Formation of Polycrystalline Boron Carbide B4C with Elevated Fracture Toughness

We have studied the processes of formation of high-density polycrystalline boron carbide B₄C during sintering under high pressure (5.5 GPa) and at high temperature (2200 K). Study of the structure by transmission electron microscopy showed that under the indicated conditions for sintering B₄C powders with original particle size range 2–80 µm, processes of primary recrystallization occur, as a result of which interlayers of nanodispersed grains (10–50 nm) are formed in the near-surface regions of the original coarse grains. The microhardness of such material is 41 GPa, and the fracture toughness is 7.2 MPa · m^1/2.__________Translated from Poroshkovaya Metallurgiya, Nos. 1–2(441), pp. 90–101, January–February, 2005.     

Thin-layer nanocrystalline composite coatings: Production,structure, and properties

To obtain a nanocrystalline structural state of materials based on aluminum and titanium, high degrees of deformation are employed: rolling with considerable reduction; and shear under high pressure (5 GPa). The nanocrystalline materials obtained are used to create thin-layer composites, with nanocrystalline silicon between the uniform layers. Measurements show that the microhardness of the composites after the application of high pressure is 2.5 (for Al–Si) and 6 (for Ti–Si) times that of the initial material, while optimal practical properties are retained. The nanocrystalline composites obtained may be recommended for ultrahard thin-layer coatings on narrow or stressed local sections of components and for local corrosion protection.     

高能球磨对碳纳米管增强铝基烧结含油轴承性能的影响

采用粉末冶金工艺制备铝粉、球磨铝粉和球磨混合粉末(添加2wt％ MWCNTs的铝粉)烧结含油轴承,测定烧结样品的性能,并研究高能球磨对粉末性能及样品性能的影响规律和机理.结果表明,球磨后粉末颗粒尺寸减小并呈薄片状或层状,MWCNTs被剪断分散后包裹在铝粉颗粒中.粉末球磨易于原子扩散促进烧结颈的形成,提高烧结样品压溃强度...     

Behavior of oxide film at the interface between particles in sintered Al powders by pulse electric-current sintering

The microstructure of the bonding interfaces between particles in aluminum (Al) powder sintered specimens by the pulse electric-current sintering (PECS) process was observed, using conventional transmission electron microscopy (CTEM) and high-resolution transmission electron microscopy (HRTEM). The behavior of oxide film at the interface between Al particles and its effect on properties of the sintered specimens were investigated. The results showed there were two kinds of bonding interfaces in the sintered specimens, namely, the direct metal/metal bonding and the metal/oxide film layer/metal bonding interface. By increasing the fraction of the direct metal/metal bonding interfaces, the tensile strength of the sintered specimens increased, and the electrical resistivity decreased. By increasing the loading pressure at higher sintering temperatures or increasing the sintering temperature under loading pressure, the breakdown of oxide film was promoted. The broken oxide film debris was dispersed in aluminum metal near the bonding interfaces between particles.     

热压技术在新型P/M金属基复合材料中的应用

反应烧结是合成颗粒增强金属基复合材料 (MMCs)的新工艺。与传统P M工艺相比 ,通过化学反应在金属基体中产生增强颗粒有诸多优势。由Al TiO2 B经反应烧结制备的Al (Al2 O3+TiB2 )MMCs便是这样一个例子。本文用热压技术将多孔Al (Al2 O3+TiB2 )MMCs致密化并得到了很好的力学性能。Al ( 10 2 %Al2 O3+9 2 %TiB2 ) (体积分数 )的弹性模量、抗弯强度和弯曲最大应变分别为 10 5GPa、50 9MPa和 5 0 %。如果复合材料的成分改变成Al ( 5 4 %Al2 O3+4 9%TiB2 ) (体积分数 ) ,其性能则分别为 89GPa ,311MPa和 8 2 %。文中亦将热压Al (Al2 O3+TiB2 )MMCs的结果与用其他致密化技术处理此材料的结果 ,以及热压其他类似材料的结果进行了比较。     

Al+TiC_(0.47)+B系掺杂制备多晶立方氮化硼烧结过程分析

应用X射线衍射分析技术，对以Al＋TiCO0.47＋B系掺杂制备多晶立方氮化硼时各物相在高温高压烧结过程中发生的化学反应、物相变化和新生物相在烧结过程中的作用，以及CBN颗粒在烧结过程中的变化等进行了研究与探讨。结果表明，Al＋TiC0.47＋B和CBN经高压高温烧结后，在CBN颗粒间形成由AlN、AlB2、TiN、TiB2和TiC等难熔化合物组成的粘结相。其中的AlN相有效抑制了CBN的六方化。     

