电弧喷涂含陶瓷颗粒铝基复合涂层的微结构和性能

采用5052半硬铝带分别包覆Al_2O_3、SiC、B_4C、TiC陶瓷颗粒制备的粉芯丝材进行电弧喷涂试验,制备了含陶瓷颗粒的铝基复合涂层。利用光学显微镜、XRD分析了涂层的微观组织和相结构,测试了复合涂层的显微硬度、耐磨性及耐腐蚀性。研究结果表明,制备的铝基复合涂层中含有一定数量的未熔陶瓷颗粒,涂层较为致密,无明显缺陷。含陶瓷铝基涂层的物相主要由Al和所添加的陶瓷相构成,其中在含B_4C陶瓷涂层中还存在Al_3BC、Al_4C_3和AlB_2等新相。陶瓷颗粒的加入有利于提高铝基复合涂层的显微硬度,其中B_4C的加入使涂层中基体相显微硬度提高了1.5倍,这是由于B_4C陶瓷和Al反应生成Al_3BC、Al_4C_3和AlB_2硬质相。复合涂层的耐磨性均优于纯铝涂层,摩擦磨损的形式主要为粘着磨损。动电位极化腐蚀试验表明,含SiC和TiC陶瓷涂层具有较低的腐蚀电流,耐蚀性较好,含SiC陶瓷的复合涂层出现了明显的钝化现象。     

不同pH值环境中镍铝青铜冷喷涂涂层腐蚀磨损性能

镍铝青铜合金是铸造大型舰艇螺旋桨的主要材料,海洋污损生物在合金表面附着引起的pH值变化会加速镍铝青铜合金的腐蚀磨损,威胁到舰艇安全航行.采用冷喷涂技术制备了较为致密的镍铝青铜涂层,厚度约300 μm,利用扫描电镜、光学显微镜观察了涂层的微观形貌,重点研究了涂层在pH值分别为3、7和11,质量分数为3.5%的NaCl溶液中的电化学腐蚀性能和腐蚀磨损行为.实验结果表明:pH值为3的环境中,镍铝青铜基体发生了选相腐蚀,表面疲劳裂纹主要分布在富Cu的α相上,涂层上的犁削沟槽加深发生了磨粒磨损,涂层耐腐蚀性能优于基体;pH值为7的环境中,基体发生了黏着磨损,表面有片层状金属剥落,涂层中的孔隙收容了大量磨屑避免了剧烈的三体摩擦,表面犁削沟槽较浅,涂层致钝电位高于基体,耐腐蚀性能变差.pH值为11的环境中,基体发生了表面疲劳磨损,涂层磨痕上有较深的犁削沟槽,沟槽内有微裂纹,涂层耐腐蚀性能优于基体.总体来说,在不同pH值环境中,涂层由于在冷喷涂过程中发生了冷加工硬化并且涂层上的孔隙收容了磨屑,导致涂层的耐腐蚀磨损性能增强.      

S355海洋钢表面微弧氧化复合膜层耐蚀性能

采用激光熔覆与微弧氧化技术相结合在海洋钢表面制备了复合膜层.运用扫描电子显微镜（SEM）、能谱仪（EDS）和X射线衍射仪（XRD）表征复合膜层的微观结构,采用极化曲线、电化学阻抗谱、腐蚀磨损实验和浸泡腐蚀实验等测试方法研究膜层在质量分数3.5%的NaCl水溶液中腐蚀行为,并与熔覆涂层和基体进行对比.结果表明:复合膜层主要分为内致密层和外疏松层,疏松层主要由γ-Al₂O₃组成,致密层主要由α-Al₂O₃组成,与基底层结合较好,复合膜层表面硬度最大能达到HV_0.2 1423.3,比熔覆涂层高47.6%,其硬度较S355海洋钢有显著提升.基体在腐蚀和磨损交互作用中主要以腐蚀加速磨损为主,涂层在交互作用中主要以磨损加速腐蚀为主,在经过微弧氧化处理后,膜层的自腐蚀电位负移,钝态电流密度上升,抗磨蚀性能明显提高.熔覆涂层的浸泡腐蚀方式以点蚀为主,复合膜层腐蚀较轻微,阻抗模值最大能达到105.3 Ω·cm2,比熔覆层提高两个数量级,这表明复合处理可进一步提高涂层的耐腐蚀性.      

