The oxidation behavior of gas-atomized Al and Al alloy powder green compacts during heating before hot extrusion and the suggested heating process

The oxidation behavior of gas-atomized Al and Al alloy powder green compacts during heating prior to hot extrusion compaction was studied at laboratory and industrial scales by TGA, DSC, DTA, EDX, TEM and XRD methods. The effect of the heating of green compacts on the mechanical properties of the powder-extruded samples was assessed. Significant oxidation of Al and Al alloy powder green compacts takes place in the solid state during heating in air. The onset and intensity of oxidation were affected by the Mg content, the surface area of the powder and the volume of the powder green compacts. An exothermic heat associated with the oxidation of Al and Al alloy powders resulted in intense overheating of bulky powder green compacts during heating in air. The samples extruded from the powder green compacts heated in air exhibited reduced strength. The loss in strength was especially pronounced in the case of Mg-containing Al alloy powders. Mg diffuses from a powder metallic core toward the native Al₂O₃ surface layer present on as-atomized Al alloy powders; it reacts with oxygen present in air and in the Al₂O₃ surface layer where the MgO phase forms, eventually resulting in the depletion of Mg from the powder core. Materials extruded from Al powders depleted of Mg do not exhibit effective Al-Mg solid solution strengthening or strengthening by Mg-containing precipitates. Economically viable approaches to avoiding the detrimental effects of powder oxidation during the heating of green compacts prior to hot working consolidation are discussed.     

Improvement of thermally grown oxide layer in thermal barrier coating systems with nano alumina as third layer

A thermally grown oxide (TGO) layer is formed at the interface of bond coat/top coat. The TGO growth during thermal exposure in air plays an important role in the spallation of the ceramic layer from the bond coat. High temperature oxidation resistance of four types of atmospheric plasma sprayed TBCs was investigated. These coatings were oxidized at 1000 °C for 24, 48 and 120 h in a normal electric furnace under air atmosphere. Microstructural characterization showed that the growth of the TGO layer in nano NiCrAlY/YSZ/nano Al₂O₃ coating is much lower than in other coatings. Moreover, EDS and XRD analyses revealed the formation of Ni(Cr,Al)₂O₄ mixed oxides (as spinel) and NiO onto the Al₂O₃ (TGO) layer. The formation of detrimental mixed oxides (spinels) on the Al₂O₃(TGO) layer of nano NiCrAlY/YSZ/nano Al₂O₃ coating is much lower compared to that of other coatings after 120 h of high temperature oxidation at 1000 °C.     

The effect of native Al2O3 skin disruption on properties of fine Al powder compacts

In the presented study we characterize how various powder metallurgical routes (extrusion, forging, and HIP/sintering) affect the fracture of native Al₂O₃ layer present on the surface of ultra-fine atomized Al powders. It is shown that the different distribution, morphology and interconnectivity of in situ introduced Al₂O₃ dispersoids strongly affect the thermal stability and mechanical and thermal properties of subsequent powder compacts.     

添加氧化镧的纳米Al2O3-13%TiO2等离子喷涂涂层高速摩擦磨损性能

以添加了少量氧化镧的团聚纳米Al2O3-13%TiO2粉末为原料,利用等离子喷涂技术制备了纳米陶瓷涂层。在MMS-1G型高速摩擦磨损试验机进行了摩擦磨损试验,利用扫描电镜和能谱仪对磨损表面进行了表征。结果表明:涂层组织呈现出典型的层状结构特征,界面结合良好。在高速摩擦磨损试验中,随着载荷的增加,涂层摩擦因数下降,而涂层微裂纹扩展引起涂层剥落,导致磨损率升高。     

The effect of compaction temperature and pressure on the pores in nanostructured metal compacts prepared by magnetic pulsed compaction

The pore behavior of nano Al compacts prepared by magnetic pulsed compaction in a temperature range from 20°C to 300°C and under pressure of 0.7 and 1.6 GPa was studied by small angle neutron scattering (SANS) analysis. The size distribution and volume fraction of pores in the compact were determined by corrected scattering curves using a direct model fitting, under the assumption that the pores were spherical in shape. The densities obtained from the SANS analysis were well matched with those of direct measurement in the range of 76–95% of theoretical density. As the compaction temperature increased from 20°C to 300°C, the size of pores increased from 1.69 to 2.31 nm; however, the number of pores decreased from 169 to 71 (1/cmŲ) in the case of 1.6 GPa compaction pressure. As a result, the density increased as the compaction temperature increased.     

