Australian Dental Journal. April, 1979
90
Volume 24, No. 2
The effect of carbon on the metallography of a nickel base removable partial denture casting alloy A. J. Lewis. M.D.Sc. (W.A.), D.D.Sc. (W.A.), D.D.S. (NU), B.Sc. (W.A.), F.R.A.C.D.S., F.I.C.D.
Senior Lecturer in Prosthodontics, The University of Western Australia
ABSTRACT-SIX special nickel-chromium alloys were prepared in which the carbon levels were adjusted above and below that of a commercial nickel base dental casting alloy.
(Received ,for publication November, 1976)
Introduction Carbon levels in the nickel base casting alloys are invariably lower than those in the cobalt base series. Additional strength is achieved in nickel base alloys by precipitation of an intermetallic compound designated gamma prime. Carbon values for nickel alloys lie in the range 0.05-0.20 per cent whilst the minimum stated value for a cobalt alloy is generally 0.25 per cent. The commercial nickel base dental casting alloy* included in this study had a carbon content of 0 . 1 7 per cent. The presence of carbon in association with suitable carbide formers results in the development of a series of carbide structures which add substantially to the mechanical behaviour of the alloy. This paper reports an investigation on the effects, particularly on microstructure, of graded carbon levels within a basic nickel chromium alloy.
Materials and methods Alloy preparation The basic alloy contained 80 per cent nickel and 20 per cent chromium melted and poured in a vacuum. A * Ticonium Premium 100.
carbon level of 0.03 per cent was found in the basic alloy but it was considered too low for the formation of an effective carbide system. A high carbon master alloy was obtained by holding a large piece of carbon rod beneath the surface of the molten basic alloy. The carbon-level of this master alloy was then determined using a combustion method. Special alloys were then produced by adding known amounts of this master alloy to standard charges of the basic nickel chromium alloy. Table 1 lists the carbon levels found in the series of six alloys.
TABLE1 Carhon levels nithin the formuluted ulloys Alloy No.
I
..
..
2 3 4 5 6
..
..
.. ..
.. .. ..
.. .. ..
Level attempted
Level achieved
(Per cent) 0.06 0.10 0.20 0.50 1 .00 2.00
(Per cent) 0.06 0.12 0.26 0.34 0.75 I.46
91
Australian Dental Journal, April, 1979 TABLE2 Mechanical properties of the alloys tested Alloy ~~
Basealloy.. Alloy I , , Alloy 2 . . Alloy 3 , . Alloy 4 . . Alloy 5 . . Alloy 6 . . T.P. ..
Elongation
0 ' "0 stress
(Per cent) 18.2 9.4 4.8 2.9 2.3 0.9 1 .0 0.9
MPa I32 221 260 278 307 354 43 1 517
U.T.S.'
~
.. ,. .. .. .. .. .. ..
MPa 308 336 337 355 358 390 465 543
Ultimate tensile strength
P
-.: ,-
Fig. I.-Structure of Alloy 2 showing two finely lamellar structures. x 500.
Fig. ;.-Structure of Alloy 3 showing the grain boundary, lamellar and island components. x200; inset x500.
Tensile and elongation tests Six specimens conforming to the Australian Standard T28 were prepared from each of the six alloys. The wax patterns were sprued in pairs parallel to the long axis of the casting ring and invested in a silica bonded mould material. A standard 34 g charge of alloy was cast into the mould heated to a temperature of 1000°C. Tensile tests were made on four specimens from each group. Ultimate tensile strength values and 0.1 per cent proof stress values were determined. Elongation estimates were determined as previously described.'
Results The values obtained from the tests are shown in Table 2 and are compared with those obtained for the base alloy (nickel 80 per cent, chromium 20 per cent) and a commercial alloy* (T.P.)* previously reported.' In the basic nickel-chromium alloy, grain boundaries were delineated by continuous deposits of primary carbide and the austenitic matrix was devoid of detail. In Alloys 1 and 2 the continuous intergranular deposits were visible, but in addition islands of primary carbide were seen (both interdendritically and intergranularly) more prominently in Alloy . Frequently areas with a classically lamellar morphology were present (Fig. I), which were ill-defined when compared with those seen at higher carbon concentrations. The continuous intergranular deposits were still a significant feature in Alloy 3 with its higher carbon
Metallography Representative samples were cut from the two remaining specimens in each group mounted, polished and etched at I . 5 V for 15 sec in a solution containing nitric acid 10 ml, glacial acid 5 ml and water 85 ml.