Characterization of Tribological and Mechanical Properties of the Si3N4 Coating Fabricated by Duplex Surface Treatment of Pack Siliconizing and Plasma Nitriding on AISI D2 Tool Steel

Silicon nitride (Si₃N₄) coating was deposited on AISI D2 tool steel through employing duplex surface treatments—pack siliconizing followed by plasma nitriding. Pack cementation was performed at 650 °C, 800 °C, and 950 °C for 2 and 3 hours by using various mixtures to realize the silicon coating. X-ray diffraction analyses and scanning electron microscopy observations were employed for demonstrating the optimal process conditions leading to high coating adhesion, uniform thickness, and composition. The optimized conditions belonging to siliconizing were employed to produce samples to be further processed via plasma nitriding. This treatment was performed with a gas mixture of 75 pct H₂-25 pct N₂, at the temperature of 550 °C for 7 hours. The results showed that different nitride phases such as Si₃N₄-β, Si₃N₄-γ, Fe₄N, and Fe₃N can be recognized as coatings reinforcements. It was demonstrated that the described composite coating procedure allowed to obtain a remarkable increase in hardness (80 pct higher with respect to the substrate) and wear resistance (30 pct decrease of weight loss) of the tool steel.

     

Microstructural Characterization and Properties Evaluation of Ni-Based Hardfaced Coating on AISI 304 Stainless Steel by High Velocity Oxyfuel Coating Technique

The present study concerns a detailed investigation of microstructural evolution of nickel based hardfaced coating on AISI 304 stainless steel by high velocity oxy-fuel (HVOF) deposition technique. The work has also been extended to study the effect of coating on microhardness, wear resistance and corrosion resistance of the surface. Deposition has been conducted on sand blasted AISI 304 stainless steel by HVOF spraying technique using nickel (Ni)-based alloy [Ni: 68.4 wt pct, chromium (Cr): 17 wt pct, boron (B): 3.9 wt pct, silicon (Si): 4.9 wt pct and iron (Fe): 5.8 wt pct] of particle size 45 to 60 ??m as precursor powder. Under the optimum process parameters, deposition leads to development of nano-borides (of chromium, Cr₂B and nickel, Ni₃B) dispersion in metastable and partly amorphous gamma nickel (??-Ni) matrix. The microhardness of the coating was significantly enhanced to 935 VHN as compared to 215 VHN of as-received substrate due to dispersion of nano-borides in grain refined and partly amorphous nickel matrix. Wear resistance property under fretting wear condition against WC indenter was improved in as-deposited layer (wear rate of 4.65 × 10^?7 mm³/mm) as compared to as-received substrate (wear rate of 20.81 × 10^?7 mm³/mm). The corrosion resistance property in a 3.56 wt pct NaCl solution was also improved.     

Friction Stir Surfacing Route: Effective Strategy for the Enhancement of Wear Resistance of Titanium Alloy

Titanium alloys are widely used in aerospace applications due to their properties like high strength to weight ratio, good corrosion and creep resistance. Poor wear resistance of these alloys limits their use in tribological applications. Friction surfacing technique is now recognized as an effective solution to surface engineer the light weight high strength alloys to make them suitable for general engineering applications involving wear and corrosion. The present work pertains to a study on wear resistance of surface coating of boron carbide on Ti–6Al–4V alloy using friction surfacing technique. Coating was formed by placing the boron carbide powder into the holes of predetermined depth on the surface and was characterized by metallography, electron probe micro analysis and dry sliding wear testing. The present study revealed that titanium alloy could be friction surfaced with boron carbide powder. The coating exhibited excellent wear resistance, which is attributed to the formation of strong metallurgical bond with the substrate. In the present work an attempt has also been made to compare the wear behaviour of surface composite layer on titanium alloy with that of conventionally used engineering materials such as mild steel and austenitic stainless steel. Wear data clearly revealed that wear resistance of friction stir surfaced composite layer is better than that of mild steel and stainless steel. This study demonstrated that friction stir surfacing is an effective strategy for the enhancement of wear resistance of titanium alloys.     

