Structure and properties of cast Ti-Al-Si alloys

ma e ials A icle S uc u e and P ope ies o Cas Ti-Al-Si Alloys Anna Knaislo á1, Pa el No ák1,* , Jiˇ íLinha 1, I o Szu man 2, Ka eˇ ina Sko nico á2, Jan Juˇ ica 2and Tomᚡ Cegan 2   Ci a ion: Knaislo á, A.; No ák, P.; Linha , J.; Szu man, I.; Sko nico á, K.; Juˇ ica, J.; ˇ Cegan, T. S uc u e and P ope ies o Cas Ti-Al-Si Alloys. Ma e ials 2021,14, 813. h ps://doi.o g/10.3390/ ma14040813 Academic Edi o : Sa a Biamino Recei ed: 22 Decembe 2020 Accep ed: 2 Feb ua y 2021 Published: 8 Feb ua y 2021 Publishe ’s No e: MDPI s ays neu al wi h ega d o ju isdic ional claims in published maps and ins i u ional a il- ia ions. Copy igh : © 2021 by he au ho s. Licensee MDPI, Basel, Swi ze land. This a icle is an open access a icle dis ibu ed unde he e ms and condi ions o he C ea i e Commons A ibu ion (CC BY) license (h ps:// c ea i ecommons.o g/licenses/by/ 4.0/). 1Depa men o Me als and Co osion Enginee ing, Uni e si y o Chemis y and Technology, P ague, Technická5, 166 28 P ague 6, Czech Republic; [email p o ec ed] (A.K.); [email p o ec ed] (J.L.) 2Depa men o Non-Fe ous Me als, Re ining and Recycling, Facul y o Ma e ials Science and Technology, VSB—Technical Uni e si y o Os a a, 17. lis opadu 15, 708 33 Os a a-Po uba, Czech Republic; [email p o ec ed] (I.S.); [email p o ec ed] (K.S.); [email p o ec ed] (J.J.); [email p o ec ed] (T. ˇ C.) *Co espondence: [email p o ec ed] Abs ac : In e me allic compounds based on Ti-Al- (Si) a e a ac i e ma e ials wi h good he mal s abili y and low densi y. Howe e , he p oduc ion o hese ma e ials is qui e complica ed. Pa ially modi ied con en ional me hods o mel ing me allu gy a e mos o en used due o a ailabili y, possible high p oduc i i y, and ela i ely low p oduc ion cos s. The e o e, some echnologies o he p oduc ion o in e me allics based on Ti-Al a e cu en ly a ailable, bu wi h ce ain disad an ages, which a e caused by poo cas ing p ope ies o ex eme eac i i y o he mel wi h c ucibles. Some sho comings can be elimina ed by modi ying he mel ing echnology, which con ibu es o inc easing he cos o he p ocess. The wo k deals wi h he p epa a ion o Ti-Al-Si in e me allic compounds wi h di e en con en s o aluminum and silicon, which we e p oduced by cen i ugal cas ing in an induc ion acuum u nace Linn Supe cas -Ti an. This p ocess could con ibu e o he comme cial use o hese alloys in he u u e. Fo his esea ch, he TiAl15Si15(in w .%) alloy was selec ed, which ep esen s a balanced a io o aluminides and silicides in i s s uc u e, and he TiAl35Si5 alloy, which due o he lowe silicon con en allows be e mel ing condi ions, especially wi h ega d o he mel ing empe a u e. This alloy was also in es iga ed a e HIP (“Ho Isos a ic P essing”) ea men . Keywo ds: in e me allics; cas ing; Ti-Al based alloys 1. In oduc ion Ex ensi e es ing o alloys based on in e me allic compounds o he Ti-Al sys em, which has been ca ied ou since he 1980s, has enabled hei comme cial applica ion in he ae ospace and au omo i e indus ies. TiAl alloys a e cu en ly used comme cially, mainly o he p oduc ion o u bocha ge s and ai c a engine blades [1–3]. Fou classes o alloys based on in e me allic compounds o he Ti-Al sys em a e p e- sen ed in [ 4 ]. The i s gene a ion is based mainly bina y alloys wi h di e en aluminum con en s in he ange o 42–48 a .