' Lewis, A. J.-The
effect of aluminium on the metallography of a nickel base removable partial denture casting alloy. Austral. D. J., 23: 6, 488491 (Dec.) 1978.
D
* Ticwium Premium 100.
92
Australian Dental Journal, April, 1979
Fig. 3.-Structure of Alloy 4 showing the completed development of a network. x 200.
Fig. 4.-Structure of Alloy 5. x 500.
Fig. 5.-Structure of Alloy 6 . x 500.
Fig. 6.-Structure of the nickel base removable partial denture casting alloy. x 500.
content but the island carbides were more prominent and tended to form a continuous network (Fig. 2). In Alloy 4 the island structures were continuous, and formed an intergranular and interdendritic network. In addition, fine particles of carbide were precipitated within the matrix during cooling (Fig. 3). In Alloy 5 the continuous primary carbides became strongly lamellar and the fine discrete particles visible in the matrix (Fig. 3 ) consolidated in areas adjacent to the intergranular and interdendritic boundaries (Fig. 4). Note a carbide depleted zone along the eutectic carbide chain. The structure of Alloy 6 (Fig. 5) is a further development. Almost continuous deposits of highly exaggerated lamellar structures are visible and the matrix regions appear choked with precipitated carbide particles. The carbide depleted zone previously described continues to be a stable feature of the microstructure. Figure 6 shows the structure of the commercial alloy, and although many of the details previously described
are recognizable, in this case the picture is complicated by the concomitant precipitation of gamma prime.
Discussion Carbide exerts a profound influence on the mechanical behaviour of nickel base alloys and thus has shown differences in the microstructure of the alloys as cast without heat treatment. A definite morphological pattern develops as the carbon concentration increases. At low carbon levels the simple grain boundary carbide deposits progressively thicken and may develop into a fine lamellar form at certain locations; at higher concentrations the lamellae become more massive and more abundant until, as in Alloy 6 , their presence, both intergranularly and interdendritcally, is the dominant feature. The islands of precipitated carbide which develop at relatively low carbon concentrations and straddle the
93
Australian Dental Journal, April, 1979
grain boundaries, slowly increase in extent, eventually forming an intergranular and interdendritic network together with a fine carbide precipitation within the matrix which coarsens as the carbon content increases. Initially, these coarser particles consolidate lateral to the lamellar structures in the intergranular and interdendritic locations until, finally, at the highest carbon concentration studied, they are so numerous that they appear to fill completely the areas of matrix (Fig. 5). The solid solubility of carbon in pure nickel decreases from approximately 5 0 per cent at the eutectic temperature of 1315°C to about 0 . 2 per cent at room temperature;z and generally is less soluble in nickel alloys. The primary carbides develop shortly before solidification from the remaining liquid in which carbon concentrations would tend t o be high, hence their intergranular and interdendritic distribution. Two classes of carbides found in nickel alloys similar to the dental alloy studied are the MCt and M,,C,. Their dense, closely-packed structures make them
strong and table.^ The MC tends to form from a combination of carbon with tantalum, columbium, titanium and vanadiun in that order and with decreasing stability. The M,,C, forms predominately when chromium is present, as in the alloys noted in this study. It was found that with an increase in carbon and hence carbides a significant effect on mechanical behaviour was observed (Table 2). Percentage elongation decreased as 0 . 1 per cent proof stress and ultimate tensile strength increased although the latter does not increase as much.
t “M” represents any metal combining with carbon t o form a carbide
Dental School, The University of Western Australia, 179 Wellington Street, Perth, W.A. 6000.
Wood, D. R.-Control of quality in the production of nickel alloys and castings. J . Inst. Metals, 85: 319-329, 1 9 5 6 5 . Sims. C. T., and Hagel, W. C.-The superalloys. New York, John Wiley and Sons, 1972 (p. 54).
Conclusion This study has demonstrated the pattern of carbide development associated with progressive increases in carbon content in a series of six nickel chromium alloys. The carbon content is critical since it influences the production and distribution of carbides, which have been shown to alter the mechanical properties, of the nickel chromium alloys, that are dependent upon the development of gamma prime. Furthermore, it has been shown that the attainment of suitable strength is invariably associated with an unacceptable level of ductility.