Wear and oxidation resistance of Al2O3 particle dispersed Al3Ti composite with a nanostructure prepared by pulsed electric current sintering of mechanically alloyed powders

Al₃Ti-matrix composite layers containing Al₂O₃ particles were formed on Ti substrate by pulsed electric current sintering (PECS) of mechanically alloyed (MA) powders to improve the wear and oxidation properties of the Ti substrate. Reducing the grain size of each element by MA makes the combustion synthesis of Al₃Ti possible at a lower temperature. The grain size formed by the combustion synthesis of Al–Ti–Al₂O₃ powder mechanically alloyed for 720 ks was about 10 nm and its growth during sintering was suppressed by the existence of Al₂O₃. The densification behavior of the powder was investigated quantitatively. The obtained Al₃Ti/Al₂O₃ composite layer showed better wear and oxidation resistance than the monolithic Al₃Ti layer.     

The mechanical milling of Al/TiO2 composite powders

The processing of Al/TiO₂ composite powders produced by high-energy mechanical milling leads to production of a range of valuable, titanium-based materials. They include Ti(Al,O)/Al₂O₃ and Ti_xAl_y(O)/Al₂O₃ composite powders, bulk composites and Ti₃Al/TiAl alloy powders, and corresponding bulk materials. The strength of the Ti(Al,O)/Al₂O₃ and Ti_xAl_y(O)/Al₂O₃ composites is moderate, but their high-temperature oxidation resistance is exceptionally high, making the titanium-based composite powders favorable feedstock materials for protective coatings. The hardness of the Ti(Al,O)/Al₂O₃ and Ti₃Al(O)/Al₂O₃ composites is also very high (10–16 GPa). For more information, contact D.L. Zhang, University of Waikato, Waikato Centre for Advanced Materials, Department of Materials and Process Engineering, Private Bag 3105, Hamilton, New Zealand; 011-64-7-838-4783; fax 011-64-7-838-4835; e-mail d.zhang@waikato.ac.nz.     

The Effect of Cr on the Oxidation of Ni–10 at% Al in 1 atm O2 at 900–1000°C

The oxidation of three Ni–xCr–10Al alloys with a constant Al content of 10 at% and containing 3, 5, and 10 at% Cr was investigated at 900–1000°C in 1 atm of pure oxygen and compared to the behavior of Ni–10Al. At both temperatures, an external NiO scale overlying a zone of internal-oxide precipitates formed on Ni–10Al and Ni–3Cr–10Al: in addition, a discontinuous Al₂O₃ layer formed at the front of the internal oxidation for Ni–3Cr–10Al. An exclusive external scale of Al₂O₃ formed at most places on Ni–5Cr–10Al at 900°C, while, at some sites, the same alloy formed an outer NiO layer overlying an internal oxidation zone. The scales formed on Ni–5Cr–10Al at 1000°C were complex, but eventually a protective Al₂O₃ layer developed either at the alloy surface or beneath a region containing a mixture of different oxides. Finally, an exclusive external Al₂O₃ layer formed on Ni–10Cr–10Al at both temperatures. Thus, the addition of sufficient chromium to Ni–10Al produced a classical third-element effect, inducing the transition between internal and external oxidation of aluminum under a constant Al content. A possible mechanism for the effect of chromium on the oxidation of Ni–10Al is discussed on the basis of an extension to ternary alloys of a criterion first proposed by Wagner for the transition between internal and external oxidation of the most-reactive component in binary alloys.     

