Evaluation of interfacial bonding in dissimilar materials of YSZ-alumina composites to 6061 aluminium alloy using friction welding

The interfacial microstructures characteristics of alumina ceramic body reinforced with yttria stabilized zirconia (YSZ) was evaluated after friction welding to 6061 aluminum alloy using optical and electron microscopy. Alumina rods containing 25 and 50 wt% yttria stabilized zirconia were fabricated by slip casting in plaster of Paris (POP) molds and subsequently sintered at 1600 °C. On the other hand, aluminum rods were machine down to the required dimension using a lathe machine. The diameter of the ceramic and the metal rods was 16 mm. Rotational speeds for the friction welding were varied between 900 and 1800 rpm. The friction pressure was maintained at 7 MPa for a friction time of 30 s. Optical and scanning electron microscopy was used to analyze the microstructure of the resultant joints, particularly at the interface. The joints were also examined with EDX line (energy dispersive X-ray) in order to determine the phases formed during the welding. The mechanical properties of the friction welded YSZ-Al₂O₃ composite to 6061 alloy were determined with a four-point bend test and Vickers microhardness. The experimental results showed the degree of deformation varied significantly for the 6061 Al alloy than the ceramic composite part. The mechanical strength of friction-welded ceramic composite/6061 Al alloy components were obviously affected by joining rotational speed selected which decreases in strength with increasing rotational speed.     

Microstructural impact on intergranular corrosion and the mechanical properties of industrial drawn 6056 aluminum wires

The influence of individual manufacturing steps during industrial wire drawing processes on the mechanical and corrosion properties of the 6056 aluminum alloy was investigated. These steps demonstrated an essential influence on the microstructure, and thus, the susceptibility to intergranular corrosion (IGC). No clear correlation between IGC susceptibility and hardness was observed. Although the highest resistance against intergranular attack was determined for those alloys in the solution annealed condition, pitting corrosion was identified to occur. Subsequent artificial aging of the solution annealed and quenched wires reintroduced IGC susceptibility; this phenomenon was attributed to the occurrence of galvanic coupling between the noble Cu-phases, located on the grain boundary, and the anodic grain boundary area.     

Exudation of Ag and Cu in internally oxidized Ag-Sn-In-(Cu) alloys

Growth and structure of pure Ag layers are investigated on the surface and at grain boundaries of Ag alloys that have been internally oxidized. Two Ag-Sn-In and an Ag-Sn-In-Cu alloy were studied by ion polishing, field emission scanning electron microscopy and optical microscopy. As already known from previous studies Ag atoms diffuse towards the surface. In the present study it is shown that grain boundaries can act as a sink for diffusing Ag atoms in competition with the surface. In the quaternary alloy Ag and Cu diffuse towards the surface simultaneously. The Cu diffusion is confined by the nucleation of CuO particles. The progress of the internal oxidation front is discussed in conjunction with the nucleation of pure Ag crystallites and CuO particles at grain boundaries.     

TEM investigations of intergranular stress corrosion cracking in austenitic alloys in PWR environmental conditions

Abstract

Analytical transmission microscopy has been used to investigate the initiation of stress corrosion cracking in Inconel 600 subjected to constant load testing under simulated pressured water reactor primary water conditions. The observations revealed that intergranular attack proceeded by the development of a zone of polycrystalline chromia along the boundary plane intersecting either the free surface or a blunted, open crack in contact with the free surface. Ni-rich metal particles were interspersed within the chromia. Conversely, open cracks were filled with nanocrystalline NiO and large compound particles of spinel and NiO, indicating a difference in potential between closed, attacked boundaries and open cracks. Open cracks appeared to have initiated by fracture of the chromia zones, such fracture being strongly dependent on boundary geometry with respect to loading direction. The observations suggest that stress corrosion crack initiation and propagation is dependent on diffusion of oxygen through the porous oxides. Dislocations and stress could enhance diffusion as chromia was observed along slip planes at the arrested tips of blunt cracks.     