%. These alloys show e y low duc ili y and educed esis ance o c eep and oxida ion a high empe a u es [ 1 , 4 ]. In o de o imp o e he p op- e ies, second gene a ion TiAl alloys ha e been de eloped. Rep esen a i es o his class include TiAl48Mn2Nb2 and TiAl48C 2Nb2 alloys, which a e s ill comme cially applied oday o he blades o low-p essu e pa s o a combus ion u bine. Addi ionallyincluded ones a e TiAl47W2Si0.5 alloy and modi ied TiAl47W2Si0.5B0.5 alloy. Du ing he de elop- men o second-gene a ion TiAl alloys, he posi i e e ec o bo on on he alloy s uc u e was demons a ed. Bo on alloying has p o en o be an e ec i e me hod o p e en ing he uncon ollable g ow h o indi idual alloy g ains du ing hea ea men and he mal exposu e du ing se ice [ 1 , 4 ]. Due o he pe sis ing limi a ion in e ms o duc ili y, a hi d gene a ion o TiAl alloys has been de eloped. This gene a ion o alloys has been de eloped p ima ily o ho o ming echnology. These TiAl-based alloys a e highly alloyed mainly wi h niobium and small amoun s o ca bon and bo on. Due o hei excellen mechanical Ma e ials 2021,14, 813. h ps://doi.o g/10.3390/ma14040813 h ps://www.mdpi.com/jou nal/ma e ials Ma e ials 2021,14, 813 2 o 14 p ope ies and esis ance o high empe a u e oxida ion and c eep up o 800 ◦ C, hey become sui able ma e ials o a ious componen s in gas u bines, je and au omobile engines [ 3 , 4 ]. Due o he g owing demands on hese alloys wi h ega d o ae ospace com- ponen s, which should be able o wi hs and e e -inc easing empe a u es, he la es ou h gene a ion o TiAl-based alloys is being de eloped, ep esen ed by Ti46Al8Ta (a .%) alloy. This compound is alloyed wi h an alum, which signi ican ly educes he di usion in he ma e ial o i s s eng hening, e en a low cooling a es [4]. Mo e han en yea s ago, TiAl48Nb2C 2 alloy was i s comme cially applied o he GEnx engine’s low-p essu e u bine blades. The lowe cen i ugal load o he ligh e TiAl blades educes he o al weigh o he disc, which con ibu es o a signi ican pa o he sa ing o he o al weigh o he mo o , up o 100 kg. Ob ained knowledge om he ope a ion show ha he GEnx engine deli e s a 20% educ ion in uel consump ion, a 50% educ ion in noise and an 80% educ ion in NO X emissions compa ed o p e ious engines in he same se ies, which use con en ional nickel alloy blades. Today, app oxima ely 190 000 TiAl blades a e applied o he low-p essu e pa s o combus ion u bines i ed o Boeing 787-s and 747-8s ai c a engines. Fu he mo e, his applica ion o TiAl alloy is also planned o new LEAP engines [ 1 – 3 ]. In addi ion o he GEnx engines al eady men ioned, he applica ion o s abilized β -TiAl alloy o he p oduc ion o LPT blades o PW1100 G engines has now expanded. Rolls-Royce has also announced he applica ion o TiAl blades o low p essu e combus ion u bine pa s o i s u u e engines. Fu he mo e, he applica ion o TiAl alloys is conside ed o co e s and blade holde s and u bine dampe s. To da e, howe e , he TiAl blades o he low-p essu e pa s o he combus ion u bine o he GEnx engine a e he only la ge-scale comme cial applica ion in he ae ospace indus y [1,2]. One o he mos success ul applica ions o TiAl alloys om he poin o iew o he au- omo i e indus y is he p oduc ion o u bocha ge s o he Japanese au omo i e company Mi subishi Mo o s Inc. These u bocha ge s a e success ully used o Mi subishi Lance p oduc ion ca s. In 2002, o ged exhaus al es o in e nal combus ion engines made o γ - TiAl alloy we e applied o Fo mula 1 ca s, which oday do no mee he equi ed s anda ds and he e o e had o be wi hd awn. Due o he a o able p ope ies o alloys, such as low densi y, high s eng h and a igue esis ance, he e is an e o o modi y he p oduc ion echnology and e-applica ion o exhaus al es o he au omo i e indus y [5,6]. The abo e comme cial and u u e applica ions show a s ong in e es in he u he de elopmen and subsequen applica ion o hese alloys in he ae ospace and au omo- i e indus ies. Ex ensi e esea ch in o TiAl-based alloys aims o educe engine weigh and imp o e engine pe o mance. The cu en s a e