Microstructural behaviour of spark plasma sintered composites containing bimodal micro- and nano-sized Al2O3 particles

Spark plasma sintering (SPS) has been recognised, in the recent past, as a very useful method to produce metal matrix composites with enhanced properties if compared with traditional powder metallurgy techniques. Obviously, the materials final properties are strongly related to the reinforcement types and percentages as well as to the processing parameters employed during synthesis. The present paper analyses the effect of microscopic and nanometric alumina particles, blended to pure aluminium in different combinations, on the final properties of metal matrix composites produced via SPS. A strong variation in the microstructural behaviour has been observed by varying the nano to micro alumina particles percentage blended with aluminium ones.     

THE FABRICATION OF COMPONENTS FROM ALUMINIUM REINFORCED WITH SILICA FIBRES

Abstract

The salient fabrication parameters involved in the hot pressing of composites of aluminium reinforced with silica fibres have been studied and their effects evaluated. The roles of temperature, atmosphere, and time in promoting bonding are much as would be anticipated from general powder-metallurgy considerations. The effect of pressure is modified by mechanical damage to the fibres at high pressures. This damage can be minimized by ensuring good fibre alignment.

Methods have been developed, analogous to those used with fibre reinforced plastics, for the fabrication of simple filament-wound shapes, such as rings, and of more complex shapes. A few trials have been made on pressure-welding and soldering techniques for forming lap-joints between pieces of aluminium-silica.     

High Strength and Thermal Stability of Multilayered Cu/Al Composites Fabricated Through Accumulative Roll Bonding and Cryorolling

Multilayered Cu/Al composites with high strength and thermal stability were successfully fabricated by combining accumulative roll bonding (ARB) and cryorolling. The microstructure, tensile properties, and thermal stability of the multilayered Cu/Al composites subjected to cold rolling and cryorolling were analysed. Subsequent cryorolling can be used to modify interfacial flatness and local necking, induce the formation of high-density stacking faults in the Cu matrix, and enhance interfacial bonding strength, which improves the mechanical properties of ARB composites. The initial lamellar structure is gradually transformed into serious mixing with an increase in annealing temperature, accompanied by the formation of excessive Cu–Al intermetallic compounds (IMCs). Cryorolled samples exhibited higher thermal stability than cold-rolled samples. At low annealing temperature, high-density stacking faults induced by cryorolling facilitated the transition from low-angle grain boundaries to high-angle grain boundaries, which led to the formation of ultra-fine grains. For the samples annealed at high temperatures, cryorolling led to the effective inhibition of Cu–Al IMC formation and growth due to the genetic effect of less heat input.

Graphical Abstract

     

Meeting the challenges: the early days and start-up of EPMA

Abstract

Magnesium (with density, ρ?=?1.74?g cc^??1) being ～35% lighter than aluminium and ～75% lighter than steel is an attractive and a viable candidate for the fabrication of lightweight structures. Being the designers' choice for weight critical applications, extensive research efforts are underway into the development of magnesium metal matrix composites (Mg-MMCs) through various cost effective fabrication technologies. In recent years, there has been a progressive advancement in utilising the microwave energy to consolidate powder materials, and the present study accentuates the use of energy efficient and environment friendly microwave sintering process to synthesise magnesium based composite materials. The processing advantages of the innovative and cost effective microwave assisted bidirectional rapid sintering technique followed by hot extrusion are first briefly introduced. Subsequently, the properties of various Mg-MMCs containing nanosized ceramic/metal particles, synthesised using this technique, are presented. Special emphasis has been made on the commending mechanical properties displayed by the nanoparticle reinforced Mg composites (Mg-MMNCs). Finally, an account of continuing research initiatives related to the development of novel lightweight Mg composites containing amorphous reinforcement is also highlighted.     

Fabrication of in situ TiB2–TiC reinforced steel matrix composites by spark plasma sintering

Abstract

In situ TiB₂ and TiC particulates reinforced steel matrix composites have been fabricated using cheap ferrotitanium and boron carbide powders by spark plasma sintering (SPS) technique. The sintering behaviour and the formation mechanism of the composite were studied. The results show that when the composite was sintered at 1050°C for 5 min, the maximum relative density and hardness of the composite are 99·2% and 83·8 HRA respectively. The phase evolution of the composite during sintering indicates that the TiB₂ and TiC reinforcements were formed in situ as follows: first, the solid/solid interface reaction between Fe₂Ti and B₄C, resulting in the formation of a small amount of TiB₂ and TiC below 950°C; second, the solid–liquid solution precipitation reaction in the Fe–Ti–B–C system, resulting in the formation of the main TiB₂ and TiC reinforcements at ～1000°C.     

