Journal of Oral Rehabilitation, 1979, Volume 6, pages 291-309
Evaluation of alternative alloys to precious ceramic alloys 1. Mechanical properties
J.-M. MEYER, J. PAYANflwiJ.-N. NALLY
School of Dentistry,
University of Geneva, Switzerland, and Faculty of Dental Surgery, University of Aix-Marseille, Marseille, France
Summary In the first part of this study, the microstructures and the mechanical properties of precious, semi-precious, and nonprecious dental casting alloys for the porcelainbaked-to-metal technique have been determined. The semi-precious alloys contained only 50 % gold, and palladium, silver, and some base metals. The nonprecious alloys were of the nickel-chromium type. Discs and miniaturized tensile bars have been cast and tested either in the as cast condition, or after a simulation of the various porcelain bakes. Proof stress, ultimate tensile strength, elongation, and plastic stiflfness have been measured and results compared by use of analyses of variance. The microstructure examination shows that the simulation of the porcelain bakes improves the homogeneity ofthe precious and semi-precious alloys. Simultaneously, the mechanical properties of the same alloys are also improved. One semi-precious alloy, still under development at the time of these tests, has its mechanical characteristics markedly downgraded by the thermal treatments. The nickel-chromium alloys exhibit the best range of mechanical properties for the porcelain-baked-to-metal technique, when considering the three most relevant properties: proof stress, plastic stiflfness, and modulus of elasticity. Introduction The porcelain-baked-to-metal technique has become widely favoured by the dental practitioners after an introductory period of about 10 years. The patients also have been seduced by its obvious aesthetic superiority, and have asked increasingly for this type of restoration. Because of its success and by the 'supply and demand' law, this technique became more expensive, and the dramatic rise of the gold price only made the situation worse. These economic factors did partly restrain the diffusion of the technique. The characteristics of the market as well as the requests of the practitioners created two lines of research among alloy producers; on one hand, the dental gold alloys manufacturers developed alloys with a reduced gold content, and on the other hand, the Correspondence: Dr J.-M. Meyer, Unite de Technologie des Materiaux Dentaires,Ecole de Medecine Dentaire, 19 rue Bartbelemy-Menn, 1211 Geneva 4, Switzerland.
0305-182X/79/0700-0291 $02.00 19
© 1979 Blackwell Scientific Publications 291
292
J.-M. Meyer, J. Payan andJ.-N. Nally
producers of cobalt-chromium alloys for partial prosthetics made use of their technology to create nonprecious alloys, generally based on a nickel-chromium composition, for the porcelain-baked-to-metal technique. This evolution led to the availability of a constantly increasing number of semi-precious and nonprecious alloys. The disparate range in each of these two categories makes the choice for the practitioner more and more diflficult. The comparison of the fundamental properties of these new types of alloys with those, already known, of dental gold alloys, is a suitable method of furnishing the practitioner with the elements of an objective evaluation of these materials. This is the aim of this series of three papers, which shall be dealing successively with the mechanical properties of the new alloys, their resistance to tarnish and corrosion, the precision of their casting and the strength of the porcelain bonding. The selection of the properties to be studied results from the early experience gained in clinics and in dental laboratories, and from the evaluation of problems, which may arise with the decrease or the complete disappearance of the gold content, the basic metal of the alloys which have set, up to now, the practical standards of the porcelain-baked-to-metal technique. It should be pointed out that the only specification available at this time is the British Standard BS 3366: Part 2: 1976 Dental base metal casting alloys. Part 2: Inlay, crown and bridge alloys, and that a subcommittee of the American National Standards Committee Z 156 has been working on this subject since 1971. A first draft, limited to precious alloys only, has already been written (Progress report ofthe Subcommittee on Porcelain-Metal Systems, 25 October, 1976). Tht precious ceramic alloys usually have a gold content higher than 80% (Anderson, 1972). Their fusing temperature is higher than that of conventional dental gold casting alloys, and their coefficient of thermal expansion lies close to that of low fusing dental porcelains and below that of conventional gold alloys. Finally, their rigidity is slightly above that of conventional gold alloys: 110 000 MN/m^ versus 95 000 MN/m^ (Craig & Peyton, 1975). The semi-precious ceramic alloys usually do not contain more than 50 % gold and 30% palladium, with a balance of silver and nonprecious metals. Platinum is no longer present. The specific gravity of this type of alloy decreases sharply, whereas the modulus of elasticity increases slightly, up to 117 000 MN/m^ (Tuccillo, 1973). Mechanical properties and bonding with the porcelain still depend, as for the precious ceramic alloys, upon the presence of nonprecious elements in small amounts, like indium, iron or tin (Tuccillo, 1974). Their working properties (casting, finishing, porcelain build-up, soldering, polishing) can generally be compared with those of the precious ceramic alloys and therefore important changes in laboratory techniques are not necessary. The nonprecious ceramic alloys, however, have a composition fundamentally different from that of the two preceding types. They do not contain gold, platinum, palladium and silver, the main constituents are nickel and chromium, usually with molybdenum. Additions of aluminium, manganese, iron, silicon, boron or beryllium can be found beside the three basic components (Meyer, 1977). The alloy composition should therefore produce a profile of properties markedly diflferent from those of the two other categories of alloys. For example, the disappearance of precious metals can negatively influence their resistance to tarnish and corrosion in the mouth. The presence of nickel and chromium increases the fusing temperature, the hardness, and the oxidizability. The following consequences can therefore be anticipated: the fusing
Alternative alloys to precious ceramic alloys. 1
293
technique will be modified, the compensation of the cooling shrinkage will be more diflficult to obtain, and along with it the precision of the fit of castings; finishing procedures will be less easy to perform; finally, the heavier build-up of superficial oxides during the various firings involved in the technique may impair the bonding strength of the porcelain and decrease the solderability. The wide range of properties associated with the porcelain-baked-to-metal technique, as well as the varying influence ofthe alloy compositions upon these properties, have led the authors to study these kinds of alloys by means of a series of in vitro experiments, prior to a comparative clinical evaluation. Materials and methods The following mechanical characteristics of ceramic alloy are the most clinically significant: the highest possible yield strength, a high modulus of elasticity, and a low ductility (Meyer, 1976). Proof stress at 0-2%, ultimate tensile strength, elongation and 'plastic stiflfness', have been measured in this study by means of tensile testing of cast specimens. The plastic stiflfness has been defined elsewhere (Paddon & Wilson, 1972) as the relationship between the stress and the early plastic strain of an alloy. Its numerical value is the slope of the line traced through the stresses corresponding to plastic strains of 0-1, 0-2, 0-5 and 1-0%, as measured on the tensile curve and plotted on a logarithmic scale of these strains. This value has a definite advantage: for a given alloy in a given state, it is far less dependent on variations from the casting technique than the other mechanical properties, and it gives a valuable insight on the ductility of alloys. The hardness has been measured on the same samples used to reveal the microstructure. Alloys studied For each category of alloys (precious, semi-precious and nonprecious), two products of various origins were selected (Table 1). Alloy E was still under development at the time of this study, and it has never been put on the market because of its various deficiencies. States of the alloys The mechanical properties and the microstructures were first studied on specimens prepared