Potential of the Co-Cr-Ni-Nb as a Dental Alloy System

H. Mohammed-Al Tahawi, DDS,MScD,PhD,FICD,DICOI,FADM
College of Dentistry, King Saud University, P.O. Box 60169, Riyadh 11545, Saudi Arabia  

In an alloy design study, Mohammed-AI Tahawi1 et al indicated that coherent intermetallic compound precipitation is the mechanism of choice for strengthening cobalt-base alloys. A study by Drapier2 et al indicated that tantalum, tungsten, molybdenum, titanium, and niobium {Nb) react with cobalt to produce coherent Co3X, where X is one of the above five elements. They could not detect Co3Nb by X-ray diffraction due to the extreme fineness of Co3Nb. A study by Koster3 showed that tantalum, niobium, and titanium raise the stacking fault energy (SFE) of cobalt and stabilize the ductile face centered cubic phase. Rideout4 et al showed that a given atomic percent of tantalum or niobium will have an equal effect on the precipitation of the embrittling sigma phase. The twelve criteria in Table 1 were used for select­ ing the most appropriate strengthening agents from the five elements. These criteria indicated that tan­ talum is the most efficient strengthener, followed by niobium. Several studies5-8 showed that the addition of tantalum to a 40Co-30Ni-30Cr alloy base produced strong, ductile, heat-treatable alloys characterized by a slow rate of work hardening.

The purpose of this investigation was to determine whether Nb has strengthening characteristics similar to those of tantalum. Studies on the strengthening abilities of tungsten, titanium, and molybdenum are currently in progress.

Fourteen quaternary alloys were prepared by adding varying quantities of niobium, 0-20 weight percent, to a 40Co-30Ni-30Cr alloy base. The ter­ nary alloy elements, which were high purity and supplied in small shots, were melted by induction in an argon atmosphere to a clean mirror surface. Then niobium (melting point, 2415°C) was added. Niobium dissolved rapidly in the ternary alloy. The quarternary alloy was then heated to its casting temperature and cast.

Tensile bars were prepared according to Amer­ ican Dental Association Specification No. 14 for cobalt alloys. The bars were sprued horizontally and invested in a phosphate bonded investment.* The investment was heated to 1800°F, and it was heat soaked for one hour before castings were made.

Tensile properties were determined by the use of a Universal testing machine and a strain gauge extensometer.** The 0.2% offset yield strength, ultimate tensile strength, and elongation were determined in a conventional manner. Four tensile specimens were tested from each alloy.

Since the atomic weight of niobium is 92.90 and that of tantalum is 180.95, the weight of one atomic percent of tantalum is twice that of niobium. Accordingly, the effect of one weight percent of niobium may be compared to that of two weight percent of tantalum.

Tensile Testing:

Elongation

The effect of adding niobium on the elongation of the alloy base is shown in Figure 1 and is superimposed on the effect of equal atomic per-cents of tantalum.

Figure 1, Effect of adding equal atomic weights of niobium (black) and tantalum (white) on the ductility (reported as 95% confidence intervals).

The addition of up to 2.9 w/o Nb or 5.8 w/o Ta produced alloys A2B2 through A5B5. This addition caused a mild decrease in the elongation of the alloy base. Hence, up to these concentrations, both Ta and Nb have similar effects on elongation.

Increasing tantalum to 12.3 w/o gradually decreased the elongation to 18%. Increasing niobium drastically reduced the elongation to 12% at 3.7 w/o Nb, then to 6% at higher concentration. Such a iow elongation was obtained only when the tantalum concentration was higher than 13 w/o in Alloy A]2B12. ln other words, niobium concentrations higher than 3% have a similar effect to that of more than 12% tantalum: a four-fold effect rather than the anticipated two-fold. Increasing niobium to higher than 8% reduced elongation below 3%. Elongation values of 2.2, 2.4, 1.9, and 0.9% were obtained when niobium concentrations were 10.7, 13.8, 16.7, and 19.3 w/o respectively.

Offset Yield Strength

The effect of adding niobium and tantalum on the 0.2% offset yield strength is shown in Figure 2. While increasing tantalum from 0 - 16.7% raised the yield strength from 44 x 103 to 108 x 103 psi, the addition of niobium had no effect up to 4.6 w/o in A7B7. When the niobium concentration was increased further, the yield strength increased at an extremely slow rate from 44 x 103 to 62 x 103 psi at 6.9 w/o. At an equal atomic weight of Ta, the yield strength was 99 x 103 psi. Yield strength was increased to 73 x 103 psi at 10.7% niobium, then deteriorated to 64 x 103 and 50 x 103 psi when the niobium concentrations were 16.7 and 19.4 w/o respectively.

Ultimate Tensile Strength

The effect of adding tantalum and niobium on ultimate tensile strength is shown in Figure 3. The addition of tantalum increased the ultimate tensile strength from 80 x 103 psi for the alloy base to 133 x 103 psi at 16.7 w/o Ta. The addition of equivalent atomic concentrations of niobium had no effect on ultimate tensile strength. The addition of 10.7 w/o and 13.7 w/o niobium raised the ultimate tensile strength to 88 x 103 and 91 x 103 psi respectively. Increasing niobium concentrations to 16.7 and 19.3 w/o reduced ultimate tensile strength to 79 x 103 and 56 x 103 psi respectively.

