Gamma titanium-aluminide (y-TiAl) based alloys are potentially suitable as advanced structural materials for high performance applications such as components in aerospace vehicles, aircraft turbine, and automotive engines due to their unique and superior properties at high temperatures. Hot pressing combined with mechanical alloying (MA) has been reported as favorably as fabrication route to produce various alloys from elemental powders since they offer high densification, particularly for consolidation of reactive elements, as well as elements with significantly different melting points. In this study, formation and densification behavior of y-TiAI alloys was investigated using mechanical alloying and hot pressing at temperatures ranging from 950-110° DC

Astrawinata, Rizal

M. Noor, Ahmad Fauzi

Usman, H.

English

Repository@USM

FABRICATION OF y-TiAl INTERMETALLIC ALLOYS VIA HOT PRESSINGTECHNIQUEH. Usman, A. Fauzi M. N., and R. AstrawinataSchool ofMaterials and Mineral Resources Engineering, Universiti Sains Malaysia, 14300 NibongTebal, PenangABSTRACTGamma titanium-aluminide (y-TiAl) based alloys are potentially suitable as advanced structuralmaterials for high performance applications such as components in aerospace vehicles, aircraftturbine, and automotive engines due to their unique and superior properties at high temperatures.Hot pressing combined with mechanical alloying (MA) has been reported as favourably as fabricationroute to produce various alloys from elemental powders since they offer high densijication,particularly for consolidation of reactive elements, as well as elements with signijicantly differentmelting points. In this study, formation and densijication behavior of y-TiAI alloys was investigatedusing mechanical alloying and hot pressing at temperatures ranging from 950-1100 DC. The resultingphases were analyzed using X-Ray diffraction (XRD, the physical and mechanical properties werealso determined.. The hot pressing and MA was found to be effective to obtain densijication up to 99% at hot pressing conditions of1100 °c and 60 min, with y-TiAI phase as dominant phase.Keywords: y-TiAl, mechanical alloying (MA), hot pressing, densification.INTRODUCTIONGamma-TiAl intermatallic alloys are potentially suitable as advanced structural materials for highperformance applications such as components in aircraft turbine engines, aerospace vehicles andautomotive engines due to low density, high strength and high elastic modulus, excellent creep andoxidation resistance at high temperatures (Noda, 1998 & Wilshire, 1994). However, in processing ofy-TiAl intermatallic alloys using powder metallurgy route , the presence of reactive elements such astitanium, aluminum and chromium has become a challenge in order to achieve fully densifiedcomponents.Literature showed that vacuum hot pressing, which combines pressure and sintering process, has beensuccessfully used to compact superalloy powders (Gessinger, 1984). Similarly, hot pressing has beenused to consolidate mechanically alloyed y-TiAl alloys resulting in the production of fully dense bulkalloys with fme grained microstructures (Dutkiewicz et. al., 2003 & Szewczak et al., 1999). Recentwork carried out to study densification behaviors of conventional titanium alloy powder (Ti-4Al-4V)using hot pressing can also produce compacts with high relative density (Kim & Yang, 2001).In this study, hot pressing was used to fabricate y-TiAl intermatallic alloy. Effects of hot pressingtemperature and holding time on densification, as well as the phases obtained are reported.EXPERIMENTAL PROCEDUREPowders oftitanium (size under 83.81 J.U1l and purity of99.7 %, STREM Chemicals), aluminium (sizeunder 72.53 J.U1l and purity of 99 %, STREM Chemicals) and chromium (size under 50.92 J.U1l andpurity of 99 %, STREM Chemicals) were used as starting materials. The powders were initially mixedto prepare a composition of Ti-48Al-2Cr (in at.%). The mixing and milling were carried out at roomtemperature in the planetary mill (Fritsch Pulverisette P5/4) at a rotation speed of 125 rpm for millingtime of 3 hrs. The milling jars and milling media are made from steel. A ratio of ball to powderweight was 10:1 (Szewczak & Wyrzykowski, 1999). A small amount (1 wt. %) of methanol wasadded to avoid the sticking of powders to milling media (Calderon, 2002). The milled powders werehot pressed at 950-1100 °C and a holding time range of 1-2 h under 29.24 MPa in vacuum condition.214A schematic process of uniaxial die hot-pressing is shown on Fig. 1. A cylindrical die assembly ismade from high strength graphite. Interior surfaces of that are protected from reaction with thepowders by a boron nitride coating, which has high-temperature lubrication and mold releaseproperties (Mao et ai., 2003). The dimensions of hot pressed samples were approximately 33 mm indiameter and 5 mm in thickness. Density measurements were measured on samples by usingArchimedes's Method. Hot pressed samples were characterized by X-ray diffraction (XRD) usingCuK radiation on a Philips PW 1877. Rockwell hardness tests were carried out using RockwellHardness Tester, LECO.Fig. 1. A schematic process of uniaxial die hot-pressingRESULTS AND DISCUSSIONFig. 