Evaluation of the Potential of the Ternary Aluminides in Nb-Ti-Al system for Hiyh Temperature Application

D0I:10.13374/i.issn1001-053x.1992.02.008 北京科技大学学报 第14卷第2期 Vo1,14No,2 1992年3月 Journal of University of Science and Technology Beijing March 1992 Evaluation of the Potential of the Ternary Aluminides in Nb-Ti-Al system for Hiyh Temperature Application 元 Chne Guoliang(陈国良)·Sun Zuqing(孙祖庆)·Xie Xishan(谢锡善)· Yao Kefu(姚可夫)·Ren Yunrong(任允蓉)·Yang Wangyue(杨王)" ABSTRACT:Being aimed to developing intermetallic base structural materials to be worked at above 1000C,a systematic investigation has been performed of the fundamental crystallographic structure,phase constitution map,density- composition dependence,oxidation resistance and general mechanical behaviour for the ternary intermetallic alloys in the Nb-Ti-Al system.Promissing resu- Its have been obtained at the initial stage in the current program.The 1100C high temperature strength at the strain rate of 10-2/s and 10-/s for the lernary compound Nb'TiAls (y)base alloys are generally much higher than that of the binary compound TiAl (y),especially in the yi+a2 phase region, it is as high as twice as that of TiAl.The densities are 4.1-4.79g/cm3, depending on relative amount of the intermetallics involved and the density difference between the intermetallics.The oxidation resistance at the tempe- rature of 900-1200C of the compound NbTiAls is relatively low but still acceptable.However,by adjusting the relative amount of constituents and adding a small amount of alloying elements it can be singnificantly improved,The room temperature brittleness remains to be solved. KEY WORDS:aluminide,high temperature alloy,Nb-Ti-Al system,evalution Advanced aero-propulsion design now being considered will require materials with high temperature strength,creep resistance,oxidation resistance and ther- momechanical stability,in excess of the range,where nickel based superalloys 1391一10一21收稿 ·材料系(Dept,of Materials Science and Engineering). 157

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are now being used.The specific properties of ordered intermetallic compounds attract the attention of materials scientists.Various aluminides,such as NisAl, TiAl and TisAl have been studied and tremendous improvement of properties has been achieved.Attention now is also being directed to refractory metal compounds containing Ta,Mo,Nb and Zr,in order to develop structural mate- rials for even higher temperature application. Our present research project is aimed toward serching the possibility of de- veloping a novel intermetallic material to be worked at above 1000C,with emphasis on high melting point,low density and relatively simple crystallogra- phic structure,Unfortunately the melting points for most binary aluminides are lower than 1700C.However it is interesting to note that the melting points of ternary aluminides may be high enough,2.On the other hand,the Ti-Al system offers a rich variety of phase and microstructure,Many of its equili- brium phases are disordered solution with wide compositional range or simple structure based on the ordered solution.In addition,since the third alloying element Nb in Ti-Al alloys enhaces the ductility of TiAl or TisAl,a conside- rable interest has developed in Nb-Ti-Al ternary system.Based on the consi- deration mentioned above.Nb-Ti-Al ternary system was chosen for study. Systematic work was performed of the fundamental crystallographic structure, phase distribution,density-composition dependence,oxidation resistance and general mechanical behaviour.The work is in progress.The resulus obtained in the initial stage are presented in this paper. 1 Experimental Procedure The alloys investigated in the present study were within a wide range of Ti 10 8 B 1380℃1300，C0 1200℃ 50h 60 70h1200℃ 1000℃2h Yom肥50七 2h 10h F.C. 908070 605040302010A1 ,Nb,% Fig.1 Compositions of experimental Fig.2 Homogenizing treatment alloys flow chart 158