90
Volume 24, No. 2
The effect of carbon on the metallography of a nickel base removable partial denture casting alloy A. J. Lewis. M.D.Sc. (W.A.), D.D.Sc. (W.A.), D.D.S. (NU), B.Sc. (W.A.), F.R.A.C.D.S., F.I.C.D.
Senior Lecturer in Prosthodontics, The University of Western Australia
ABSTRACT-SIX special nickel-chromium alloys were prepared in which the carbon levels were adjusted above and below that of a commercial nickel base dental casting alloy.
(Received ,for publication November, 1976)
Introduction Carbon levels in the nickel base casting alloys are invariably lower than those in the cobalt base series. Additional strength is achieved in nickel base alloys by precipitation of an intermetallic compound designated gamma prime. Carbon values for nickel alloys lie in the range 0.05-0.20 per cent whilst the minimum stated value for a cobalt alloy is generally 0.25 per cent. The commercial nickel base dental casting alloy* included in this study had a carbon content of 0 . 1 7 per cent. The presence of carbon in association with suitable carbide formers results in the development of a series of carbide structures which add substantially to the mechanical behaviour of the alloy. This paper reports an investigation on the effects, particularly on microstructure, of graded carbon levels within a basic nickel chromium alloy.
Materials and methods Alloy preparation The basic alloy contained 80 per cent nickel and 20 per cent chromium melted and poured in a vacuum. A * Ticonium Premium 100.
carbon level of 0.03 per cent was found in the basic alloy but it was considered too low for the formation of an effective carbide system. A high carbon master alloy was obtained by holding a large piece of carbon rod beneath the surface of the molten basic alloy. The carbon-level of this master alloy was then determined using a combustion method. Special alloys were then produced by adding known amounts of this master alloy to standard charges of the basic nickel chromium alloy. Table 1 lists the carbon levels found in the series of six alloys.
TABLE1 Carhon levels nithin the formuluted ulloys Alloy No.
I
..
..
2 3 4 5 6
..
..
.. ..
.. .. ..
.. .. ..
Level attempted
Level achieved
(Per cent) 0.06 0.10 0.20 0.50 1 .00 2.00
(Per cent) 0.06 0.12 0.26 0.34 0.75 I.46
91
Australian Dental Journal, April, 1979 TABLE2 Mechanical properties of the alloys tested Alloy ~~
Basealloy.. Alloy I , , Alloy 2 . . Alloy 3 , . Alloy 4 . . Alloy 5 . . Alloy 6 . . T.P. ..
Elongation
0 ' "0 stress
(Per cent) 18.2 9.4 4.8 2.9 2.3 0.9 1 .0 0.9
MPa I32 221 260 278 307 354 43 1 517
U.T.S.'
~
.. ,. .. .. .. .. .. ..
MPa 308 336 337 355 358 390 465 543
Ultimate tensile strength
P
-.: ,-
Fig. I.-Structure of Alloy 2 showing two finely lamellar structures. x 500.
Fig. ;.-Structure of Alloy 3 showing the grain boundary, lamellar and island components. x200; inset x500.
Tensile and elongation tests Six specimens conforming to the Australian Standard T28 were prepared from each of the six alloys. The wax patterns were sprued in pairs parallel to the long axis of the casting ring and invested in a silica bonded mould material. A standard 34 g charge of alloy was cast into the mould heated to a temperature of 1000°C. Tensile tests were made on four specimens from each group. Ultimate tensile strength values and 0.1 per cent proof stress values were determined. Elongation estimates were determined as previously described.'
Results The values obtained from the tests are shown in Table 2 and are compared with those obtained for the base alloy (nickel 80 per cent, chromium 20 per cent) and a commercial alloy* (T.P.)* previously reported.' In the basic nickel-chromium alloy, grain boundaries were delineated by continuous deposits of primary carbide and the austenitic matrix was devoid of detail. In Alloys 1 and 2 the continuous intergranular deposits were visible, but in addition islands of primary carbide were seen (both interdendritically and intergranularly) more prominently in Alloy . Frequently areas with a classically lamellar morphology were present (Fig. I), which were ill-defined when compared with those seen at higher carbon concentrations. The continuous intergranular deposits were still a significant feature in Alloy 3 with its higher carbon
Metallography Representative samples were cut from the two remaining specimens in each group mounted, polished and etched at I . 5 V for 15 sec in a solution containing nitric acid 10 ml, glacial acid 5 ml and water 85 ml.
