Synthesis of Al-TiAl 3 compound by reactive deposition of molten Al droplets and Ti powders

Transcription

1 Synthess of Al-TAl 3 compound by reactve deposton of molten Al droplets and T powders Peng-yun WANG 1, He-jun LI 1, Le-hua QI 2, Xang-hu ZENG 2, Han-song ZUO 1 1. C/C Compostes Technology Research Center, State Key Laboratory of Soldfcaton Processng, Northwestern Polytechncal Unversty, X an , Chna; 2. School of Mechancal Engneerng, Northwestern Polytechncal Unversty, X an , Chna Receved 15 February 2011; accepted 9 Aprl 2011 Abstract: The Al-TAl 3 compound materals were prepared by depostng molten Al droplets onto the T powders layer at 350 C. The results show that the stochometrc TAl 3 phase s the only T-Al ntermetallc compound formed durng the reacton process. The mcrostructure analyss shows that the TAl 3 partcles dsperse n an Al matrx. The results of the compressve tests of the specmens at room temperature show that the reacted Al-TAl 3 compound has hgher compressve strength of MPa and fracture stran of 13.5 %. Key words: T-Al ntermetallcs; TAl 3 ; molten Al droplets; synthess; compressve propertes; 1 Introducton Recently, ordered T-Al ntermetallc compounds, especally the stochometrc compounds TAl 3, TAl(γ) and T 3 Al(α 2 ), have attracted extensve attenton of many researchers, because of ther excellent propertes, such as the superor specfc stffness, hgh meltng pont, good oxdaton resstance at hgh temperature[1]. Therefore, these T-Al based ntermetallc alloys are consdered as one of the most promsng canddate materals as the structural components used at hgh temperature[2 3]. Among these compounds, TAl 3 has the lowest densty of 3.4 g/cm 3, the hghest mcro hardness of kg/mm 2 and the best oxdaton resstance even at C[4]. However, the applcatons of TAl 3 n the engneerng and aerospace felds are lmted by ts poor ductlty. In addton, the loss of ductlty at ambent temperature s usually accompaned by a change of fracture mode from ductle transgranular to brttle ntergranular or to brttle cleavage[5]. Despte the fact that a lot of toughenng strateges have been developed to mprove ther toughness, machnng qualty s stll a dffcult problem to tackle. Near-net shape manufacturng technology s consdered as one of the best choces for preparng such materals[6 7]. As an emergng near-net shape manufacturng technology, the unform droplet-based spray formng (UDS) technque has attracted more and more attentons[7 14]. Durng ths process, molten metal are dspersed nto unform mcro-droplets by a vbraton of a rod mmerged n the metals melt or by an nert gas pulse. Successve droplets are delvered precsely onto a movable substrate n X, Y, and Z drectons to buld a three-dmensonal (3D) component layer by layer n the lght of computer-aded desgnng (CAD) nformaton. Therefore, UDS technque can accomplsh rapd prototypng (RP) of products. Addtonally, the UDS technque could be theoretcally appled to any metal that can be contaned n a crucble n ts molten state[11]. The UDS technque had been appled for the solder prntng[15] and preparng metal droplets wth hgher meltng pont, for example, Al alloys. Al-4.5%Cu (mass fracton) droplets wth 295 μm n sze and Al-4.3% Fe droplets wth 250 μm n dameter were fabrcated by UDS technque[12]. Besdes, a droplet-based rapd prototype technque for makng metallc structural components drectly from ther CAD models was developed by ORME et al[13 14]. A 3D Al parts prepared by depostng molten Al droplets had been Foundaton tem: Project (2008AA03A238) supported by the Natonal H-Tech Research and Development Program of Chna; Project ( ) supported by the Natonal Natural Scence Foundaton of Chna; Project (B08040) supported by the Program of Introducng Talents of Dscplne to Unverstes of Chna Correspondng author: He-jun LI; Tel: ; Fax: ; E-mal: lhejun@nwpu.edu.cn