High-performance Ni3Al synthesized from composite powders

Specimens of Ni₃Al + B of high density (>99.3 Pct RD) and relatively large dimension have been synthesized from composite powders through processes of replacing plating and electroless Ni-B plating on Al powder, sintering, and thermal-mechanical treatment. The uniformly coated Ni layer over fine Al or Ni core particles constituting these coating/core composite powders has advantages such as better resistance to oxidation relative to pure Al powder, a greater green density as a compacted powder than prealloyed powder, the possibility of atomically added B to the material by careful choice of a suitable plating solution, and avoidance of the expensive powder metallurgy (PM) equipment such as a hot isostatic press (HIP), hot press (HP),etc. The final Ni₃Al + B product is made from Ni-B-Al and Ni-B-Ni mixed composite powders by means of traditional PM processes such as compacting, sintering, rolling, and annealing, and therefore, the dimensions of the product are not constrained by the capacity of an HIP or HP. The properties of Ni₃Al composite powder metallurgy (CPM) specimens tested at room temperature have been obtained, and comparison with previous reports is conducted. A tensile elongation of about 16 Pct at room temperature was attained.     

Mechanical and microstructural properties of titanium/hydroxyapatite functionally graded material fabricated by spark plasma sintering

ABSTRACT

In this article, different functionally graded material specimens were made from titanium powder and hydroxyapatite (HA) submicron particles. The spark plasma sintering method was applied to fabricate the specimens. Two kinds of starting powder mixture were used: mixed powder and ball-milled powder. Percentage of HA was changed from 0 vol.-% to 40 vol.-% in the different number of layers (2, 3 and 5-layer). The effects of the number of the layers and ball-mill process were investigated on microstructure, microhardness, compressive strength and fracture surface. The results show that the grain size has been enhanced by increasing the amount of HA in the layers. In addition, the Vickers microhardness has been first increased by enhancing the percentage of HA, while it has been decreased in the layers with a higher amount of it. Furthermore, the highest compressive strength could be achieved in the five-layer samples. Moreover, specimens with ball-milled powder have higher microhardness and less compressive strength.     

Si对Ti3SiC2/Al复合材料的摩擦性能影响

MAX相因其独特的晶体结构和优良的自润滑性能受到广泛的关注,作为新型固体润滑剂添加到金属基复合材料可获得良好的摩擦磨损性能.采用铝粉、硅粉、Ti₃SiC₂粉为原材料,通过冷压成型无压烧结法制备了Ti₃SiC₂增强铝硅基复合材料.研究发现,复合材料的组成物相除Al、Si及外加Ti₃SiC₂相之外,在烧结的过程中生成了Al₂O₃和Al₄C₃随着硅元素含量质量分数的增加,大的圆球形Ti₃SiC₂逐渐减小,摩擦系数和磨损失重均呈现先减小后增大的规律.当硅的含量为12.5 %时,摩擦系数最低,其值为0.18,磨损失重也达最低;随着硅含量的增加,材料的硬度先增大后趋于平缓,致密度呈线性下降趋势.通过磨损表面SEM分析初步揭示了金属基自润滑的机理,复合材料的磨损机制为磨粒磨损、剥层磨损和氧化磨损并存.      

Synthesis of iron aluminides from elemental powders: Reaction mechanisms and densification behavior

