Microstructure evolution of directionally solidified Ti-45Al-8.5Nb-(W, B, Y) alloys

Intermetallic Ti-45Al-8.5Nb-(W, B, Y) alloys were directionally solidified at constant growth rates (V) ranging from 10 to 400 μm/s under the temperature gradient G = 3.8 × 10³ K/m. Quenching was performed at the end of directional solidification (DS) experiments. Microstructure evolution was investigated by analyzing the microstructures formed at the quenching interfaces and in the DS regions. The primary dendritic arm spacing (λ) decreases with increasing growth rate according to the relationship λ ∝ V^−0.36. Both the width of columnar grain (λw) and the interlamellar spacing (λ_s) decrease with increasing growth rate according to the relationships λw∝V−1.13 and λ_s ∝ V^−0.32, respectively. Lamellar microstructure initially disappears from the dendrites at the growth rate of 100 μm/s and subsequently from the interdendritic regions when the growth rate is up to 200 μm/s. The B2 particles can precipitate in the interdendritic regions.     

Formation and characterization of aluminide coatings on alloy 800 substrate

Fe-Ni-Cr based Alloy 800 substrates were pack aluminized at 1273 K for 8 hours in argon atmosphere. The cross-section of aluminized substrates indicated formation of multilayer of aluminides as revealed by microscopy, electron probe microanalysis and X-ray diffraction. The outermost layer (~ 150 μm) was found to consist of FeAl + Fe₂Al₅ type phases, while the adjoining layer (~ 250 μm) was composed of an FeAl type phase. The innermost layer (~ 60 μm), which was a solid solution zone, was found to consist of ~ 43 at.% Fe, 38 at.% Cr, 11 at.% Ni containing about 6 at.% Al. In microhardness test, a hardness variation of 213 to 1098 Knoop hardness number along the cross-section was obtained. Scratch test along the cross-section at load levels ranging from 0.9 to 10 N with a loading rate of 30 N/min showed a maximum penetration depth of 12 μm indicating a good adherence of aluminide coatings.     

Influences of different prior heat treatments on the microstructural and mechanical properties of Cu-Fe filamentary composites

Cu-6 wt.% Fe and Cu-12 wt.% Fe filamentary composites were prepared by casting and cold drawing. And a different heat treatment of quenching and aging or homogenizing was introduced before cold drawing process, respectively. The microstructure was observed and the tensile strength measured for the composites at different drawing strains. The quenching and aging or homogenizing prior to drawing deformation refine the as-cast microstructure and result in the increase in interface density in the drawn microstructure. The drawn alloys with the homogenizing treatment show smaller filament spacing than those with the quenching and aging treatment because homogenizing results in smaller and more dispersive primary Fe dendrites before drawing deformation. The heat treatments can improve the strength of the composites by increasing precipitation strengthening and interface strengthening levels. With the reduction in filament spacing during drawing deformation, the strength of the alloys with smaller initial size of Fe dendrites increases more obviously.     

低碳钢表面激光熔覆不锈钢的组织和性能

采用JHM-1GY-400型脉冲Nb∶YAG固体激光器和316L不锈钢粉末在20低碳钢表面制备了激光熔覆层。利用OM、XRD、SEM等表征方法分析了不锈钢熔覆层的物相组成和显微组织,并分别利用旋转摩擦试验机和电化学工作站对熔覆层和基材的耐磨损和耐腐蚀性进行了研究。试验结果表明,不锈钢熔覆层厚度约为50 μm,由γ相(奥氏体)和α相(铁素体)组成,其显微组织主要包括细小的树枝晶、粗大的胞状晶以及平面晶;不锈钢熔覆层表面硬度约为基材的2倍,摩擦因数比基材低0.0418,磨损量更低,不锈钢熔覆层比基材具有更高的耐磨性。与基材相比,不锈钢熔覆层具有更低的自腐蚀电流和更高的自腐蚀电位,其耐腐蚀性能更优异。     

Effects of microstructure on native oxide scale development and electrical characteristics of eutectic Cu-Cu6La alloys

A combination of electron microscopy, focused ion beam and conductive atomic force microscopy techniques have been used to study the microstructure, oxide scale development, and electrical behavior of a Cu-9 at.% La alloy. The as-cast alloy exhibits a eutectic microstructure comprising 30 vol.% Cu rods in a Cu₆La matrix. The eutectic colonies exhibit a singular orientation relationship with [0 1 0] Cu₆La parallel to 〈0 1 1〉 Cu along the rod axis, and it is shown that this corresponds to lattice matching of the two phases along this direction (∼0.02% misfit). Oxidation of the alloy at 100 °C to accelerate formation of a native oxide scale led to the development of a Cu₂O layer less than 25 nm thick on the Cu rods and a La-doped Cu₂O scale up to 1 μm thick on the Cu₆La matrix. The La-doped regions of the scale are more conductive despite being much thicker, which is consistent with previous contact resistance data obtained for this alloy. The mechanisms responsible for the formation of this non-equilibrium oxide scale structure and for the enhanced electrical conductivity of the La-doped regions are discussed.     