Synthesis and characterisation of Al2O3/Ni-type composites obtained by spark plasma sintering

This paper presents the influence of sintering on the structure, morphology and compressing strength of alumina/nickel composite compacts obtained by spark plasma sintering (SPS). Al₂O₃/Ni composites were prepared by SPS in argon atmosphere at temperatures in the range of 1000–1200 –C with a holding time of 2, 5 and 10?minutes. The heating rate was 200 C?min^?1. These composites have been characterised by X-ray diffraction, SEM and EDX. The relative density and compressive strength of the as-obtained compacts were determined. The results showed that the alumina particles are uniformly dispersed in a quasi-continuous Ni network, and there was no sign of phase changes during sintering. The maximum strength of the alumina/nickel composite with a content of 75 vol. ? Al₂O₃ and 25 vol. ? Ni was about 240?MPa for the samples sintered at 1200?C for 10?minutes.

Special block from the conference RoPM2017 guest edited by Ionel Chicinas, Technical University, Cluj-Napoca.     

Metal Matrix Composites Produced by Spray Codeposition

Abstract

A new technique for producing metal matrix particulate composites, consisting of the spray deposition of the metal matrix and the particles, is described. The deposit is removed from the substrate and hot rolled to produce composite strip for examination and testing. It is shown that up to 36 vol.-% of SiC, Al₂0₃, chilled iron, graphite, and sand particles, and mixtures of these, 75–120 μm in size, can successfully be incorporated in aluminium and Al–5Si alloy matrixes. The friction properties of some of the composites were shown to be particularly promising. High coefficients of friction of ～0·6 were obtainable under dry contact conditions, and these were remarkably constant with time. The friction properties compared very favourably with conventional asbestos based and sintered friction materials which showed lower coefficients of friction that increased with time. PM/0310     

Aluminium/polyimide adhesion

We present a theoretical study of the interactions between isolated aluminium atoms and a polyimide surface and between polyimide and aluminium surface. The effect of surface modification on adhesion is also discussed. Self-consistent geometry optimization and molecular orbital calculations have been carried out within CNDO approximation in a cluster model framework. Our results suggest that aluminium atoms react preferentially with five and six-fold rings of polyimide when those chemical groups are present on the surface. The compound formation is accompanied by charge transfer from the metal atoms to polyimide and charge rearrangement among the polyimide atoms. Madelung potential calculations also suggest considerable core level shifts at polyimide atoms far from the reaction site. The adhesion of polymide to aluminium surface is predicted to be somewhat weaker than that of aliminium to polyimide.     

Novel technique for synthesis and consolidation of aluminium nitride nanopowders

Abstract

In this paper, the use of a microwave plasma method for the synthesis of aluminium nitride nanopowders is described. The powders were consolidated to near theoretical densities using the unique rapid consolidation technique, plasma pressure consolidation (P²C), developed by MMI. Rapid consolidation of nanopowders is an ideal requirement for better mechanical and thermal properties in the consolidated part, as it retains the fine microstructure preventing anomalous grain growth. Microwave plasma synthesis resulted in aluminium nitride nanopowders (85–200 nm), which were consolidated to near theoretical density using P²C in <5 min without sintering additives. The effect of yttria (3 wt-%) as a sintering additive on the thermal conductivity (TC) of aluminium nitride was also evaluated and compared with TC values obtained from additive free AlN consolidated samples.     

Production of NiAl–matrix composites by reactive sintering

Abstract

This work was devoted to the development of NiAl–matrix composite and its production by reactive sintering powder metallurgy. Various types of reinforcement (aluminium oxide, silicon and tungsten carbides, titanium silicide) were tested. The best chemical compatibility and the highest hardness and wear resistance were achieved by Al₂O₃ fibres. Electroless nickel plating pretreatment of Al₂O₃ fibres improves both distribution of fibres and hardness of the composite. However, it strongly reduces the wear resistance, probably due to phosphorus content in the nickel coating. In situ formation of NiAl–Al₂O₃ composites by reactive sintering of a pressed powder mixture of Ni, Al and NiO was unsuccessful. Only a small amount of cubic γ-Al₂O₃ was detected after reactive sintering and hence no significant hardness increase was observed.     