AZ31镁合金包埋渗Al涂层的显微组织与性能

采用包埋渗法在AZ31镁合金表面制备Al涂层.利用X射线衍射(XRD)、背散射电子成像(BSEI)和能谱仪(EDS)等技术对Al涂层的物相、显微组织和成分进行表征分析.结果表明,包埋粉的成分对涂层的物相、显微组织和厚度有显著影响.对于以AlCl3粉末为活化剂的样品,涂层较厚,涂层从表面到内部由呈梯度分布的相和组织构成,...     

Fabrication of High Performance PM Nanocrystalline Bulk AA2124

AA2124 nanopowders <100 nm in particle size and 20 nm internal structure produced by high energy ball milling of gas-atomized micronpowders ~45 μm in particle size and 700 nm internal structure were processed in to bulk rods. The micro- and nanopowders were hot compacted using uniaxial pressing for preliminary densification at 0.7T _m of the alloy. Selected intact hot compacts (HCs) were promoted for warm severe plastic deformation via equal channel angular pressing (ECAP) at the minimum possible deforming temperature for final densification. Effect of the fabrication method of the consolidated powders was investigated. A combined processing via HC/ECAP produced bulk nanostructured rods 2.5 μm and 50-60 nm in grain size for the micro- and nanopowder consolidates, respectively. The powder properties controlled the degree of densification and mechanical behavior during the hot compaction stage, which influenced strongly the deformation behavior during subsequent ECAP. At the end of HC/ECAP one pass, the HC stage was responsible for about 83 and 95% of the total grain coarsening encountered for the micro- and nanopowder HCs, respectively. Throughout the various consolidation stages employed, the ball-milled (BM) nanopowder consolidates exhibited 2/3 the grain growth and displayed almost twice the hardness and compressive strength values of the gas-atomized micronpowder ones. Influence of BM and HC on the Al₂O₃ layer formed around the individual powder particles was also investigated.     

Optimization of Composition in Ni(Al)-Cr2O3 Based Adaptive Nanocomposite Coatings

A promising Ni(Al)-Cr₂O₃-Ag-CNT-WS₂ self-lubricating wear-resistant coating was deposited via atmospheric plasma spray of Ni(Al), nano Cr₂O₃, nano silver and nano WS₂ powders, and CNTs. Feedstock powders with various compositions prepared by spray drying were plasma sprayed onto carbon steel substrates. The tribological properties of coatings were tested by a high temperature tribometer in a dry environment from room temperature to 400 °C, and in a natural humid environment at room temperature. It was found that all nanocomposite coatings have better frictional behavior compared with pure Ni(Al) and Ni(Al)-Cr₂O₃ coatings; the specimen containing aproximately 7 vol.% Ag, CNT, and WS₂ had the best frictional performance. The average room temperature friction coefficient of this coating was 0.36 in humid atmosphere, 0.32 in dry atmosphere, and about 0.3 at high temperature.     

Oxidation of Cr2AlC Between 900 and 1200 °C in Air

High-purity, dense Cr₂AlC compounds were synthesized by hot pressing a mixture of CrC _X (x = 0.5) and Al powders. Oxidation at temperatures between 900 and 1200 °C in air for up to 480 h resulted in the formation of a thin Al₂O₃ layer. The consumption of Al to make the Al₂O₃ layer led to the enrichment of Cr immediately below the Al₂O₃ layer, resulting in the formation of an underlying Cr₇C₃ layer. At the same time, carbon escaped from the Cr₂AlC into the air. During the initial stage of oxidation, oxygen diffused inward to form the Al₂O₃ layer, which vastly improved the oxidation resistance of Cr₂AlC from the initial stage of oxidation.     

Effect of nano Al (Scx−1Zrx) precipitates on the mechanical and corrosion behavior of Al‐2.5 Mg alloys

Microanalytical, mechanical, and corrosion studies were undertaken to investigate the effect of nano‐precipitates of Al(Sc_x−1Zr_x) on the mechanical and corrosion characteristics of Al 2.5 alloy containing 0, 0.15, 0.3, 0.6, and 0.9 wt% of Sc with 0.15 wt% Zr. Addition of 0.3% Sc significantly increased the yield strength due to small precipitates sizes (5–19 nm) and the high coherency of the nano‐particles. Largest contributor to the strength was grain boundary strengthening caused by pinning of grain boundary precipitates. The alloys showed a good resistance to corrosion in 3.5 wt% neutral chloride solution. The alloy offered a high passivation tendency because of homogeneous coherent nano Al(Sc_x−1Zr_x) precipitates. The nano precipitates interfaces and homogeneously distributed Al₃Sc precipitates offer a high degree of corrosion resistance to Al 2.5 Mg Sc alloys compared to conventional aluminum alloys, such as Al 6061 and Al6013.     