Pressureless sintering and hot pressing experiments were conducted on elemental powder compacts of Fe-15.8 wt pct Al and Fe-32 wt pct Al, corresponding approximately to the compositions of stoichiometric Fe₃Al and FeAl, respectively. Upon heating near the melting point of aluminum, an exothermic reaction was initiated in the compacts, resulting in synthesis of the desired compounds with reaction times on the order of seconds. Thermal analysis and microstructural observations indicate the formation of a transient liquid phase during rapid exothermic compact heating. The mechanisms shown to be responsible for microstructural development include initial compound formation in the solid state, appearance of an aluminum-rich liquid at the aluminum particle sites, iron dissolution accompanied by outward spreading of the liquid, and subsequent precipitation of the iron-rich compounds. Apparent enthalpies of formation,ΔH _f ^°(298), estimated from reaction temperature measurements were −18 and −31.8 kJ/mol for Fe₃Al and FeAl, respectively. The influences of heating rate, green density, and aluminum particle size on sintered density were studied for pressureless reaction sintering in vacuum. The effects of processing variables on densification were explained as the net result of swelling during heating and subsequent shrinkage due to the transient liquid phase. Near full density Fe₃Al and FeAl compounds were obtained through the application of external pressures near 70 MPa during reaction in a hot press. These alloys were partially ordered, chemically homogeneous, and exhibited an equiaxed grain structure with an average grain size below 10μm. The Fe₃Al material exhibited significantly higher fracture strength and somewhat lower ductility than coarse-grained wrought material of the same composition.     

The influence of the atmosphere on the sintering of aluminum

Alloys of Al, Al-0.15Mg, and Al-12Sn made using air atomized aluminum powder and pressed to green densities of 75 to 98 pct were sintered under argon or nitrogen. Sintering in argon is only effective at high green densities when magnesium is present. In contrast, highly porous aluminum can be sintered in nitrogen without the need for magnesium. The oxygen concentration in the gas is reduced by the aluminum through a self-gettering process. The outer layers of the porous powder compact serve as a getter for the inner layers such that the oxygen partial pressure is reduced deep within the pore network. Aluminum nitride then forms, either by direct reaction with the metal or by reduction of the oxide layer, and sintering follows.     

Bulk nanocrystalline aluminum 5083 alloy fabricated by a novel technique: Cryomilling and spark plasma sintering

Dense, bulk nanocrystalline aluminum 5083 alloy was fabricatedvia a combined technique: cryomilling (mechanical milling at cryogenic temperature) to achieve the nanocrystalline Al 5083 powder and spark plasma sintering (SPS) to consolidate the cryomilled powder. The results of X-ray diffraction analysis indicate that the average grain size in the SPS consolidated material is 51 nm, one of the smallest grain sizes ever reported in bulk Al alloys produced by powder metallurgy derived methods. In contrast, transmission electron microscopy (TEM) analysis revealed a bimodal grain size distribution, with an average grain size of 47 nm in the fine-grained regions and approximately 300 nm in the coarse-grained regions. Nanoindentation was used to evaluate the mechanical properties and the uniformity of the consolidated nanocrystalline Al 5083. The hardness of the material is greatly improved over that of the conventional equivalent, due to the fine grain size. The mechanisms for spark plasma sintering and the microstructural evolution are discussed on the basis of the experimental findings.     

Synthesis and characterisation of Ti6Al4V/xTa alloy processed by solid state sintering

ABSTRACT

This work investigates the effect of Ta particle addition into a Ti6Al4V alloy processed by solid state sintering. The volume fraction of Ta ranged between 0 and 30?vol.-%. The sintering kinetics of powder mixes are evaluated by dilatometry. Sintered materials are characterised by SEM and XRD, and their mechanical properties are obtained from microhardness and compression tests. Sintering behaviour and final microstructure are affected by Ta particles, which slow down the densification, lower the temperature of α-to-β phase transition and stabilise the β phase. Mechanical properties, as microhardness, Young’s modulus and yield stress, depend on the microstructure reached after sintering and on the residual porosity. An equation expressing the Young’s modulus of Ti6Al4V/xTa alloy as function of x and porosity is proposed and validated. The materials with at least 20?vol.-% of Ta exhibited a high strength to modulus ratio, which is suitable for orthopaedic implants.     

Properties of Glass‐Ceramics Synthesized from Blast Furnace Slag with Different Sizes

Glass‐ceramics were made from ground blast‐furnace slag and other additives using a conventional ceramic‐sintering route. The weight percent of blast furnace slag reached more than 90% in raw material. The size influence of slag powders on the properties of glass‐ceramics was investigated. The main crystalline phase of glass‐ceramic is melilite. A high microhardness between 4.4 to 5.2 GPa and a bending strength between 74 to 90 MPa as well as a density between 2.5 to 2.8 g/cm³ were obtained. The result indicated that the obtained glass‐ceramics present desirable properties for building materials, and this may be an attractive method for making useful and marketable products from the waste of iron production.     

