Microstructural properties of superalloys investigated by nanoindentations in an atomic force microscope

The microstructure of nickel-base superalloys with differently shaped γ′ precipitates determines their excellent high-temperature behaviour. A reliable quantitative determination of volume fraction and particle size distribution (PSD) of these precipitates is difficult, since the size of the precipitates is often below 100 nm. With an atomic force microscope (AFM), sections through specimens are analysed with a resolution in the nanometre range, which allows a quantitative determination of the γ′ volume content and PSD for different superalloys. Thus, determined volume fractions for the γ′ phase in the superalloys CMSX-6 and Waspaloy amount to 56% and 27%, respectively. A combination of an AFM with a nanoindentation system also allows the measurement of local mechanical properties such as hardness and elasticity. These quantities are determined for the first time directly on the superalloys CMSX-6 and Waspaloy for the γ′ and matrix phases, separately. The γ′ phase in both alloys shows a significantly higher but different hardness than the matrix phase, whereas the moduli of elasticity are similar. A depth dependence of the hardness was found for very small indentations.     

低碳钢与紫铜搅拌摩擦焊接头显微组织分析

用搅拌摩擦焊焊接了4 mm厚的Q235低碳钢板和T2紫铜板,得到了内部无缺陷、外观成形良好、无变形的对接接头.研究表明,低碳钢紫铜接头的显微组织与其所受到的热和力作用大小有关.在接头钢侧,轴肩挤压区有较大的变形,组织发生动态再结晶后形成了细小的等轴晶铁素体;在探针附近的热影响区,经历的应变较小,组织则由变形的先共析铁素体和侧板条铁素体组成.在接头铜侧热影响区的晶粒受热长大,而热力影响区的铜由于发生了动态再结晶,晶粒细小.在焊核区,上部主要由钢、铜薄层交叠组成;焊核中下部为钢、铜及钢与铜形成的化合物交叠组成的漩涡状条带,其中铜条带的组织为细小的等轴晶,钢条带的组织为细小的等轴晶或先共析铁素体 侧板条铁素体组织.     

Microstructural response of pure copper to cryogenic rolling

A high-resolution electron backscatter diffraction technique was applied to quantify grain-structure development and texture evolution during/after cryogenic rolling of pure copper. Microstructure evolution was found to be a complex process involving mainly geometrical effects associated with strain and discontinuous recrystallization but also including limited twinning and grain subdivision. Recrystallization was deduced to be static in nature and probably occurred during static storage of the material at room temperature after cryogenic rolling. The texture contained a pronounced {1 1 0}〈1 1 2〉 brass component; this observation was interpreted in terms of the suppression of cross-slip at cryogenic temperatures. In general, cryogenic rolling was found to be ineffective for the formation of a nanocrystalline structure in pure copper.     

Microstructural and mechanical investigation of aluminum tailor-welded blanks

The push to manufacture lighter-weight vehicles has forced the auto industry to look to alternative materials than steel for vehicle body structures. Aluminum is one such material that can greatly decrease the weight of vehicle body structures and is also consistent with existing manufacturing processes. As in steel structures, cost and weight can be saved in aluminum structures with the use of tailored blanks. These blanks consist of two or more sheets of dissimilar thicknesses and/or properties joined together through some type of welding process. This enables the design engineer to “tailor” the blank to meet the exact needs of a specific part. Cost savings can be gained by the elimination of reinforcement parts and the stamping dies used to manufacture them. Weight savings can be attained based on the fact that one thicker piece is more efficient than a welded structure and therefore can allow for down-gauging of parts. Although tailor-welded blanks (twbs) offer both potential weight and cost benefits, the continuous weldline and thickness differential in twbs can often result in difficulty in stamping. This problem is more severe in aluminum because of its limited formability as compared with typical drawing-quality steels. Additionally, welding of steel twbs tends to increase the strength of the weld material, which helps prevent failure in the weld during forming. Aluminum twbs do not experience this increase in strength and therefore may have a greater tendency to fail in the weld. In this study, several aspects of twbs manufactured from 6111-T4, 5754-O, and 5182-O aluminum alloys were analyzed and compared with those of a more conventional steel twb. The effect of gauge mismatch on the formability of these blanks is discussed as well as the overall potential of these blanks for automotive applications.     