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compositions among TisAl (aa),TiAl (y),NbTiAla(y1),NbAls()and TiAls (in Nb-Ti-Al ternary system,with Al being from 25at%to 75at%,Nb 0at% to 25at%and Ti (25-75)at%.The actual compositions are shown in Fig.1. The bulk samples for structure study were prepared from 4NAl,2N8Nb and 3NTi by arc melting in a gettered argon atmosphere in a small button of 50g.In excess 5 remelts were performed to ensure mixing of the alloys,The chemical anlysis of the alloy having the NbTiAls composition confirmed the gaseous content was not to be of quantity to significantly influence phase stru- cture(Table 1).The samples for property study were melted by a vacuum induction furnace.The macrosegregation was investigated by a electronprobe at the different location of the ingot,The result showed that the composition from the edge to the centre of the ingot was reasonably homogenous.In orde to eliminate the microsegregation homogenization was performed,The homno- genizing processing flow chat can be seen in Fig.2.Smnall metallographic and Tablel Analysed gaseous content in NbTiAla alloy metho d N,Ppm O,ppm non-comsumablc arc melting <100 65 vacuum induction melting <100 473 mechanical samples were then made by electric spark cutting,Phase identifi- 的 cation was determined by means of microscopy (SEM TEM),microanalysis (EDX),structure analysis (selected area diffraction,microiiffraction,conver- gent beam diffraction and X-ray diffraction).Compessive test was performed on a Gleeble machine at various temperatures up to 1100C.The strain rate was chosen at 10-2/s and 10-5/s (corresponding to the strain rates for hot de- formation and creep respectively).Tensile test was also performed for part of the alloys on a MTS machine.Fracture mode under tcnsile condition was observed by SEM.Static oxidation test and oxidation kinetics measurement were carried out in high temperature furnace in air at from 900 to 1200C 2 Results and Discussion 2,1 Microstructure Studies Limited work has been done on the phase diagram of Nb-Ti-Al ternary system.Relatively little has been concluded on the central area of the phase diagram,especially on the presence of the ternary intermetallic compound NbTiAls.Published reports are subject to controversy1-.Zazharovt2, claimed the existence of NbTiAls having a tetragonal lattice unit of a= 159

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0.8418nm and c=0,4538nm.In contrast,Jewett et alt 4)disproved the above rcsult,Their published Nb-Ti-Al phase diagram at 1200C indicated that the alloy with nominal composition of NbTiAls was located within the dual-phase ficld y+n.In order to resolve the apparent conflict between the previous studies,the phase structure and morphology of the sample having NbTiAls com- position,annealed,water quenched from 1200,900,600C and splat quenched respectively,were investigated.The results from X-ray diffraction were some- time ambiquous indeed,the diffraction pattern may be indexed by two phases e+n.Optical and electronic microscopy revealed the existence of a needle-like morphology.However,EDX analysis clearly showed that the compositions of the matrix and the needle-like area were essentially the same,the needle-like mor- phology may be caused by twinning or a diffusionless transformation in solid state at intermedium temperature range (Fig.3).Importantly,indexing the electron di- ffraction patterns from the matrix undoubtly proved to be presence of NbTiAls ternary compound.Fig.4 shows a series of EDP obtained by tilting the thin foil sample from 20 to -35.Using the lattice pararmeter of a=0.8383nm and c=0.4673nm of tetragonal lattice unit,the reciprocal lattice spacing and iheir mutual angles were calculated.It was found that all the patterns could be indexed by the calculated spacings and angles within very small deviation.The Fig.3 Back scallered clectron micrograph of the alloy having stoichiomctric composition of NbTiAl3, microanalysis confirmed the composition range of the phase as 15-20 at%Nb, 20-25at%Ti and 56-62at%Al roughly.Based on the analysis by electron diffra- ction the X-ray diffraction peaks can also be indexed by same crystallogra- phic parameters as the above within a reasonable measuring error,except one or two weak peaks of n phase,which may be caused by a minor segregation.Our work on the electron channel enhaneing analysis [s)show- ed that the addition of Nb occupied the site of Ti sublattice in TiAl.The 160

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10 7,5 24 Fig.4 a.TEM morphology of single phasc,b.electron diffraction pattern obtained by Lilling the specimen from +20 to -35 and keeping (421)fized. 161

