Precipitation hardening of Cu-Fe-Cr alloys part II Microstructural characterisation

This research is part of a project whose scope was to investigate the engineering properties of new non-commercial alloy formulations based on the Cu rich corner of the Cu-Fe-Cr ternary system with the primary aim of exploring the development of a new cost-effective high-strength, high-conductivity copper alloy. Promising properties have been measured for the following alloys: Cu-0.7wt%Cr-0.3wt%Fe and Cu-0.7wt%Cr-2.0wt%Fe. This paper reports on the microstructural characterisation of these alloys and discusses the mechanical and electrical properties of these alloys in terms of their microstructure, particularly the formation of precipitates. These alloys have evinced properties that warrant further investigation. Cost modelling has shown that Cu-0.7wt%Cr-0.3wt%Fe is approximately 25% cheaper to produce than commercial Cu-1%Cr. It has also been shown to be more cost efficient on a yield stress and % IACS per dollar basis. The reason for the cost saving is that the Cu-0.7%Cr-0.3%Fe alloy can be made with low carbon ferro-chrome additions as the source of chromium rather than the more expensive Cu-Cr master-alloy. For applications in which cost is one of the primary materials selection criteria, it is envisaged that there would be numerous applications in both cast and wrought form, where the Cu-0.7%Cr-0.3%Fe alloy would be more suitable than Cu-1%Cr.     

Precipitation hardening of Cu-Fe-Cr alloys part I Mechanical and electrical properties

This research is part of a project whose scope was to investigate the engineering properties of new non-commercial alloy formulations based on the Cu rich corner of the Cu-Fe-Cr ternary system with the primary aim of exploring the development of a new cost-effective high-strength, high-conductivity copper alloy. The literature indicated that Cu rich Cu-Cr and Cu-Fe alloys have been thoroughly investigated. A number of commercial alloys have been developed and these are used for a variety of applications requiring combinations of high-strength, high-conductivity and resistance to softening. Little evidence was found in the literature that the Cu rich corner of the Cu-Fe-Cr system had previously been investigated for the purpose of developing high-strength, high-conductivity copper alloys resistant to softening. The aim of these present investigations was to explore the possibility that new alloys could be developed that combined the properties of both sets of alloys, ie large precipitation hardening response combined with the ability to stabilise cold worked microstructures to high temperatures while at the same maintain high electrical conductivity. To assess thefeasibility of this goal the following alloys were chosen for investigation: Cu-0.7wt%Cr-0.3wt%Fe, Cu-0.7wt%Cr-0.8wt%Fe, Cu-0.7wt%Cr-2.0wt%Fe. This paper reports on the mechanical property investigation which indicated that the Cu-0.7wt%Cr-0.3wt%Fe, and Cu-0.7wt%Cr-2.0wt%Fe alloys were worthy of further investigation.     

Quaternary Cu-0.7%Cr-0.3%Fe-X alloys

This research is part of a project whose scope was to investigate the engineering properties of new non-commercial alloy formulations based on the Cu rich corner of the Cu-Fe-Cr ternary system with the primary aim of exploring the development of a new cost-effective high-strength, high-conductivity copper alloy. The aim of the present work was to increase the electrical conductivity and strength of the Cu-0.7wt%Cr-0.3wt%Fe alloy through selective minor additions (0.15 wt%) of elements expected to promote precipitation of dissolved Fe: Ti, B, P, Ni & Y. Such quaternary alloys with reduced Fe in solid solution would be expected to have properties equivalent to or better than those of the Cu-1%Cr reference alloy (Alloy Z). The investigation showed that none of the trace element additions significantly improved the size of the age hardening response or the peak aged electrical conductivity of Alloy A, although further work is required on the influence of Ti. Additions of P and B were detrimental. Other trace additions had little or no effect apart from causing some slight changes to the precipitation kinetics. The mechanical properties of the Cu-0.7%Cr-0.3%Fe alloy made with less expensive high carbon ferrochrome were found to be inferior to those of the equivalent alloy made with low carbon ferrochrome.     

A comparative study of precipitation at interphase boundaries in Fe-Cu-Ni and Fe-Au-Ni alloys

This paper reports the results of a comparative study on the formation of interphase boundary nucleated precipitates in an Fe-2 wt% Cu-2 wt% Ni and an Fe-4 wt% Au-2 wt % Ni alloy. High-speed dilatometry has been used to determine the kinetics of the austenite-ferrite transformation in these alloys and the resultant microstructures have been investigated using both light microscopy and transmission electron microscopy. The differences in precipitate morphology observed are discussed in terms of the reaction kinetics of the two alloys.     

