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Improvement of the Mechanical Properties of Al-Si Alloys

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Improvement of the Mechanical Properties of Al-Si Alloys by TiC Nanoparticles KONSTANTIN BORODIANSKIY, ALEXEY KOSSENKO, and MICHAEL ZINIGRAD Al-Si alloy A356 was modified by TiC nanoparticles. First, the nanoparticles were mechano- chemically activated together with aluminum powder. Next, the activated particles were hot extruded in a home-made extruder. Finally, nanoparticles thus prepared in the aluminum matrix were added to the liquid Al-Si alloy,
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  Improvement of the Mechanical Properties of Al-Si Alloysby TiC Nanoparticles KONSTANTIN BORODIANSKIY, ALEXEY KOSSENKO, and MICHAEL ZINIGRADAl-Si alloy A356 was modified by TiC nanoparticles. First, the nanoparticles were mechano-chemically activated together with aluminum powder. Next, the activated particles were hotextruded in a home-made extruder. Finally, nanoparticles thus prepared in the aluminum matrixwere added to the liquid Al-Si alloy, which was then cast into sand molds. A comparison of themicrostructure and mechanical properties of the modified alloy thus produced with those of thealloy without the nanoparticles demonstrated that the grain size of the modified alloy decreased.The mechanical properties determined after T6 heat treatment indicated unusual behavior,where the elongation of the modified alloys increased by 20 to 50 pct in different regions of thecast, while the tensile strength remained unchanged and the hardness increased by 18 pct. Anelectron microscopy study revealed concentration of dislocations near grain boundaries in themodified alloy samples. These grain boundaries serve as obstacles to dislocation motion. It wastherefore concluded that the improvement in the mechanical properties of the aluminum alloymodified by TiC nanoparticles was caused by the grain-size-strengthening mechanism.DOI: 10.1007/s11661-013-1850-4   The Minerals, Metals & Materials Society and ASM International 2013 I. INTRODUCTION M ETAL  strengthening is one of the main challengesencountered in materials technology. Traditionally, suchstrengthening was achieved by alloying with otherchemical elements or compounds, but relatively highfractions of relatively expensive materials were neededto obtain the required technological characteristics, andthe results achieved were still far from satisfactory. Thegrowing interest in Al-Si cast alloys is due to theirwidespread and extensive use in automotive, aerospace,and transport systems. One of these alloys is A356 alloy,which has excellent castability, mechanical characteris-tics, and physical properties. The strength of the alloy can be improved by alloying, [1 – 6] by means of masteralloys, by heat treatment, or by applying ultrasound that affects the crystallization process. [7 – 9] There are reportsof TiB 2  particles being utilized, which serve as crystal-lization nuclei and cause refinement of the metalgrains. [10,11] Small addition of strontium and sodiuminto Al-Si casting alloys enhance the formation of refined eutectic colonies. [12,13] Another relatively new strengthening technology issemisolid metal processing (SSM), which was initiallydeveloped by researchers at MIT in the 1970s. [14] In theSSM approach, metals are treated by a mixture of  fine solid nonmetal particles dispersed in the liquid. [15 – 18] Nanotechnology is also used in the metal-strengtheningprocesses. It would be logical to expect that nanosizedparticles would cause similar changes in mechanicalproperties, but that their influence would be ‘‘milder’’and more balanced. Indeed, it has been established thatthe mechanical properties of Al A356 could be improvedby adding Al 2 O 3  nanoparticles in a minute concentra-tion of 1 wt pct during casting. [19] In the current article, we report the influence of ceramic nanoparticles on the mechanical properties of Al-Si alloy A356. One of the serious technologicalproblems associated with the introduction of theseparticles into the molten metal is the poor wettabilityof the former by the latter. We also report here a methodfor solving this problem by severe mechanical loading of the metal matrix with titanium carbide nanoparticlespretreated by mechanochemical activation. The assump-tion that the grain-size-strengthening mechanism acts inthe process is discussed in the current article. II. EXPERIMENTAL Al-Si alloy A356 (Rheinfelden Alloys GmbH) wasused as a bulk material. The composition of the alloy isgiven in Table I.Titanium carbide nanoparticles (Inframat  AdvancedMaterials, 99.9 pct, crystallite size 20 nm) weremechanochemically activated with aluminum powder(Strem, 99 pct+, 20 to 40 mesh) in a Retsch PM 100planetary ball mill. The milling speed was 400 rpm, andthe activation time was 5 minutes. The powder mixtureobtained was hot extruded at 623 K (350   C) in a home-made extrusion container with an extrusion ratio of 17.4. KONSTANTIN BORODIANSKIY, Researcher, is with theLaboratory for Metal and Ceramic Coatings and Nanotechnology,Materials Research Center, Ariel University, Science Park, 40700Ariel, Israel, and also with Department of Chemistry, Bar-IlanUniversity, 52900 Ramat Gan, Israel. Contact e-mail: konstantinb@ariel.ac.il ALEXEY KOSSENKO, Researcher, and MICHAELZINIGRAD, Professor and Head, are with the Laboratory for Metaland Ceramic Coatings and Nanotechnology, Materials