Influence of time on the microstructure of AISI 321 austenitic stainless steel in salt bath nitriding

Influence of nitriding time on the microstructure and microhardness of AISI 321 austenite stainless steel was investigated, using a complex salt bath heat-treatment at low temperature, 430 °C. Experimental results revealed that after salt bath nitriding, a modified layer was formed on the surface of substrate with the thickness ranging from 2 μm to 30 μm with changing treating time. The nitrided layer depth thickened extensively with increasing nitriding time. The growth of the nitrided layer takes place mainly by nitrogen diffusion according to the expected parabolic rate law. Scanning electron microscopy and X-ray diffraction showed that in 321 stainless steel subjected to complex salt bathing nitrided at such temperature for less than 8 hours, the main phase of the nitrided layer was expanded austenite (S phase) by large. When the treatment time is prolonged up to 8 hours and more, S phase is formed and subsequently transforms partially into CrN, and then the secondary CrN phase precipitated. With treating time prolonged, more CrN precipitates formed along the grain boundaries in the outer part. In the inside part between the some CrN and the substrate, there is still a broad single S phase layer. All treatments can effectively improve the surface hardness.     

The oxidation of two ternary Fe-Cu-10 at.% Al alloys in 1 atm of pure O2 at 800-900 °C

The oxidation of two ternary Fe-Cu-Al alloys containing 10 at.% Al (Fe-65Cu-10Al and Fe-30Cu-10Al) has been studied at 800-900 °C under 1 atm O₂. Under all conditions both alloys show an initial faster stage during which Fe-65Cu-10Al corrodes more rapidly at 800 °C than at 900 °C, while Fe-30Cu-10Al follows nearly identical kinetics at both temperatures. As oxidation proceeds, a continuous alumina layer is eventually established on the surface of the two alloys, thus decreasing significantly their oxidation rates. Altogether, the Fe-rich alloy Fe-30Cu-10Al oxidizes slightly faster than the Cu-rich alloy Fe-65Cu-10Al at both temperatures. The possible reasons for the decrease in the critical Al content needed to form external alumina scales for the Cu-rich alloy in comparison with binary Cu-Al alloys are examined.     

Microstructure, residual stresses and shear strength of diamond-steel-joints brazed with a Cu-Sn-based active filler alloy

Brazing of diamonds is important in grinding technology. The brazing parameters can strongly influence the grinding tool's performance. In this work a Cu-Sn-based active filler alloy (73.9 Cu-14.4 Sn-10.2 Ti-1.5 Zr, wt.%) was applied to join monocrystalline block-shaped diamonds onto a stainless steel substrate using three different brazing temperatures (880, 930 and 980 °C) and two different dwell times (10 and 30 min), respectively. The characteristics of the joints were investigated by means of scanning electron microscopy and energy dispersive X-ray spectroscopy (microstructure and phase composition), by Raman-spectroscopy (residual stress) as well as by shear testing (bond strength). The microstructural investigations revealed an intermetallic interlayer of type Fe₂Ti at the steel-filler alloy interface, which grew with increasing brazing temperatures and longer dwell durations. The brazing parameters strongly affected the residual stresses in the diamond. Compressive residual stresses with a maximum value of − 350 MPa were found in the samples brazed at 880 and 930 °C, whereas tensile stresses of maximum + 150 MPa were determined in samples joined at 980 °C. The effect of the brazing parameters on the shear strength is very pronounced. The shear strength decreased from (321 ± 107) MPa at 880 °C, 10 min to (78 ± 30) MPa at 980 °C, 30 min.     