2 154 Peng-yun WANG, et al/progress n Natural Scence: Materals Internatonal 21(2011) studed by CAO et al[9]. They also attempted to depost molten Al droplets onto T powder bed wth the result that the two elements exothermcally reacted, ntermetallc beads were produced[7]. By repeatng ths process, a 3D structural component wth T-Al ntermetallcs would be eventually fabrcated. Addtonally, a heterogeneous structural component could be fabrcated by selecton of fall pont of molten Al droplets. To date, there s lttle reported nformaton about the synthess and mechancal propertes of T-Al ntermetallcs prepared through the UDS technque. The am of the present work was to nvestgate the synthess technology of T-Al ntermetallcs through molten Al droplets depostng on T powders layer. The reacton mechansm by thermodynamc calculaton and compressve propertes of fabrcated T-Al ntermetallcs were also dscussed. 2 Expermental A schematc dagram of UDS apparatus for molten Al droplet generaton and deposton s shown n Fg.1. Durng ths process, 30 g pure Al rod was placed n a graphte crucble and heated to 950 C, whch was mplemented n a glove box full of % purty argon gas to avod oxdaton of Al lqud. A nozzle of 0.5 mm n dameter was located at the bottom of the crucble. Molten Al droplets were ejected out of the nozzle under an argon gas pulse that was controlled by a solenod valve. The pressure and duraton tme of the argon gas pulse were MPa and ms, respectvely. The frequency range of droplet formaton was occurrng at rates between 1 and 10 Hz. The as-receved T powders (99.3%, mass fracton) passed through the seve of 325 meshes and were deposted onto the substrate to make a powder bed. The powders were fed by ultrasonc vbraton[16]. The dstance between the nozzle and the surface T powders bed was approxmately 10 mm. Two experments under condtons of T powders at room temperature and heated to 350 C were mplemented, respectvely. The mcrostructure and phase dstrbuton of reacton product n the T-Al system were dentfed by X-ray dffracton (XRD, X Pert Pro MPD, PANalytcal BV, the Netherlands), optcal mcroscope (OM) and feld emsson scannng electron mcroscope (FESEM, SUPRA55, ZEISS, Germany) wth back scatter electron (BSE) detector. The approxmate composton of reacton product was dentfed by an energy dspersve spectroscopy (EDS). The determnaton of T-Al ntermetallcs n reacton product and the reacton mechansm between molten Al droplet and T powders were nvestgated by a smultaneous TG-DTA (TGA/SDTA851 e, Mettler Toledo, USA) machne. In the frst DTA test, about 12 mg sample of reacton product was heated at the rate of 30 C/mn from room temperature to C. In the second DTA test, approxmate 17 mg sample was heated wth the rate of 10 C /mn from room temperature to C. The operaton was conducted under argon atmosphere. The mechancal propertes of the specmens machned from the product were evaluated. Rectangular specmens wth dmenson of 1.6 mm 1.6 mm 2.4 mm were used for the compresson tests. The tests were mplemented on an Instron 5848 mcrotester wth a statc load of 2 kn at an ntal stran rate of s 1. Fg.1 Schematc dagram of unform droplet-based spray formng process for alumnum alloy

