MICROSTRUCTURE AND PHASE TRANSFORMATIONS IN DENTAL GOLD ALLOYS DETERMINATION OF A COHERENT PHASE DIAGRAM Katsuhiro Yasuda d- Kunihiro Hisatsune Department of Dental Materials Science Nagasaki University School of Dentistry Nagasaki, Japan Electron microscopy has a lot to offer materials scientists in helping them to understand the dependence of mechanical and electrochemical properties on the microstructures resulting from phase transformations in dental alloys. A coherent phase diagram gives us effective information as an aid to developing improved dental alloys. This brief article describes the process of construction of a coherent phase diagram for the AuX - (Ag0.24Cu0.76)1_x section of the gold-copper-silver ternary system, which is the essential system of dental gold alloys, using conventional transmission electron microscopy coupled with selected area electron diffraction. 50 (' Gold Bull., 1993, 26 (2) INTRODUCTION - WHY DO WE range. The modulated structure grows into a 'tweed- NEED A PHASE DIAGRAM? like' structure which is composed of a l and a2 disor- dered phases, both fcc in structure, due to phase sepa- ration by further ageing above T. According to American Dental Association (ADA) While AuCull ordering is formed from a copper- Specification No. 5 and International Standard ISO rich portion in the modulated structure by ageing 1562, dental gold alloys for casting are classified below T, the a2 disordered and AuCui long-pe- into four types, I, II, III and IV, by their gold and riod ordered phases are formed by prolonged ageing platinum metals content and their mechanical proper- in the higher temperature range below T. The Cu3Au ties. Among other things, the Type IV alloys are de- ordering is also induced by ageing in the lower tem- signed to be age-hardenable by an appropriate heat perature range below T, and the a 2 disordered, Cu3- treatment. Au and AuCui ordered phases are also formed by As is prescribed in ADA specification No. 5, the lengthy ageing in this temperature range. Type IV alloys can be softened by quenching after being heated for 0.6 ks (10 min.) at 973 K (700 °C) at In Figure 1, thick solid lines with shading indi- which temperature a single-phase solid solution is cate the locus of the maximum hardness values deter- formed in the alloys. The quenching tends to suppress mined by isothermal ageing. The dot-and-dash-line phase transformation by preventing diffusion of sol- shows the critical temperature for AuCui ordering ute atoms in the crystal lattice. Subsequent hardening in the alloy. The broken line represents the phase is achieved by slow cooling from 723 K (450 °C) to boundary between the region where the three phases 523 I{ (250 °C) in 1.8 ks (30 min.) at a constant cool- AuCui, al and a2 coexist and the Cu3Au-AuCul -a2 ing rate, without referring to a critical temperature for three-phase region; it also shows the critical tempera- phase transformation in each particular alloy. The hard- ture for Cu3Au ordering. ening heat-treatment seems to be based on the fact Another broken line with arrows represents the that the critical temperature of the Type IV dental gold cooling curve of the hardening treatment prescribed alloys usually exists between 723 and 523 K. However, in ADA specification No. 5 (slow cooling from 723 to the actual critical temperature for phase transformation 523 I{ in 1.8 ks at a constant cooling rate). varies among the alloys, depending upon their composition. Moreover, the type of phase trans- formation is different at each tem- perature in an alloy. Microstructures and mechanical properties are af- fected by the types and sequence of the phase transformations in the alloy. For instance, Figure 1 represents a Time-Temperature-Transformation (T-T T) diagram in the Au-49.7at.- %Cu-15.8at.%Ag alloy, as determined by X-ray and electron diffraction as well as transmission electron micros- copy [1]. It is clear that the type of the phase transformation varies at tem- peratures above and below the critical temperature, T, a modulated struc- Figure 1 ture induced by spinodal decomposi- Time-Temperature-Transformation diag am of Au-49.7at. %Cu- tion being formed in the initial stage 15.8at. %Ag alloy and the cooling curve prescribed in ADA of ageing throughout the temperature specification No. 5 for dental gold casting alloys ome. Gold Bull., 1993, 26 (2) 51 From Figure 1, it is clear that the sequence of — predict what phases are in equilibrium for selected phase transformations is a complex process depend- alloy compositions at required temperatures; ing upon ageing temperatures and times, even if the — determine the chemical composition of each phase; ageing is performed at a constant temperature, be- — calculate a volumetric ratio of each phase that is cause the sequence of phase transformations in this formed in the alloy and alloy involves dual mechanisms of ordering and — predict phase transformations which give rise to phase separation. Spinodal decomposition is also at- changes in mechanica) and electrochemical prop- tributed to the age-hardening in the initial stage of erties as welt as biological characteristics. Figure 2 Ag Isothermal section ofa plausible phase diagram of the Au-Cu-Ag ternasy system at 573 K al+a2 20 80 AuCu II + a 2 t..' 60 1 40 cfk -. > a2 60;' 40 AuCu 1 + a 2 } s AuCu II + a 2 80 - 20 y ` , Cu 20 t 40 60 80 Au ai / AuCu I Au in at.