' Lewis, A. J.-The
effect of aluminium on the metallography of a nickel base removable partial denture casting alloy. Austral. D. J., 23: 6, 488491 (Dec.) 1978.
D
* Ticwium Premium 100.
92
Australian Dental Journal, April, 1979
Fig. 3.-Structure of Alloy 4 showing the completed development of a network. x 200.
Fig. 4.-Structure of Alloy 5. x 500.
Fig. 5.-Structure of Alloy 6 . x 500.
Fig. 6.-Structure of the nickel base removable partial denture casting alloy. x 500.
content but the island carbides were more prominent and tended to form a continuous network (Fig. 2). In Alloy 4 the island structures were continuous, and formed an intergranular and interdendritic network. In addition, fine particles of carbide were precipitated within the matrix during cooling (Fig. 3). In Alloy 5 the continuous primary carbides became strongly lamellar and the fine discrete particles visible in the matrix (Fig. 3 ) consolidated in areas adjacent to the intergranular and interdendritic boundaries (Fig. 4). Note a carbide depleted zone along the eutectic carbide chain. The structure of Alloy 6 (Fig. 5) is a further development. Almost continuous deposits of highly exaggerated lamellar structures are visible and the matrix regions appear choked with precipitated carbide particles. The carbide depleted zone previously described continues to be a stable feature of the microstructure. Figure 6 shows the structure of the commercial alloy, and although many of the details previously described
are recognizable, in this case the picture is complicated by the concomitant precipitation of gamma prime.
Discussion Carbide exerts a profound influence on the mechanical behaviour of nickel base alloys and thus has shown differences in the microstructure of the alloys as cast without heat treatment. A definite morphological pattern develops as the carbon concentration increases. At low carbon levels the simple grain boundary carbide deposits progressively thicken and may develop into a fine lamellar form at certain locations; at higher concentrations the lamellae become more massive and more abundant until, as in Alloy 6 , their presence, both intergranularly and interdendritcally, is the dominant feature. The islands of precipitated carbide which develop at relatively low carbon concentrations and straddle the
93
Australian Dental Journal, April, 1979
grain boundaries, slowly increase in extent, eventually forming an intergranular and interdendritic network together with a fine carbide precipitation within the matrix which coarsens as the carbon content increases. Initially, these coarser particles consolidate lateral to the lamellar structures in the intergranular and interdendritic locations until, finally, at the highest carbon concentration studied, they are so numerous that they appear to fill completely the areas of matrix (Fig. 5). The solid solubility of carbon in pure nickel decreases from approximately 5 0 per cent at the eutectic temperature of 1315°C to about 0 . 2 per cent at room temperature;z and generally is less soluble in nickel alloys. The primary carbides develop shortly before solidification from the remaining liquid in which carbon concentrations would tend t o be high, hence their intergranular and interdendritic distribution. Two classes of carbides found in nickel alloys similar to the dental alloy studied are the MCt and M,,C,. Their dense, closely-packed structures make them
strong and table.^ The MC tends to form from a combination of carbon with tantalum, columbium, titanium and vanadiun in that order and with decreasing stability. The M,,C, forms predominately when chromium is present, as in the alloys noted in this study. It was found that with an increase in carbon and hence carbides a significant effect on mechanical behaviour was observed (Table 2). Percentage elongation decreased as 0 . 1 per cent proof stress and ultimate tensile strength increased although the latter does not increase as much.
t “M” represents any metal combining with carbon t o form a carbide
Dental School, The University of Western Australia, 179 Wellington Street, Perth, W.A. 6000.
Wood, D. R.-Control of quality in the production of nickel alloys and castings. J . Inst. Metals, 85: 319-329, 1 9 5 6 5 . Sims. C. T., and Hagel, W. C.-The superalloys. New York, John Wiley and Sons, 1972 (p. 54).
Conclusion This study has demonstrated the pattern of carbide development associated with progressive increases in carbon content in a series of six nickel chromium alloys. The carbon content is critical since it influences the production and distribution of carbides, which have been shown to alter the mechanical properties, of the nickel chromium alloys, that are dependent upon the development of gamma prime. Furthermore, it has been shown that the attainment of suitable strength is invariably associated with an unacceptable level of ductility.
