Work softening characterization of alumina dispersion strengthened copper alloys

The effect of cold rolling on the hardness and microstructure of alumina dispersion strengthened copper (ADSC) alloys and oxygen free pure copper was investigated. With increased cold rolling deformation, a work softening phenomenon can be observed in ADSC alloys. The higher the alumina content is, the more difficult it is to observe the work softening phenomenon. This phenomenon is not observed in oxygen free pure copper, even after cold rolling 90%. The microstructural changes of the ADSC alloys were analyzed by TEM as a function of deformation. Initially, with an increase of the cold rolling deformation, a large number of dislocation cells are formed. Then, with further increased deformation, both a decrease of dislocation density and the formation of the smaller dislocation cells (subgrains) within the larger dislocation cells or the elongated bands are observed. Finally, as deformation attains a certain value, the smaller dislocation cells (subgrains) begin to coalesce, and the work softening phenomenon appears. All of these changes are the result of the interaction between the alumina particles and the dislocation line segments. In order to analyze the work softening mechanism for the ADSC alloys, a model is introduced in the paper.     

Electrical performance of ZrC ceramics

In the present study it was possible to provide evidence that ZrC ceramics – regarding their electrical-mechanical and thermal properties – are an attractive substitute for tungsten and molybdenum in the field of refractory materials and heating elements. For this purpose, the heating behavior of ZrC materials was investigated in a specific test stand under high vacuum conditions (10⁻⁶ mbar) at temperatures above 2000 °C. In addition, the applicability of ZrC as an electrical heating element was investigated in terms of geometric design and long-term behavior. Examinations after the heating tests did not reveal any significant changes in the material.     

纳米Al2O3粒子浓度对弥散强化铜合金退火行为的影响

通过力学性能、金相和TEM观察对低、中浓度Cu-Al2O3弥散强化铜合金的退火行为进行了研究.结果表明:低浓度弥散强化铜合金具有一定的抗高温软化性能,500℃退火后发生再结晶,900℃退火后已基本完全再结晶,屈强比约为56%.中浓度合金抗高温软化性能较好,900℃退火后,合金仍然以回复过程为主,金相尺度下不能看到再结晶晶粒,屈强比可达70%.弥散强化铜合金优越的抗高温软化性能归功于铜基体内均匀弥散分布的纳米Al2O3粒子.Al2O3粒子尺寸在10～20nm内时,粒子间距＞200nm时,阻碍晶界迁移能力较差,粒子间距＜90nm时,可显著阻碍晶界的迁移,合金抗高温软化性能优越.     

Brazing perovskite ceramics with silver/copper oxide braze alloys

In this study the feasibility of eutectic braze alloys based on the silver/copper oxide (Ag/CuO) system were investigated for use in joining lead magnesium niobate (PMN) ceramics. PMN was successfully brazed in air at 1050 and 1100°C using Ag/CuO alloys. The brazed samples had an average four-point-bend fracture strength of approximately 40 percent of the average monolithic PMN strength (with an actual value of 19 ± 11 MPa). The fracture strength was relatively constant for all brazing conditions tested. No significant reaction product layer was observed at the silver/PMN interface and the electrical properties of the PMN were changed only slightly by presence of the braze alloy interlayers.     

Precipitation behaviour of Al-Mg-Si alloys with high silicon content

The precipitation behaviour of three Al-Mg-Si alloys has been studied by differential scanning calorimetry (DSC) and transmission electron microscopy. Seven exothermic peaks are usually observed in DSC thermograms of Al-Mg-Si alloys with high silicon contents. These peaks are believed to be caused by the formation of clusters of silicon and magnesium atoms, Guinier-Preston zones, small precipitates with an unknown structure, β″ phase, B′ phase, silicon precipitates and β phase. The silicon content has no obvious effect on the precipitation of β″ and B′ phases in these alloys, but it has a pronounced influence on the formation of silicon precipitates. The cold-rolling reduction before solution heat treatment is also found to affect the precipitation process appreciably. This revised version was published online in November 2006 with corrections to the Cover Date.     