Study of the Corrosion Behavior of Nanocrystalline Ni-P Electrodeposited Coating

The corrosion behavior of electrodeposited Ni-P coating in 5 pct (in weight) NaCl solution has been studied. The microstructure of the Ni-P electrodeposit displayed a layered structure composed of nanocrystalline grains revealed by transmission electron microscopy (TEM). With the increase in polarization potential and bath temperature, the corrosion rate as well as the weight loss was significantly increased. Under observation by a field-emission scanning electron microscope (FESEM), the pitting evolved into larger sizes with a diameter of approximately 30 μm as the temperature and potential were increased to 50 °C and 600 mV_SCE, respectively. Cracks and delamination among the layered structure were found inside the deepened corrosion pit. The increase in surface P content and the trace of the formation of phosphate were also observed by using X-ray photoelectron spectroscopy (XPS) analysis.     

HVAF喷涂La2O3改性WC-20Cr3C2-11NiMo涂层的耐磨耐蚀性能

为进一步提升高质量WC涂层的耐磨性、耐海水腐蚀性和耐海水气蚀性。采用大气超音速火焰喷涂(HVAF)在0Cr13Ni5Mo基体上制备稀土La₂O₃改性WC-20Cr₃C₂-11NiMo涂层。通过显微硬度测试、平面孔隙测试、摩擦磨损实验、电化学实验和模拟海水超声波气蚀实验,测试涂层的显微硬度、孔隙率、摩擦因数、摩擦磨损性能、耐海水腐蚀性能和耐海水气蚀性能,分析La₂O₃对WC-20Cr₃C₂-11NiMo涂层耐磨耐蚀性能的影响。结果表明,改性后的涂层显微硬度提升到1400 HV0.2左右,平均孔隙率降低约48.6%;涂层磨损质量降低约33%,摩擦因数降低约30%,摩擦磨损表面微凹坑和微裂纹明显减少;电化学自腐蚀电位明显右移,电化学自腐蚀电流密度明显减小;涂层的气蚀质量损失减少约20%,气蚀坑洞明显减少和变小。HVAF喷涂La₂O₃改性后的WC-20Cr₃C₂-11NiMo涂层硬度略微提升,致密性、耐磨性、耐海水腐蚀性和耐海水气蚀性得到明显提升,除表面疲劳磨损外,表面摩擦磨损机理从严重磨粒磨损转变为轻微磨粒磨损,气蚀机理主要为流体冲击波侵蚀。     

Effects of TiC composite coating on electrode degradation in microresistance welding of nickel-plated steel

Electrode degradation has been studied during series-mode microresistance welding of thin-sheet nickel-plated steel to nickel. The main focus of the study was the effects of a TiC metal matrix composite coating. The results indicated that electrode degradation was caused predominantly by material loss due to pitting (as a result of the fracturing of local bonds between the electrode tip and sheet) and also by microscopic extrusion or plastic deformation (as a result of the softening of electrode tip regions). The composite coating improved tip life by about 70 pct, mainly because the TiC particles contained in the coating discouraged local bonding between the electrodes and sheets, and probably also improved the resistance to surface extrusion. It was also found that the use of an oxide-dispersion-strengthened copper alloy (Cu-Al₂O₃) improved tip life by only about 15 pct compared to the conventional precipitation-strengthened Cu-Cr-Zr electrode alloy.     

Studies on Nb Microalloying of 13Cr Super Martensitic Stainless Steel

The effect of Nb microalloying on microstructure, mechanical properties, and pitting corrosion properties of quenched and tempered 13?pct Cr-5?pct Ni-0.02?pct C martensitic stainless steels with different Mo and N contents was investigated. The microstructure, density, and dispersion of high-angle boundaries, nanoscale precipitates, and amount of retained austenite were characterized by using electron backscattered diffraction, transmission electron microscopy, and X-ray diffraction to correlate with properties. The results show that the combined effects of lowering nitrogen content in 13?pct Cr-5?pct Ni-1~2?pct Mo-0.02?pct C steels to 0.01?wt pct, and adding 0.1?pct Nb are to decrease the amount of Cr-rich precipitates, as Nb preferentially combines with residual carbon and nitrogen to form carbonitrides, suppressing the formation of Cr₂N and Cr₂₃C₆. Austenite grain refinement can be achieved by Nb microalloying through proper heat treatment. If the nitrogen content is kept high, then Cr-rich precipitates would occur irrespective of microalloying addition. The NbN would also occur at high temperature, which will act as substrate for nucleation of coarse precipitates during subsequent tempering, impairing the toughness of the steel. It was shown that the addition of Nb to low interstitial super martensitic stainless steel retards the formation of reversed austenite and results in the formation of nanoscale precipitates (5 to 15?nm), which contribute to a significant increase in strength. More importantly, the pitting corrosion resistance was found to increase with Nb addition. This is attributed to suppression of Cr-rich precipitates, which can cause local depletion of Cr in the matrix and the initiation of pitting corrosion.     