Structural heterogeneity and its effects on the properties of an in situ TiB2–TiC/Cu composite synthesised by hot pressing

ABSTRACT

In situ TiB₂ and TiC reinforced copper matrix composites with tailored heterogeneous structure were fabricated via high-energy ball milling of Cu, TiH₂ and B₄C powders followed by hot pressing. The microstructures of both ball-milled powders and hot-pressed composites were compared. Although the dislocation density of Cu matrix was changed after hot pressing, the mode of distribution of ceramic phases in the Cu matrix was noted to transmit from the ball-milled powders to hot-pressed composites in case of the TiH₂ particles synthesised by the in situ reactions. The structural inheritance between the ball-milled powders and hot-pressed composites could be used to control microstructural features and thus to tune properties. The hot-pressed TiB₂–TiC/Cu composites with tailored heterogeneous structure exhibited better performance than those of homogeneous counterparts.     

Assessment of consolidation of oxide dispersion strengthened ferritic steels by spark plasma sintering: from laboratory scale to industrial products

Abstract

Oxide dispersion strengthened steels are new generation alloys that are usually processed by hot isostatic pressing (HIP). In this study, spark plasma sintering (SPS) was studied as an alternative consolidation technique. The influence of the processing parameters on the microstructure was quantified. The homogeneity of the SPSed materials was characterised by electron microprobe and microhardness. A combination of limited grain growth and minimised porosity can be achieved on semi-industrial compact. Excellent tensile properties were obtained compared to the literature.     

放电等离子烧结制备非连续石墨纤维/Cu复合材料

以磨碎中间相沥青基石墨纤维和铜粉为原料,通过放电等离子烧结(spark plasma sintering,SPS)制备非连续石墨纤维/Cu复合材料,对石墨纤维表面进行镀钛金属化处理,以改善材料的界面结合状况.研究SPS工艺参数、铜粉粒度搭配、石墨纤维表面镀钛以及石墨纤维含量对石墨纤维/Cu复合材料致密度及热导率的影响.结果表明,将平均粒度为12和80 μm的铜粉按1∶2的质量比搭配,再与表面镀钛石墨纤维按1∶1的体积比混合,采用35 MPa先加压后送热的加压方式,于895℃下进行放电等离子烧结,可获得致密度达99.6％、热导率为364 W/(m·K)的石墨纤维/Cu复合材料,是1种很有潜力的电子封装材料.石墨纤维表面镀覆的极薄Ti镀层,可使复合材料在二维平面方向上的热导率从196 W/(m·K)提高到364 W/(m·K).     

Spark plasma sintering of cryomilled copper powder

Abstract

The densification and sintering behaviour of a cryomilled copper powder (grain size of 17±2 nm and dislocation density of 6·26±0·04×10¹⁶ m^?2) were investigated and compared to those of an atomised copper powder with the same mean particle size in order to highlight the effect of the nanostructure on spark plasma sintering (SPS). Oxygen and nitrogen contamination of the cryomilled powder gives rise to extensive degassing during SPS up to 400°C. The cryomilled powder is more resistant to plastic deformation than the atomised one, but the huge density of dislocations and grain boundary activates sintering at low temperature. Densification is therefore promoted by deformation in the atomised powder and by sintering shrinkage in the cryomilled one. As a consequence, in the SPS conditions investigated, the atomised specimen is densified but not sintered, while the cryomilled one is effectively sintered and consequently densified.     

Solid reaction between Al and B4C

Abstract

In the present work, solid reaction products formed by a mixture of aluminium and boron carbide (B₄C) powder heat treated under low pressure were studied. No significant reaction between B₄C and Al could be detected for reaction temperatures of 600°C or less. However, at 640°C, and over a short treatment time (1 h), a solid reaction took place between boron carbide and aluminium with the formation of A1₃BC and AlB₂. Meanwhile, over a longer treatment time (36 h), the detectable product phase between boron carbide and aluminium was AlB₁₂.     

Electrode reactions in dc electroslag remelting of steel rod

Abstract

The non-metallic inclusion content increased significantly when a steel rod of Fe-Ni was remelted by dc electroslag remelting. The silicon content increased slightly. The manganese and sulphur contents did not change. The total aluminium content in the ingot was max. 0·7%, while that in the electrode was only 10 ppm. The aluminium cations Al³+ in the slag are reduced to metallic aluminium at the slag/electrode interface, while O² - anions are oxidised to dissolved O in the metal pool. This Al and O subsequently recombine to form alumina inclusions in the metal pool. The inclusion content was dependent on the alumina content in the slag. When a rod of plain carbon steel was remelted, however, the increase in nonmetallic inclusion content was as little as one-tenth of that for the remelted Fe-Ni rod. The non-metallic inclusion content was independent of the polarity of the electrode.     