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Fig.5 TEM morphology and electron diffraction pattern showing the cxistcnc: of yi+y dual phasc region results from the systematic investigation by clectron diffraclion and microana- lysis of thin foil sample indicated that with increasing Nb content the phase having Llo structure and gradually transformed to an phass (denoted as the y phase,,having higher degree of order in terms of selective distribution of Nb atom in a specific location,(101)superlattice spot,which should not appear ◆ in Llo structure can be clearly seea in Fig.5,a micro-diffraction pattern.As a further development of ordering the unit cell of the structure is refitted as y1.The TEM study has proved the existence of the y+y dual phase range. However no evidence has been obtained from X-ray or electron diffraction pattern to indicate the presence of a y+y two phase region.This may su- ggest that the ordering from to be thermodynamic secondary transformation. An estimated parlial phase constitution map was established as shown in Fig.6. The results from rapidly solidified samples further confirmed the existence of NbTiAls phase.Since splat-quenching significantly reduces segregation,TEM clearly shows that,from the foil thinned from the area near surface of the 162

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sample,only equiaxed single phase is presented (Fig.7),which is identified as NbTiAls by electron diffraction.The rcsullt from X-ray diffraction indicates there is no peaks of n phase,the positions of all the peaks were in excellent agreement with the calculated values of the crystallographic parameters of NbTiAls.The phase constitution map for rapid solidified state was also obta- ined and it appearcd that all the single phase fields moved towards high Al contents, ￥y1 Ti d2 TizAl TiAl NbTiAl3 0706 Ordered d (Ti,Nb)Al; 1020304050 8 008070605040332010A1 Nb, Fig.6 Phase constitution map for the Fig.7 TEM morphology of rapidly solidified area concerned in Nb-Ti-Al single r1 phase and selected area system for the annealed state clectron diffraction pattern 2,2 Density-composition dependence Fig.8 shows the density map,the bands on the map indicate iso-density. For the single phase yi alloy the density varies from 4.1 to 4.79g/cms as the composition changes.In the multi-phase ragions the density depends on the relative amount of the intermetallics involved and the density difference be- tween the intermetallics.Since the density of y:approximates to that of aa the iso-density line between these phases is almost parallel to the tie line be- twcen these two phases.It is interesting to note that,in spite of a large con- tent of Nb,the density of the alloys studied are rather low. 2.3 Oxidation resistance The static isothermal oxidation maps from 900 to 1200C has been obtai- ned.Fig.9 shows the map of 1100c as an example.It can be seen that the oxidation resistance of the alloy having ihe stoicheometric composition of NbTiAls compound is relatively low,but still comparable with that of the 163

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i 405060708090 30 40 2 2030 67心 2030405060708090 A Nb 9080706050403020 10A1 b,9080706050403c2a104L Nb.% Nb,9% No density Nbwt%=100-x-y Fig.9 Oxidation map at 1100c oxidation 1 4.384.39 y=47.5-0.4x 2 4.274.284.29 y=48.7一0.4y resista血ce1eve1(g/m2h)】-0.17~0.3: 3 4.234.214.73 y=51.2一0.4x 且-0.4~0.7；I-0.7-1.5IV-8.4~12.5 4.114.12~4.07 y=53.1-0.36g 3,98~3.97~3.98 y=55.0-0.35x 6 3.75-3.74×3.71 y=54.8-0.12x 3.52-3.56 y=54.0-0.1x 4.66-4,63-4.73 y=48.42一0.55x x=Ti wt%y=Al wt% Fig.8 Deasity-composition map for the area concerned in Nb-Ti-Al system binary compound TiAl,The relative content of the constituents greatly influ- ence the oxidation rate.Unless the content of Nb or Ti is too high,in gene- ral,the higher the content of Al,the higher the oxidation resistance.Alloys in the range of (55-64)at%AI and moderate Nb (6at%-14 at%)also show very low weight gain at 1100 and 1200C,which is much better than TiAl,say,by Table 2( Compresive strength at Table 3 Representive data of 1100℃(MPa) oxidation resistance (mg/cm3)for No phasc e=10-2/s e=10~5/s 100h 7 No y1 440 92 phase 1000℃ 1100℃ 1200℃ 14 y1' 300 39 7 (NbTiAl3) 38.5 125.0 40.0 17 y 190 38 14 Y1 15,8 4.29 17,3 % Y1 335 66 6 Y1′+y 0.66 6.53 15.9 63 162 25 63 0.e0 1.46 22.9 66 y1+Y+a2230 55 17 Y(TiAI) 35.3 84,2 116,4 71 y1+7 322 64 Y1+7 2.94 4.25 22.9 Y1+a2 450 68 65 Y1+C2 2,71 6.97 24.1 市 Y1+c2 336 66 70 Y1+a2 1,21 1.84 31.7 164