Process of electroless deposition of zinc and lead on aluminum in electrolyte melt by contact exchange method

The mechanism of electroless deposition of lead and zinc and their codeposition on an aluminum substrate in a chloride electrolyte melt at temperatures below the melting point of aluminum has been studied. It is shown that lead forms a dense thick (up to 60 μm) porousless layer on aluminum during the contact exchange, whereas zinc coatings are not more than 30 μm thick and have a number of pores a few nm to 1 μm in diameter penetrating to the aluminum substrate. When lead and zinc are deposited simultaneously, the whole aluminum surface is covered with a two-phase coating lead precipitates as round spots with zinc between them; areas with deposited lead are free of zinc and vice versa.     

Structure and properties of nanostructured A357 alloy produced by melt spinning compared with direct chill ingot

The properties of nanostructured A357 ribbons produced by melt spinning were investigated using field emission gun scanning electron microscope, X-ray diffraction pattern, differential scanning calorimetry and microhardness testing in comparison with those fabricated by direct-chill (DC) casting. The solidification time and cooling rate of 46 μm thick melt-spun ribbon were estimated to be 9.13 × 10⁻⁶ s and 1.17 × 10⁷ K s⁻¹, respectively. The results show that the nanostructure of A357 ribbons exhibits the enhanced solid-solubility of Si in Al matrix to 2.00 wt.% and the existence of ultra-fine and homogenous dendritic structure having a dendrite arm spacing of about 200 nm. The nano-sized spherical eutectic Si crystals having a size of 50 nm were also observed. All these structural factors increase the microhardness of the ribbon which is twice as high as that of DC casting.     

Elastic properties of Ni and Ni + Mo coatings electrodeposited on stainless steel substrate

Adhesion coefficient and Young's modulus of Ni and Ni + Mo coatings electrochemically deposited on stainless steel were examined by applying vibrating reed technique. It was shown that adhesion coefficient of the Ni coating slightly decreases (about 8%) with increasing layer thickness (5-40 μm). Young's modulus E_f of these coatings at room temperature was found to be about 130 GPa. The relative adhesion coefficient of the Ni layer decreases with increasing temperature (300-600 K) in relation to the thinnest examined layer (5 μm). Young's modulus of the Ni + Mo coatings decreases with increasing Mo content; for 9 wt.% of Mo E_f = 40 GPa and for 32 wt.% of Mo E_f = 23 GPa.     

Interfacial reactions and mechanical properties between Sn-4.0Ag-0.5Cu and Sn-4.0Ag-0.5Cu-0.05Ni-0.01Ge lead-free solders with the Au/Ni/Cu substrate

Sn-4.0Ag-0.5Cu (SAC) and Sn-4.0Ag-0.5Cu-0.05Ni-0.01Ge (SACNG) lead-free solders reacting with the Au/Ni/Cu multi-layer substrate were investigated in this study. All reaction couples were reflowed at 240 and 255 °C for a few minutes and then aged at 150 °C for 100-500 h. The (Cu, Ni, Au)₆Sn₅ phase was formed by reflowing for 3 min at the interface. If the reflowing time was increased to 10 min, both (Cu, Ni, Au)₆Sn₅ and (Ni, Cu, Au)₃Sn₄ phases formed at the interface. The AuSn₄ phase was found in the solder for all reaction couples. An addition of Ni and Ge to the solder does not significantly affect the IMC formation. After a long period of heat-treatment, the thickness of the (Cu, Ni, Au)₆Sn₅ and (Ni, Cu, Au)₃Sn₄ phases increased and the intermetallic compounds (IMCs) growth mechanism obeyed the parabolic law and the IMC growth mechanism was diffusion-controlled. The mechanical strengths for both the soldered joints decreased with increasing thermal aging time. The SACNG/Au/Ni/Cu couple had better mechanical strength than that in the SAC/Au/Ni/Cu couple.     

Effect of laser cladding process parameters on clad quality and in-situ formed microstructure of Fe-TiC composite coatings

Over the past decade, researchers have demonstrated interest in tribology and prototyping by the laser cladding process. In-situ laser cladding enables the formation of a uniform clad by melting the powder to form desired composition from pure powder component.In this research pure Ti, graphite, and Fe with max particle sizes of 40 μm (0.04 mm) are used for in-situ laser cladding on a steel substrate. The effects of laser parameters on the quality of an in-situ formed TiC-Fe based composite clad are investigated. Laser parameters have an important role in clad quality and crack formation. They affect the bonding between clad/substrate and cooling rate. Diverse microstructures have been detected in the clad. Finally a model is developed in order to explain the formation and morphology of TiC. The melting and solidification stages of TiC formation and matrix confirm the suggested model.TiC particles increase the clad hardness to an average of four times greater than substrate's hardness. Experimental methods such as, XRD and SEM are used for phases characterization.     