o p oduc ion echnology o TiAl in e me allics is he esul o se e al decades o signi ican esea ch and de elopmen in academia and indus y a ound he wo ld. Today, esea ch is mainly conce ned wi h imp o ing he alloy i sel and p oduc ion echnologies o achie e he equi ed p ope ies adap ed o speci ic applica ions, and TiAl-based alloy componen s will be mo e compe i i e in he u u e and hei po en ial can be ully exploi ed [2,7]. The p ocessing echnologies o in e me allics, which a e conside ed o al eady applied in indus ial p axis, include mos commonly he me hods o mel ing me allu gy. Fo he p ocessing o in e me allics, he ExoMel p ocess had been de eloped [ 8 ], es ed o Fe-Al in e me allics and could be applicable also o he p ocessing o Ti-Al based alloys. This p ocess comp ises a me hod o cha ging ini ial componen s in he u nace in o de o u ilize he hea gene a ed by he eac ions be ween he aluminum and ansi ion me als, which o m he in e me allics. O he wise, acuum induc ion mel ing has become a s anda d in p ocessing o Ti-Al based in e me allics [ 9 – 14 ]. The ma e ial solu ion o he mel ing c ucible is also a p oblem in he case o Ti-based in e me allics due o ex eme eac i i y o he mel s [ 15 ]. The ce amics based on y ia (y ium oxide) is widely applied [ 16 ], bu e en in such a case he mel could be con amina ed by y ium. Calcium zi cona e c ucibles could be a solu ion [ 17 ]. Howe e , se e al eams succeeded wi h using g aphi e as he c ucible ma e ial o Ti-based in e me allics [ 18 , 19 ]. Due o a poo cas abili y o hese ma e ials, Ma e ials 2021,14, 813 3 o 14 cen i ugal cas ing is o en applied [ 20 – 22 ]. In o de o heal he in e nal po osi y o he cas ings, ho isos a ic p essing (HIP) is equi ed in some cases [23,24]. This wo k deals wi h he addi ion o silicon in o he TiAl alloys. Silicon is a sui able alloying elemen ha imp o es he esis ance o in e me allics TiAl o oxida ion and c eep a high empe a u es. I has a e y low solubili y in he gi en in e me allic and he e o e o ms a s able silicide Ti 5 Si 3 wi h i anium. I is an in e me allic phase wi h a high mel ing poin (2130 ◦ C) and a low speci ic g a i y. Fu he mo e, silicon has a posi i e e ec on he o ma ion o an adhesi e and compac oxide laye , which in he case o in e me allics based on Ti-Al-Si consis s o a mix u e o TiO 2 and Al 2 O 3 and SiO 2 , which heals unwan ed po es. Fo hese easons, in e me allics a e esis an e en a highe empe a u es [ 25 – 28 ]. Recen ly de eloped Ti-Al-Si alloys ha e been p epa ed by powde me allu gy p ocesses, especially eac i e sin e ing [ 26 , 29 ] and he echnology consis ing o mechanical alloying and spa k plasma sin e ing [ 30 ]. Howe e , hese me hods, e en hough hey enable apid p epa a ion and almos ee choice o he chemical composi ion o he p oduc , a e no so sui able o mass indus ial p oduc ion. This pape aims o es he acuum induc ion mel ing wi h cen i ugal cas ing as he p ocessing ou e o hese ma e ials and desc ibes he p ope ies o he p oduc . 2. Ma e ials and Me hods Expe imen al alloys TiAl15Si15 and TiAl35Si5 (w .%) we e p epa ed by acuum induc ion mel ing wi h cen i ugal cas ing (CC). Fo he p epa a ion, he Linn Supe cas - Ti an de ice was used. The mel ing ope a ions we e pe o med a VSB-Technical uni e si y o Os a a, depa men o Non- e ous me als, e ining and ecycling. As aw/inpu ma e ials, Ti (g ade 2, ound ba , diame e 10 mm, heigh up o 60 mm), Al (99.99 w .%, pieces up o 20 mm × 20 mm × 20 mm) and Si (99.99 w .