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.     

Effect of doping with Zr on the properties of the deformation-processed Cu-Fe in-situ composites

Effect of doping with alloying element Zr on the structure, the electrical resistivity and the strength of deformation-processed Cu-Fe in-situ composites were studied respectively by scanning electron microscope (SEM), transmission electronic microscope (TEM), material test system (MTS) and resistance measuring apparatus. The experimental results show that the ultimate tensile strength (UTS) and the conductivity of Cu-11.5% Fe-Zr wire cold drawn to the drawing strain η = 7.57 with intermediate heat treatments were observed to be 824 MPa and 61.4% IACS respectively, and those of Cu-11.5% Fe were 752 MPa and 64.6% IACS. Doping Zr can improve the thermal stability of Cu-Fe composites. The strength of Cu-Fe-Zr wire does not drop more rapidly at higher annealing temperatures (above 300°C) than that of Cu-Fe wire.     

微量Cu和形变热处理对Al-Fe合金性能的影响

观察Al-Fe合金的显微组织并测量其力学性能和导电性能,研究了Cu元素和形变热处理对其性能的影响。结果表明：在铸态Al-Fe-Cu合金组织中,Cu元素在基体内均匀分布,而Fe元素在晶界处偏析;挤压态的Al-0.7Fe-0.2Cu合金其性能最优：导电率为59.90%IACS,抗拉强度为108 MPa,硬度为31.2HV;随着退火温度的提高Al-0.7Fe-0.2Cu合金的抗拉强度急剧降低,在400℃退火时其抗拉强度最低(100 MPa),伸长率最高(31.3%);在250℃退火时导电率出现峰值(62.61%IACS)。在退火Al-0.7Cu-0.2Cu合金中有许多细小针状的θ(Al₂Cu)相析出,并与位错交互缠结。随着退火温度的提高合金中的位错密度降低,晶粒细化。     

加工硬化效应对Cu-3.2Ni-0.75Si合金时效组织和性能的影响

利用显微硬度法研究了Cu 3 .2Ni 0 .75Si合金不同时效组织的加工硬化效应对合金组织和性能的影响。研究表明 ,Cu 3 .2Ni 0 .75Si合金中Ni2 Si相的大小和分布对合金时效硬化效应产生显著的影响 ,4 5 0℃×8h时效组织加工硬化效应最大 ,变形量为 80 %时 ,显微硬度增幅在Hv60左右 ;5 5 0℃× 8h时效组织随变形量增加其硬度变化最平缓 ,变形量为 80 %时 ,显微硬度增幅仅为Hv1 0左右。随着变形量的增加 ,合金的导电率缓慢下降 ,80 %变形后 ,4 5 0℃× 4h、4 5 0℃× 8h和 5 0 0℃× 8h的时效组织导电率均下降 6%IACS左右 ,而5 5 0℃× 8h时效组织的导电率变化不大     

冷轧Cu基原位复合材料组织演变及性能研究

采用冷拉拔与冷轧相结合的变形方式将Cu-15Cr-0.24Zr合金制备成纤维增强复合薄板材料.采用扫描电子显微镜(SEM)对不同变形量下Cr纤维的形态进行观察,利用拉力试验机和电阻率测试仪对不同变形量下的薄板材料进行抗拉强度和电阻率测试.结果表明:冷拔变形量为5.18时,枝晶Cr已经具备了纤维的基本形貌,但纤维不连续,呈竹叶状;冷轧变形量为91.6%时,形成了均匀、连续的Cr纤维,并且由竹叶状演变为板条状;在冷轧变形量为91.6%时,成功制备出抗拉强度大于800 MPa,导电率大于70%IACS的Cu-15Cr-0.24Zr薄板材料.     

Thermophysical Properties of Aluminum 1060 Fabricated by Equal Channel Angular Pressing

Equal channel angular pressing (ECAP) has the advantage of enabling an ultrafine grain size. Aluminum 1060 is used as a power plant material because of its favorable electrical properties. However, the weak strength of aluminum limits its application. In this study, the thermal conductivity and electrical conductivity of Al 1060 made by ECAP was investigated. ECAP was conducted through the die having a channel angle of 90° and a corner angle of 20° at a temperature of 473 K with a strain rate of 2 mm · s⁻¹. The specimen was then processed with 1 to 8 passes by the route Bc method with 90° rotation. In the case of eight passes, the grain size was reduced to as small as 300 nm. As a result of the ECAP, the tensile strength was raised from 75 MPa to 134 MPa, while the electrical conductivity did not show a significant difference after eight passes. The thermal conductivity gradually decreased with ECAP passes, because of the decreased grain size by ECAP.     

