Dental Materials - Advances in Biomaterials Science and Biomedical Applications - page 524

Figure 10 shows the SADP, key diagram, and HRTEM bright field image of ST 1123K -WQ [11].
The SADP and the key diagram in Fig. 10 (a) and (b) indicate that the FCC α phase and the
three variants of the L1 0 -type ordered β′ phase are superimposed. The L1 0 -type ordered β′
phase is precipitated in the FCC matrix after ST. The streaks on the reflection spots of the
L1 0 -type ordered β′ phase indicate that the shape of the β′ phase is similar to a thin plate.
The HRTEM bright field image in Fig. 10 (c) shows that several nanometer-sized thick plate-
shaped β′ phase with two variants (N 1 , N 2 ), whose c-axes are normal to the electron beam,
are precipitated in the matrix.
It is well known that it is difficult to make a single phase by quenching after high temperature
ST; the supersaturated vacancies help the solutes to diffuse more easily and also help to form
more clusters, G-P zones, and metastable phases during quenching after high temperature ST.
The precipitated β′ phase of ST 1123K -WQ shown in Fig.9 (d), (e) and Fig. 10 (c) is like a thin plate
with a nanometer-scale thickness. These images also suggest that the formation of the β′ phase
is diffusion controlled. Generally, the formation of precipitates can be considered to be order-
disorder transition, diffusionless transformation (martensitic transformation), or diffusional
transformation. In the as-solutionized Ag-Pd-Cu-Au alloy used in this case, the dependence
of microstructural changes in the precipitated β′ phase on both the cooling rate after ST and the
ST temperature show that the precipitated L1 0 -type ordered β′ phase is formed during the
cooling process and that the growth of the β′ phase is influenced by the diffusion process. The
hardness increases with an increase in the cooling rate after ST, and consequently, the hard‐
ness of ST 1123K -WQ increases significantly by quenching after ST (Fig. 6). The fine β′ phase in
ST 1123K -WQ is densely precipitated in the matrix. The hardness of ST 1123K -AC increased only
slightly, while the hardness of ST 1123K -FC decreases, as only coarse β′ phases are precipitated in
the matrix of ST 1123K -AC and ST 1123K -FC. Thus, the increase in hardness may be strongly affected
by the presence of finely precipitated β′ phase. The coherent precipitation of β′ phases with
long and short axes of around 100 nm and 10 nm, respectively, also occured during ST, although
the amount of β′ phase decreases with an increase in the ST time. The effect of solid solution
hardening in the α, α 1 , and α 2 phases is lower than that exerted by the precipitation harden‐
ing due to β′ phases.
3.1.2. Hardening behavior of Ag-20Pd-12Au-14.5Cu alloy fabricated by liquid rapid solidification
An Ag-20Pd-14.5Cu-12Au alloy with a single α phase can be fabricated using a liquid rapid
solidification (LRS) method that employs a melting mechanism, as shown schematically in Fig.
11 [12]. The critical temperature for the order-disorder transformation in the Cu-Pd binary
phase diagram is below 1023 K, and hence, at 1023 K, the Cu-rich phase α 1 and Ag-rich phase
α 2 decompose, as shown in the Ag-Cu binary phase diagram. Figure 12 shows TEM micro‐
graphs of an Ag-20Pd-14.5Cu-12Au alloy fabricated by the LRS method [13]. No precipita‐
tion is observed in the matrix.
As shown in Fig. 12 (b) and (c), the LRS alloy consists of a single α phase with face centered
cubic structure (FCC). The tensile properties of the as-received Ag-20Pd-14.5Cu-12Au alloy
(AS), AS subjected to ST at 1123 K for 3.6 ks in vacuum (ST AS/3.6 ks ), LRS alloy (LRS), and LRS
alloy subjected to ST at 1123 K for 3.6 ks in vacuum (ST LRS/3.6 ks ) are shown in Fig. 13 [13]. The