%, pieces up o 10 mm × 10 mm × 10 mm) we e used. The o al olume o he mel was 100 cm 3 . Used g aphi e c ucible was made o isos a ic p essed g aphi e—SGV5-G B 527 XN. Fo each alloy, a sepa a e c ucible was used. G aphi e cas ing o m (inne diame e 20 mm, heigh 225 mm) wi h no-p ehea ing was used. P io o mel ing, he de ice was e acua ed se e al imes and illed wi h A (99.9999%). Mel ing was ealized unde low p essu e o p o ec i e gas. Ro a ing speed du ing cas ing was 400 pm. By his way, cylind ical samples we e ob ained. The selec ed TiAl35Si5 alloy was also subjec ed o ho isos a ic p essing (HIP), which ook place in a u nace a a empe a u e o 1260 ◦ C and a p essu e o 190 MPa o 4 h wi h hea ing a 10 ◦ C/min. This echnology was chosen o minimizing he in e nal po osi y o he cas ing. Cas alloys TiAl15Si15 and TiAl35Si5 we e subjec ed o X- ay di ac ion analysis using a di ac ome e PANaly icalX’Pe P o ((PANaly ical, Almelo, Ne he lands) ollowed by e alua ion in X’Pe HighSco e 3.0 so wa e package (PANaly ical, Almelo, Ne he lands) using PDF-2 2018 da abase o iden i y he phase composi ion. Me allog aphic cu s we e p epa ed o s udy he mic os uc u e o expe imen al alloys. Samples we e e ched by K oll’s agen (5 mL HNO 3 , 10 mL HF, 85 mL H 2 O) due o he obse ing o he mic os uc u e wi h a Nikon Eclipse MA200 ligh mic oscope (Nikon, Tokyo, Japan) using he NIS Elemen s (Labo a o y Imaging, P ague, Czech Republic) p og am. The indi idual phases and chemical composi ion we e examined using a TESCAN VEGA 3LMU scanning elec on mic oscope in backsca e ed elec ons egime (TESCAN, B no, Czech Republic) wi h an Ox o d Ins umen s X-max 20 mm 2 EDS analyze (Ox o d Ins umen s, High Wycombe, UK). The po osi y o he alloys was de e mined by he image analysis o he op ical mic og aphs by ImageJ 1.53 so wa e. F om he mechanical es s, Vicke s ha dness wi h a load o 5 kg (HV 5) was mea- su ed om 10 inden ion in each sample. Tes s o comp essi e s eng h we e conduc ed by he means o he uni e sal es ing de ice LabTes 5.250SP1-VM (p oduced by Labo Tech, Opa a, Czech Republic). Values o ul ima e ensile s eng h in comp ession we e de e - mined om he measu ed loading cu es. T ibological p ope ies was de e mined by he TRIBO es e ball-on-disc ibome e (T ibo echnic, Clichy, F ance) wi h subsequen e alua ion in he TRIBO echnic p og am. The wea es s ook place a a load o 5.0 N, wi h Ma e ials 2021,14, 813 4 o 14 a o al pa h o 20,000 mm and a displacemen speed o 15 mm/s, which was pe o med by a sphe ical body o Al 2 O 3 wi h a diame e o 6 mm as he s a ic pa ne . The eccen ici y o he de ice was se o 5 mm and o his eason he ab asion a ea on he sample eached a leng h o 10 mm. The whole measu emen ook place a oom empe a u e and he esul was he measu ed wea a e o he sample and he a e age coe icien o ic ion. The esul ing wea acks we e subsequen ly examined using he scanning elec on mic oscope wi h he EDS analyze desc ibed abo ein o de o app oach he wea mechanism. High- empe a u e p ope ies o alloys we e in es iga ed by cyclic oxida ion es s a 800 ◦ C and 1000 ◦ C o a o al leng h o 400 h. The leng h o one oxida ion cycle was 50 h (8 cycles pe 50h). A he end o each cycle, he samples we e emo ed om he u nace. A e spon aneous cooling in ai , hey we e weighed by an analy ical scale Pionee Plus (Ohaus, Pa sippany, NJ, USA) wi h an accu acy o 0.0001 g and placed back in he u nace a he se es empe a u e. The oxida ion a e was de e mined as he weigh gain ha esul ed om he o ma ion o an oxide laye on he su ace o he indi idual expe imen al alloys. The phase composi ion o he o med oxide laye s was de e mined by X- ay di ac ion analysis using a PANaly icalX ´ Pe P o di ac ome e and he mic os uc u e o he oxide laye s was examined using SEM-EDS. 3. Resul s 3.1. Mic os uc u e and Phase Composi ion Mic os uc u e o bo h es ed alloys consis s o i anium silicide (Ti 5 Si 3 ) pa icles in i