from fresh alloys, in the 'as cast' (AC) condition. All the alloys were then tested with cast specimens which received a cycle of thermal treatments simulating the various firings necessary to the porcelain build-up. This cycle included: (1) oxidation for 10 min at 980°C, under vacuum and followed by air cooling; (2) one firing (opaque porcelain) for 6 min at 960°C, under vacuum and followed by air cooling; (3) two firings (gingival porcelain) for 4 min each at 960°C, under vacuum and followed by air cooling; (4) one firing (glazing) for 3 min at 950°C, under normal pressure and followed by air cooling. The particular state of these speciriiens is later referred to as 'simulated bakes' (SB). The precious and semi-precious alloys were also tested with cast specimens prepared from the remelted sprue buttons and casting bars of the preceding castings. These specimens were thermally treated as above, and their state is referred to as 'remelted with simulated bakes' (R/SB). 19*
294
J.-M. Meyer, J. Payan and J.-N. Nally
Table 1. Tested alloys Code
Alloys
Precious alloys A Armator II Jelenko 'O' Semi-precious alloys E E 1179 (experimental) C Cameo
Manufacturer Usine Genevoise de Degrossissage d'Or(UGDO) - Switzerland J.F. Jelenko Co., USA
Au 82-Pt 11-9Pd ] -S-Ag 2-5 other: In Au 88-Pt 5Pd 5-Ag 1 others: In-Fe-Sn
(a)
UGDO-Switzerland
Au 50-5-Pd 30-5 Ag 13-In 6 Au 50-Pd 30 Agl3 others: In-Fe-Sn
(b)
J.F. Jelenko C c USA
Nonprecious alloys T Ticon
Ticonium, USA
W
Bego, W. Germany
Wiron S
Composition (weight %)
Ni 70-Cr 16 Mo 5-Mn 4 others: Al-Si-Fe Be-C Ni 70-Cr 16 Mo 4-8-Mn 3-5 others: Al-Si-B
(b)
(b)
(c)
(b)
(a) Meyer (1971); (b) Personal communication from the manufacturer; (c) Bunisset (1972).
Finally, an additional state was tested for alloys A and E: previous castings were regenerated with 50% of fresh alloy, and the specimens prepared from this regenerated alloy received the same cycle of thermal treatments. This state is referred to as 'regenerated (50% fresh alloy) with simulated bakes' (50/SB). Preparation of the testing specimens The testing bars were cast from machined acrylic models. The model was that described by Gettleman & Harrison (1969); it was chosen because of its small dimensions, of considerable importance when testing precious alloys (Fig. 1). This testing bar model was designed according to the ASTM specification No. E8-66 'Standard methods of tension testing of metallic materials' (ASTM Standards, Part 31, p. 201220, 1967), and had a gauge length/diameter ratio of 4:1, which is close to the ratio recommended by the British Standard 18:1962 'Methods for tensile testing of metals', as cited by Fenner (1965). The threaded ends ofthe original model were replaced by 25-8 15 -12-5 in ro
All values in mm
Fig. 1. Design of the tensile bar, according to Gettleman & Harrison (1969).
Alternative alloys to precious ceramic alloys. 1
295
two flat, parallel surfaces to ensure an effective gripping during the tensile test. Each tensile bar cast in precious alloys weighed between 4-5 and 5 grams, plus the weight of the casting bars and sprue button (Figs 2 and 3).
Fig. 2. Typical spruing design for three tensile bars and one disc.
Hardness tests were made on discs measuring 10 mm in diameter and 2-3 mm in thickness. Wax models of these discs were prepared in a metal mould. Three tensile bars and one disc were invested* together in standardized conditions. The overall weight of such a standard casting ranged between 45 and 50 grams for precious alloys, including sprues and button. After casting and removal ofthe investment, the testing specimens were cut oflf from the sprues and given an ultrasonic cleaning. When needed, the thermal treatments described above were performed at this stage. The discs were then embedded in an acrylic mould and metallographically polished. Testing procedures Tensile testing was performed with an Instron testing machinet at a crosshead speed of 2 mm/min. Tensile curves were traced with a magnification of x 100 for the elongation. Since the machine was not equipped with an extensometer, the elongation was measured directly on the stress-strain curve (Fig. 4). Vickers hardness was determined under a load of 1 kgj. * Ceramigold, Whip-Mix Co., U.S.A. t Instron Model 1114, High Wycombe, Bucks., England. + Microdurometer Type 249A, Hauser, Bienne, Switzerland.
