Effect of Tantalum Additions to a Cobalt-Chromium-Nickel

Transcription

1 Effect of Tntlum Additions to Coblt-Chromium-Nickel Bse Alloy A. P. ROWE, W. C. BIGELOW, nd K. ASGAR University of Michign, School of Dentistry, Ann Arbor, Michign 48104, USA An investigtion by electron diffrction, trnsmission nd scnning electron microscopy, nd energy-dispersive X-ry nlysis hs shown tht T dditions to Co-Cr-Ni-bse lloy strengthen by ordering nd by formtion of coherent -CoST precipitte. However, incresing T content increses the proportion of the hexgonl phse nd decreses ductility. The effect of dditions of tntlum (T) to coblt (Co) -chromium (Cr) - nickel (Ni) lloy hs been studied for severl yers t the University of Michign. Mechnicl properties of series of these lloys hve been reported by Mohmmed nd Asgr.1 Those results showed tht n lloy contining 13% T would hve n ultimte tensile strength of 124,000 psi, yield strength of 90,000 psi, nd n elongtion of 10%. There is shrp chnge in these properties round 12 to 13% T. In the present study, this system ws exmined by electron diffrction, trnsmission nd scnning electron microscopy, nd energy-dispersive X-ry nlysis to identify the microstructure ssocited with these mechnicl properties. Mterils nd Methods Specimens were prepred by conventionl investment csting techniques with the use of wx pttern, phosphte-type investment, nd centrifugl csting from zircon crucible under rgon. The structures of the s-cst specimens re extremely sensitive to the cst- This investigtion ws supported by USPHS Reserch Grnt DE from the Ntionl Institute of Dentl Reserch, Ntionl Institutes of Helth, Bethesd, Md, nd is bsed on disserttion submitted in prtil fulfillment of the PhD requirements, University of Michign, Received for publiction August 24, ing cross-section, which of course influences the cooling rte. Therefore, one series of specimens ws cst in the form of 2.5-mm pltes; nother ws cst in 0.6-mm pltes. The pltes were then ground down mechniclly to bout 0.15 mm nd electropolished in perchloric cid-ethnol to thicknesses suitble for electron microscopy. Results nd Discussion Alloy compositions contining up to 16.7% T consist primrily of fce-centered cubic mtrix phse nd n interdentritic hexgonl phse. Sometimes minor mounts of of phse re present. Depending on the T content nd the csting conditions, there lso my be fine precipittes of -Co3T, f3-co3t, nd y-co2t. Figure 1 shows the distribution of the (hexgonl) phse in these smples s reveled by scnning electron microscopy. The light res hve been identified from electron diffrction ptterns s the phse, which evidently dissolves more slowly in the etching solution thn does the surrounding t phse. The numbers on the microgrphs indicte the nominl, s weighed, T contents in weight percentges. The ctul compositions re somewht different. The tble lists the results of energy-dispersive X-ry nlysis of these lloys. For ech specimen type the first row lists the weight percentges of ech component s weighed for the csting; the second row indictes the ctul compositions s determined by nlysis. The cstings contined n verge of 19%, more T, 8% more Cr, 9% less Co, nd 6% less Ni thn weighing indicted. The remining two rows for ech csting represent verge compositions of the fcecentered cubic () nd hexgonl () phses. 325

2 ~~~~~~1 0 i 1 ~~~~~~~13 FIG 1.-Scnning electron microgrphs show distribution of : phse in Co-Cr-Ni-T lloys. Top, 0.6-mm cstings; right, 2.5-mm cstings. 326

3 Vol 53 No. 2 CO-CR-NI-TA ALLOY 327 TABLE ENERGY DISPERSIVE X-RAY ANALYSES OF CO-CR-NI-TA ALLOYS Phse Csting Frction of 11 Ni Co Cr T Thin cstings 9.1% T 13.0% T 13.8% T Thick cstings 9.1% T 13.0% T 13.8% T 16.7% T Note: (0.6 mm) (2.5 mm) , All vlues reported s weight percentges Anlyses of phse re not vilble for ech of the cstings, but typicl nlysis is 23% Ni, 27% Co, 24% Cr, nd 25% T. As the csting cooled, the phse ws the first to solidify. Its T content rnged from 3 to 13% for the vrious compositions, but ws lwys less thn the overll T content. As the temperture decresed, the remining, T-rich (40 to 45%) mteril solidified in hexgonl structure, despite the fct tht T dditions re reported to fvor the fce-centered cubic structure.2 The Co-T binry phse digrm,3 for exmple, shows no hexgonl phse for T content greter thn 7% ḞACTORS AFFECTING DUCTILITY.-The system studied here showed n incresing mount of hexgonl phse with incresing T content, nd it is in this phse tht filure is initited. Figure 2 is scnning electron microgrph of longitudinl section of tensile br pulled to filure nd shows crcks in the phse. Figure 3 is nother such specimen in which there is evidence of pulling prt t the - interfce. Frcture surfces of the high-t, low-ductility specimens suggest tht filure hs occurred by interfce shering; the dendritic structure ws left (Fig 4). The thinner cstings showed slightly more,b phse thn did the thicker ones, nd the phse contined less T. In short, there ws less time for formtion nd T segregtion to occur before the solidus ws reched. Also, in the thinner specimens, the distribution of nd phses ws on much finer scle thn tht in the thicker cstings of the composition (Fig 1). STRENGTHENING MECHANISMS.-At 9.1 % T (chrge composition), both the nd / phses were solid solutions (Fig 5). At 13%,