anium aluminide (TiAl) ma ix, see Figu es 1and 2. In addi ion o hese phases, g aphi e was also de ec ed in TiAl15Si15 alloy (Figu e 1) as a esul o equi ed o e hea ing o his high-silicon alloy and co esponding in e ac ion o g aphi e wi h he mel . The size (equi - alen diame e ) o silicide pa icles eaches app oxima ely 50 and 10 in cas TiAl35Si5 and TiAl15Si15 alloys, espec i ely. The silicide pa icles exhibi mos ly sha p-edged mo phol- ogy in bo h alloys in as-cas s a e. Ho isos a ic p essing o he TiAl35Si5 cas ing led o pa ial sphe oidiza ion o he Ti 5 Si 3 pa icles, leading o ound shape o he silicide. The po osi y o all es ed alloys de e mined by image analysis is e y low. As-cas alloys each he po osi y o abou 1 ol. %, a e HIP he po osi y dec eased o 0.7 ol. %. Ma e ials 2021, 14, x FOR PEER REVIEW 5 o 15 Figu e 1. XRD pa e ns o he es ed alloys. Figu e 2. Op ical mic og aphs o he es ed alloys: (a) TiAl15Si15 in as-cas s a e, (b) TiAl35Si5in as-cas s a e, (c) TiAl35Si5 a e ho isos a ic p essing (HIP). Figu e 1. XRD pa e ns o he es ed alloys. Ma e ials 2021,14, 813 5 o 14 Ma e ials 2021, 14, x FOR PEER REVIEW 5 o 15 Figu e 1. XRD pa e ns o he es ed alloys. Figu e 2. Op ical mic og aphs o he es ed alloys: (a) TiAl15Si15 in as-cas s a e, (b) TiAl35Si5in as-cas s a e, (c) TiAl35Si5 a e ho isos a ic p essing (HIP). Figu e 2. Op ical mic og aphs o he es ed alloys: ( a ) TiAl15Si15 in as-cas s a e, ( b ) TiAl35Si5in as-cas s a e, ( c ) TiAl35Si5 a - e ho isos a ic p essing (HIP). 3.2. Mechanical and T ibological P ope ies The ha dness o he Ti-Al-Si alloys inc eases wi h he amoun o silicon (Table 1). TiAl15Si15 alloy eaches a highe ha dness han TiAl35Si5 alloy. Ho isos a ic p essing has no had an e ec on he ha dness o he ma e ial, TiAl35Si5 alloy has he same ha dness be o e HIP and a e HIP. Values o ul ima e comp essi e s eng h inc ease wi h he amoun o aluminum and HIP has no an e ec on hese alues. TiAl35Si5 alloys ha e he highes alues due o i s s uc u e, which is la gely o med by ela i ely oughe aluminides TiAl. Table 1. Mechanical p ope ies o es ed alloys. Mechanical P ope ies TiAl15Si15 (CC) TiAl35Si5 (CC) TiAl35Si5 (CC + HIP) Ha dness HV 5 459 ±15 375 ±16 374 ±7 Ul ima e comp essi e s eng h [MPa] 1205 ±80 1867 ±75 1801 ±47 T ibological p ope ies we e es ed unde he condi ions o a d y sliding wea agains alumina as he s a ic ic ion pa ne . The esul s indica e ha he ic ion coe icien , as well as he wea a e, a e in luenced by he amoun s o silicon and aluminum in he alloy. The high-silicon ma e ial (TiAl15Si15) exhibi s he highes ic ion coe icien (Table 2) and wea a e. Obse a ion o he wea ack by scanning elec on mic oscope in backsca e ed elec ons egime e ealed ha his alloy ends o a s ong emo al o he silicide pa icles Ma e ials 2021,14, 813 6 o 14 om he su ace due o hei b i le na u e (Figu e 3a). This phenomenon can be seen also in he case o he low-silicon (FeAl35Si5) ma e ial, bu in much lowe ex en (Figu e 3b,c). The wea a all o he es ed ma e ials is mos ly ab asi e, whe e he ex ac ed ha d silicide pa icles ac as ab asi e. The e a e no isible signs o oxida ion o he ma e ial in he wea ack. The in luence o HIP p ocessing on he ic ion coe icien , wea a e, and mechanism o he wea damage is almos negligible. Table 2. T ibological p ope ies o es ed alloys. T ibological P ope ies TiAl15Si15 (CC) TiAl35Si5 (CC) TiAl35Si5 (CC + HIP) ic ion coe icien [–] 0.555 ±0.006 0.453 ±0.004 0.462 ±0.003 wea a e [mm3m−1N−1] (4.17 ±0.09) ×10 −4(1.25 ±0.02) ×10 −4(1.21 ±0.04) ×10 −4 Ma e ials 2021, 14, x FOR PEER REVIEW 7 o 15 Figu e 3. Mo phology o he wea acks (SEM-BSE) on: (a) TiAl15Si15 in as-cas s a e, (b) TiAl35Si5 in as-cas s a