An investigation on the microstructure and oxidation behavior of laser remelted air plasma sprayed thermal barrier coatings

NiCrAlY bond-coat was coated on Inconel 718 substrate by air plasma spraying (APS) followed by APS ZrO₂-8 wt.%Y₂O₃ as top-coat. Using CO₂ laser of different energy densities, ceramic top-coat surface was remelted. Laser remelting with high energy density (4 J/mm²) produced a dense microstructure over the whole thickness of top-coat, while low energy density (0.67 J/mm²) laser remelting produced a ~ 50 μm thick dense layer on the top-coat surface. It was found that the volume fraction of monoclinic phase decreased from 9% in as-sprayed coating to 4% and 3% after laser remelting with high and low energy density respectively. After isothermal oxidation at 1200 °C for 200 h, the thickness of oxide layer (TGO) in the sample produced by low energy density laser remelting was ~ 5.6 μm, which was thinner than that of oxide layer in as-sprayed (~ 7.6 μm) and high energy density laser remelted (~ 7.5 μm) samples. A uniform and continuous oxide layer was found to develop on the bond-coat surface after low energy density laser remelting. Thicker oxide layer containing Cr₂O₃, NiO and spinel oxides was observed in both as-sprayed and high energy density laser remelted coatings. After cyclic oxidation at 1200 °C for 240 h, the weight gain per unit area of as-sprayed coating was similar to that of high energy density laser remelted coating while a significantly smaller weight gain was found in low energy density laser remelted coating.     

Influence of rotating magnetic field on the microstructure and phase content of Ni-Al alloy

Rotating magnetic field is introduced in the production process of Ni-Al precursor alloy of skeletal Ni catalyst. The results showed that the big dendrites of Al₃Ni₂ disappeared, the size of Al₃Ni₂ decreased from 64.5 μm to 37.2 and 35.5 μm, phase content of Al₃Ni₂ decreased while Al₃Ni increased after applying field current of 80 A and 140 A, respectively. The change of phase content is probably caused by the increase of surface area between the Al₃Ni₂ phase and fluid which is favorable to the peritectic reaction.     

Slurry based multilayer environmental barrier coatings for silicon carbide and silicon nitride ceramics — II. Oxidation resistance

In part I of this study, the dip-coat processing of mullite/gadolinium silicate (Gd₂SiO₅) environmental barrier coatings (EBCs) applied on α-SiC and SN282™ Si₃N₄ through alcohol based and sol based slurries was presented. Here, the performance of selected EBCs by evaluating their oxidation resistances during thermal cycling in simulated combustion (90% H₂O-balance O₂) environment between 1350 °C and RT for up to 400 cycles is being reported. Oxidation of un-coated α-SiC was severe, leading to aligned and layered porous silica scale formation (~ 17 μm thick) on its surface with frequent scale spallation when exposed to 100 cycles. Mullite/Gd₂SiO₅/B₂O₃ (83.5/11.5/5 wt.%) EBCs remained adherent to α-SiC substrate with an underlying porous silica layer formed at substrate/coating interface, which was ~ 12 μm after 100 cycles, ~ 16 μm after 200 cycles, and ~ 25 μm after 400 cycles. In contrast, α-SiC substrate coated with mullite/Gd₂SiO₅ (88/12 wt.%) EBC had only limited oxidation of ~ 10 μm even after 1350 °C/400 cycles. The sol based mullite/Gd₂SiO₅ (88/12 wt.%) EBC on α-SiC substrate after 400 cycles was adherent, but showed more interfacial damages (~ 20 μm after 400 cycles) though it had increased coating density. However, the mullite/Gd₂SiO₅ (88/12 wt.%) EBC (alcohol based) delaminated from the SN282™ Si₃N₄ substrate after 1350 °C/100 cycles, because of the formation of interconnected interfacial voids and hairline cracks. Parabolic growth kinetics for the underlying silica was observed for both the alcohol and sol based coated samples.     