Microstructural stability during cyclic loading of multilayer copper/copper samples with nanoscale twinning

The response to cyclic deformation has been studied for Cu/Cu multilayer material consisting of columns of closely spaced, parallel nanotwins. The fatigue life under stress-controlled cycling is greatly improved over that of coarse-grained Cu. Nanotwinning provides significant strengthening, which is unchanged by fatigue or severe compression. Observations by focused ion beam microscopy showed that the microstructure is quite stable under deformation. Localized deformation from indentation produced shear bands and apparently some loss of nanotwinned area but no decrease in hardness.     

铜铝异种金属激光焊接头组织特征及力学性能

铜铝异种金属薄板的激光焊加热和冷却速度快,焊缝区温度梯度和成分浓度梯度大,接头微观组织具有特殊性.通过激光焊接试验、拉伸测试、焊缝横截面及断口SEM和DES分析,研究了铜铝薄板的激光搭接焊接头组织分布特征及其对性能的影响.结果表明,在高温度梯度和浓度梯度下焊缝区形成了组织形貌不同的区域,分别是平行排列的板条状过共晶组织区、板条状过共晶组织与共晶组织混合区(简称混合区)、片层状的共晶区以及枝晶状的亚共晶区.拉伸剪切断裂在混合区.随着焊接速度的增加,混合区的宽度变窄,混合区的过共晶组织由等轴状向板条状转变,试件承受的最大剪切力随之增大.     

Microstructural characteristics of immersion tin coatings on copper circuitries in circuit boards

Immersion tin coating, which is used as a lead-free surface finish, is deposited on the surface of copper circuitries on circuit boards by a replacement reaction. The characteristics of immersion tin coatings and the formation of tin whiskers and the intermetallic compound (IMC) are described. A Cu₆Sn₅ phase forms at the beginning of the immersion-plating process and expands until all of the tin has been transformed into a copper-tin alloy. The IMC layer becomes thicker and the volume of the pure tin layer decreases during storage. The tin involved in the formation of whiskers must originate from the tin layer of the immersion tin coating. Therefore, the formation of tin whiskers stops when all of the tin has been expended. Moreover, relatively larger whiskers grow on thicker coating layers, which contain more tin.     

Microstructural investigation of friction stir welded 7050-T651 aluminium

The grain structure, dislocation density and second phase particles in various regions including the dynamically recrystallized zone (DXZ), thermo-mechanically affected zone (TMAZ), and heat affected zone (HAZ) of a friction stir weld aluminum alloy 7050-T651 were investigated and compared with the unaffected base metal. The various regions were studied in detail to better understand the microstructural evolution during friction stir welding (FSW). The microstructural development in each region was a strong function of the local thermo-mechanical cycle experienced during welding. Using the combination of structural characteristics observed in each weld region, a new dynamic recrystallization model has been proposed. The precipitation phenomena in different weld regions are also discussed.     

Atomic scale investigation of Cr precipitation in copper

The early stage of chromium precipitation in copper was analyzed at the atomic scale by atom probe tomography (APT). Quantitative data about the precipitate size, three-dimensional shape, density, composition and volume fraction were obtained in a Cu–1Cr–0.1Zr (wt.%) commercial alloy aged at 713 K. Surprisingly, nanoscaled precipitates exhibit various shapes (spherical, plates and ellipsoid) and contain a large amount of Cu (up to 50%), in contradiction to the equilibrium Cu–Cr phase diagram. APT data also show that some impurities (Fe) may segregate along Cu/Cr interfaces. The concomitant evolution of the precipitate shape and composition as a function of the aging time is discussed. Special emphasis is given to the competition between interfacial and elastic energy, and to the role of Fe segregation.     

