/mol)、高的氢扩散系数(1.57×10-9 cm^2/s)、高临界载荷(78.4 N),表现出良好的抗弯力学性能和氢渗透性能,这与其多元掺杂导致的微观结构特征有关。     

Structure and mechanical properties of as-cast Ti–5Nb–xFe alloys

In this study, as-cast Ti–5Nb and a series of Ti–5Nb–xFe alloys were investigated and compared with commercially pure titanium (c.p. Ti) in order to determine their structure and mechanical properties. The series of Ti–5Nb–xFe alloys contained an iron content ranging from 1 to 5 mass% and were prepared by using a commercial arc-melting vacuum-pressure casting system. Additionally, X-ray diffraction (XRD) for phase analysis was conducted with a diffractometer, and three-point bending tests were performed to evaluate the mechanical properties of all specimens. The fractured surfaces were observed by using scanning electron microscopy (SEM). The experimental results indicated that these alloys possessed a range of different structures and mechanical properties dependent upon the various additions of Fe. With an addition of 1 mass% Fe, retention of the metastable β phase began. However, when 4 mass% Fe or greater was added, the β phase was entirely retained with a bcc crystal structure. Moreover, the ω phase was only detected in the Ti–5Nb–2Fe, Ti–5Nb–3Fe and Ti–5Nb–4Fe alloys. The largest quantity of ω phase and the highest bending modulus were found in the Ti–5Nb–3Fe alloy. The Ti–5Nb–2Fe alloy had the lowest bending modulus, which was lower than that of c.p. Ti by 20%. This alloy exhibited the highest bending strength/modulus ratio of 26.7, which was higher than that of c.p. Ti by 214%, and of the Ti–5Nb alloy (14.4 ) by 85%. Additionally, the elastically recoverable angles of the ductile Ti–5Nb–1Fe (19.9°) and Ti–5Nb–5Fe (29.5°) alloys were greater than that of c.p. Ti (2.7°) by as much as 637% and 993%, respectively. Furthermore, the preliminary cell culturing results revealed that the Ti–5Nb–xFe alloys were not only biocompatible, but also supported cell attachment.