Microstructure,formation mechanism and properties of plasma-sprayed Cr7C3—CrSi2—Al2O3 coatings

The Cr₇C₃—CrSi₂—Al₂O₃ composite coatings were prepared by plasma spraying Cr₇C₃—CrSi₂—Al₂O₃ and Al—Cr₂O₃—SiC composite powders, respectively. The microstructure, formation mechanism and properties of the two Cr₇C₃—CrSi₂—Al₂O₃ composite coatings obtained by plasma spraying were investigated, and the reaction mechanism of the Al—Cr₂O₃—SiC system was explored. The results show that the coating obtained by plasma spraying Al—Cr₂O₃—SiC composite powders had thinner lamella and more tortuous interlayer interface, and the in-situ synthesized Cr₇C₃, CrSi₂ and Al₂O₃ in the coating were all nano-crystallines. Compared with the Cr₇C₃—CrSi₂—Al₂O₃ coating prepared by plasma spraying Cr₇C₃—CrSi₂—Al₂O₃ composite powders, the plasma-sprayed Cr₇C₃—CrSi₂—Al₂O₃ coating obtained from Al—Cr₂O₃—SiC composite powders had higher density, higher microhardness (increased by 20%), better fracture toughness and lower wear rate (reduced by 28%).     

Effects of Plasma-spraying Powers on Microstructure and Microhardness of In-Situ Nanostructured FeAl2O4 Composite Coatings

In-situ nanostructured FeAl₂O₄ composite coatings were prepared using plasma spraying of Al/Fe₂O₃ composite powders applying different spraying powers. The effects of plasma-spraying powers on microstructure and property of FeAl₂O₄ composite coatings were investigated. The results indicated the composite coatings had the microstructure with thin lamellar splats rich in FeAl₂O₄ as matrix, and dispersed granules rich in Fe and thin lamellar splats rich in Al₂O₃ as second phases. The reaction degree of Al/Fe₂O₃ composite powders increased while applying spraying power of 25-30 kW and then decreased while applying spraying power of 30-40 kW, which first resulted in the increase and then in the decrease of the Al₂O₃ content. The coating prepared by applying spraying power of 30 kW had the maximum microhardness, which was attributed to the maximum Al₂O₃ content present in the coating and the most uniform microstructure of the coating.     

Processing and microstructure of alumina-based composites

A study of powder structure and its effect on the sintering tendency of certain alumina-based ceramic systems, that is, Al₂O₃-SiO₂ and Al₂O₃-ZrO₂, was carried out to improve their mechanical strength and fracture toughness. The compacting behavior and the sintering characteristics were optimized through control of various parameters such as composition, compaction pressure, sintering temperature, and time. Best densification was obtained for mixtures prepared using very fine and deagglomerated alumina powders.     

High-Temperature Scaling of Cu–Al and Cu–Cr–Al Alloys: An Example of a Non-Classical Third-Element Effect