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.     

控温铸型连铸Cu-Ni-Si合金的加工工艺与组织性能的关系及其机理

采用控温铸型连铸(temperature controlled mold continuous casting,TCMCC)技术制备C70250铜合金带坯,对带坯进行冷轧及不同温度和时间的时效处理,研究加工工艺与微观组织、力学性能及导电性能的关系,并揭示其机理。结果表明:TCMCC制备的C70250铜合金带坯具有粗大的柱状晶组织,横向晶界较少,经变形量97.5%的冷轧后形成了沿轧向的纤维条带状组织。当时效温度为450℃、时效时间为60min时,合金的抗拉强度为758MPa、导电率为54.5%IACS;与传统制备工艺相比,抗拉强度提高了5.3%,导电率提高了36.3%,实现了强度和导电率的同步提升。该条件下合金保留了纤维条带状组织并均匀析出了大量尺寸为6~10nm的Ni2Si相,通过加工硬化和Orowan强化共同作用提高了合金的强度;且溶质原子得到充分析出,横向晶界较少,显著提高了C70250铜合金的导电性能。     

Microstructures and properties of ultrafine-grained pure titanium processed by equal-channel angular pressing and cold deformation

Equal-channel angular pressing (ECAP) has been used to refine the grain size of commercially pure (CP) titanium as well as other metals and alloys. CP-Ti is usually processed at about 400 degrees C because it lacks sufficient ductility at lower temperature. The warm processing temperature limits the ability of the ECAP technique to improve the strength of CP-Ti. We have employed cold deformation following warm ECAP to further improve the strength of CP-Ti. Ti billets were first processed for eight passes via ECAP route Bc, with a clockwise rotation of 90 degrees between adjacent passes. The grain size obtained by ECAP alone is about 260 nm. The billets were further processed by cold deformation (cold rolling) to increase the crystalline defects such as dislocations. The strength of pure Ti was improved from 380 to around 1000 MPa by the two-step process. This article reports the microstructures, microhardness, tensile properties, and thermal stability of these Ti billets processed by a combination of ECAP and cold deformation.     

Toward High Strength and High Electrical Conductivity in Super‐Aligned Carbon Nanotubes Reinforced Copper

The miniaturization of electronic products is drawing higher demand in the strength and conductivity of conductors. This work demonstrates the possibility of substantially increasing the dislocation density in copper to enhance the strength of super‐aligned carbon nanotubes (SACNTs) reinforced copper matrix composites (SACNT/Cu) without compromising the electrical conductivity. High strain is introduced into pure copper and SACNT/Cu by accumulative roll‐bonding (ARB) process up to 16 cycles at ambient temperature. SACNTs with initial laminated distribution turn out to be dispersed uniformly with maintained directional arrangement inside the copper matrix after ARB, which can then effectively block the motion of dislocations. Therefore, large number of dislocations propagated by large strains can be accumulated without subdivision. The accumulated dislocations will result into strain hardening, which is the major strengthening mechanism in SACNT/Cu after ARB. Furthermore, the contribution of dislocations to resistivity increase is little, thus maintaining high electrical conductivity. As a result, a high tensile strength (505 MPa) combined with a high electrical conductivity (90% IACS) is achieved in large‐sized composite sheet.

     

Microstructure and mechanical properties of ZA62 Mg alloy by equal-channel angular pressing

The Mg-6Zn-2Al alloy was processed by ECAP and microstructure and mechanical properties of the alloy before and after ECAP were studied. The results revealed that the microstructure of the ZA62 alloy was successfully refined after two-step ECAP (2 passes at 473 K and 2-8 passes at 423 K). The course bulk interphase of Mg₅₁Zn₂₀ was crushed into fine particles and mixed with fine matrix grains forming “stripes” in the microstructure after the second step of ECAP extrusion. A bimodal microstructure of small grains of the matrix with size of ∼0.5 μm in the stripes and large grains of the matrix with size of ∼2 μm out of stripes was observed in the microstructure of samples after 4-8 passes of ECAP extrusion at the second step. The mechanical properties of the alloy studied were significantly improved after ECAP and the highest yield strength and elongation at room temperature were obtained at the samples after 4 and 8 ECAP passes at the second step, respectively. Tensile tests carried out at temperature of 473 K to 573 K and strain rate of 1 × 10⁻³ s⁻¹ to 3 × 10⁻² s⁻¹ revealed that the alloy after 8 ECAP passes at the second step showed superplasticity and the highest elongation and strain rate sensitivity (m-value) reached 520% and 0.45, respectively.     