2 shows densification behavior of the samples densified over hot pressing temperatures at threedifferent holding times. Generally, increasing the temperature of hot pressing would increase thedensification. A densification of about 99 % could be achieved with a temperature of 1100 DC withholding times of 60 min and 120 min. Therefore hot pressing is seen as a process potentially toproduce highly densified compact. However, influence of holding time is also crucial. Short holdingtime (in this work, 60 min and 120 min) produced good densification. In fact, as plotted in Fig. 2, itcan be seen that a holding time of 100 min had almost achieved high densification, and increasingtemperature increased densification slightly (98.3 % to 99.2 % when temperature increased from 950DC to UOO DC). With 60 min and 950 DC, lower densification attained, and increasing temperaturesubstantially increased derisification. However, a low temperature with longer holding time (950 DC,120 min) did not produce good densification due to melting of aluminium, hence leaked out ofgraphite die. At this stage, the leaking from die need to be overcome.XRD patterns ofTi-48Al-2Cr samples consolidated via hot pressing at various hot pressing conditionsare shown on Fig. 3. It was determined that on Fig 3a (for all holding times), Fig 3b and 3c (holdingtime of 60 and 100 min.) the samples had formed intermetallic phases of y-TiAl and cx.2-ThAl with thefirst strongest line belonging to y-TiAl, except for Fig. 3c (holding time of 100 min) in which the firststrongest line belongs to cx.z-ThAl. In addition to y-TiAl and cx.2-ThAl, there is also another phase,which might be a p (B2) phase. The presence of Pphase can be understood due to the addition ofalloying element of Cr (Jewett, 1997). Chromium and other alloying elements such as Mo, Nb, Mnand V are recognized as Pstabilizing elements (Shao & Tsakiropoulus, 1999).21599.50 .99.0098.5098.00~ 97.50:?j 97.00~] 96.5096.0095.5095.0094.50HZOlIOO108010601040LOZO100098096094.00 +---r----,---r---,---~--,--------;r------,---;940Temperature (oC)Fig. 2. Effect of hot pressing temperatures and holding time on densificationNote:y: y-TiAlu: uz-ThAl13 (B2): TisAhCrzFig. 3. XRD Patterns ofTi48Al-2Cr samples hot pressed at (a) 950°C, (b) 1000 °c and (c) HOD °c216On the other hand, by increasing holding time up to 120 min Fig. 3b, there is no y-TiAl phase presentand only one peak matches with uz-ThAl phase. At Fig. 3c (holding time of 120 min.) the phases ofy-TiAl and Uz-Ti3Al are also not present.Effect of hot pressing temperature on hardness is shown on Fig. 4. In this case, by increasing hotpressing temperature the hardness of samples increases until reaching a maximum hardness at 1100DC, holding time of 60 min. From Fig. 2 and Fig.3 it was found that as hot pressing temperatureincreases, at certain holding time, the increase of densification as well as the phase formed hadresulted in the increase of hardness.72.00 , :71.0070.00~ 69.00~~ 68.00~:Ii 67.0066.0065.00 1...... 60minJ.. j80·IOOmin"h··120min112011001080106010401020100098096064.00 +------,---,---..,......---,-------,----,---.,-------,----;940Temperature (oC)Fig. 4. Hardness of samples versus temperature hot pressed at various holding timeCONCLUSIONSThe hot pressing combined with mechanical alloying is effective to produce near dense bulk samplesof y-TiAl intermetallic. Densification up to 99 % and high hardness can be achieved at hot pressingconditions of 1100 DC, 60 min and the present phases are y-TiAl, uz-Ti3Al and ~ (B2) with y-TiAl asdominant phase.ACKNOWLEDGEMENTSThe authors would like to acknowledge IICA for AUN/SEED-Net for research fund undercollaborative research program, for support from USM's FRGS grant and are also grateful toCeramics Technology Group, SIRIM Berhad for performing the hot pressing.REFERENCES1. Calderon, H.A., Garibay-Febles, V., Umemoto, M. and Yamaguchi, M., 2002. Mechanicalproperties ofnanocrystalline Ti-AI-X alloys. Mater. Sci. and Eng., A329-331, p.l96-205.2. Dutkiewicz, 1. and Maziarz, W., Heinrich, H. and Kostorz, G., 2003. Structure of Ti-Al-Nbintermetallics produced by mechanical alloying and hot-pressing technique. Mater. Chem.And Phys. Vol. 81, p. 414-416.3. Gessinger, G.H., 1984. Powder metallurgy of superalloys, 1st edition, Switzerland: BBCBrown Boveri & Co. Ltd.4. Jewett, TJ., Ahrens, B. and Dahms, M., 1997. Stability of TiAl in the Ti-Al-er system.Materials Science and Engineering, A225, p. 29-37.2175. Kim, K.T., and Yang, H.C. 2001. Densification behavior of titanium alloy powder during hotpressing. Mater. Sci. and Eng., A313, p. 46-52.6. Mao, S.x., McMinn, N.A. and WU, N.Q. 2003. Processing and mechanical behavior ofTiAllNiA1 intermetallic composites produced by cryogenic mechanical alloying. Mater. Sci.and Eng. A363, p.275-289.7. Noda, T., 1998. Aplication of cast gamma TiAI for automobiles. Intermetallics. Vol. 6, p.709-713.8. Shao, G. and Tsakiropoulus, P., 1999. Solidification structures of Ti-Al-Cr alloys. J.Intermetallics, Vol. 7, p. 579-587.9. Szewczak, E., Presz, A, Witek, A, Wyrzykowski, J.W. and Matyja, H. 1999. Microstructureand composition of mechanically alloyed and hot pressed Ti-AI alloys. NanostructuredMaterials, Vol. 12,p. 167-170.10. Szewczak, E., and Wyrzykowski, J.W., 1999. Influence of The mechanical Alloyingparameters on Crystallite Size of Ti-AI Powders. Nanostructured Materials, Vo1.l2, p.I71-174.II. Wilshire, W., 1994. Recent developments in materials for high temperature service. RAMM,p.l-IO.218

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Fabrication Of y-TiAl Intermetallic Alloys Via Hot Pressing Technique.

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