Metallographic Examination:

A 500 magnification of the microstructure of the alloys containing 0 w/o, 2.9 w/o, and 6.5 w/o niobium are depicted in Figure 4. The addition of 2.9 and 6.5 w/o niobium caused an evident refinement of the dendritic structure of the alloy. In addition to the dendrites and the matrix, a third fine phase appeared in the matrix of the 6.5 w/o Nb alloy.

A 1250 magnification of the alloys containing 0.0, 2.9, and 3.8 w/o niobium is shown in Figure 5. The precipitate represented by the fine phase of Figure 4 is detectable in the 3.8 w/o Nb alloy.

Microstructures of the alloys containing 6.5, 10.7, and 16.7 w/o Nb at the higher magnification
are shown in Figure 6. It is evident that the precipitate increased in volume and conglomerates to form continuous plates surrounding the dendrites and occupying the centers of the interdendritic matrix. At 16.7% the precipitate completely obliterates the matrix.

The mechanical testing results showed that the addition of up to 3 w/o niobium had an extremely mild effect on the three tensile properties. A slight decrease in ductility was noticed while the yield and ultimate strengths remained rather constant. Metaliographic examination of these alloys, Figures 4 and 5, showed that the addition of niobium caused refinement of the dendritic structure, hence the grains, of the alloys. The larger atomic radius of niobium, 1.46Å, also suggests its efficiency as a solid solution hardener for a Co- Cr-Ni alloy where the average atomic radius is 1.26Å. Concentrations between 3-4 w/o niobium caused drastic reduction in ductility with a negligi-ble gain in both the yield and ultimate strength. The microstructure of alloys containing concentrations higher than 4 w/o niobium, Figure 6, suggested the precipitation of a globular, structureless precipitate in the interdendritic space.

The study by Drapier2 showed that it was not possible to obtain an X-ray diffraction from residues extracted from niobium-bearing alloys due to the extreme fineness of the precipitate. Extreme fineness implies a coherent precipitate. Coherent precipitates were found to be efficient strengtheners with minimal sacrifice in ductility.1 The behavior of the niobium-bearing alloys in this study shows that the precipitate has the opposite effects of a coherent one.

The drastic reduction in ductility may suggest that the interdendritic precipitate is one of the electron compounds termed sigma, pi, R, etc. The precipi­ tate cannot be an electron compound, however, since the electron hole theory indicates that both niobium and tantalum have the same electron4 hole number. Excessive electron hole compound precipitation occurred in alloys containing higher than 13 w/o tantalum6. An equivalent atomic weight of niobium causing excessive sigma formation would be 6.5 w/o. The mechanical properties deteriorated at much lesser niobium concentrations, however.

Since the interdendritic precipitate is neither coherent alpha-Co3Nb nor an electron compound, the only other phase, it may be, is an incoherent inter-metallic compound of niobium with either cobalt or nickel. Since the precipitate embrittles the alloys sig­ nificantly, the incoherent intermetallic compound is expected to be a Laves compound. Laves com­ pounds are known to have more embrittling effects than the electron compounds. Saito and Beck10 reported that cobalt and niobium form several com­ pounds. The hexagonal Laves MgNi2 type Co2Nb phase was composed of 74.5 - 75.2 atomic percent cobalt. The latter phase parameters reported were a = 4.70Å,c= 15.45Å, and c/a = 3.259.

Wallbaum11 reported the cubic Laves MgCu2 type Co2Nb phase with a = 6.758Å. Piearcey et al12 reported the hexagonal Laves MgZn2 type Co2Nb with a = 5.190Å, c = 8.384Å, and c/a = 1.615. To increase the dilemma, a study by Chung et al13 showed that the only precipitate they observed in a 38-40Co, 37-40Ni, and 17Cr containing niobium is the orthorhombic beta - Ni3Nb. The lattice parameters were a = 5.096Å, b = 4.211 Å, and c = 4.578Å. The phase precipitated on the (11) matrix planes and had a plate-like shape. Accord­ ingly, the inter- dendritic plate-like phase observed at moderate concentrations of niobium may be safely called Laves phase.

At concentrations higher than 19 w/o niobium, the eutectic of Figure 7 was observed. At such high concentrations, there is enough niobium in the alloy to form more than one type of Co2 Nb or even one Co2Nb phase and one beta-Ni3Nb phase. The Nb-Ni phase diagram14 shows a eutectic at 40.2 atomic percent Nb. The resulting phases are Ni3Nb and NiNb.

The experimental results obtained from this inves­ tigation indicate that niobium does not form stable coherent intermetallic compounds in 40Co-30Cr-30Ni alloys. It was concluded that the precipitate resulting from the addition of niobium is a Laves compound  resulting from the reaction between niobium and cobalt or nickel. Hence, the addition of niobium caused major deterioration of the ductil­ ity with no improvement or with deterioration of the strength properties. At excessively higher concen­ trations of niobium, a eutectic was formed.