Formation of aluminium nitride during sintering of powder injection moulded aluminium

Abstract

A detailed transmission electron microscopy study of the structure of aluminium nitride formed during sintering of powder injection moulded aluminium is presented. A polycrystalline layer formed on Al particle surfaces exposed to a nitrogen atmosphere. This layer consisted of fine, rod-like crystallites of hexagonal AlN typically aligned normal to the Al surface. A double layer of AlN separated by a thin layer of Al was observed at the interfaces between Al grains. In this report, the structure of the nitride is characterised and its influence on sintering is discussed.     

Al2O3细粉对MgO系浇注料性能的影响

本文研究了Al2O3细粉的加入对MgO系浇注料性能的影响。在110℃×24h烘干,1500℃×3h烧成制度下,通过改变基质中Al2O3细粉的加入量,对材料物理性能和力学性能进行测试。结果表明：3％～5％Al2O3细粉的加入能显著改善MgO系浇注料的综合烧结性能：显气孔率15％～17％;体积密度3．01～3．03g／cm^3;耐压强度和抗折强度分别高达85～92MPa和12．4—14．5MPa;当Al2O3细粉的加入量大于5％时,由于材料线变化率和显气孔率的过度增大,引起材料结构的松散,导致材料强度的降低和抗渣性能的下降如：显气孔率大于20％;体积密度小于2．90g／cm^3;耐压强度和抗折强度则分别小于70MPa和10MPa。     

热处理工艺对B4C/2519A复合材料相组成及显微组织的影响

采用XRD、金相、SEM和EDS等测试手段对无压浸渗法制备的B4C/2519A复合材料热处理前后的物相组成、显微组织和微区成分进行研究.结果表明,热处理前的B4C/2519A复合材料中以A1相为主,其次是B4C、Al3BC和AlB2 相;经900℃×25 h热处理后主要为Al3BC相,其次为B4C、A1和AlB2相.随...     

Distinguishing the influence of aluminium and vanadium additions on microstructural evolution and densification behaviour during the sintering of ti6Al,ti4V and ti6al4v

In this work, the influence of pure Al (Ti6Al) and V (Ti4?V) powder additions on the sintering behaviour of a coarse CP-Ti powder compact was investigated. Pure Al melts and spreads to form intermetallic layers at the CP-Ti surface, causing swelling and prevention of sintering and shrinkage below 1050°C. Ti4?V compacts do not swell and begin to sinter at 990°C. The sinter rate for both Ti4?V and Ti6Al are similar at 1200°C and higher than the CP-Ti compact alone. Aluminium melting distributes this element better than the dispersed V particles, leading to more rapid homogenisation. When both pure Al and V are present (Ti6Al4?V), the sintering rate at 1200°C is similar to that of Ti6Al and Ti4?V. However, swelling is increased and homogenisation is slower, resulting in a reduced sintering aid effect compared to Ti6Al and Ti4?V.     

Response to Thermal Exposure of Ball-Milled Aluminum-Borax Powder Blends

Aluminum-borax powder mixtures were ball milled and heated above 873 K (600 °C) to produce Al-B master alloys. Ball-milled powder blends reveal interpenetrating layers of deformed aluminum and borax grains that are increasingly refined with increasing milling time. Thermal exposure of the ball-milled powder blends facilitates a series of thermite reactions between these layers. Borax, dehydrated during heating, is reduced by Al, and B thus generated reacts with excess Al to produce AlB₂ particles dispersed across the aluminum grains starting at 873 K (600 °C). AlB₂ particles start to form along the interface of the aluminum and borax layers. Once nucleated, these particles grow readily to become hexagonal-shaped crystals that traverse the aluminum grains with increasing temperatures as evidenced by the increase in the size as well as in the number of the AlB₂ particles. Ball milling for 1 hour suffices to achieve a thermite reaction between borax and aluminum. Ball milling further does not impact the response of the powder blend to thermal exposure. The nucleation-reaction sites are multiplied, however, with increasing milling time and thus insure a higher number of smaller AlB₂ particles. The size of the AlB₂ platelets may be adjusted with the ball milling time.     

Structure and Properties of an Ni3Al Intermetallic Compound Formed as a Result of Spark Plasma Sintering of the Powder Mixtures Prepared by Various Methods

Metallurgist - The paper described the structure and mechanical properties of nickel aluminide (Ni3Al) obtained by spark plasma sintering of the two compositions of nickel and aluminum powder...