296
J.-M. Meyer, J. Payan and J.-N. Nally
Fig. 3. Typical casting.
Ultimate tensile strength 330
—
-
•
•
•
•
•
—
•
—
•
—
•
—
•
— ~ \ ^ . . . — ^
*
300
/
/
(kg:
200 /
1
153
100
/Proof stress 0-2 7
n 1
0
1
0-5
1-0 Elongation
•
Length (mm)
Fig. 4. Load vs crosshead travel curve (tensile curve) for alloy T after simulated bakes (SB state).
Alternative alloys to precious ceramie alloys. 1 Results and discussion Microstructure
:
•
-/;
..
297
;
Typical microstructures of the tested alloys are shown at the same enlargement and for the various states in Fig. 5: (a) precious alloys; (b) semi-precious alloys; and (c) nonprecious alloys. ' : : These photomicrographs indicate that: (1) alloy E exhibits larger grains than those of the other precious and semi-precious alloys; ' .
^ Q-l mm Fig. 5(a) Typical microstructures of precious alloys.
298
J.-M. Meyer, J. Payan and J.-N. Nally
"
^i
t•
AC V,
X, k
*
/
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>-
'
^
• , • ' • •
,
/
.
SB
f
;••
0-1 mm Fig. 5(b) Typical microstructures of semi-precious alloys.
(2) the simulation of the porcelain bakes improves the homogeneity of precious alloys and of the semi-precious alloy C; (3) the simulated bakes do not show any apparent influence on the microstructure of the nonprecious alloys. Hardness Table 2 shows the Vickers hardness of the tested alloys, as measured in their various states. These results are the mean of twenty measurements made on two discs for the precious and semi-precious alloys, and of ten measurements made on one disc for the nonprecious alloys. The coefficient of variation is given for every mean.
Alternative alloys to precious ceramic alloys. 1
299
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Evaluation of alternative alloys to precious ceramic alloys 1. Mechanical properties
J.-M. MEYER, J. PAYANflwiJ.-N. NALLY
School of Dentistry,
University of Geneva, Switzerland, and Faculty of Dental Surgery, University of Aix-Marseille, Marseille, France
Summary In the first part of this study, the microstructures and the mechanical properties of precious, semi-precious, and nonprecious dental casting alloys for the porcelainbaked-to-metal technique have been determined. The semi-precious alloys contained only 50 % gold, and palladium, silver, and some base metals. The nonprecious alloys were of the nickel-chromium type. Discs and miniaturized tensile bars have been cast and tested either in the as cast condition, or after a simulation of the various porcelain bakes. Proof stress, ultimate tensile strength, elongation, and plastic stiflfness have been measured and results compared by use of analyses of variance. The microstructure examination shows that the simulation of the porcelain bakes improves the homogeneity ofthe precious and semi-precious alloys. Simultaneously, the mechanical properties of the same alloys are also improved. One semi-precious alloy, still under development at the time of these tests, has its mechanical characteristics markedly downgraded by the thermal treatments. The nickel-chromium alloys exhibit the best range of mechanical properties for the porcelain-baked-to-metal technique, when considering the three most relevant properties: proof stress, plastic stiflfness, and modulus of elasticity. Introduction The porcelain-baked-to-metal technique has become widely favoured by the dental practitioners after an introductory period of about 10 years. The patients also have been seduced by its obvious aesthetic superiority, and have asked increasingly for this type of restoration. Because of its success and by the 'supply and demand' law, this technique became more expensive, and the dramatic rise of the gold price only made the situation worse. These economic factors did partly restrain the diffusion of the technique. The characteristics of the market as well as the requests of the practitioners created two lines of research among alloy producers; on one hand, the dental gold alloys manufacturers developed alloys with a reduced gold content, and on the other hand, the Correspondence: Dr J.-M. Meyer, Unite de Technologie des Materiaux Dentaires,Ecole de Medecine Dentaire, 19 rue Bartbelemy-Menn, 1211 Geneva 4, Switzerland.