On the Prospects of Using Nanoindentation and Wear Test to Study the Mechanical Behavior of Fe-Based Metallic Glass Coating Reinforced by B4C Nanoparticles

In this study, Fe-based metallic glass was served as the matrix in which various ratios of hard B₄C nanoparticles as reinforcing agents were prepared using a high-energy mechanical milling. The feedstock nanocomposite powders were transferred to the coatings using a high-velocity oxygen fuel process. The results showed that the microstructure of the nanocomposite coating was divided into two regions, namely a full amorphous phase region and homogeneous dispersion of B₄C nanoparticles with a scale of 10 to 50 nm in a residual amorphous matrix. As the B₄C content is increased, the hardness of the composite coatings is increased too, but the fracture toughness begins to be decreased at the B₄C content higher than 20 vol pct. The optimal mechanical properties are obtained with 15 vol pct B₄C due to the suitable content and uniform distribution of nanoparticles. The addition of 15 vol pct B₄C to the Fe-based metallic glass matrix reduced the friction coefficient from 0.49 to 0.28. The average specific wear rate of the nanocomposite coating (0.48 × 10⁻⁵ mm³ Nm⁻¹) was much less than that for the single-phase amorphous coating (1.23 × 10⁻⁵ mm³Nm⁻¹). Consequently, the changes in wear resistance between both coatings were attributed to the changes in the brittle to ductile transition by adding B₄C reinforcing nanoparticles.

     

Friction Stir Processing for Enhancement of Wear Resistance of ZM21 Magnesium Alloy

Present work pertains to surface modification of the magnesium alloy using friction stir processing (FSP). Silicon carbide and boron carbide powders are used in the friction stir processing of the ZM21 Magnesium alloy. Coating was formed by FSP of the alloy by placing the carbide powders into the holes made on the surface. Surface coating was characterized by metallography, hardness and pin-on-disc testing. Friction stir processed coating exhibited excellent wear resistance and is attributed to grain boundary pinning and dispersion hardening caused by carbide particles. Surface composite coating with boron carbide was found to possess better wear resistance than coating made with silicon carbide. This may be attributed to formation of very hard layer coating of boron carbide reinforced composite on the surface of magnesium alloy. In the present work an attempt has also been made to compare the wear behaviour of surface composite layer on ZM21 Mg alloy with that of conventionally used engineering materials such as mild steel and austenitic stainless steel. Wear data clearly shows that wear resistance of friction stir processed composite layer is better than that of mild steel and stainless steel. This work demonstrates that friction stir processing is an effective strategy for enhancement of wear resistance of magnesium alloys.     

Synergistic effects of wear and corrosion for Al2O3 particulate-reinforced 6061 aluminum matrix composites

Wear corrosion of alumina particulate-reinforced 6061 aluminum matrix composites in a 3.5 wt pct NaCl solution with a revised block-on-ring wear tester has been investigated. The studies involved the effects of applied load, rotational speed, and environments (dry air and 3.5 pct NaCl solution) on the wear rates of materials. Also various specimens with Al₂O₃ volume fractions of 0, 10, 15, and 20 pct were employed in this work. Electrochemical measurements and electron micrographic observations were conducted to clarify the micromechanisms of wear corrosion in such metal matrix composites. Experimental results indicated that the wear rate of monolithic 6061 Al in either dry wear or wear corrosion was reduced by adding alumina reinforcements. However, the effect of volume fraction on wear rate is only minor in dry wear, while it is significant in the case of wear corrosion. Wear-corrosion tests also showed that the corrosion potential shifted to the active side and the current density for an applied potential increased with the decrease of Al₂O₃ volume fraction in the materials and the increase in applied load and rotational speed. Although the incorporation of reinforcement in these aluminum matrix composites was deterimental to their corrosion resistance, the influence on wear corrosion was favorable.     