高强高导铜合金设计思路及其应用

综述了高强高导铜合金的设计思路，分析了不同高强高导铜合金的强化机制和导电机制，并指出了高强高导铜合金的发展动向。     

Microstructural and mechanical stability of Cu-6 wt. % Ag alloy

The microstructural and mechanical stability of Cu-6 wt. % Ag alloy obtained by cold rolling combined with intermediate heat treatments have been investigated. The stress-strain responses and fracture behavior of Cu-6 wt. % Ag alloy were examined and correlated with the microstructural change caused by thermo-mechanical treatments. The deformation bands stabilized by silver precipitates were observed in heavily rolled Cu-6 wt. % Ag alloy. The highly deformed microstructure stabilized by silver filament was observed to be unstable at temperatures above 200 °C. The strength of Cu-6wt.%Ag alloys were found to decrease remarkably if they were heat-treated above 300°C. The fracture surfaces of Cu-Ag two phase alloys showed typical ductile type fracture. The electrical conductivity did not change appreciably up to the aging temperature of 200°C and increased rapidly at temperatures above 300°C. The increase of the conductivity and the decrease of the strength can be associated with the microstructural coarsening of heavily deformed linear band structure. The difference of the UTS and the conductivity between the rolling direction and the direction perpendicular to the rolling direction (on the rolling plane) were found to be relatively small.     

Age hardening of rapidly quenched Al-Zr-B alloys

Ternary alloys from the Al-Zr-B system were prepared as thin foils by a rapid quench technique. These foils were annealed isothermally as well as isochronally at various temperatures (150 to 550° C). The microstructures show that at high temperature, the grain growth is significantly retarded by the grain boundary pinning of boride precipitates. A strong age hardening is also a characteristic phenomenon in these alloys. It is found that microstructure and microhardness largely depend upon the zirconium/boron ratio of the alloy, indicating that the ratio determines the types of compounds occurring in this alloy system. It appears that in these alloys, high concentration of boron and the low ratio of zirconium/boron together yield stable precipitates at high temperatures.     

TEM observatiion of a Cu-Mg age-hardenable alloy

The precipitation processes in Cu-Be, Cu-Co, Cu-Fe alloys have been thoroughly investigated; however, much less attention has been paid to studying the Cu-Mg system. In this work the decomposition of Cu-3.5 wt% Mg alloy during ageing was investigated by means of transmission electron microscopy (TEM). The microstructure of Cu-3.5 wt% Mg alloy aged at 340° C is characterized by the presence of fine dispersed coherent precipitates. On continued ageing the coherent precipitates disappear and a new transition phase with oblate octahedron morphology grows. At temperatures above 34O° C the equilibrium phase is formed by discontinuous precipitation. Ageing of Cu-3.5 wt% Mg alloy at temperatures above 450° C results in the formation of the equilibrium Cu₂Mg phase.     

Effect of prior cold work on mechanical properties, electrical conductivity and microstructure of aged Cu-Ti alloys

The mechanical properties, electrical conductivity and microstructure of Cu-2.7wt%Ti and Cu-5.4wt%Ti alloys have been studied in different conditions employing hardness and resistivity measurements, tensile tests and optical, scanning and transmission electron microscopy. Ageing of undeformed as well as cold worked alloys raises their hardness, strength and electrical conductivity. The hardness increased from 120 VHN for solution treated Cu-2.7Ti to 455 VHN for ST + cold worked + peak aged Cu-5.4Ti alloy. While tensile stength increased from 430 to 1450 MPa, the ductility (elongation) decreased from 36 to 1.5%. A maximum conductivity of 25% International Annealed Copper Standard (IACS) for Cu-2.7Ti and 14.5% IACS for Cu-5.4Ti is obtained with the present treatments. Peak strength was obtained when the solution treated alloys are aged at 450°C for 16 hours due to precipitation of ordered, metastable and coherent , Cu₄Ti phase having body centred tetragonal (bct) structure. While mechanical properties of Cu-Ti alloys are comparable, electrical conductivity is less than that of commercial Cu-Be-Co alloys.     