0305-182X/79/0700-0291 $02.00 19
© 1979 Blackwell Scientific Publications 291
292
J.-M. Meyer, J. Payan andJ.-N. Nally
producers of cobalt-chromium alloys for partial prosthetics made use of their technology to create nonprecious alloys, generally based on a nickel-chromium composition, for the porcelain-baked-to-metal technique. This evolution led to the availability of a constantly increasing number of semi-precious and nonprecious alloys. The disparate range in each of these two categories makes the choice for the practitioner more and more diflficult. The comparison of the fundamental properties of these new types of alloys with those, already known, of dental gold alloys, is a suitable method of furnishing the practitioner with the elements of an objective evaluation of these materials. This is the aim of this series of three papers, which shall be dealing successively with the mechanical properties of the new alloys, their resistance to tarnish and corrosion, the precision of their casting and the strength of the porcelain bonding. The selection of the properties to be studied results from the early experience gained in clinics and in dental laboratories, and from the evaluation of problems, which may arise with the decrease or the complete disappearance of the gold content, the basic metal of the alloys which have set, up to now, the practical standards of the porcelain-baked-to-metal technique. It should be pointed out that the only specification available at this time is the British Standard BS 3366: Part 2: 1976 Dental base metal casting alloys. Part 2: Inlay, crown and bridge alloys, and that a subcommittee of the American National Standards Committee Z 156 has been working on this subject since 1971. A first draft, limited to precious alloys only, has already been written (Progress report ofthe Subcommittee on Porcelain-Metal Systems, 25 October, 1976). Tht precious ceramic alloys usually have a gold content higher than 80% (Anderson, 1972). Their fusing temperature is higher than that of conventional dental gold casting alloys, and their coefficient of thermal expansion lies close to that of low fusing dental porcelains and below that of conventional gold alloys. Finally, their rigidity is slightly above that of conventional gold alloys: 110 000 MN/m^ versus 95 000 MN/m^ (Craig & Peyton, 1975). The semi-precious ceramic alloys usually do not contain more than 50 % gold and 30% palladium, with a balance of silver and nonprecious metals. Platinum is no longer present. The specific gravity of this type of alloy decreases sharply, whereas the modulus of elasticity increases slightly, up to 117 000 MN/m^ (Tuccillo, 1973). Mechanical properties and bonding with the porcelain still depend, as for the precious ceramic alloys, upon the presence of nonprecious elements in small amounts, like indium, iron or tin (Tuccillo, 1974). Their working properties (casting, finishing, porcelain build-up, soldering, polishing) can generally be compared with those of the precious ceramic alloys and therefore important changes in laboratory techniques are not necessary. The nonprecious ceramic alloys, however, have a composition fundamentally different from that of the two preceding types. They do not contain gold, platinum, palladium and silver, the main constituents are nickel and chromium, usually with molybdenum. Additions of aluminium, manganese, iron, silicon, boron or beryllium can be found beside the three basic components (Meyer, 1977). The alloy composition should therefore produce a profile of properties markedly diflferent from those of the two other categories of alloys. For example, the disappearance of precious metals can negatively influence their resistance to tarnish and corrosion in the mouth. The presence of nickel and chromium increases the fusing temperature, the hardness, and the oxidizability. The following consequences can therefore be anticipated: the fusing