Ni-SiO2复合镀层电沉积制备及组织性能研究

利用直流电沉积方法在Zr-4合金表面制备了Ni-SiO₂复合镀层,采用场发射扫描电镜、显微硬度计、电化学工作站、摩擦磨损试验机等研究复合镀层的表面形貌、显微硬度、耐腐蚀性及摩擦磨损性能。研究结果表明:与单一的Ni镀层相比较,Ni-SiO₂复合镀层的显微硬度值有所提升,表面更为均匀,Ni-SiO₂复合镀层的耐腐蚀性能和耐磨性能也得到明显提升。且当SiO₂颗粒添加量为10 g/L时,复合镀层的综合性能较优。      

Corrosion Behavior of Alloy 625 in PbSO4-Pb3O4-PbCl2-ZnO-10?Wt?Pct CdO Molten Salt Medium

Corrosion behavior and degradation mechanisms of alloy 625 under a 47.288 PbSO₄-12.776 Pb₃O₄-6.844PbCl₂-23.108ZnO-10CdO (wt pct) molten salt mixture under air atmosphere were studied at 873?K, 973?K, and 1073?K (600?°C, 700?°C, and 800?°C). Electrochemical impedance spectroscopy (EIS), open circuit potential (OCP) measurements, and potentiodynamic polarization techniques were used to evaluate the degradation mechanisms and characterize the corrosion behavior of the alloy. Morphology, chemical composition, and phase structure of the corrosion products and surface layers of the corroded specimens were studied by scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) and X-ray map analyses. Results confirmed that during the exposure of alloy 625 to the molten salt, chromium was mainly dissolved through an active oxidation process as CrO₃, Cr₂O₃, and CrNbO₄, while nickel dissolved only as NiO in the system. Formation of a porous and nonprotective oxide layer with low resistance is responsible for the weak protective properties of the barrier layer at high temperatures of 973?K and 1073?K (700?°C and 800?°C). There were two kinds of attack for INCONEL 625, including general surface corrosion and pitting. Pitting corrosion occurred due to the breakdown of the initial oxide layer by molten salt dissolution of the oxide or oxide cracking.     

Stellite12 和 Stellite20 涂层的耐磨性和耐腐蚀性能对比研究

利用超音速火焰喷涂技术 (High-velocity-oxy-fuel, HVOF) 在 F316 不锈钢表面制备 Stellite 12 和 Stellite 20 两种钴基合金涂层, 对比研究了涂层的摩擦磨损性能以及在不同浓度硫酸溶液中耐腐蚀性。 采用 HT2101 销盘磨 损试验仪, 进行了摩擦磨损试验。 利用电子探针仪 (EPMA) 观察涂层的微观组织形貌, WDS 波谱仪分析涂层微 区成分。 采用三电极体系在 CHI660C 电化学工作站上测试 Stellite12 和 Stellite20 涂层在不同浓度硫酸溶液中的极 化曲线, 并与 F316 不锈钢进行对比。 结果表明, HVOF 工艺制备的 Stellite12 和 Stellite20 涂层均匀致密, 无裂纹 等缺陷, 显微硬度分别为 750 HV0.3 和 1000 HV0.3; Stellite 12、 Stellite 20 涂层的摩擦系数约 0.55~0.6。 磨损机理 主要为磨粒磨损, 伴随一定的粘着磨损。 Stellite 12 的磨损量为 Stellite 20 的两倍。 在室温条件下, 两种涂层在质 量分数 20 % 和 50 % 硫酸溶液中腐蚀电位均低于 F 316 的腐蚀电位, F 316 具有较好的耐腐蚀性; 在 80% 硫酸溶 液中, 这两种涂层的腐蚀电位均高于 F 316 的腐蚀腐蚀电位, F 316 不锈钢耐蚀性相对较差。     

Microstructure and corrosion behavior of as-cast and heat-treated Al-4.5 Wt pct Cu-2.0 wt pct Mn alloys

The microstructure and corrosion behavior of as-cast and heat-treated Al-4.5 pct Cu-2.0 pct Mn alloy specimens solidified at various cooling rates were investigated. The equilibrium phases Al₆Mn and θ-Al₂Cu, which are observed in the conventionally solidified alloy in the as-cast condition, were not detected in rapidly solidified (melt-spun) material. Instead, the ternary compound Al₂₀Cu₂Mn₃ was present in addition to the α phase, which was present in all cases. The morphological and kinetic nature of corrosion was investigated metallographically and through potentiostatic techniques in 3.5 wt pct NaCl aqueous solution. Corrosion of the as-cast material was described by two anodic reactions: corrosion of the intermetallic phases and pitting of the α-Al solid solution. The corrosion rate increased with cooling rate from that for the furnace-cooled alloy to that for the copper mold-cast alloy and, subsequently, decreased in the rapidly solidified alloy. In the heat-treated material, corrosion could be described by two anodic reactions: corrosion of Al₂₀Cu₂Mn₃ precipitate particles and pitting of the α-Al matrix. S.M. Skolianos, formerly Graduate Student, Department of Metallurgy, University of Connecticut     

