Erosion Characteristics of Aluminum-based Metal Matrix Composites in Slurry Environments

第18卷材料科学与工程增刊Vo1.18Materials Science & EngineeringSep.2000Erosion Characteristics of Aluminum-based Metal MatrixComposites in Slurry EnvironmentsTu JiangpingDepartment of Chemical Engineering, Hiroshima University, Kagamiyama 14-1,Higashi-Hiroshima 739 -8527, JapanDepartment of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, China[Abstract] The erosion resistance of the Al,B,O33 whisker reinforced AC4C Al composites in water andsaline slurry were investigated using a jet-in-slit rig. Erosion tests were performed at slury velocities between6.4 m/sto 15.2 m/s and at normal impact angle. The detachment of flake and dislodgement of whisker wereidentified as the major mechanisms of material removal in slury environments. The composites showed bettererosion resistance due to the protection of the matrix by the whisker at low slurry velocities. Because of reducedfracture strain, the erosion rates of the composites were generally greater than that of the unreinforced alloy athigh slury velocities. Owing to interfacial reaction which resulted in decrease in hardness and fracture strain,the T6 treatment for the composites had a deleterious influence on the erosion resistance. By considering thematerial removal processes in the water slurry, a simple rationalization of the inverse dependence of slurryerosion rate on Hεp was obtained. In the saline slurry, there exists a strong synergistic effect between erosion andcorrosion. The volume loss of the composites was enhanced through cracking of flakes and detaching of whiskerinduced by stress and corrosion.[Key Words] slurry erosion, erosin-corrosion, flow-induced corrosion, metal matrix composites1. Introductionsuffers from impingement of solid erodent particlessuspended in a carrier liquid. For the metal matixAluminum alloy matrix composites (MMCs) arecomposites, the improvement in mechanicalused in many engineering applications due to highproperties due to secondary processing does notspecific strength and superior wear characteristics.lead to a proportionate increase in erosion resistanceThe potential applications for these composites121. The erosion mechanisms of the compositesinclude a variety of marine structures, such asinvolve not only the basic ductile or brittleimpellers and agitators, which are subjected tmechanisms operative in single-phase materials, butslurry erosion and erosion-corrosion in marinealsoeffectsrelatedto microstructuralenvironments. Therefore, an understanding of thecharacterisation P The most important parameter inslurry erosion and erosion-corrosion characteristicsdeciding the performance of the composites inof the material is needed before MMCs can beerosive conditions is whether the reinforcementapplied to fabricate the wear-resistant structuralparticle or fiber undergoes fracture or not 4. Incomponents.practi中国煤化工rosive, and theSlurry erosion occurs when surface materialcombC N M H Grosion resulted●266●in total materials loss that was much greater thanh, water quenched to room temperature, aged at 160the additive effects of each process taken alone,C for 6 h and then air cooled. After extractionthus showing a strong synergism between erosionfrom the composites, the whisker surfaces wereand corrosion (51. It has been reported 16-8| that theexamined using scanning electron microscopy andcomposites suffered from higher weight loss thanX-ray diffraction. The erosion test specimensthe base aloy during corrosion and erosion-having discs 2 mm thick and 20 mm in diameterwere prepared. All the specimens were polishedcorrosion.with 1000 mesh SiC emery paper and degreasedIn some ceramic-aluminum systems, there existwith acetone before each experiment.chemical interaction between the ceramic phase and2.2. Erosion testsmatrix at high temperature 9. An excessiveErosiontestswere conducted at roomchemical reaction would degrade the reinforcementtemperature using a jet-in-slit rig, which has beenstrength and create a brittle and weak interface.described previously 川，Fig. 1 shows a velocityConsequently，the mechanical properties andprofile for the liquid-particle flow in the test sectionerosion resistance of the composites are consideredof the jetin-slit rig. In this rig. the specimen, whichto be changed 101. The purpose of this work is tois mounted on the test section directly upper theinvestigate the erosion characteristics of as-cast andnozzle with a distance of 2 mm to the outlet of theT6-treated AlgB,O3。 