Alternative alloys to precious ceramic alloys. 1
293
technique will be modified, the compensation of the cooling shrinkage will be more diflficult to obtain, and along with it the precision of the fit of castings; finishing procedures will be less easy to perform; finally, the heavier build-up of superficial oxides during the various firings involved in the technique may impair the bonding strength of the porcelain and decrease the solderability. The wide range of properties associated with the porcelain-baked-to-metal technique, as well as the varying influence ofthe alloy compositions upon these properties, have led the authors to study these kinds of alloys by means of a series of in vitro experiments, prior to a comparative clinical evaluation. Materials and methods The following mechanical characteristics of ceramic alloy are the most clinically significant: the highest possible yield strength, a high modulus of elasticity, and a low ductility (Meyer, 1976). Proof stress at 0-2%, ultimate tensile strength, elongation and 'plastic stiflfness', have been measured in this study by means of tensile testing of cast specimens. The plastic stiflfness has been defined elsewhere (Paddon & Wilson, 1972) as the relationship between the stress and the early plastic strain of an alloy. Its numerical value is the slope of the line traced through the stresses corresponding to plastic strains of 0-1, 0-2, 0-5 and 1-0%, as measured on the tensile curve and plotted on a logarithmic scale of these strains. This value has a definite advantage: for a given alloy in a given state, it is far less dependent on variations from the casting technique than the other mechanical properties, and it gives a valuable insight on the ductility of alloys. The hardness has been measured on the same samples used to reveal the microstructure. Alloys studied For each category of alloys (precious, semi-precious and nonprecious), two products of various origins were selected (Table 1). Alloy E was still under development at the time of this study, and it has never been put on the market because of its various deficiencies. States of the alloys The mechanical properties and the microstructures were first studied on specimens prepared from fresh alloys, in the 'as cast' (AC) condition. All the alloys were then tested with cast specimens which received a cycle of thermal treatments simulating the various firings necessary to the porcelain build-up. This cycle included: (1) oxidation for 10 min at 980°C, under vacuum and followed by air cooling; (2) one firing (opaque porcelain) for 6 min at 960°C, under vacuum and followed by air cooling; (3) two firings (gingival porcelain) for 4 min each at 960°C, under vacuum and followed by air cooling; (4) one firing (glazing) for 3 min at 950°C, under normal pressure and followed by air cooling. The particular state of these speciriiens is later referred to as 'simulated bakes' (SB). The precious and semi-precious alloys were also tested with cast specimens prepared from the remelted sprue buttons and casting bars of the preceding castings. These specimens were thermally treated as above, and their state is referred to as 'remelted with simulated bakes' (R/SB). 19*
294
J.-M. Meyer, J. Payan and J.-N. Nally
Table 1. Tested alloys Code
Alloys
Precious alloys A Armator II Jelenko 'O' Semi-precious alloys E E 1179 (experimental) C Cameo
Manufacturer Usine Genevoise de Degrossissage d'Or(UGDO) - Switzerland J.F. Jelenko Co., USA
Au 82-Pt 11-9Pd ] -S-Ag 2-5 other: In Au 88-Pt 5Pd 5-Ag 1 others: In-Fe-Sn
(a)
UGDO-Switzerland
Au 50-5-Pd 30-5 Ag 13-In 6 Au 50-Pd 30 Agl3 others: In-Fe-Sn
(b)
J.F. Jelenko C c USA
Nonprecious alloys T Ticon
Ticonium, USA
W
Bego, W. Germany
Wiron S
Composition (weight %)
Ni 70-Cr 16 Mo 5-Mn 4 others: Al-Si-Fe Be-C Ni 70-Cr 16 Mo 4-8-Mn 3-5 others: Al-Si-B
(b)
(b)
(c)
(b)
(a) Meyer (1971); (b) Personal communication from the manufacturer; (c) Bunisset (1972).