Enhancement of wear and corrosion resistance of cast A356 aluminium alloy using friction stir processing

The present work pertains to investigation carried out on the feasibility of locally modifying the surface properties of cast aluminium alloy A356 using friction stir processing (FSP). The friction stir processed zone was characterized by metallography, electron micro probe analysis, hardness, dry sliding wear and potentio dynamic polarization testing. Hardness mapping showed that stir zones experienced increase of 40% compared to the as-cast metal. Further uniform micro-hardness was observed in the friction stir processed zone, which was not the case with as-cast A356 aluminum alloy. The FSP of cast A356 alloy exhibited excellent wear resistance, which is attributed to break-up of the coarse silicon rich eutectic particles, dendrite structure and homogenous distribution of fine Si particulates throughout the α-Al matrix due to intense plastic deformation and mixing during friction stir processing. The friction stir processed zone was also found to have adequate corrosion resistance. This work demonstrates that friction stir processing is an effective strategy for enhancement of wear and pitting corrosion resistance of as cast aluminum alloys     

Effect of different mo contents on tensile and corrosion behaviors of CD4MCU cast duplex stainless steels

The tensile and corrosion behaviors of CD4MCU cast duplex stainless steels with different Mo contents of 0, 2, and 4 pct, respectively, were examined in the present study. The polarization and the in-situ slow-strain-rate (SSR) tests were conducted in a 3.5 pct NaCl+5 pct H₂SO₄ aqueous solution to quantify the resistances to pitting corrosion and stress corrosion cracking (SCC) with different Mo contents. The addition of Mo, which is a strong ferrite stabilizer, affected the microstructure of the present alloy and, eventually, the tensile and corrosion behaviors in a complex manner. The tensile properties of CD4MCU cast duplex stainless steel, for example, were found to be determined by the volume fraction of hard ferritic phase, the presence of the second precipitates of soft austenitic phase in the ferrite matrix, and the shape of the austenitic phase. The addition of 2 pct Mo was detrimental to the corrosion properties of CD4MCU cast duplex stainless steel due to the significant increase in the volume fraction of ferritic phase. With the addition of 4 pct Mo, however, the resistances to pitting corrosion and SCC recovered to those of the specimen without Mo. The relationship between the microstructural evolution and the tensile and corrosion behavior of CD4MCU cast duplex stainless steels with different Mo contents was discussed based on the micrographic and fractographic observations.     

Aluminum matrix composites: Fabrication and properties

Aluminum alloy matrix composites containing 1 to 30 wt pct of fibrous and particulate nonmetals varying in size from 0.06 μm to 840 μm were fabricated. The composites were cast into cylindrical molds for friction and wear tests, hot extrusion and tensile tests. The distribution of the nonmetals in the cast ingots was homogeneous. Friction and wear tests were done on a pin (52100 bearing steel) and dish type machine without lubrication. It was found that composites containing ∼10 wt pct or more of SiC, TiC, Si₃N₄, Al₂O₃, glass, solid waste slag, and silica sand wear less than the pure matrix alloy, but have slightly higher average coefficients of friction. Wear in composites containing soft particles, especially MgO and boron nitride was higher than the pure matrix alloy. The average coefficient of friction of all the composites was in the range of 0.35 to 0.58. Increasing the sliding velocity reduced this range to ∼ 0.4 to 0.45. The longitudinal tensile properties of the extruded composites (with the exception of loss of ductility in some cases) are comparable to that of the matrix alloys. Improvements in strength or ductility were noted. For example, addition of 15 wt pct of 3 μm size Al₂O₃ particles raised the yield and ultimate strength of the Al-4 pct Cu-0.75 pct Mg alloy matrix from 227 to 302 MPa, and 356 to 403 MPa, respectively. The corresponding percent elongation decreased from 25.8 to 12.5. The fact that the various composites can be readily cast and hot formed suggests a variety of engineering applications. AKIRA SATO, formerly Visiting Scientist at Massachusetts Institute of Technology, Cambridge.