Microstructural analysis of multilayered titanium aluminide sheets fabricated by hot rolling and heat treatment

This study is concerned with the microstructural analysis of multilayered or bulk Ti aluminide sheets fabricated by the self-propagating high-temperature synthesis (SHS) reaction using hot rolling and heat treatment. Multilayered Ti/Al sheets were prepared by stacking thin Ti and Al sheets alternately, and a good Ti/Al interfacial bonding was achieved after rolling at 500 °C. When these sheets were held at 1000 °C, spheroidal TiAl₃ phases were formed by the SHS reaction at Ti/Al interfaces and inside Al layers. Microstructural analysis on the hot-rolled, multilayered Ti/TiAl₃ sheets revealed that intermetallic phases such as TiAl₂, TiAl, and Ti₃Al were formed at Ti/TiAl₃ interfaces due to interaction between Ti and TiAl₃ and that pores formed in the TiAl₃ layer were significantly reduced during hot rolling. When multilayered Ti/Ti aluminide sheets were heat treated at 1000 °C, Ti₃Al, TiAl, and TiAl₂ were grown as Ti and TiAl₃ were consumed. As the heat treatment proceeded, TiAl grew further, eventually leading to the fabrication of multilayered sheets composed of Ti₃Al and TiAl. Bulk Ti aluminide sheets, having a lamellar structure of Ti₃Al and TiAl, instead of multilayered sheets, were also fabricated successfully by heat treatment at 1400 °C. This fabrication method of the bulk sheets had several advantages over the method by hot forging or rolling of conventional cast Ti aluminides. From these findings, an idea to fabricate multilayered or bulk Ti aluminide sheets by hot rolling and heat treatment is suggested as an economical and continuous fabrication method, and the formation and growth mechanisms of interfacial phases are elucidated in this study.     

A THERMAL-ETCHING STUDY OF THE NICKEL/ALUMINA INTERFACE

Abstract

Nickel-alumina compacts have been thermally etched. Sections taken through the surface revealed that characteristic dihedral angles tended to develop at the lines of intersection between the alumina particles and the nickel surface. The angles seen in the section were measured and from these the dihedral angles and hence the surface and interfacial energies in the system were estimated. It is suggested that the results of Humenik and Kingery (J. Amer. Ceram. Soc., 1954, 37, 18) on the equilibrium of a liquid-nickel drop on alumina are probably not accurate, and in view of this the agreement between the two sets of results is satisfactory. It is concluded that solid nickel does not “wet” alumina, and that this is why it is difficult to sinter nickel-alumina composites to theoretical density. The addition of 5% aluminium to the nickel results in no significant improvement.     

镀钨碳纳米管增强铜基复合材料的制备及性能

采用羰基热分解法对多壁碳纳米管表面进行镀钨处理,并以镀钨碳纳米管和电解铜粉为原料,进行机械球磨混粉和放电等离子体烧结,制备了镀钨碳纳米管/铜基复合材料.采用场发射扫描电镜观察了粉体和复合材料的组织形貌,并对复合材料物相进行了X射线衍射分析.探讨了镀钨碳纳米管含量和放电等离子体烧结温度对复合材料致密度、抗拉强度、延伸率和电导率的影响.结果表明,镀钨碳纳米管质量分数为1%和烧结温度为850℃时,复合材料的致密度、抗拉强度和电导率最高.与烧结纯铜相比,复合材料的抗拉强度提高了103.6%,电导率仅降低15.9%.     

Tribological property of in situ directly nitrided and sintered Al/AlN composite

Abstract

An in situ directly nitrided and sintered Al/AlN composite was developed using a chemical reaction between aluminium and nitrogen gas at temperatures below 823 K. It is clear from SEM and TEM observation that the composite has good bonding of the in situ formed AlN to the aluminium matrix. The composite shows an unusually low friction coefficient (0·004–0·008) under oil lubrication, and also has excellent wear resistance and antiseizure properties when compared with hard anodised aluminium alloys. AlN particles protruding from the matrix create oil microgrooves and which work as oil pits to enhance the formability of oil films. Therefore, it is easy to form hydrodynamic lubrication with continuous oil films on a sliding surface.