Welding solidification cracking susceptibility and behavior of a Ni-28W-6Cr alloy

Welding solidification cracking of alloys is associated with the range of solidification temperature that can be greatly affected by the amount of refractory metals and other additives. In this work, solidification cracking of Ni-28W-6Cr alloy with high W content was studied by gas tungsten arc welding, showing that the welding current, alloying elements and precipitates all affect the cracking susceptibility. The lengths of cracks increase linearly with the welding current in the range from 150 to 250 A. The relatively high cracking susceptibility is mainly attributed to the high content of Si, which tends to segregate with other elements including W, Cr, Mn as films or components with low melting point in the last solidification stage and weaken the binding force of grain boundaries. Moreover, the existence of precipitated continuous eutectic M_6C carbides in the grain boundaries also acts as nucleation sites of crack initiation, and the cracks often propagate along solidification grain boundary.     

Filament formation during elevated temperature deformation of high purity ultrafine-grained aluminum

Ultrafine-grained aluminum materials were processed by hot isostatic pressing of aluminum nanopowders (99.7 wt.% purity). Quasi-static compression tests were carried out at a strain rate of 2 × 10^− 4 s^− 1 at 200 °C. Scanning electron microscopy investigations of fracture surfaces or cavities that were formed during straining reveal the presence of filaments. The number and dimensions of the filaments depend on the shielding effect of the native amorphous alumina film that forms on the surface of the nanoparticles in the starting powder. After crystallization of the amorphous, extensive filament formation is observed.     

Microstructural response on the cracking resistance of alloy 600

Precipitation of chromium rich carbides promotes the development of a Cr-depleted zone which in turn provided a weak path for the intergranular crack propagation. The role of low temperature anneals on the intergranular cracking resistance (IGC) of alloy 600 was investigated using modified wedge opening loading specimens heat treated at 930, 800 and 600 °C and exposed to high purity water pressurized with hydrogen at 300 °C. Mill annealing at 930 °C did lead to IGC susceptible microstructures. In this condition the alloy 600 exhibited the least crack growth rates (da / dt) of the order of 1.86 × 10^− 12 m/s and characterized the substantial work hardening ahead of the crack front. In contrast, annealing at 600 °C (HT600) resulted in increasing IGC susceptibilities. Under these conditions, crack growth rates, da / dt, as high as 7.10 × 10^− 10 m/s were found (HT600). Accordingly, significant interactions between the slip bands and the crack path lead to crack bifurcation into the slip planes and cavity formation.     

数值模拟在超级钢焊接中的应用

本文综述了超级钢焊接温度场的数值模拟方法及研究现状,以及超级钢焊接热影响区微观组织的预测方法及研究现状,论述了超级钢的强化机制及细晶原理。超级钢作为一种性能好、成本低的新型材料,有着广阔的应用前景,但其焊接工艺尚不完善。数值模拟方法能够准确的模拟超级钢焊接温度场,预测超级钢焊接热影响区微观组织,这对了解超级钢焊接机理有很大帮助,并为优化焊接工艺提供指导和依据。     

High temperature processing of Mg-Zn-Y alloys containing quasicrystal phase for high strength

A process to obtain high strength in a Mg-Zn-Y alloy containing quasicrystalline phase is described. The process involves solutionizing at a high temperature, precipitation of the quasicrystal phase during extrusion, followed by ageing. Tensile yield strengths of over 350 MPa are obtained with grain sizes of 14-20 μm.     

Structural variations in heat treated low alloy steel forgings

A study has been made of the microstructures of two low alloy Cr–Mo–Ni–V steel forgings in the quenched and tempered condition. Optical metallography of variously etched specimens, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) and X-ray diffraction were used to characterize the tempered martensite microstructures. Variations in prior austenite grain size and martensite lath structure have been related to the observed different carbide distributions. These arise from small differences in composition, mainly vanadium content, and the consequent response to heat treatment. The metallographic results can account for the differences in tensile and fracture toughness properties observed between the two materials as well as between the two ends of the same forging.     