e, (c) TiAl35Si5 a e HIP. 3.3. High-Tempe a u e P ope ies The oxida ion a e was de e mined om he weigh gain ob ained due o he o - ma ion o oxides on he su ace o he base ma e ial du ing he mal exposu e. The weigh gain can be obse ed on he kine ic cu es in Figu e 4 o 800 °C and in Figu e 5 o 1000 °C, when he s udied alloys we e weigh ed, including scaled-o (delamina ed) oxides. Fo a mo e de ailed iew o he delamina ion o oxide laye s, kine ic cu es we e gene - a ed only o delamina ed oxides, in Figu e 6 o 800 °C, and in Figu e 7 o 1000 °C. Figu e 3. Mo phology o he wea acks (SEM-BSE) on: ( a ) TiAl15Si15 in as-cas s a e, ( b ) TiAl35Si5 in as-cas s a e, (c) TiAl35Si5 a e HIP. Ma e ials 2021,14, 813 7 o 14 3.3. High-Tempe a u e P ope ies The oxida ion a e was de e mined om he weigh gain ob ained due o he o ma ion o oxides on he su ace o he base ma e ial du ing he mal exposu e. The weigh gain can be obse ed on he kine ic cu es in Figu e 4 o 800 ◦ C and in Figu e 5 o 1000 ◦ C, when he s udied alloys we e weigh ed, including scaled-o (delamina ed) oxides. Fo a mo e de ailed iew o he delamina ion o oxide laye s, kine ic cu es we e gene a ed only o delamina ed oxides, in Figu e 6 o 800 ◦C, and in Figu e 7 o 1000 ◦C. Ma e ials 2021, 14, x FOR PEER REVIEW 8 o 15 Figu e 4. Dependence o o al weigh gain o Ti-Al-Si alloys (including delamina ed oxides) on cy- clic oxida ion du a ion (800 °C; 400 h). Figu e 5. Dependence o o al weigh gain o Ti-Al-Si alloys (including delamina ed oxides) on cy- clic oxida ion du a ion (1000 °C; 400 h). Figu e 4. Dependence o o al weigh gain o Ti-Al-Si alloys (including delamina ed oxides) on cyclic oxida ion du a ion (800 ◦C; 400 h). Ma e ials 2021, 14, x FOR PEER REVIEW 8 o 15 Figu e 4. Dependence o o al weigh gain o Ti-Al-Si alloys (including delamina ed oxides) on cy- clic oxida ion du a ion (800 °C; 400 h). Figu e 5. Dependence o o al weigh gain o Ti-Al-Si alloys (including delamina ed oxides) on cy- clic oxida ion du a ion (1000 °C; 400 h). Figu e 5. Dependence o o al weigh gain o Ti-Al-Si alloys (including delamina ed oxides) on cyclic oxida ion du a ion (1000 ◦C; 400 h). Ma e ials 2021,14, 813 8 o 14 Ma e ials 2021, 14, x FOR PEER REVIEW 9 o 15 Figu e 6. Dependence o he weigh o delamina ed oxides o Ti-Al-Si alloys on he ime o cyclic oxida ions (800 °C; 400 h). Figu e 7. Dependence o he weigh o delamina ed oxides o Ti-Al-Si alloys on he ime o cyclic oxida ions (1000 °C; 400 h). Cyclic oxida ion es s a 800 °C showed good oxide laye adhesion o all alloys. The highes inc ease in oxida ion a e was shown by he TiAl15Si15 alloy a bo h p ocess empe a u es. This alloy con ained la ge sha p-edged silicide pa icles in i s s uc u e and in pa icula i was con amina ed by ca bon. A e cyclic oxida ion es s o his alloy, he e was mos likely a be e oxygen pe meabili y o he base ma e ial du ing exposu e due o he di usion o ca bon om he s uc u e a a highe empe a u e. Fo his eason, he e was a cons an g ow h o he oxide laye and con inuous oxida ion o he whole ma e ial, which was demons a ed o his alloy a e cyclic oxida ion es s a a empe - a u e o 1000 °C, when he whole base ma e ial was comple ely oxidized. The esul ing oxide laye is po ous, bu e en a a empe a u e o 1000 °C i did no unde go massi e delamina ion (Figu e 7). In he case o cas alloys TiAl35Si5 and TiAl35Si5 a e HIP, he esul s o cyclic oxida ion es s a 800 °C showed he o ma ion o a hin and well-adhe ing oxide laye , which p o ec ed he base ma e ial om subsequen oxida ion. The weigh gain was p ac ically ze o h oughou he oxida ion es s (see Figu e 4). The Figu e 6. Dependence o he weigh o delamina ed oxides o Ti-Al-Si alloys on he ime o cyclic