3 Peng-yun WANG, et al/progress n Natural Scence: Materals Internatonal 21(2011) Results and dscusson The dameter of an alumnum droplet ejected was about 1mm n ths case. The deposted molten Al droplets soldfed nstantly and cannot nfltrate nto T powders bed and enwrap T partcles at room temperature. When molten Al droplets were deposted onto the T powder layer at 350 C, an exothermc reacton between Al lqud and T powders occurred, whch nduced the propagaton of a combuston wave. The XRD pattern of reacton product s shown n Fg.2. It can be found that a new phase of TAl3 forms due to the volent reacton between Al lqud and T powders, and the peak of the TAl3 phase s very strong. In addton, stll redundant Al and trace amount of T exst n the reacton product. The reasons for ths phenomenon wll be gven below. Fgure 3(a) shows the morphology of the T powders as receved. The T partcles exhbt rregular shape and are 5 50 μm n sze. Fg.3(b) shows the morphology of Al balls collected n the UDS process. The sze of Al balls s unform and about 1 mm n dameter. The optcal mcrostructure mages of the reacton procuct are shown n Fgs.3(c) and (d). Whte rregular T powders wth the sze of 40 μm nearly reacted completely, except for separated gray cores. And the grey area represents resdual Al. Accordng to the EDS results of the product n Fg.4, brght whte phase 155 should be TAl3 derved from the smaller T partcles enveloped by Al lqud. Fg.4(a) shows a BSE mage of the product. Three knds of phases marked wth dfferent colors are found n the sample. By EDS and XRD, the three phases could be characterzed as T (A), TAl3 (B) and Al (C). The T-rch phases gve rse to the brghtest mage because T has the hghest atomc number (22) of the elements present. Al-rch areas gve rse to the darkest mage because t has the lowest atomc number (13). TAl3 phases present gray mage due to T atoms replaced by Al atoms from lqud alumnum. Fg.2 XRD patterns of T powders (a) and reacton compound (b) of T powders and Al droplets Fg.3 Morphologes of T powders(a), soldfed Al droplets(b), and reacton compound ((c), (d))

4 156 Peng-yun WANG, et al/progress n Natural Scence: Materals Internatonal 21(2011) Fg.4 BSE mage of reacton compound(a) and EDS patterns correspondng to spots A(b), B(c) and C(d) DTA had been employed to dentfy the compounds of TAl 3 and dscuss the reacton mechansm between lqud Al droplet and T partcles. The DTA curve of the product contanng TAl 3 s shown n Fg.5(a). A sharp endothermc peak at C was due to the melt of Al, whch s consstent wth the redundant Al n the XRD pattern. Another endothermc peak s located at C, whch approaches to the meltng pont of TAl 3 (1 340 C). Ths ndcates the exstence of TAl 3 n reacton product. The offset of peaks towards hgh temperature n the DTA curve may result from the larger heatng rate of 30 C /mn. To understand the reacton between Al lqud and T powders, the sample of the second DTA test s a mxture of T powders and pure Al balls collected n UDS experment as shown n Fg.3(b). Fg.5 (b) shows the DTA curve of the mxture of Al balls and T powders. Durng the sample beng heated, two endothermc peaks and two exothermc peaks appear, respectvely. However, the peak refers to the reacton between lqud Al and sold T partcles s at C. There were sold T partcles and lqud Al at ths temperature. The possble reactons are as follows: () l TAl3 T + 3Al (1) T + Al() l TAl (2) 2T + Al() l T2Al (3) Accordng to the bnary T-Al phase dagram[17], T 2 Al cannot exst stably. So the possble TAl 3 and TAl Fg.5 DTA curves of reacton compound (a) and mxture of T powders and soldfed Al droplets (b)