Microscopic characterization of individual grain boundaries in Cu-III-VI2 chalcopyrites

The role of grain boundaries in polycrystalline Cu-III-VI₂ absorber material for thin film photovoltaics has not been fully understood and is currently under discussion. Recently, intensive efforts have been devoted to the characterization of the properties of individual grain boundaries using microscopic techniques, including Kelvin probe force microscopy (KPFM). KPFM provides local electronic information by measuring the surface potential in addition to the topography. We introduce the KPFM method and present simulations assessing the technique's limitations with respect to spatial resolution regarding the measurement of grain boundary properties. KPFM studies of individual GBs in the Cu(In,Ga)Se₂ materials system are reviewed and critically discussed, considering also results from other microscopic characterization techniques.     

Resistance to cleavage cracking and subsequent shearing of high-angle grain boundary

In a previous experimental study, it was observed that the break-through process of a cleavage front across a high-angle grain boundary can be highly nonuniform. While the central part of the boundary can be cleaved quite smoothly, the rest parts must be sheared apart. In this paper, the trapping effect of grain boundary shearing is analyzed in considerable detail. Before the shearing is completed, the crack flanks are locally pinned together and a bridging stress must be provided. The bridging stress has a negative contribution to the local stress intensity at the cleavage front segment that penetrates across the grain boundary, and thus the crack growth driving force must be increased. A closed-form equation is derived to relate the overall fracture resistance to the fracture mode through an energy analysis.     

Mechanical behavior and tensile/compressive strength asymmetry of ultrafine structured Ti–Nb–Ni–Co–Al alloys with bi-modal grain size distribution

Bulk ultrafine structured metallic materials with the bi-modal grain size distribution exhibit both high strength and good ductility. Here we show a new family of bi-modal Ti–Nb–Ni–Co–Al alloys. Their microstructure consists of an ultrafine structured eutectic matrix and relatively coarse β-Ti dendrites. Chemical modification of the parent Ti–Nb–Ni–Cu–Al system significantly affected type and volume fraction of matrix phases. In its turn, this influenced the deformation behavior. The unexpected “double yielding” behavior as well as tensile/compressive fracture strength asymmetry of the designed alloys are discussed in detail. For that, the microstructure alterations under compressive and tensile loading was in situ and ex situ analyzed by scanning electron microscopy.     

Evolution of precipitated phases during prolonged tempering in a 9%Cr1%MoVNb ferritic-martensitic steel: Influence on creep performance

Ferritic-martensitic steels of the 9%Cr1%Mo type have been extensively used in power plant components, heat exchangers, piping and tubing, etc., due to an excellent combination of properties such as creep resistance, toughness and resistance to oxidation at high temperatures. In these steels the stabilizing role of MX carbonitrides (M = Nb, V; X = C, N) is one of the main factors responsible for the resistance under creep conditions. The control of precipitation and coarsening of MX phases during prolonged, high temperature tempering or post-weld heat treatment is then a key point to obtain the desirable microstructure and hence, to achieve high temperature resistance under service conditions.In the present contribution we report the evolution of the precipitated phases during heat treatment at 780 °C for increasing times in the range 40 min to 7 h for an ASTM A213 T91 steel. The Nb and V contents in solid solution were determined as a function of the time of treatment and maxima were observed for 5 and 5.66 h, respectively. Creep tests to rupture were also conducted at 600 °C - 190 MPa for as-treated specimens. A maximum creep rate was observed to occur in coincidence with the maximum values of Nb and V contents in solid solution. We suggest possible relationships between the observed second phase evolution and the creep resistance behavior.     