Enhancement of strength and electrical conductivity for a dilute Al-Sc-Zr alloy via heat treatments and cold drawing

Developing heat-resistant conductors with high strength and high electrical conductivity is a key issue in the electrical conductor industries, as the ever-increasing power transmission poses higher requirement on the thermal stability of electrical conductor wires. Dilute Al-Sc-Zr alloys are considered as promising candidates due to the excellent heat resistance and high electrical conductivity, but the low strength always limits their application on electrical wires. Yet, few efforts on process design have been made in dilute Al-Sc-Zr alloys to enhance the strength. Here, various kinds of processing paths via combination of cold drawing, ageing and/or annealing were conducted to improve the strength and electrical conductivity of a dilute Al-Sc-Zr alloy. Results show that enhanced strength and electrical conductivity were obtained after cold drawing + ageing or pre-ageing + cold drawing + annealing treatments processes. Optimal properties (194 MPa in ultimate tensile strength and 61% IACS in electrical conductivity) were obtained through cold drawing followed by ageing. Microstructure evolution which affects strength and electrical conductivity was systematically investigated using TEM and 3DAP. The enhanced strength was mainly attributed to the suitable interactions between strain strengthening and precipitation strengthening. The enhancement in electrical conductivity was caused by precipitation of solute atoms and recovery of defects. These results provide foundations for the processing design of Al-Sc-Zr conducting wires with good properties and push forward their potential application in heat resistant conductor industries.     

Mechanical Properties,Fatigue Life,and Electrical Conductivity of Cu–Cr–Hf Alloy after Equal Channel Angular Pressing

Structure, mechanical, and service properties of a Cu–Cr–Hf alloy after quenching, equal‐channel angular pressing (ECAP), and subsequent aging have been studied. The positive effects of ultrafine‐grained structure formation (grain/subgrain size of ≈200 nm) during ECAP and strengthening particles precipitation upon subsequent aging at 450 °C on the mechanical and fatigue properties of the alloy are shown. Ultrafine‐grained Cu–Cr–Hf alloy after aging shows increasing in the fatigue limit on the basis of 10⁷ cycles from 185 to 375 MPa relative to that of the initial coarse‐grained state. The alloy after ECAP and aging also exhibits sufficient elongation to failure (11.4%) and good electrical conductivity (78%IACS).     

Effect of multidirectional forging on microstructures and tensile properties of a particulate reinforced magnesium matrix composite

A particulate reinforced magnesium matrix composite prepared with stir casting was subjected to multidirectional forging (MDF). The results showed that after 1 MDF pass the grain size of matrix in the composites decreased compared with as-cast composite, and increased with increasing the MDF temperature from 370 °C to 450 °C. With increasing the MDF passes at 370 °C, the particle distribution of the composite was improved until 3 MDF passes while the grain size of matrix in the composite reached a minimum after 4 MDF passes. Both the yield strength and the ultimate tensile strength of the composite were enhanced with increasing the MDF passes.     

Effect of prior cold work on age hardening of Cu-4Ti-1Cd alloy

This research is part of a project whose scope was to develop high strength ternary alloys based on Cu-Ti system with the primary aim of substituting them for toxic and expensive Cu-Be alloys. In this pursuit, age hardening behaviour of Cu-4Ti-1Cd alloy has already been investigated and the present paper reports the investigations on the influence of prior cold work by rolling of 50, 75 and 90% on the age hardening of a Cu-4Ti-1Cd alloy using hardness and tensile tests and optical as well as transmission electron microscopy. As a result of cold work followed by aging, hardness of the alloy increased from 237 Hv in solution treated condition to 425 Hv on 90% cold work and peak aging. Similarly, yield and tensile strengths of the alloy reached maxima of 1037 and 1252 MPa respectively on 90% deformation and peak aging. The microstructure of the deformed alloy exhibited elongated grains and deformation bands. The maximum strength on peak aging was obtained due to precipitation of ordered, metastable and coherent β^l, Cu₄Ti phase in addition to high dislocation density and deformation twins. Both hardness and strength of the alloy decreased on overaging due to the formation of incoherent and equilibrium β, Cu₃Ti phase. However, the morphology of the discontinuous precipitation was changed to globular shape due to large deformations and overaging.     