Hardness and modulus of the lamellar microstructure in PST-TiAl studied by nanoindentations and AFM

A nanoindenting atomic force microscope (NI-AFM) allows quantitative measurements of the modulus of elasticity and the hardness on a very local scale. This technique was used to study the elastic and plastic deformation properties of different lamellae in a polysynthetically twinned TiAl crystal. The results show, that the hardness determined at a maximum indentation force of 1000 μN is higher in α₂ lamellae than in γ lamellae. Hardness variations were also observed between γ lamellae of different orientation. The shape of impressions left from indentations in γ lamellae deviate clearly from the shape of triangular Berkovich indenters. The irregular shape of these indents, as imaged with the atomic force microscope, indicate a strong anisotropic plastic deformation mode in γ domains. Pop-ins or yield-points, which mark the transition from elastic to plastic deformation, were observed frequently on γ lamellae and less frequently on α₂ lamellae. The modulus in the investigated <11-20> direction of α₂ lamellae is significantly lower than the modulus of γ lamellae. This corresponds with calculations of the indentation modulus from the elastic stiffness constants of these phases.     

Microstructural evolution in copper subjected to severe plastic deformation: Experiments and analysis

The evolution of microstructure and the mechanical response of copper subjected to severe plastic deformation using equal channel angular pressing (ECAP) was investigated. Samples were subjected to ECAP under three different processing routes: B_C, A and C. The microstructural refinement was dependent on processing with route B_C being the most effective. The mechanical response is modeled by an equation containing two dislocation evolution terms: one for the cells/subgrain interiors and one for the cells/subgrain walls. The deformation structure evolves from elongated dislocation cells to subgrains to equiaxed grains with diameters of ∼200–500 nm. The misorientation between adjacent regions, measured by electron backscatter diffraction, gradually increases. The mechanical response is well represented by a Voce equation with a saturation stress of 450 MPa. Interestingly, the microstructures produced through adiabatic shear localization during high strain rate deformation and ECAP are very similar, leading to the same grain size. It is shown that both processes have very close Zener–Hollomon parameters (ln Z ∼ 25). Calculations show that grain boundaries with size of 200 nm can rotate by ∼30° during ECAP, thereby generating and retaining a steady-state equiaxed structure. This is confirmed by a grain-boundary mobility calculation which shows that their velocity is 40 nm/s for a 200 nm grain size at 350 K, which is typical of an ECAP process. This can lead to the grain-boundary movement necessary to retain an equiaxed structure.     

Laboratory scale investigation into the corrosion of copper in a sulphur-containing environment

The effect of temperature and gas composition on the corrosion rate and corrosion by-product of copper foil was studied by exposing it to sulphur (S₂), S₂ + hydrochloric acid (HCl) and hydrogen sulphide. The temperature was varied from 80 to 140 °C. Copper foil reacted with S₂ to form CuS, Cu₉S₈ and Cu_1.8S. Corrosion rates ranged from 9.6 μm/h at 110 °C to 0.5 μm/h at 140 °C. The presence of HCl caused pitting and enhanced the corrosion rate above 112 °C. Cu₂S formed when copper was exposed to hydrogen sulphide gas. Sulphide scale that formed was friable and non-adherent.     

Microstructural and mechanical properties of friction stir welded aluminum/copper lap joints

This paper focuses on the microstructural and mechanical properties of the friction stir welding (FSW) of 1060 aluminum alloy to a commercially pure copper. A number of FSW experiments were carried out to obtain the optimum mechanical properties by adjusting the rotational speed and welding speed in the range of 750–1500 rpm and 30–375 mm/min, respectively. Various microstructures with different morphologies and properties were observed in the stir zone. The results indicated that Al₄Cu₉, AlCu and Al₂Cu are the main intermetallic compounds formed in the interfacial region. The effect of formation of hard and brittle intermetallic phase at the interface of the joints on the shear strength of the joint is discussed.     

Microstructural investigation and mechanical properties of dissimilar friction stir welded magnesium alloys

Abstract

Dissimilar joints of magnesium alloys were obtained by friction stir welding. Detailed microstructure and texture examinations were performed on the joints. Significant difference in microtexture distribution and microstructural features is observed between crown and stir zones in the dissimilar joints. However, an overall effect of these factors on mechanical properties of layered joints is not obvious as both the up and middle samples present quite similar yield strength and strain hardening behaviour. Moreover, both samples fracture in the Mg–3Al–1Zn alloy side. The fracture of the middle sample starts at the boundary of transition and stir zones, while the up sample starts in the crown zone side.     