Elucidating the microstructural and tribological characteristics of NiCrAlCoCu and NiCrAlCoMo multicomponent alloy clad layers synthesized in situ

High-entropy alloys have received considerable attention owing to their unique structures and properties. In this work, two multicomponent alloy clad layers were synthesized from two equimolar powder mixtures of NiCrAlCoCu and NiCrAlCoMo via an in situ reaction on the surface of the AISI 1050 medium carbon steel using the gas tungsten arc welding (GTAW) method. The microstructure, constituent phases and tribological properties of the clad layers were examined under a field-emission scanning electron microscope (FESEM), an X-ray diffractometer (XRD) and a pin-on-disc rotating tribometer. Experimental results indicate that the microstructure of the NiCrAlCoMo clad layer comprises AlFe_0.23Ni_0.77, Co₆Mo₆C₂ and Fe₆₃Mo₃₇. The NiCrAlCoCu clad layer has a simple microstructure that consists of only BCC and FCC solid solution phases, with lattice constants of a = 0.288 nm and a = 0.362 nm, respectively. In the NiCrAlCoMo clad layer, the complex geometric effect caused by the vein-shaped reinforcement results in strong mechanical interlocking, which can prevent detachment of the reinforcement during rubbing. Therefore, the wear performance of the NiCrAlCoMo clad layer greatly exceeds that of the NiCrAlCoCu clad layer.     

Effect of periodic in situ deposition of silica layer on the long term operation of a micro-gas turbine

Tetraethylorthosilicate (TEOS) diluted in methanol was periodically supplied during operation of a 13 kgf (max. thrust)-class small gas turbine to deposit silica layers on the turbine blades and its effect on the degradation of the turbine blades upon the long term cyclic operation (1 cycle: 30/10 min run/pause at 20,000 rpm) was tested. The exhaust gas temperature (EGT) was set at 800 °C. For comparison, two other gas turbines were also tested without feeding TEOS at the EGT of 800 °C and 700 °C, respectively. The blades were inspected optically and metallographically before and after the test. The supply of TEOS produced a white silica layer indicating a very low level of the apparent density all around the surface of the blades, some of which became dense at the end to a thickness of 5-10 μm. The underlying substrate Inconel 713 was effectively protected, while the blades operated at the EGT of 800 °C, but not in situ coated, were severely degraded in terms of the surface oxidation and microstructural evolution of the substrate. The microstructure of the protected substrate was comparable with that of the unprotected but operated at the EGT of 700 °C. This result was discussed based on the effect of the formation of dense silica layer on the blade by comparing with the previously obtained results.     

Microstructural evolution of twin-roll cast Mg–3Al–0.5Mn–0.2Mm alloys during warm rolling and subsequent annealing

Microstructure of twin-roll casted (TRC) Mg–3Al–0.5Mn–0.2Mm (or AM30 + 0.2Mm) alloy strips consisted of columnar dendrites in the surface and equiaxed dendrites in the center regions, as well as widely dispersed fine primary intermetallic compounds located in the interdendritic region. Warm rolling of the TRC strips developed both deformation or shear bands and homogeneously dispersed fine particles. No evident dynamic recrystallization (DRX) was found in the TRC sheets during warm rolling. The dispersed fine particles seemed to retard DRX. The warm-rolled TRC sheets showed equiaxed fine grains with an average size of around 8 μm after annealing at 350 °C for 60 min. The TRC sheets had superior yield and tensile strengths to ingot cast (IC) samples. Elongation was similar to both TRC and IC samples.     

A high thermal gradient directional solidification method for growing superalloy single crystals

The experiments here were conducted at withdrawal rates of 3 mm/min and 1 mm/min using a CMSX-6 and a CMSX-4 superalloy, respectively. The process was assessed in terms of the thermal gradient (G_L), structural refinement, microsegregation and porosity distribution, and compared to those using a Bridgman process. The G_L of the process was 200–236 K/cm, which was 10–12 times higher than that in the Bridgman process. A more refined microstructure was produced having average primary and secondary dendrite arm spacing values as low as 243 μm and 72 μm, as well as 272 μm and 76 μm in the CMSX-6 and the CMSX-4 castings, respectively. The diameter of γ′ phase in the dendrite core of CMSX-6 and CMSX-4 castings was reduced from 0.8 μm to 0.3 μm and from 1.2 μm to 0.6 μm, respectively. The average areas of (γ′ + γ) eutectic pools became smaller and more homogeneously distributed. The mean pore sizes in the castings were reduced by 57% and 43% for the CMSX-6 and CMSX-4 superalloys, respectively, and the area fractions of the pores in the CMSX-6 and CMSX-4 samples were 16% and 12% of those produced in the Bridgman samples. The segregation coefficients of the major alloying elements were closer to unity than those in the Bridgman process, which indicates that the composition distribution is more uniform.