Continuous precipitation in Cu-rich Cu-Ti binary and Cu-Ti-Al ternary alloys

An investigation has been carried out to study precipitation in a Cu-4% Ti binary and a Cu-2.1% Ti-2.4% Al ternary alloys which develop modulated microstructures upon ageing. Precipitate-free zones near the grain boundaries were observed on ageing immediately below a critical temperature representing the transition from the coherent to the incoherent region of the coherent phase diagram. In addition, a pronounced tendency for elastic interaction between the precipitate particles was detected, giving rise to aligned groups of precipitates containing a maximum of eight or nine particles in each group. At least near this critical temperature, decomposition occurs in the metastable region of the coherent phase diagram rather than in the spinodal region. The formation of the modulated structures in these alloys is, therefore, considered to arise not necessarily through spinodal decomposition. Continuous precipitation in a Cu-2.1% Ti-5% Al alloy is shown to result in spherical particles with no alignment. The effect of Al addition on the coherent solvus temperature is discussed in terms of a vacancy solute interaction mechanism.     

The presence and consequences of precipitatefree zones in an aluminium-copper-lithium alloy

The addition of lithium to aluminium alloys has the potential for providing a class of high-strength alloys with exceptional properties suitable for aerospace applications. Potential candidates are precipitation hardenable and belong to the Al-Li-Cu family. The intrinsic microstructural features have a pronounced influence on the mechanical response of these alloys. In this work, the mechanisms responsible for the formation of precipitate-free zones along grain boundaries in precipitation-strengthened lithium-containing aluminium alloys were examined. The influence of grain morphology and the nature and type of precipitate coverage at the grain boundary in controlling the formation of these zones was analysed. The presence and influence of these zones along the grain boundaries on mechanical properties was studied for an Al-4.5Cu-1.21 Li alloy. It was found that while strength is comparable with existing high-strength alloys, the ductility decreases due to the presence of precipitate-free zones. The degradation in ductility is attributed to the particular mode of plastic deformation of this alloy, and to the restriction of plastic deformation in narrow planar zones along the grain boundaries. Fracture occurs when a critical local strain is reached in these zones. The overall consequences of precipitate-free zones along grain boundaries on mechanical properties are discussed in the light of competing effects involving the nature of matrix-strengthening precipitates, grain-boundary particles and deformation characteristics. Previous address: Materials Modification Inc., Falls Church, Virginia 22044, USA Previous address: Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.     

Precipitates-interaction capture of nano-sized iron-rich precipitates during copper solidification

Nano-sized iron-rich precipitates reinforced copper alloys achieve excellent mechanical properties. Capture mechanism of iron-rich precipitates into copper grains during solidification was described but needs further validation. Here, Cu-1.5Fe-0.5Co (wt-%) alloy is fabricated by gravity casting. Iron-rich precipitates in nano and submicron scale (mostly?<?100?nm) are well dispersed in copper grain interior. Traditional pushing/engulfment transition (PET) models are used to interpret the capture process of iron-rich precipitates during copper solidification, but all fail to match the experimental results. The precipitates-interaction capture mechanism is most reasonable for describing the capture process.     

Infrared brazing Ti-6Al-4V and SP-700 alloys using the Ti-20Zr-20Cu-20Ni braze alloy

Great efforts have been made in brazing high-strength α-β titanium alloys below their beta-phase transformation temperature in order to obtain optimized mechanical properties. The brazing temperature of the cold roll-bonded Ti-20Zr-20Cu-20Ni foil is roughly 70 °C lower than that of Ti-15Cu-15Ni filler metal. Moreover, the detrimental Cu-Ni and Cu-Ni-Zr rich Ti phases can be greatly reduced or eliminated by properly choosing the brazing thermal cycle. This research demonstrates the potential application of Ti-20Zr-20Cu-20Ni foil in brazing titanium alloys.     

高强高导铜银合金的研究现状及发展趋势

Cu-Ag合金作为先进的导体材料,广泛应用于微电子、交通、航空航天及机械制造等工业领域。回顾了近年来高强高导Cu-Ag合金的主要研究进展。针对Cu-Ag合金的导电性和力学性能,主要从合金设计中的Ag成分设计、微合金化和加工工艺中的制备方法、热处理及变形处理等方面进行评述。分析了Cu-Ag合金的成分设计原则,比较了上述几种加工工艺的特点,并提出大塑性变形将会是一种非常有前景的制备高强高导Cu-Ag合金及其它合金的加工工艺。最后指出了现阶段研究中存在的问题及未来发展的趋势。     

大变形Cu-Fe原位复合材料研究

研究了Cu-1Fe(质量分数/%)合金经形变热处理后的组织与性能.结果表明,采用合适的中间热处理工艺,可以明显地提高Cu-Fe合金的强度及导电性,使之具有高强度与高导电性的良好结合.     