Microstructural investigation of PGM‐based alloy coatings for oxidation protection

It is well known that Pt addition significantly improves the resistance of aluminide coatings to high‐temperature oxidation and hot corrosion, which has led to the widespread application of Pt modified aluminide coatings on the superalloy components of advanced gas turbine engines. Other platinum group metals (PGMs) such as Ir and Ru attract researchers for high temperature applications. In this study, oxidation properties of Pt‐Ir and Pt‐Ru based alloy coatings were investigated. Pt, Ru, and Ir were electroplated on a directionally solidified Ni‐base superalloy DZ125. The cyclic oxidation test revealed that both Pt‐Ir and Pt‐Ru alloys exhibited good oxidation performance. The effect of substrate alloy and coating compositions on microstructural changes during cyclic oxidation tests were discussed.     

The corrosion of copper as a tool in the investigation of the reaction kinetics

The copper electrode reaction kinetics have been studied in acid CuSO, solutions. Dissolved oxygen was used to remove the reaction intermediate Cu⁺ from the electrode. This shifted the equilibrium potentials of the two partial reactions in an opposite direction allowing a somewhat unusual interpretation of impedance measurements. The charge transfer resistance of the Cu²⁺ → Cu⁺ reaction was always higher than that of the Cu⁺ → Cu reaction. In the neighbourhood of the open circuit potential an additional “surface step” in the latter reaction made it rate-controlling. The slow diffusion of cuprous ions was manifested in an inductive impedance component in this region.     

A photo-electrochemical investigation of semiconducting oxide films on copper

p-type copper oxides formed by electrochemical and chemical methods are compared using voltammetry and photo-electrochemical techniques. Cupric and cuprous films exhibit characteristic photo-current signals, the sign and magnitude of which are a function of band curvature. Semi-conductor properties and aspects of the band structure of the surface films have been determined from observations of the spectral dependence of photo-assisted charge transfer. The positions of voltammetry peaks and the standard potentials of oxide formation are correlated with the flat band potentials of Cu₂O and CuO. The inhibition of copper oxidation by illumination and the corrosion of copper in solutions containing O₂ are discussed with regard to derived semiconductor models.     

Microstructure and texture investigation of chemically deposited copper coatings

Abstract

This work reports electron microscopy and X-ray investigations of the structure and texture of thin copper coatings that were chemically deposited (_chemCu) from trilonic electolytes. The relationships between the structure and texture of _chemCu, and the structure and texture of electrochemically deposited copper coatings have been established. It has been determined that chemically deposited copper coatings have very fine crystal structure, with areas of coherent scattering that are orders of magnitude smaller than their counterparts in matt copper coatings (_mattCu) that are electrochemically deposited from simple (non-complex) sulphuric-acid electrolyte. Additionally, in _chemCu, the axis of preferred orientation is in the <111> direction, while in _mattCu it is in the <110> direction. Alternatively, bright copper coatings (_brightCu) (electrodeposited from similar sulphuric acid electrolyte but with brightener additives), which are also characterised with extremely fine crystal structure have mixed <311> plus <110> texture, at a <311> dominant axis of preferable orientation. The values of microhardness of the copper coatings are arranged in the following sequence: HV_mattCu<HV_chemCu<HV_brightCu, opposite to the crystal size (D) and the internal stress (rms) in these three types of coatings – D_mattCu> D_chemCu>D_brightCu and rms_mattCu>rms_chemCu≧rms_brightCu. The data obtained for D and rms are in good agreement with the Hall–Petch rule on HV-grain size relationship.     

铜连续挤压的有限体积法数值模拟

对铜母线连续挤压过程的几何模型进行简化,基于MSC.Superforge软件平台,对铜母线连续扩展成形过程进行了数值模拟,确定了压下量为铜杆料直径的25%时为压实轮对铜杆压下量的最佳值,分析了铜连续挤压成形过程中坯料在挤压轮沟槽内的温度分布,指出在铜的连续挤压过程中,坯料的温度上升主要源于坯料的塑性变形.结果显示,铜坯料作用在腔体挡料块上的压力高达528～600 MPa,在坯料镦粗段前偶尔会发生折叠回流,这是造成连续挤压产品表面产生较大气泡和冷拔时断线质量缺陷的重要原因之一.为了避免金属流动造成的产品质量缺陷并提高腔体的使用寿命,应对腔体、挤压轮进行优化设计.