oxida ions (800 ◦C; 400 h). Ma e ials 2021, 14, x FOR PEER REVIEW 9 o 15 Figu e 6. Dependence o he weigh o delamina ed oxides o Ti-Al-Si alloys on he ime o cyclic oxida ions (800 °C; 400 h). Figu e 7. Dependence o he weigh o delamina ed oxides o Ti-Al-Si alloys on he ime o cyclic oxida ions (1000 °C; 400 h). Cyclic oxida ion es s a 800 °C showed good oxide laye adhesion o all alloys. The highes inc ease in oxida ion a e was shown by he TiAl15Si15 alloy a bo h p ocess empe a u es. This alloy con ained la ge sha p-edged silicide pa icles in i s s uc u e and in pa icula i was con amina ed by ca bon. A e cyclic oxida ion es s o his alloy, he e was mos likely a be e oxygen pe meabili y o he base ma e ial du ing exposu e due o he di usion o ca bon om he s uc u e a a highe empe a u e. Fo his eason, he e was a cons an g ow h o he oxide laye and con inuous oxida ion o he whole ma e ial, which was demons a ed o his alloy a e cyclic oxida ion es s a a empe - a u e o 1000 °C, when he whole base ma e ial was comple ely oxidized. The esul ing oxide laye is po ous, bu e en a a empe a u e o 1000 °C i did no unde go massi e delamina ion (Figu e 7). In he case o cas alloys TiAl35Si5 and TiAl35Si5 a e HIP, he esul s o cyclic oxida ion es s a 800 °C showed he o ma ion o a hin and well-adhe ing oxide laye , which p o ec ed he base ma e ial om subsequen oxida ion. The weigh gain was p ac ically ze o h oughou he oxida ion es s (see Figu e 4). The Figu e 7. Dependence o he weigh o delamina ed oxides o Ti-Al-Si alloys on he ime o cyclic oxida ions (1000 ◦C; 400 h). Cyclic oxida ion es s a 800 ◦ C showed good oxide laye adhesion o all alloys. The highes inc ease in oxida ion a e was shown by he TiAl15Si15 alloy a bo h p ocess empe a u es. This alloy con ained la ge sha p-edged silicide pa icles in i s s uc u e and in pa icula i was con amina ed by ca bon. A e cyclic oxida ion es s o his alloy, he e was mos likely a be e oxygen pe meabili y o he base ma e ial du ing exposu e due o he di usion o ca bon om he s uc u e a a highe empe a u e. Fo his eason, he e was a cons an g ow h o he oxide laye and con inuous oxida ion o he whole ma e ial, which was demons a ed o his alloy a e cyclic oxida ion es s a a empe a u e o 1000 ◦ C, when he whole base ma e ial was comple ely oxidized. The esul ing oxide laye is po ous, bu e en a a empe a u e o 1000 ◦ C i did no unde go massi e delamina ion (Figu e 7). In he case o cas alloys TiAl35Si5 and TiAl35Si5 a e HIP, he esul s o cyclic oxida ion es s a 800 ◦ C showed he o ma ion o a hin and well-adhe ing oxide laye , which p o ec ed he base ma e ial om subsequen oxida ion. The weigh gain was p ac ically ze o h oughou he oxida ion es s (see Figu e 4). The o ma ion o a s able oxide laye wi h p o ec i e e ec s esul s in a dec ease in he oxida ion a e due o he slowing down o he di usion o oxygen h ough he oxide laye . In he cyclic oxida ion Ma e ials 2021,14, 813 9 o 14 es s, which ook place a a empe a u e o 1000 ◦ C, he e was al eady an inc ease in he oxide laye wi h insu icien p o ec i e e ec , and he e o e a massi e delamina ion o his laye ollowed in bo h TiAl35Si5 alloys, which allowed u he oxida ion o he base ma e ial. Poo adhesion o he p o ec i e laye can be de ec ed in Figu e 5. Alloys show epe i i e pa abolic g ow h o he oxide laye . Due o he insu icien p o ec ion o he base ma e ial by he espec i e su ace oxides, he oxida ion a e is con olled by he chemical eac ion o oxygen wi h he su ace o he ma e ial. The non-compac ness o he laye du ing his exposu e is pa ly due o he insu icien amoun o silicon in he alloys. The phase composi ion o he o med oxide laye s o he s udied Ti-Al-Si alloys was de