% Cu 3Au [ AuCu II ageing in the alloy [1, 2]. During slow cooling from Accordingly, the following research projects are being 723 to 523 K, the harderving occurs mainly by conducted by our laboratory in the Nagasaki Uni- spinodal decomposition and to a lesser extent by Au- versity School of Dentistry: Cu H ordering. The spinodal decomposition, however, — Construction of a coherent phase diagram for the is thought to be an undesirable mechanism of age- Au-Cu-Ag ternary system. hardening for dental gold alloys, because brittleness — Phase transformations and the associated micros- will be caused by the formation of a modulated tructural changes in Au-Cu-Ag ternary alloys. structure. — Age-hardening and the related phase transforma- Thus, several questions arise from the heat-treat- tions in Au-Cu-Ag ternary alloys and commercial ment method prescribed in ADA specification No. 5 dental gold alloys. as well as in the ISO specification for dental gold — Interfacial configurations induced by phase trans- alloys. It is important, therefore, to precisely deter- formation in Au-Cu-Ag ternary alloys. mine the critical temperature of phase transforma- This review article attempts to explain how to deter- tions and to construct a phase diagram for each dental mine the coherent phase diagram of a section in the gold alloy. Generally, a phase diagram gives us effec- Au-Cu-Ag ternary system on the basis of studies using tive information to enable us to: electron microscopy. 52 (` Gold Bull., 1993, 26 (2) CONSTRUCTION OF A COHERENT PHASE DIAGRAM OF THE 873 AuX-(Ago.24Cuo.i6) ► -X SECTION IN THE Au-Cu-Ag TERNARY SYSTEM 773 á} According to the hypothesis of Allen and Cahn [3], a roJ 673 coherent phase must be metastable and, in the presente á of the incoherent phase, must be unstable; coherency E 573 strain must also be present in the coherent multi-phase structure, and dislocation must be visible at the inter- faces with the incoherent phase to reduce coherency 473 strain. Thus, it is thought that grain boundary products are equilibrium incoherent phases, while phases 0 20 40 60 80 formed in the grain interior are metastable coherent Ag.24CU 76 Au (at%) phases. A coherent phase diagram, therefore, gives us more effective information to predict phase transfor- Figure 3 mations related to age-hardening characteristics in an Experimental coherent phase diagram of the alloy system than an incoherent phase diagram. Aux-(Ago.24Cuo.76)1-xpseudobinary section in the As the first step of our systematic study, a coher- Au-Cu Ag ternary system ent phase diagram of the Au-(Ago.24Cuo.76) 1 - X sec- tion in the Au-Cu-Ag ternary system was examined by means of transmission electron micros- copy (TEM) and selected area electron dif- 873 fraction (SAED), since the compositions of most commercial dental gold alloys are in a region close to the section as seen in 773 Figure 2. The experimental result obtained is shown in Figure 3 [4]. A theoretical coher- ent phase diagram of the same vertical sec- d 673 tion in the Au-Cu-Ag ternary system is also al represented in Figure 4, which is calculated d using the cluster variation approximation by E ,_' 573 Yamauchi [5]. Although there are slight differences between the locations of the ex- perimental and theoretical phase boundary lines, both coherent phase diagrams repre- 473 sent a two-phase region which was formed by phase separation, ordering regions of 1 two types, i.e. Cu3Au and AuCu ordered Ag.:iCuae Au (at %) phases, and coexisting regions associated with these two or three phases. In the theo- Figure 4 retical coherent phase diagram, the long- Theoretical coherent phase diagram of 'the period superstructure of the AuCuII phase Aux-(Ago.24Cuo.76)1-x pseudobinary section in the Au-Cu-Ag cannot be distinguished from the AuCul ternary system tetragonal superstructure, because all the Gold Bull, 1993, 26 (2) 53 Table 1 Alloy Composition of alloy Composition of the experimental Lattice parameter of Speci- (at.%) alloys and their lattice parameters solid solution (nm) in solid solution mens Code Au Cu Ag a 3G 48.5 48.5 3 - b 6G 46.9 47,5 5.6 0.3890 c 9G 45.5 45,5 9 - d 16K 43.0 43.3 13.9 0.3903 e 17G 36.2 41.4 22.4 0.3915 f 28G 33.4 35.5 31.1 0.3839 g 18K 53.1 35.6 11.3 0.3838 h 14K 34.5 49.8 15.7 0.3875 i 60A 60 20 20 0.4003 i 50A 50 25 25 0.3986 k 40A 40 30 30 0.3960 1 19K 58.9 31.2 9,9 0.3957 m 12K 27.4 55.2 17.4 0.3850 possible compounds formed must be superstructures crostructuresandSAEDpatternsclearlyindicates the of the lattice of the parent disordered fcc phase on the coexistence of Cu3Au and AuCuil ordered phases basis of the cluster variation approximation [6].