Finally, an additional state was tested for alloys A and E: previous castings were regenerated with 50% of fresh alloy, and the specimens prepared from this regenerated alloy received the same cycle of thermal treatments. This state is referred to as 'regenerated (50% fresh alloy) with simulated bakes' (50/SB). Preparation of the testing specimens The testing bars were cast from machined acrylic models. The model was that described by Gettleman & Harrison (1969); it was chosen because of its small dimensions, of considerable importance when testing precious alloys (Fig. 1). This testing bar model was designed according to the ASTM specification No. E8-66 'Standard methods of tension testing of metallic materials' (ASTM Standards, Part 31, p. 201220, 1967), and had a gauge length/diameter ratio of 4:1, which is close to the ratio recommended by the British Standard 18:1962 'Methods for tensile testing of metals', as cited by Fenner (1965). The threaded ends ofthe original model were replaced by 25-8 15 -12-5 in ro
All values in mm
Fig. 1. Design of the tensile bar, according to Gettleman & Harrison (1969).
Alternative alloys to precious ceramic alloys. 1
295
two flat, parallel surfaces to ensure an effective gripping during the tensile test. Each tensile bar cast in precious alloys weighed between 4-5 and 5 grams, plus the weight of the casting bars and sprue button (Figs 2 and 3).
Fig. 2. Typical spruing design for three tensile bars and one disc.
Hardness tests were made on discs measuring 10 mm in diameter and 2-3 mm in thickness. Wax models of these discs were prepared in a metal mould. Three tensile bars and one disc were invested* together in standardized conditions. The overall weight of such a standard casting ranged between 45 and 50 grams for precious alloys, including sprues and button. After casting and removal ofthe investment, the testing specimens were cut oflf from the sprues and given an ultrasonic cleaning. When needed, the thermal treatments described above were performed at this stage. The discs were then embedded in an acrylic mould and metallographically polished. Testing procedures Tensile testing was performed with an Instron testing machinet at a crosshead speed of 2 mm/min. Tensile curves were traced with a magnification of x 100 for the elongation. Since the machine was not equipped with an extensometer, the elongation was measured directly on the stress-strain curve (Fig. 4). Vickers hardness was determined under a load of 1 kgj. * Ceramigold, Whip-Mix Co., U.S.A. t Instron Model 1114, High Wycombe, Bucks., England. + Microdurometer Type 249A, Hauser, Bienne, Switzerland.
296
J.-M. Meyer, J. Payan and J.-N. Nally
Fig. 3. Typical casting.
Ultimate tensile strength 330
—
-
•
•
•
•
•
—
•
—
•
—
•
—
•
— ~ \ ^ . . . — ^
*
300
/
/
(kg:
200 /
1
153
100
/Proof stress 0-2 7
n 1
0
1
0-5
1-0 Elongation
•
Length (mm)
Fig. 4. Load vs crosshead travel curve (tensile curve) for alloy T after simulated bakes (SB state).
Alternative alloys to precious ceramie alloys. 1 Results and discussion Microstructure
:
•
-/;
..
297
;
Typical microstructures of the tested alloys are shown at the same enlargement and for the various states in Fig. 5: (a) precious alloys; (b) semi-precious alloys; and (c) nonprecious alloys. ' : : These photomicrographs indicate that: (1) alloy E exhibits larger grains than those of the other precious and semi-precious alloys; ' .
^ Q-l mm Fig. 5(a) Typical microstructures of precious alloys.
298
J.-M. Meyer, J. Payan and J.-N. Nally
"
^i
t•
AC V,
X, k
*
/
»
>-
'
^
• , • ' • •
,
/
.
SB
f
;••
0-1 mm Fig. 5(b) Typical microstructures of semi-precious alloys.
(2) the simulation of the porcelain bakes improves the homogeneity of precious alloys and of the semi-precious alloy C; (3) the simulated bakes do not show any apparent influence on the microstructure of the nonprecious alloys. Hardness Table 2 shows the Vickers hardness of the tested alloys, as measured in their various states. These results are the mean of twenty measurements made on two discs for the precious and semi-precious alloys, and of ten measurements made on one disc for the nonprecious alloys. The coefficient of variation is given for every mean.
Alternative alloys to precious ceramic alloys. 1
299
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