Atomistic simulations of metallic microstructures

This paper reviews recent results in the simulation of the mechanical response of metallic microstructures at the atomic level. The role of the grain boundary network in deformation process is the concentration of this paper as studied by virtual tensile and nanoindentation tests. The grain boundary network is found to contribute to plastic deformation through the process of dislocation nucleation, absorption and transmission, as well as grain boundary accommodation mechanisms such as grain boundary sliding and migration. The microstructural grain boundary network is also critical to the nucleation and propagation of cracks. The challenges and opportunities in this area are discussed.     

The effect of test temperature on the intergranular cracking of Nb-A286 alloy in low cycle fatigue

The continuous low cycle fatigue behaviors of a Fe-base superalloy, Nb-modified A286 alloy, have been evaluated at the test temperatures of 650°C and 350°C under various total strain ranges. It was found that the change of the slope in the Coffin–Manson plot was closely related to the fatigue cracking with the test temperature. In the high temperature low cycle fatigue (HTLCF) of Nb-A286 alloy, the fatigue cracking exhibited the intergranular mode at 650°C and the transgranular mode at 350°C. The intergranular fatigue cracking at 650°C was due to the precipitation of the phase at the grain boundary assisted by the applied stress during low cycle fatigue. It is investigated whether the precipitate at the grain boundary provides the site for the grain boundary cavitation, which induces the intergranular cracking in low cycle fatigue. This is confirmed by the results of low cycle fatigue at 25°C after heat treatment which forms the phase at the grain boundary.     

Influence of processing parameters on lattice parameters in laser deposited tool alloy steel

Laser aided direct metal deposition (DMD) has been used to form AISI 4340 steel coating on the AISI 4140 steel substrate. The microstructural property of the DMD coating was analyzed by means of scanning electron microscopy, transmission electron microscopy and X-ray diffractometry. Microhardness of the DMD was measured with a Vickers microhardness tester. Results indicate that DMD can be used to form dense AISI 4340 steel coatings on AISI 4140 steel substrate. The DMD coating is mainly composed of martensite and retained austenite. Consecutive thermal cycles have a remarkable effect on the microstructure of the plan view of the DMD coating and on the corresponding microhardness distribution. Orientation relationships among austenite, martensite and cementite in the DMD coating followed the ones in conventional heat treated steels. As the laser specific energy decreased, cooling rate increased, and martensite peaks broadened and shifted to a lower Bragg's angle. Also martensite lattice parameters increased and austenite lattice parameters decreased due to the above parameter change.     

A comparison of crystallization behaviors of laser spot welded Zr–Cu–Ag–Al and Zr–Cu–Ni–Al bulk metallic glasses

A novel Ni-free Zr–Cu–Ag–Al ((Zr₄₈Cu₃₆Ag₈Al₈)Si_0.75) and a Zr–Cu–Ni–Al ((Zr₅₃Cu₃₀Ni₉Al₈)Si_0.5) bulk metallic glass (BMG), for comparison, were employed for Nd:YAG laser spot welding with three pre-selected energy inputs, including a low (6.2 J), a medium (8.0 J) and a high (9.2 J) energy input. After the welding process, the microstructure evolution, glass-forming ability (GFA) and mechanical properties of the welded samples were determined by a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and the Vicker's microhardness test.     

Metallurgical characterization of pulsed current gas tungsten arc,friction stir and laser beam welded AZ31B magnesium alloy joints

This paper reports the influences of welding processes such as friction stir welding (FSW), laser beam welding (LBW) and pulsed current gas tungsten arc welding (PCGTAW) on mechanical and metallurgical properties of AZ31B magnesium alloy. Optical microscopy, scanning electron microscopy, transmission electron microscopy and X-Ray diffraction technique were used to evaluate the metallurgical characteristics of welded joints. LBW joints exhibited superior tensile properties compared to FSW and PCGTAW joints due to the formation of finer grains in weld region, higher fusion zone hardness, the absence of heat affected zone, presence of uniformly distributed finer precipitates in weld region.