whisker / AC4C Alnozzle. is immersed in the slury. The water streamcomposites in water and saline slurry. The efects ofis jetted from the nozzle with an intemal diametermechanical properties and flow-induced corrosionof 1.8 mm, and the slurry is drawn up and mixedon the erosion resistance of the composites arewith the waterjet stream. The slurry then impingesdiscussed.on the surfaces of the test specimens and isexhausted through the slit between the specimen2. Experimental detailsand guide plate. Before conducting the erosionexperiments, the relationship between the water2.1. Materialsflow rate and the slurry impact velocity wasThe composites containing 19.5 vol.%established.aluminum borate whiskers were fabricated bsqueeze casting technique at the molten metalpouring temperature of 760oC and preform pre-heating temperature of 700oC. A pressure (100MPa)was applied to the melt in an appropriate die. Thealuminum borate (Al|gB,O3) whisker with length10-30 um and diameter 0.5-1.0 um, density 2.93g/cm', Young's modulus 400 GPa, tensile strength 8GPa and Moh's hardness 7, was used as thereinforcement. AC4C Al alloy (Si: 6.5-7.5 wt.%,Mg: 0.2-0.45 wt.%, Fe: <0.55 wt.%, Zn: <0.35Fig.I Velocity profile for the liquid-particle flow inwt.%, Cu: <0.25 wt.%, Al: bal.) was chosen as thethe test section of the jet-in-slit rig.matrix material. Tests were conducted on thecomposites in as-cast and heat-treated (T6)，F.rosion tests were conducted in distilled waterconditions at room temperature. In the T6 treatment,中国煤化工ular silica sand (77-the specimens were solution treated at 525C for 8PYHC N M H Gin corrosive slurry●267●containing the same erodent concentration and 3.5AC4C Al composite, there was no evidence ofwt% NaCl solution (pH: 6.85). The slurry velocityreaction products after squeeze casting as comparedin this work was between 6.4 m/s to 15.2 m/s. Sincewith the as-received whisker patterm. After a T6-it was difficult to conduct in situ electrochemicaltreatment, however, there exist MgAl2O, and Al2O3measurement, flow-induced corrosion rates werein the X-ray diffraction pattern of the extracteddetermined from the volume losses of thwhisker, indicating that interfacial reaction occurredspecimens after exposure to the impact of 3.5 wt.%between the whisker and AC4C Al matrix alloyNaCl aqucous solution jet, without erodent particles.during the heat treatment process. The scanningThe damaged surfaces of the specimens wereelectron micrographs showing the morphology ofexamined using scanning electron microscopywhiskers in various states are present in Fig. 2. On(SEM) and hardness measurements were performedthe surface of the whisker extracted from T6-treatedon a transverse section of each material, usingcomposite, a lot of interfacial products and defectsVickers microhardness tester with a load of l0g.can be seen Fig. 2b). The chemical interactionsbetween the whisker and matrix alloy drastically3. Results and discussionalter the interfacial microstructure and mechanicalproperties of the whisker. Consequently, the tensile3.1. Microstructure and mechanical propertiesstrength, fracture strain and hardness of thThe micrograph of the as-cast compositecomposites have varied significantly. For therevealed a reasonably uniform distribution andcomposites, the fracture strain, ultimate tensilerandom orientation of the whisker throughout thestrength and hardness decreased after the Tmatrix. From the X-ray diffraction patterm of thetreatment (Tablc 1).extracted whisker from the as-cast Al,B,O3 12865820 arwneao (a)l810 daa(b)Fig.2 SEM micrographs of the whiskers extracted from the (曲) as-cast composite, (b) T6-treated composite.Table 1 Mechanical properties and density of the composites and AC4C AI matrix alloyUItimate tensile strengthFracture strainYoung's modulus HardnessDensityMaterials(Mpa)_(%)(Gpa)(Hv)(g/cm'Composite(as-cas)302.3296.11262.72Composite(T6)232.52.2中国煤化工AC4C Al (as-cas)191.74.2.6MYHC NMH G-●268●3.2 Slurry erosionrevealing craters, gouges and flakes. WithFor the test materials exposure to the waterincreasing slurry