基于正交试验的Cu-Ni-Si合金加工工艺研究

利用L9(34)表对Cu-3.2Ni-0.75Si-0.3Zn合金的时效工艺进行了正交试验,结果发现,各因素对合金导电率和显微硬度影响程度的主次顺序分别为:时效温度＞合金状态＞时效后冷变形量＞时效时间和时效温度＞冷变形量＞合金状态＞时效时间.在保证合金性能不降低的情况下,该合金热轧板材经60%冷变形后,可直接进行时效而省去固溶处理.本试验所得合金的性能为:抗拉强度830MPa,显微硬度220Hv,导电率51%IACS,延伸率6%;同时材料厚度＜0.120 mm.     

室温ECAP和冷轧复合变形工业纯钛的组织和性能

采用ECAP技术和常规冷轧复合变形工艺制备了高强度工业纯钛,研究了复合变形后试样的力学性能与显微组织的关系.结果表明,工业纯钛经室温单道次ECAP和冷轧复合变形后,晶粒被严重拉长,形成了明显的纤维状组织,试样的抗拉强度高达805MPa;随着冷轧变形量的增大,变形组织的细化程度和均匀性提高,使试样的强度和塑性进一步提高.位错滑移和孪生是工业纯钛室温ECAP和冷轧复合变形的主要变形机制.     

Mechanical and Electrical Properties of Cryo-worked Cu

For manufacturing the magnets of fusion machines pure copper of both high mechanical resistance and electrical conductivity is required. Though high purity copper guarantees high electrical conductivity, its mechanical properties may be not suitable for the applications in tokamaks. In this view, a new procedure developed for obtaining high purity copper with excellent mechanical strength is described in this work. Samples of oxygen free copper (OFC) have been worked by pressing in liquid nitrogen (77 K). It has been verified that the mechanical properties of the worked metal are strongly dependent on the strain rate. Very low strain rates permitted to attain values of tensile yield strength (550 MPa) significantly higher than those obtained by traditional cold-working at room temperature (450 MPa). The electrical conductivity of the cryo-worked Cu decreases with the tensile yield strength even though the hardest samples of tensile yield strength of 550 MPa exhibit still acceptable values of conductivity (about 94 % IACS at room temperature).     

Effect of grain refinement on the mechanical properties of Cu–Al–Be–B shape memory alloy

In this work, the mechanical properties of equal channel angular processing (ECAP)-processed fine- and coarse-grained Cu–11.42Al–0.35Be–0.18B shape memory alloys (wt.%) were evaluated using tensile testing. After eight passes of ECAP and subsequently quenching from 600 °C to RT, the mean grain diameter was refined from 227 μm to 42 μm with grain boundaries purified. The fine-grained alloy exhibited good mechanical properties with a high tensile strength (703 MPa) and featured deeper and closer dimples on its fracture surface. The micro cracks were more refined, and the cracks extension along the grain boundaries was improved in the fine-grained alloy. These changes can be attributed to improvement of martensite morphology, structural refinement and grain boundary purification.     

T-shaped equi-channel angular pressing of Pb–Sn eutectic and its tensile properties

Equi-channel angular pressing (ECAP) of a Pb–Sn eutectic alloy up to six passes in a T-shaped die, rather than a conventional L-shaped die, was studied for grain refinement. The effect of ECAP on the hardness and tensile properties was studied. Microstructure predominately changed in the early part of the ECAP process and became equiaxed and uniformly distributed in both the longitudinal and the transverse sections after four passes. There occurred substantial softening over the first two passes—hardness of 10 Hv, yield strength of 14.2 MPa and tensile strength of 16.3 MPa in the as-cast condition decreased upon two passes to 6 Hv, 9.7 MPa and 13.0 MPa, respectively. The ductility (% elongation) increased drastically from <50% in the as-cast condition to 150% upon two passes, and further increased to 230% after four passes. Various tensile properties and concurrent microstructural evolution were used to develop a mutual relationship among them.