ResearchCenter, Ariel University.Manuscript submitted March 17, 2013.Article published online June 29, 2013 4948—VOLUME 44A, NOVEMBER 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A  To examine the effect of nanoparticle modification,Al-Si alloy A356 ingots weighing 100 kg were melted inan industrial electric resistance furnace and superheatedto 1033 K (760   C) and then subjected to a standardindustrial modification process using sodium and degas-sing by FDU. After the degassing, a modifier containing0.03 wt pct of TiC nanoparticles (out of the total Almass) was added to the molten metal, and the mixturewas stirred for 10 minutes. The melt was poured into aspecial sand mold (Figure 1), central part of whichsolidified much more slowly than the outer perimeter.The pouring temperature was controlled at about1003 K (730   C).Samples for a macrostructure investigation wereprepared by polishing and etching with Kalling’sreagent. Microstructure studies were carried out usinga JEOL JSM 6510LV scanning electron microscope(SEM). The phases were identified by XRD analysisusing a Panalytical X’Pert Pro X-ray powder diffrac-tometer at 40 kV and 40 mA. The XRD patterns wererecorded in the 2 H  range from 20 to 100 deg (the stepsize was 0.03 deg, and the time per step was 3 seconds).The chemical composition was determined by a Spec-tromax optical emission spectrometer.The specimens produced were T6-heat treated. First,the solid solution was heat treated to dissolve the solublephases at 811 K (538   C) for 8 hours, after which thesolution was water quenched to develop supersaturationin water, and then it was artificially aged to precipitatethe solute atoms at 433 K (160   C) for 4 hours.The mechanical properties were measured using anInstron 3369 testing machine according to ASTM B 108-01 after T6-heat treatment. The modified alloys pro-duced were compared with the unmodified alloy castunder the same conditions. Hardness tests were con-ducted both on as-cast and heat-treated alloys by aRockwell hardness tester (Wilson Hardness Ltd.) beforeand after modification. The results shown are averagesof three points in different regions of the alloys.The aluminum-strengthening mechanism was deter-mined from measurements taken on a JEOL JEM 2010high-resolution transmission electron microscope (HR-TEM). Specially prepared disks with a diameter of 3 mm were punched out of thinned Al and weresubsequently jet-polished using a solution of 30 pctnitric acid and 70 pct methanol at   20   C (253 K) and10 V. III. RESULTS Figure 2 shows the macrostructure of the as-cast Al-Si alloy A356 before and after the modification process.It can be seen that the modification process withtitanium carbide nanoparticles has a profound effecton the structure of the cast alloy: the average grain sizeof the modified alloy decreased, and the structure of thematerial became finer.Grain size measurements were conducted using theSIAMS image analysis software. Grain size parameterswere determined for each zone in the field of view. Theseresults are shown in Table II.The calculated results showed that after nanoparticleaddition the grain size of the aluminum alloy decreasedby 44.5 pct.SEM images ofthemicrostructures ofa modified alloysample  vs  an unmodified sample are shown in Figure 3.Typical primary  a -aluminum grains are observed in theas-cast sample. In a modified casting sample, these a -aluminum grains become smaller, and there are smallglobular eutectic silicon crystals, which are uniformlydistributedbetweenthegrainboundaries(thewhiteareasaround the aluminum grains in the image).XRD measurements were used to reveal structuralchanges in the alloys during the modification process.XRD patterns of the modified and of the unmodifiedcast Al-Si (A356) alloys are shown in Figure 4.The XRD results indicate the presence of aluminum(JCPDS 01-071-4624) and silicon (JCPDS 01-070-5680)phases. No difference caused by the modification isobserved; therefore, no new phase has been formed in adetectable amount as a result of the modificationprocess. The very small quantity of the ceramic nano-particles introduced in the modification stage is evenlydistributed and does not comprise any detectable phase.The chemical compositions of the alloys before andafter modification were also determined by opticalemission spectroscopy and are presented in Table III.After T6 heat treatment, two specimens cut from thecenter and two from the perimeter of three differentmodified casting parts were subjected to tensile strengthtests and compared with two specimens from the centerand the perimeter of one as-cast part. Cast partperimeter crystallization rate was much higher ascompare to the crystallization rate at the central areabecause of the different mold thickness.Perimeter area average values of the tensile and yieldstrength and the elongation of the as-cast and modifiedalloys are presented in Table IV. Table I. Chemical Composition of Al A356 Si Mg Fe Ti Others Al7.020 0.354 0.121 0.125 0.1 bal. Fig. 1—Special sand mold used in the research. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 44A, NOVEMBER 2013—4949  The addition of a modifier based on titanium carbidenanoparticles increased the elongation by 20 and 50 pctin the perimeter and in the central part of the mold,respectively. The tensile strength of the parts remainedunchanged.Fractographs of the alloys after tensile testing of thespecimens exhibit ductile–brittle fracture, as shown inFigure 5. There are more dimples in the modified alloy,which are characteristic of ductile fracture. The surfacealso displays many smooth, flat areas separated bybright ridges. The flat areas are cleaved silicon particles.Hardness tests were conducted on the as-cast andheat-treated alloys before and after modification. Theresults shown below are averages of three points indifferent regions of the cast ingots. The hardness of theheat-treated as-cast alloy before modification was42.3  ±  1.7 HRB, and the hardness of the heat-treatedmodified alloy was 49.8  ±  0.3 HRB. The addition of titanium carbide nanoparticles causes the hardness toimprove by 18 pct.Electron microscopy observation was performed todetermine the mechanism of the strengthening whichoccurs in A356 alloy because of the addition of the Fig. 2—Macrostructure of A356 alloy before ( a ) and after ( b ) the modification process. The average grain size of the modified macrostructure(image b) profoundly decreased in comparison to the unmodified macrostructure (image a). Table II. Grain Size Calculations of A356 Alloy Before and After the Modification Process Using the TiC Modifier Al Alloy Analyzed Area (mm 2 ) No. of Grains on 1 cm 2 Average Grain Size (mm)Al-Si Alloy Before Modification 82.0 1485 0.259Al-Si Alloy Modified by TiC Nanoparticles 80.1 4810 0.144 Fig. 3—SEM observation of the A356 microstructure before ( a ) and after ( b ) the modification process (200 times). The modified microstructurecontains small aluminum grains (black areas) with a high concentration of eutectic silicon crystals distributed between the grain boundaries(white areas).Fig. 4—XRD patterns of the aluminum A356 alloy before and aftermodification using TiC nanoparticles. 4950—VOLUME 44A, NOVEMBER 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A  ceramic nanoparticles. Figure 6(a) shows an electronmicroscopy image of a grain boundary of the modifiedalloy. A high dislocation concentration was observednear the grain boundaries. On the other hand, notitanium or titanium carbide was found near the grainboundaries. Therefore, the particles act as crystallizationcenters and can be found in the aluminum grains, asshown in Figure 6(b).Figure 7 shows an electron microscopy image of theunmodified A356 alloy. As opposed to the modifiedalloy, here dislocations are found in the Al grains, andno TiC particles (Ti inclusions) are found in the sample. IV. DISCUSSION It is generally preferable to achieve a grain size in astructure that is as small as possible, because a smallgrain size significantly improves the mechanical proper-ties of metal castings. Significant grain refinement can beachieved by adding inoculants to the liquid metal. Theseinoculants are added in a suitable form to be uniformlydistributed throughout the liquid, and they act asnucleating agents to increase the nucleation ratethroughout the casting.Inthecurrentresearch,TiCnanoparticleswereusedasnucleating agents. Because of their very small size, thenumber of nanoparticles per unit volume is much greaterthan the number of microparticles. These particles serveas crystallization centers in the liquid aluminum andcause the formation of a fine-structured aluminum alloy.Nanoparticles have a very large surface-to-mass ratioand are, therefore, highly surface active. To preventthem from floating on the surface of the melt, thetitanium carbide particles were first mechanochemicallytreated with aluminum powder. The powder obtainedwas pressed to remove the oxide layer. Total amount of 0.03 wt pct of TiC nanoparticles out of the total Al masswas added to the molten metal.Additional experiments were conducted to determinethe optimal concentration of added nanoparticles intothe melt. Total amounts of 0.1, 0.045, and 0.01 wt pct of nanoparticles were added to the molten aluminum indifferent experiments. It was found that the mechanicalproperties in the additional experiments either were notchanged, or the changes were very slight. The addition of 0.03 wt pctwas foundasanoptimalconcentrationintheprocess. We assumed that the addition of a high quantityof nanoparticles causes the formation of large aggregatesthat are not effective in the metal-crystallization process.At the same time, low quantity of nanoparticles has noeffect on the improvement of mechanical propertiesbecause of their dissolution in the melt.Normally, increasing the tensile strength reduces theductility and  vice versa . Ductility is the ability of amaterial to withstand plastic deformation without rup-ture and is quantitatively expressed as the linearelongation of a specimen in a tensile strength test. Table III. Chemical Composition of Cast Alloys Before and After the Modification Process Using TiC Nanoparticles State of the Cast Alloy Si Mg Ti Fe Cu AlBefore Modification 6.99 0.35 0.16 0.13 0.01 baseAfter Modification 6.96 0.34 0.16 0.13 0.01 base Table IV. Mechanical Properties of A356 Alloy Before and After the Modification Process Using TiC Nanoparticles State of the Cast Alloy Tensile Strength (MPa) Yield Strength (MPa) Elongation (Percent)Center of a Cast before modification 268.3 225.0 1.9after modification 280.9  ±  1.0 221.7  ±  4.0 3.8  ±  0.7Perimeter of a Cast before modification 311.0 233.0 6.5after modification 310.4  ±  3.1 226.9  ±  2.2 9.0  ±  1.3 Fig. 5—SEM fracture micrographs of the unmodified ( a ), and modified cast alloy ( b ) (800 times). METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 44A, NOVEMBER 2013—4951
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