5 Peng-yun WANG, et al/progress n Natural Scence: Materals Internatonal 21(2011) compounds were consdered. Free energes of reactons were calculated usng the followng formula[18 19]: T t ( ) =Δ f + d + Δ H T H c T H (4) p, where Δ f H was the standard enthalpy of formaton from the elements n ther reference phases at T= t K; Δ H was the standard enthalpy of transton at the transton temperature. The calculaton of the entropy functon was as follow [18 19]: t T ΔH ( ) = + d ln p, + Tt S T S c T (5) where s the standard entropy of the substance at T= K; Δ H t /Tt was the entropy of phase change. The molar heat capactes of pure substances can be S descrbed wth suffcent accuracy over a relatvely wde range of temperature by means of the followng polynomal[19]. c p, = a + b 10 T + c 10 T + d 10 T (6) where c p, s the standard heat capacty of the substance concerned over the temperature range under consderaton; a, b, c, d were the temperature dependence of the heat capacty of the substrate. At certan temperature, the Gbbs free energy of the reacton system can be expressed as follow: T T T ΔG = ΔH TΔS (7) where H, S and T stand for the enthalpy, entropy and thermodynamc temperature, respectvely. The thermochemcal data of pure substances adopted n ths calculaton was from[19 20]. The calculaton results of reacton enthalpes and free energes at C are lsted n Table 1. It can be seen that reacton enthalpy and free energy of reacton (1) are both less than that of reacton (2) at C. So the reacton between the molten Al droplets and heated T powders took place as the reacton (1). 2 contrast wth these T-rch compounds, TAl 3 exsts as a lne compound and crystallzed n the tetragonal DO 22 unt cell. Many studes nvolvng synthess of ttanum alumndes showed that TAl 3 formed pror to the formaton of any other ttanum alumndes present n ths system[17, 21]. Furthermore, SUJATA et al[4, 22]. proved that TAl 3 was the only compound formed durng reacton between T and Al n the temperature range of K. The phenomena appeared n our experments accorded wth ths vewpont. In addton, TAl 3 layer surroundng bgger T partcles hndered drect contact between resdual lqud Al and T cores. Due to the lower temperature and shorter tme of the reacton, lqud Al could not dffuse through TAl 3 layer and react wth the T core. Therefore, the resdual Al and Table 1 Calculated results of reacton enthalpes and free energes (ΔH, ΔG) n reactons (1) and (2) Temperature/ C ΔH/ Reacton (1) Reacton (2) ΔG/ ΔH/ ΔG/ From the bnary T-Al phase dagram[17], t can be seen that several ntermetallc compounds, such as T 3 Al, TAl, TAl 2 and TAl 3, form n the T-Al system. Among these compounds, T 3 Al and TAl belong to T-rch compounds and exst over a range of composton. In Fg.6 Compressve test results for Al-TAl 3 samples: (a) Falure mode n quas-statc compresson specmen; (b) Compressve engneerng stress stran curve; (c) SEM mage of fracture surface

6 158 Peng-yun WANG, et al/progress n Natural Scence: Materals Internatonal 21(2011) trace amount of T appeared n the reacton compound and other compounds, such as TAl and TAl 2 were not formed n the T-Al system. Based upon the results of X-ray dffracton and mcroscopc observatons (SEM and OM), the reacton product can be characterzed as TAl 3 partcles dspersed n an Al matrx. Unaxal compresson tests of the Al-TAl 3 compostes were performed. Ths specmen shows an edge fracture wth an angle of about 45º wth respect to the compressve drecton(fg.6). The measured compressve stress stran curve of the sample at room temperature s shown n Fg.6. The specmen exhbts hgh compressve strength of MPa and fracture stran of 13.5%. Ths character mght be related wth the chemcal composton consstng of Al and TAl 3. The exstence of TAl 3 and Al n the specmen contrbuted to the strength and fracture stran, respectvely[1]. Fg.6 shows SEM mage of the fracture surface of the specmen after the compressve test. The Al-TAl 3 composte fractured wth hgh-densty tear edges and cleavage steps, whch were typcal feature of quas-cleavage fracture. 4 Conclusons 1) The Al-TAl 3 compound materals were prepared by unform droplet sprayng technology under the condtons of droplets temperature of 950 C and the T powders heated to 350 C. 2) The stochometrc TAl 3 was the only T-Al ntermetallc n the reacton product. Most T powders on the order of 40 μm reacted wth lqud Al completely. 3) Accordng to the calculaton results of enthalpy and free energy, the reacton mechansm was consdered as that lqud Al droplets reacted wth sold T powders by T+3Al(l) TAl 3. The fabrcated composte conssted of TAl 3 partcles dspersed n an Al matrx and exhbted hgh compressve strength of MPa and fracture stran of 13.5% at room temperature. References [1] ZHANG Y G, HAN Y F, CHEN G L, GUO J T, WAN X J, FENG D. Structural ntermetallcs [M]. Bejng: Natonal Defence Industry Press, 2001: 144, 686, 691, 833. (n Chnese) [2] LIU Y D, LIU W. 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