Rate sensitivity in the high temperature deformation of dispersion strengthened Al-Fe-V-Si alloys

The rate sensitive flow characteristics in the elevated temperature deformation of Al-Fe-V-Si alloys processed by rapid solidification/powder metallurgy route were assessed by the strain rate change tests in compression. With an ultrafine grain size, stabilized by fine dispersoids, a peak rate sensitivity index of 0.15 and normal ductility were observed in alloys containing dispersoids up to a volume fraction of 0.37. The lack of superplastic response is interpreted in terms of a high threshold stress for superplastic flow. The threshold stress assessed by an extrapolation procedure is observed to be grain size and temperature dependent. Its origin is suggested to be Zener drag limited boundary migration, which is an essential part of the superplastic flow mechanism.     

Dislocations and faults in some alloys containing copper and aluminum

By using the method of variance of the respective X-ray diffraction line profile, the effective particle size (p) and r.m.s.. strain (S ^21/2) of four alloys containing copper and aluminium (Al-0.08, 4 and 10 wt% Cu and Cu-5.87 wt% Al) at different annealing stages (starting from the cold filed stages) have been determined. From the peak shift of these samples, their average stacking fault probability = – where is the intrinsic stacking fault probability and the corresponding extrinsic stacking fault probability, respectively, were determined. Similarly, from the angular distance between the peak and the centroid of the diffraction profiles, the magnitude of + 4.5, where is the twinning fault probability, was measured. From these, the true particle sizeT, the stacking and twinning fault probabilities , and and the minimum stacking fault widthD _min have been determined. The hardness of alloys at different annealing stages was found to be dependent on the dislocation density and the stored energy in the alloys. The relationT S ^21/2 = constant was found to be valid for the alloys and from this a mechanism of grain growth with annealing has been suggested. With annealing the dislocation density and stored energy per unit volume were found to decrease until finally, in the fully annealed stages, they disappeared. The same was found to be valid for stacking and twinning fault probabilities as well as the minimum stacking fault width. A mechanism for the creation and annihilation of the intrinsic and extrinsic stacking faults has been suggested.     

镁系准晶与高性能镁合金

综述了Mg-Al-X(X=Cu,Zn,Pd,Ag,Pt)、Mg-Zn-(Ga,Al)、Mg-Zn-RE(RE=Y,以及Gd到Er的稀土元素)等系列的镁系准晶的结构、形成机理、准晶形成的成分范围,以及准晶的力学性能.介绍了近年来利用镁系准晶的优异性能,采用各种制备工艺,开发含准晶的高性能镁基材料的研究进展.     

NbMoTiVSi_x refractory high entropy alloys strengthened by forming BCC phase and silicide eutectic structure

In order to improve mechanical properties of refractory high entropy alloys,silicide was introduced and NbMoTiVSi_x(x=0,0.1,0.2,0.3,and 0.4,molar ratio) refractory high entropy alloys are prepared by vacuum arc melting.Phase composition,micro structure evolution and mechanical properties were systematically studied.Results show that the silicide phase is formed in the alloys with addition of silicon,and the volume fraction of silicide increases from 0 to 8.3 % with increasing of silicon.Microstructure observation shows that the morphology of dendrite changes from columnar to near equiaxed,eutectic structure is formed at grain boundaries and composed of secondary BCC phase and silicide phase.The average length of the primary and second dendrites decreases with the increasing of silicon.Whereas,the ratio of eutectic structure increases from 0 to 19.8 % with the increment of silicon.The refinement of microstructure is caused by heterogeneous nucleation from the silicide.Compressive tests show that the yield and ultimate strength of the alloys increases from 1141.5 MPa to 2093.1 MPa and from 1700.1 MPa to 2374.7 MPa with increasing silicon content.The fracture strain decreases from 24.7 %-11.0 %.Fracture mechanism is changed from ductile fracture to ductile and brittle mixed fracture.The improvement of the strength is caused by grain bounda ry strengthening,which includes more boundaries around primary BCC phase and eutectic structure in grain boundary,both of them is resulted from the formation of silicide.     