Cu—Ni—Fe合金在特殊涂层中的深过冷及其遗传性

以（８０％石英砂＋２０％石英玻璃粉）＋３％Ｈ３ＢＯ３（均为质量分数，下同）为坩埚涂层材料，烧结后获得厚３ｍｍ的表面光洁，无裂纹玻璃态涂层，在该涂层中熔炼Ｃｕ－３９Ｎｉ－６Ｆｅ和Ｎｉ－３０Ｃｕ－５Ｆｅ合金，分别获得了１９０Ｋ和２１０Ｋ的大过冷度，超过了晶粒骤然细化的临界过冷度△Ｔ２，以硅溶胶为粘结剂，石英玻璃粉为面涂层的铸型，浇铸后进行一次过热，使Ｎｉ－３０Ｃｕ－５Ｆｅ合金获得了１００过冷度，超过了     

Microstructure development and mechanical properties of rapidly solidified Ti–Fe and Ti–Fe–Bi alloys

Even though rapidly solidified Ti–Fe eutectic alloys may achieve good mechanical strength, increase in ductility is already a task to be accomplished. Addition of tin and arrangements of nano- and ultrafine-grain metallic materials have been shown as potential alternatives to overcome such drawback. Also, to address this problem, it seems that alternative alloy chemistries and processing routes must be adopted when manufacturing Ti-based alloys. In the present investigation, Ti–26 wt.%Fe (Ti–24.5 at.%Fe) and Ti–20 wt.%Fe–3 wt.%Bi (Ti–18 at.%Fe–0.7 at.%Bi) alloys have been prepared in a stepped copper mold using centrifugal casting. The as-cast Ti–Fe(–Bi) microstructures were formed by equiaxial arrangements of cells. Finer cell spacing (λ_c) was associated with the Ti–20 wt.%Fe–3 wt.%Bi alloy. The results include cell spacing measurements, segregation profile by X-ray fluorescence (XRF), uniaxial compression tests, optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A wide variation on the scale of the microstructure was noted especially in the case of the Ti–26 wt.%Fe alloy with the λ_c varying from 11−30 μm. This is due to the different cooling conditions of each diameter along the as-cast rod. Hall–Petch type equations are proposed relating σ_max to λ_c. Bi was dissolved in the β-Ti solid solution as well as TiFe compound formed in the cellular structure of the Ti–20 wt.%Fe–3 wt.%Bi.     

A study of the wetting,microstructure and bond strength in brazing SiC by Cu-X(X = Ti,V,Nb,Cr) alloys

In the active brazing of SiC by copper-based alloys, the effects of various active elements such as titanium, vanadium, niobium and chromium on the wetting, microstructure and bond strength are investigated. In wetting, Cu-Cr alloys have the lowest wetting angles on SiC of 10°–20° depending on the chromium content. SiC is decomposed on contact with alloy melts during brazing. Carbon and silicon released from this decomposition of SiC react with active elements to produce their carbides and suicides at the interface. The reacted layers have different microstructures depending on the brazing alloys, but Cu-Ti and Cu-Cr alloys show similar microstructure, as do Cu-V and Cu-Nb alloys. In the four-point bend tests, the brazed joints of Cu-5 at % Ti, Cu-5 at % V and Cu-5 at % Nb alloys have similar bend strengths of 86.9, 80.3 and 92.4 MPa, respectively. The brazed joints of Cu-2 at % Nb alloys show a high bend strength of 154 MPa, although the wetting angle is a little higher, at about 60°. Niobium is found as a new active element of copper-based alloys to braze SiC. Cu-Nb alloys are promising for substitution for Cu-Ti alloys.     

高强高导高耐热弥散强化铜合金的研究现状

综述了高强高导高耐热弥散强化铜合金的制备方法及其特点,比较了各种方法制备的弥散强化铜合金的性能,简述了国内外研究现状.