e mined a e cyclic oxida ion es s by X- ay di ac ion analysis. The p esence o TiO2 and Al2O3 oxides, which o med he majo componen s o he su ace oxide laye , was p o ed in all alloys. A e he mal exposu e a 800 ◦ C (Figu e 8), he p esence o i anium silicide was u he p esen ed in Ti-Al-Si alloys p oduced by cen i ugal cas ing. In he case o TiAl15Si15 (CC) alloy, a highe TiSi 2 silicide was de ec ed on he su ace, in con as o TiAl35Si5 (CC) alloys, whe e he b i le phase o Ti 5 Si 3 silicide was p esen . The su ace laye o TiAl35Si5 (CC) and TiAl35Si5 (CC+HIP) alloys was u he supplemen ed wi h a Ti 2 AlN phase, which o ms an in e media e laye be ween he base ma e ial and he esul ing oxide laye and u he inc eases i s adhesion. A e he mal exposu e a 1000 ◦ C (Figu e 9), he e was no signi ican change in he phase composi ion o he cas Ti-Al-Si alloys compa ed o he p e ious esul s. In he case o he TiAl35Si5 (CC) alloy, he AlN and TiN phases we e u he de ec ed, in which he same e ec can be expec ed as in he men ioned Ti 2 AlN phase. In he cas HIP- ea ed TiAl35Si5 alloy, he p esence o a highe i anium aluminide TiAl 3 was de ec ed, which was p obably due o he inc eased concen a ion o aluminum in he su ace laye o he alloy du ing he mal exposu e. Ma e ials 2021, 14, x FOR PEER REVIEW 10 o 15 o ma ion o a s able oxide laye wi h p o ec i e e ec s esul s in a dec ease in he oxi- da ion a e due o he slowing down o he di usion o oxygen h ough he oxide laye . In he cyclic oxida ion es s, which ook place a a empe a u e o 1000 °C, he e was al- eady an inc ease in he oxide laye wi h insu icien p o ec i e e ec , and he e o e a massi e delamina ion o his laye ollowed in bo h TiAl35Si5 alloys, which allowed u he oxida ion o he base ma e ial. Poo adhesion o he p o ec i e laye can be de- ec ed in Figu e 5. Alloys show epe i i e pa abolic g ow h o he oxide laye . Due o he insu icien p o ec ion o he base ma e ial by he espec i e su ace oxides, he oxida ion a e is con olled by he chemical eac ion o oxygen wi h he su ace o he ma e ial. The non-compac ness o he laye du ing his exposu e is pa ly due o he insu icien amoun o silicon in he alloys. The phase composi ion o he o med oxide laye s o he s udied Ti-Al-Si alloys was de e mined a e cyclic oxida ion es s by X- ay di ac ion analysis. The p esence o TiO2 and Al2O3 oxides, which o med he majo componen s o he su ace oxide laye , was p o ed in all alloys. A e he mal exposu e a 800 °C (Figu e 8), he p esence o i anium silicide was u he p esen ed in Ti-Al-Si alloys p oduced by cen i ugal cas ing. In he case o TiAl15Si15 (CC) alloy, a highe TiSi2 silicide was de ec ed on he su ace, in con- as o TiAl35Si5 (CC) alloys, whe e he b i le phase o Ti5Si3 silicide was p esen . The su ace laye o TiAl35Si5 (CC) and TiAl35Si5 (CC+HIP) alloys was u he supplemen ed wi h a Ti2AlN phase, which o ms an in e media e laye be ween he base ma e ial and he esul ing oxide laye and u he inc eases i s adhesion. A e he mal exposu e a 1000 °C (Figu e 9), he e was no signi ican change in he phase composi ion o he cas Ti-Al-Si alloys compa ed o he p e ious esul s. In he case o he TiAl35Si5 (CC) alloy, he AlN and TiN phases we e u he de ec ed, in which he same e ec can be expec ed as in he men ioned Ti2AlN phase. In he cas HIP- ea ed TiAl35Si5 alloy, he p esence o a highe i anium aluminide TiAl3 was de ec ed, which was p obably due o he in- c eased concen a ion o aluminum in he su ace laye o he alloy du ing he mal ex- posu e. Figu e 8. Phase composi ion o he su ace oxide laye o Ti-Al-Si alloys a e cyclic oxida ion a 800°C. Figu e 8. Phase composi ion o he su ace oxide laye o Ti-Al-Si alloys a e cyclic oxida ion a 800 ◦C.