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a factor of about 10 at 1200c for 100h.The representive results are shown in Table 2.Al2Oa and TiAl2 were generally detected but without the oxide of Nb,except where the content of Nb is high. Various alloying elements such as Cr,Y,Si,Co,Ce etc.were added at different level to investigate the possibility of improving the oxidation resis- tance.The results indicated that some of the alloying elements greatly redu- ced the weight gain,which could be as low as 7.2mg/cm3 at 1200C for 100h. 2.4 General mechnical behaviour The flow stress at 1100'C at the strain rate of 10-2/s are summarised in Fig.10a,with classified according to the strength level and the representive data in Table 3.It can be scen that the high temperature strength of the alloys based on yi+n or y,+a2 phases are higher than those for y-base alloys. 0.2 of 450MPa is obtained in y,+az region,the strength of the alloys within the y+n phase field reaches up to approximately 320MPa.The lowest stre- ngth area is around binary TiAl,being 190MPa,which is about 43%of that of NbTiAs.By combining the data of the measured strength and density it can be obtained that the specific strength of NbTiAls is 97MPa/(g/cm3),in A330~450Pa A 60~100MPa B200~330MPa B 39~60MPa 0 c100~200HPa C 28~39MPa 20304050607080 0 50 50 60 405060 0 20 10 V 5 b90807060504030 201rA1 Nb ,908070605040302010 b,5 Nb,% A Traasgranular B Intergranular C Mixed trasyraanular and Intergra- nular Fig.10 Compressive strength map at 1100C,a.with strain rate of 10-2/s,b.with strain ratc of 10-5/s. contrast,for TiAl that is only 50MPa/(g/cm3). Fig,10b shows the at 1100c at the strain rate of 10-5/s,claasified by 165

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three groups:>60,39-60,<39MPa.It is noted that the composition dependen- ce of strength is generally consistent with that in the case of strain rate of 10-2/s,but the alloy having NbTiAls composition is the best. A000 800 600 Fig.11 Strength-temperature dependence for the Nb-Ti-Al system 400% 1、1+92、11+a1,3、11,4、y1,5、1, 200 6、7 R.T.600 800 1n0 T/℃ Fig.12 Typical fractography a.at room temperature b.at 1100C. In the compressive test the final reduction was controlled to be 45%,with the radial real strain being approximatcly 80%.No crack was observed on the sample surface for most the alloys studied,indicating good high temperature formability.The temperature dependenc of o.2 at strain rate of 10-2/s for part of the alloys are shown in Fig.11.The result indicates that for the alloy in the phase range of y1,n,y,y+n and yi+az an abnormal tempe- rature-strength dependence existes,No data of elongation at room leperature has been obtained.From the fractography it is clearly scen that the dominant fracture mode in the tensile condition for all the alloys investigated is essenti- ally brittle cleavage.However,the fraction of intergranular fracture for 1100C test increases as the high temperature strength of alloys increases (Fig.12). 3 Conelusion The evaluation for NB-Ti-Al ternary intermetallic alloys has shown their 166

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