The oxidation of three Cu–xCr–2Al and three Cu–xCr–4Al alloys (x ≅ 0,4,8 at.%) has been investigated at 800°C in 1 atm O₂. Oxidation of a binary Cu–Al alloy containing 2.2 at.% Al produced external scales composed mainly of copper oxides with small amounts of Al-rich oxide in the inner region, while the internal oxidation of Al was almost absent. The addition of 3.9 at.% Cr to this alloy was able to decrease the oxidation rate but was insufficient to prevent the oxidation of copper. Conversely, addition of 8.1 at.% Cr to the same binary alloy promoted the rather fast formation of a protective Al₂O₃ layer in contact with the metal substrate, with a simultaneous large decrease in the oxidation rate, producing a form of third-element effect. On the contrary, all the Cu–xCr–4Al alloys formed an internal Al₂O₃ layer after an initial stage during which all the alloy components were oxidized, so that the only effect of the presence of chromium was to decrease the duration of the fast initial stage. The third-element effect due to chromium additions to Cu–2Al is related to a transition from the formation of external scales composed of mixtures of Cu and Al oxides to the external growth of Al₂O₃–rich scales as a consequence of a thermodynamic destabilization of copper oxides associated with the formation of solid solutions between Al₂O₃ and Cr₂O₃.     

Interfacial characteristics and magnetoresistive properties of reactively sputtered Fe-Al2O3-Co magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) comprised of two magnetic metal layers separated by a thin insulating oxide layer have been prepared by rf sputtering onto thermally oxidized (100) silicon wafers at room temperature. The magnetic layers with thickness of ~50 nm consisted of thin films of Fe and Co, and the material for the oxide barriers with thickness of ~10 nm was A1_,O₃. The barriers were prepared by rf reactive sputtering from pure metallic Al sources in mixed argon-oxygen atmospheres. Pt/Al₂O₃/Pt tunnel junctions were also prepared to characterize the reactively sputtered Al₂O₃ barriers. Auger electron spectroscopy was used in this study to characterize the interfacial structures of the junctions. A distinct nonlinear transport behavior, even at 300 K, in Pt/Al₂O₃/Pt junctions has revealed that the reactively sputtered A1₂O₃ is a good barrier material for tunnel junctions. Magnetoresistive measurements were made perpendicular to the film plane on the MTJs for in-plane applied magnetic fields and the results were discussed in connection with the interfacial structures of the MTJs.     

Effects of lubrication and aspect ratio on the consolidation of metal matrix composites under cyclic pressure

The effects of powder compaction under cyclic load were compared to traditional single-cycle compaction. The effect of interparticle friction as modified by lubrication and compact aspect ratio received particular attention. Mixtures of Al and Al₂O₃ were consolidated at room temperature in contained uniaxial consolidation experiments. The experiments showed that in static compaction, lubrication aids densification at low pressures but can inhibit consolidation at high pressures. Enhanced densification was observed following pressure cycling. These improvements were more pronounced in compacts having smaller aspect ratios. The efficiency of pressure cycling was reduced by the lubricant. Lubrication also decreased the effects of aspect ratio in both the static and cyclic compaction cases. Although lubrication did increase density uniformity, the resulting compact green strength was much lower. Both single and double action compaction were studied and the best green strength and density distribution were obtained with double-action compaction under cyclic pressure without lubricant.     

Al2O3-nanodiamond composite coatings with high durability and hydrophobicity prepared by aerosol deposition

We attempted the room-temperature fabrication of Al₂O₃-based nanodiamond (ND) composite coating films on glass substrates by an aerosol deposition (AD) process to improve the anti-scratch and anti-smudge properties of the films. Submicron Al₂O₃ powder capable of fabricating transparent hard coating films was used as a base material for the starting powders, and ND treated by 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES) was added to the Al₂O₃ to increase the hydrophobicity and anti-wear properties. The ND powder treated by PFOTES was mixed with the Al₂O₃ powder by ball milling to ratios of 0.01 wt.%, 0.03 wt.%, and 0.05 wt.% ND. The water contact angle (CA) of the Al₂O₃-ND composite coating films was increased as the ND ratio increased, and the maximum water CA among all the films was 110°. In contrast to the water CA, the Al₂O₃-ND composite coating films showed low transmittance values of below 50% at a wavelength of 550 nm due to the strong agglomeration of ND. To prevent the agglomeration of ND, the starting powders were mixed by attrition milling. As a result, Al₂O₃-ND composite coating films were produced that showed high transmittance values of close to 80%, even though the starting powder included 1.0 wt.% ND. In addition, the Al₂O₃-ND composite coating films had a high water CA of 109° and superior anti-wear properties compared to those of glass substrates.