Precipitation in Ni-Co-Al alloys

Continuous precipitation of in Ni-Co-Al alloys has been studied using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The precipitation sequence is observed to be a change of morphology from spheres cube plates. The lattice parameter mismatch is of the order of 0.3%. The particle growth rate has been determined over a range of temperatures for alloys containing a range of cobalt contents from 0 to 55 at %. In all cases the particle size varies att ^1/3 and the particle number density ast ^–1. The growth rate varies very little with increasing volume fraction of . The measured particle growth rate decreases with increasing cobalt content, probably as a result of partitioning of cobalt between particle and matrix. Electron micro-probe analysis, volume fraction measurements and X-ray analyses were used to determine the ( + ) boundary in the Ni-Co-Al ternary system at temperatures between 973 and 1073 K. It is clear from the position of the boundary that a significant beneficial effect of cobalt is in decreasing the solubility of the phase in for any temperature up to 1073 K. Histograms of particle size were constructed and compared with both the Lifshitz-Slyozov-Wagner and the Lifshitz-Slyozov encounter modified (LSEM) predicted distributions. The LSEM theory for particle coarsening was shown to be a much better fit to the experimental particle size distributions.     

Precipitation in Ni-Co-Al alloys

Discontinuous precipitation of the phase in Ni-Co-Al alloys has been studied in the temperature range 673 to 973 K using optical microscopy and transmission electron microscopy. Discontinuous precipitation was observed to occur in all the alloys to some degree, the extent of the precipitation decreasing with decreasing cobalt content of the alloy. Only in high cobalt alloys (greater than 38 at%) did the discontinuous transformation go to completion. The discontinuous transformation product consisted of fine straight coherent lamellae with a specific cube-cube orientation relationship to the matrix at low ageing temperatures and short times. At higher temperatures and longer times, continuous and discontinuous coarsening processes produced a coarse, less regular structure with a plate spacing ten times that of the regular structure. The overall kinetics of the discontinuous transformation were affected by the prior continuous precipitation of particles ahead of the discontinuous reaction fronts. The coarsening of these continuous particles as a function of time stopped the transformation in low cobalt alloys and continuously slowed the rate of transformation in high cobalt alloys resulting in Avrami exponents,n, less than 1. Reversion experiments and experiments on prestrained specimens yielded Avrami exponents ofn=1 lending support to the above suggestion. The major effect of cobalt in these alloys was to increase their propensity to transform discontinuously. This effect was largely due to the decrease in the rate of continuous precipitation and coarsening as a result of the increased partitioning of cobalt between and in high cobalt alloys.     

锆微合金化增强铝合金的研究进展EI北大核心CSCD

锆(Zr)元素是铝合金中研究较为深入、实际应用较为广泛的微合金元素之一。由于Zr在铝中具有低的固态扩散速率且可形成低密度、高熔点、低界面错配度的Al_(3)Zr弥散相,因此合金展现出高温下服役的潜力。然而,Al_(3)Zr粒子的弥散强化效果主要受到粒子低数量密度或体积分数的制约;此外,多元合金体系凝固、变形、热处理过程中多组元间交互作用复杂,Al_(3)Zr弥散强化与各体系中本征相强化作用往往难以兼得,上述问题均对合金的力学强度造成了不利的影响。本文综合近年来的相关报道,对含Zr铝合金中Zr的存在形式、析出和粗化行为以及强化机制进行了概述;简要介绍了复合微合金化促进Al_(3)Zr析出机理与最新研究结果;对某些体系铝合金中Zr微合金化的应用进行了归纳与总结,结合当前新型耐热铝基合金发展的新趋势,指出铝合金内Zr的微量添加对调控微结构、提升室温和高温强度的重要意义。     