velocity, the eroded surface of theslurry impact, an initial transient period duringAC4C AI alloy was characterized by deep craterswhich the volume loss increased rapidly withand a high degree of flake formation. The flakeincreasing erosion time was observed, followed by aformation occurred when plastic deformation tooksteady state period. Under steady-state conditions,place in the vicinity of the slurry impact and athe volume loss increased linearly with timestrain-hardened layer formed on the specimenthroughout the period of measurement. Fig. 3surface after multiple impact. When a critical strainrepresents the influence of the slurry velocity on theis exceeded in the deformation volume beneath thesteady-state erosion rate of the composites andsurface, the flake or the outer strain-hardened layerAC4C Al alloy in the water slurry. It can beis detached from the eroded surface by ductileobserved that the steady-state erosion rates of thematerials increase with the slurry velocity. At lowfracture, thereby causing the erosive loss of theslurry velocities, the steady-state erosion rates formaterial.the composites were, in general, smaller than thatIn the case of. the composites, the operatingfor the AC4C Al alloy. Upon increasing slurrymechanism was broadly similar to the AC4C Alvelocities more than 11.5 m/s, the steady-statematrix alloy with additional processes. Theseprocesses involve protection of the matrix by theerosion rates began to increase rapidly withwhisker at low slurry velocity, and formation ofincreasing slury velocity. Above 12.5 m/s, thfracture-formed layer and whisker dislodgmentcomposites presented higher erosion rates asunder high-velocity slurry impact. Additioncompared with the AC4C Al alloy. For every set ofAlpB,O33 whisker to the AC4C AI matrixconditions, the erosion rate of the as-cast compositeconsiderablyincreased the hardness of thwas lower than that of the T6-treated composite.composites and resulted in a reduction in the extentof plastic deformation of the matrix. Upon0.35comparing the erosion rates of the composites and一 一AC4C AI (as-cast)MMC (as-cast)unreinforced matrix alloy at low slurry velocities,MMC (T6)the steady-state erosion rates for the composites are.25ess than that for the unreinforced matrix alloy.0.20Consequently, from Fig. 4a and 4b, a ltte degree of0.15上flake is observed in the eroded surface of the as-castand T6-treated composites. It may be seen that thecraters are very fine, indicating that the materials0.05could resist the gouging process. Under the high-velocity slurry impact conditions, the surface16Slurry Velocity /ms 1morphologies of the composites exhibit severeeroded type suface: cracking and flaking, as shownFig.3 Variation of steady-state erosion rates for thein Fig. 4c. Because the whisker in the compositesmaterials with the slurry velocity.reduced the fracture strain, relatively larger numberof cracks and detached whiskers were revealed onThe sturry erosion processes observed in the中国煤化工n contras, lcalizedAC4C AI alloy specimens were associated withwhere the whiskerstheir corresponding plastic deformation, typically.CNMHG the refnecnconn●269●whisker is completely dislodged (Fig. 4d, shown byarrow mark).From the results obtained in the water slurry,strong velocity dependence of erosion rates of thetest materials is evident. The velocity exponentsobtained for the AC4C Al alloy and the as-cast andT6-treated composites at the normal impact arelisted in Table 2. The velocity exponents show aslight increase after the T6 heat treatment. The highvelocity exponents of the composites areFig.4 SEM micrographs of the eroded surfaces of theparticularly noteworthy.composites at various velocities: (a) as-cast composite, 6.4m/s, b) T6-reated composite, 6.4 m/s, (C) as-castcomposite, 15.2 m/s, (d) T6-treated composite, 15.2 m/s,showing dislodgement of whisker in the surface.Table2 Velocity exponents of the testmaterials at nor mal impact angleTest MaterialsVelocity exponent (m)AC4C AI aly (as-cast)4.1Composites (as-cast).9Composites (T6)5.0名280(间)n many erosion studies, the hardness isconsidered t0 be the key material parameter becausethe removal of ductile materials varies inverselywith the hardness 1.12131. However, except plasticdeformation, severe localized fracture anddinlodgoment of AlgB