4 328 ROWE, BIGELOW, AND ASGAR J Dent Res Mrch-April 1974 FIG 2.-Scnning electron microgrph of longitudinl section of tensile br pulled to filure. Arrow, tensile direction. the phse, which is the predominnt phse, ws well ordered but there ws no evidence of precipitte. Ordering is well-recognized strengthening mechnism; it increses the Burger's vector of disloctions nd impedes disloction motion, prticulrly if ntiphse boundries re present. At 13.8% T, fine (< 50 A) coherent precipitte ws found in the thicker cstings (Fig 6). It is described s -Co3T, lthough its unit cell dimensions correspond to fcecentered cubic Co rther thn the slightly lrger cell of -CoT. With higher concentrtions of T, the lrger cell is found, the precipitte prticles re lrger, nd t 16.7% some re no longer coherent. In fct, t this ltter composition there lso is precipitte in the hexgonl phse; it is y-co2t, which hs hexgonl structure. In the thinner, 0.6-mm specimens, precipitte ws not observed until T concentrtions incresed to more thn 13.8% (Fig 7). HEAT-TREATED SPECIMENS.-Of course none of these structures represents the equilibrium condition for this system. When the lloys FIG 3.-Scnning electron microgrph of longitudinl section of tensile br pulled to filure. Arrow, tensile direction. were het-treted t 700 C for 20 hours, equilibrium ws pproched (but not reched). Het-treted specimens with T s low s If. FicE 4.-Scnning electron microgrph of trnsverse section of 9.1% T tensile br pulled to filure. ('\-1

5 FIG 5.-Trnsmission electron microgrph of (110) phse in 9.1% T lloy..61'? -, O~~~~~~~~~~~~~~~ FIG 6.-Trnsmission electron microgrph of (110) ordered phse in 13.8% T lloy. 329

6 FIG 7.-Trnsmission electron microgrph of (100) ordered phse nd -Co3T in 13.8% T lloy. FIG 8.-Trnsmission electron microgrph of (110) phse nd -CosT in het-treted 9.1% T lloy. 330

7 Vol1 53 No. 2 9% contined fine -Co3T precipitte in some regions of the phse (Figs 8-10). Higher T compositions contined lrger precipittes (Fig 11) nd lso precipittes (/-Co3T nd -y-co2t) in the hexgonl phse. In the Co-T system, -Co3T is described s metstble phse nd -Co3T is the equilibrium phse t 700 C. SIGMA PHASE. This study hs not included CO-CR-NI-TA ALLOY 331 quntittive informtion on how much phse occurs in ech composition, but it hs suggested tht the embrittling influence of the u phse my hve been overestimted, t lest for this system. For exmple, Figure 12 shows specimen with considerble quntity of o- phse (the needles) surrounding ech islnd of hexgonl phse. Yet this is section of tensile FIG 9.-Trnsmission electron microgrph of (130) phse nd -Co3T in het-treted 9.1% T lloy.

8 FIG 10.-Trnsmission electron microgrph of (100) phse nd -CoT in het-treted 9.1% T lloy FIG 11.-Trnsmission electron microgrph of (120) in het-treted 16.7% T lloy. phse nd c-co3t

9 Vol 53 No. 2 FIG 12.-Scnning electron microgrph of 9.1%7 T lloy shows, ps nd C phses. br with n elongtion of 14.5%; nd filure hs strted in the A phse, not in the phse. Conclusions As the T concentrtion is incresed in this lloy system, strengthening is incresed first by ordering of the T in the fcecentered cubic phse nd lter by formtion CO-CR-NI-TA ALLOY 333 of fine, coherent precipitte, -Co3T. However, incresing the T concentrtion lso increses the mount of the more brittle hexgonl phse nd ductility decreses. A desirble combintion of these properties for dentl pplictions ws reported' for 13% T, where strengthening is by ordering nd elongtion is bout 10% for the 2.5-mm cstings. In prcticl situtions, where the thickness of prtil denture clsps, for exmple, my vry between 1 nd 2 mm, T concentrtions slightly less thn 13% will be needed to chieve the desired properties. The thicker portion of such csting would hve slightly different microstructure, with less hexgonl phse nd more T in ech phse; lhence, it would hve greter strength nd greter ductility thn the clsp region. The composition should be selected for optimum properties in the clsp. A slower cooling rte should improve the properties of these lloys. Apprecition is due to D. K. McCllum of the University of Michign, School of Dentistry, for some of the trnsmission microgrphs. References 1. MOHAMMED, H., nd ASGAR, K.: A New Dentl Superlloy System: II. Mechnicl Properties, J Dent Res 52: , KOSTER, W.: Uber den Einfluss der Elemente uf die polymorphe Umwndlung des Koblts, Z Metllkunde 43: , KORCHYNSKY, M., nd FOUNTAIN, R.W.: Precipittion Phenomen in Coblt-Tntlum Alloys, Trns AIME 215: , 1959.