Characterization of strengthened rapidly quenched Zr-based alloys

The nominal composition components of alloy Zr_66.4Nb_6.4Ni_8.7Cu_10.5Al₈ (Alloy A) were fabricated and characterized. The strengthening of in-situ alloys depends on the role of both the glassy matrix and the second phases. The glass transition and the crystallization kinetics were studied using DSC and X-ray diffraction as a function of element distribution. The amorphous and semi-crystalline structures were identified with the existence of nano crystals in the alloy nominal compositions. The Elastic compression modulus were found to increase with transition to crystallite phase. Where as, the microhardness decreases dramatically with the change from crystalline to amorphous phase. The compression fracture surface shows classic veins behavior. In mode of continuous heating and adiabatic annealing the glass transition, T _g , and the crystalline peak, T _p , temperatures display a strong dependency on heating rate. The activation energy for glass transition and crystallization were determined as E _g  = 226 KJ/mol based on Kissinger method, but during the isothermal process E _g  = 121 KJ/mol.     

Precipitation processes in Cu-Co-Si alloys

The precipitation of cobalt and silicon atoms from supersaturated solid solutions of Cu-Co-Si alloys was studied during ageing between 400 and 600° C by electrical resistivity, thermoelectric power and calorimetric measurements, by mechanical testing and by transmission electron microscopy investigations. It has been found that the decomposition begins with the appearance of concentration fluctuations from which cobalt precipitates first. The clustering of cobalt atoms initiates the precipitation of silicon, and so particles with the stoichiometric Co₂Si composition are finally formed. The silicon that is in excess with respect to the stoichiometry of Co₂Si is retained in the solid solution but its increase in the initial supersaturated solid solution strongly enhances the nucleation of the particles and, therefore, results in a finer precipitate structure. On the basis of calorimetric measurements the effective activation energy of both the cobalt and silicon precipitation was determined to be around 1.5 eV.     

Nanocrystalline magnetic alloys and ceramics

Magnetic properties of materials in their nanocrystalline state have assumed significance in recent years because of their potential applications. A number of techniques have been used to prepare nanocrystalline magnetic phases. Melt spinning, high energy ball milling, sputtering, glassceramization and molecular beam epitaxy are some of the physical methods used so far. Among the chemical methods, sol-gel and co-precipitation routes have been found to be convenient. Ultrafine particles of both ferro- and ferrimagnetic systems show superparamagnetic behaviour at room temperature. Coercivity(H _c ) and maximum energy product(BH) _max of the magnetic particles can be changed by controlling their sizes. The present paper reviews all these aspects in the case of nanocrystalline magnetic systems — both metallic and ceramics.     

Wear properties of oxide dispersion strengthened nickel alloys

Wear resistance of Ni superalloys and oxide-dispersed Ni alloys was studied at room temperature. The wear resistance was connected with dislocation behaviour in the alloys. Dislocations in Ni-1%Al₂O₃ and Ni-0.5%Y-1%Al₂O₃ easily cross-slipped at dispersed alumina particles. Dislocation cell structures fully developed. Easiness of the cell structure formation corresponded to the lower wear resistance of Ni-1%Al₂O₃ and Ni-0.5%Y-1%Al₂O₃ than Ni. On the other hand, the dislocations in MA6000 hardly cross-slipped at the dispersed yttoria. Cell structure was not formed. Difficulty of cell formation corresponded to the higher wear resistance of MA6000 than Sumicolloy No. 6, a kind of stellite alloy.     

Properties of high performance concrete containing fine-ground ceramics as supplementary cementitious material

This paper presents experimental work regarding the basic physical characteristics, mechanical and fracture-mechanics properties, durability characteristics, hydric and thermal properties of high performance concrete (HPC) with up to 60% of Portland cement replaced by fine-ground ceramics. Experimental results show that the amount of the ceramics in the mix is limited mainly by the resistance against de-icing salts which is found satisfactory only up to the cement replacement level of 10%. The mechanical and water transport properties are not significantly impaired by ceramic additions of up to 20%, whereas the effective fracture toughness, specific fracture energy, and chemical resistance (to MgCl₂, NH₄Cl, Na₂SO₄, HCl) are effectively maintained up to 40%. The frost resistance, water vapor transport and storage parameters and thermal properties are not significantly impaired even up to a 60% replacement level.