Phase Reaction and Diffusion Behavior between AuTi and CoTi Intermetallic Compounds
2019 Volume 60 Issue 5 Pages 631-635
Details
2019 Volume 60 Issue 5 Pages 631-635
The phase reaction and diffusion behavior of stoichiometric AuTi and CoTi intermetallic compounds were clarified using diffusion couple technique. It was found by scanning electron microscopy equipped with energy dispersive X-ray spectroscope that two intermetallic compounds of (Au,Co)Ti3 and (Au,Co)2Ti were produced near interface between the AuTi and CoTi when heat-treated at 1173 K and 1273 K, and that only (Au,Co)Ti3 was formed when heat-treated at 1373 K. The presence of these intermetallic compounds was explained by the concentration dependent diffusivity in the diffusion couple. With increasing the heat-treatment temperature from 1173 K to 1373 K, the thickness of the diffusion layer increased from 78 µm to 200 µm. The apparent activation energy for the layer growth was estimated as 127 kJ/mol. Furthermore, the diffusion path in the isothermal section at 1173 K–1373 K in the Au–Ti–Co system was also discussed.
The concerning of the possibility of Ni-toxicity1,2) and insufficient radiopaque3,4) in NiTi practical biomedical shape memory alloys (SMAs) encouraged the research and development of Ni-free biomedical SMAs equipped with high radiopaque during the past few decades. AuTi alloy has received much attention in recent year due to the required properties of superior biocompatibility and high X-ray visibility.5–8) AuTi alloy possesses a thermoelastic martensitic transformation from ordered bcc phase (B2 parent) to orthorhombic phase (B19 martensite). This alloy exhibits shape recovery ratio of 80% at elevated temperature and the maximum transformation strain of 10% which is enough for a practical application.9–12) However, AuTi alloy cannot be applied as a biomedical material since the high operating temperature i.e. the martensitic transformation start temperature (Ms) of 880 K.9,11,13) It has been reported that the Ms temperature in AuTi closed to the body temperature by an alloying of Co about 18–19 mol%.14–16) Then, it is expected that AuTi and CoTi can be formed a continuous solid solution of (Au,Co)Ti in the AuTi–CoTi pseudo-binary system.
The diffusion couple (DC) technique is a useful method for determining diffusion coefficient, diffusion path, and phase diagram in structural and functional materials.17–21) Understanding of diffusion path is applicable for predicting inter-diffusion microstructures and suggesting the initial alloy compositions to change the microstructure. These capability have potential to design the laminated SMA composite plates22,23) and combinatorial studies for high-throughput characterization of shape memory and mechanical properties of SMAs.18,24) Recently, our group have reported the diffusion behavior in the AuTi/Co system by using the laminate of AuTi/Co/AuTi fabricated by the DC method.20) The (Au,Co)2Ti and (Au,Ti)Co2 intermetallic compounds were formed near the interface after heat-treated 1073 K to 1273 K. The apparent activation energy for the growth of the diffusion layer was calculated to be 168 kJ/mol and the diffusion path between AuTi and Co was also discussed. Apart from the AuTi/Co system, the diffusion behavior in the AuTi/Fe25) and AuTi/Nb26) systems has also been studied.
Even though the Ms temperature of AuTi can be linearly changed by Co addition, the phase diagram of the AuTi–CoTi pseudo-binary system is still unclear. To obtain the information of phase diagram between AuTi and CoTi, a DC method is applied. Therefore, the objective of the present study is to clarify chemical composition, phase reaction and diffusion behavior near the interface between AuTi and CoTi alloys.
Arc melting method under Ar–1%H2 atmosphere was used to fabricate the equiatomic AuTi and CoTi alloy ingots. They were hot-forged at 1423 K for 3 h to be about 2 mm in thickness. Alloy plates, 20 mm × 20 mm × 1.5 mm were cut from the hot-forged disks by an electro-discharge machine (EDM). The bonding face of DC was fine polished to a 1 µm finish. A DC of AuTi and CoTi was made by hot-forging under a compressive stress of about 5 MPa to ensure good contact between the couple. W wires were inserted between the DC to indicate the initial interface between the AuTi and CoTi alloys. A hot-forging was conducted at 1523 K for 3 h with a heating/cooling rate of 4 K/min in Ar environment. Figure 1 shows a combination of cross-sectional optical microscope images of the DC after hot-forged at 1523 K for 3 h and the dash circles indicated the W wires. The DC was cross-sectioned and followed by heat-treatments under partial pressure at 1173 K to 1373 K for 5 h, eventually water quenched.
A combination of cross-sectional optical microscope images of the DC hot-forged at 1523 K for 3 h.
To characterize the microstructure, the chemical composition and concentration profiles near the AuTi/CoTi interfaces, optical microscope (OM), scanning electron microscope (SEM) and energy dispersive X-ray spectroscope (EDS) were carried out. The SEM-EDS measurement was performed by a HITACHI S-4300SE operated at 20 kV. The OM, SEM and EDS specimens were mechanically polished until a 1 µm finish.
Figure 2(a) illustrates a SEM micrograph near the interface of the DC of AuTi/CoTi heat-treated at 1173 K for 5 h. It is seen that the DC of AuTi/CoTi is uniformly bonded. In this SEM image, the dark side refers to the CoTi side of the DC and the bright side corresponds to the AuTi end of the DC. The inset image in Fig. 2(a) also shows an enlarged SEM image obtained from the region closed to the interface. This region contained the dark layer and the bright particles. The initial interface was identified by the line between the center of W wires which indicated by the dash line. The chemical compositions of the layer and the particles will be discussed later.
(a) SEM images near the AuTi/CoTi interface when heat-treated at 1173 K for 5 h, and (b) a concentration profile of the inter-diffusion zone obtained by EDS.
Figure 2(b) shows a corresponding average concentration profile across the inter diffusion zone (IDZ) in Fig. 2(a) as a function of distance. In the present study, the layer where Au and Co elements are diffused is called a “diffusion layer”. The dark layer closed to the interface is defined as “dark layer” and the layer containing bright particles is termed as “reactant layer”. This composition profile indicates that the diffusion layer mainly expands into the AuTi side and slightly grows into the CoTi side. The chemical composition of the dark layer was evaluated to be 25 mol%Au–74 mol%Ti–1 mol%Co. Besides, the chemical composition of the bright particles in the reactant layer was 64 mol%Au–32 mol%Ti–4 mol%Co. The composition ratios of (Au and Co) and Ti are about 1:3 and 2:1 for the dark layer and bright particles, respectively. Taking into account the phase diagram,27) the dark layer and the bright particles were considered to be intermetallic compounds of (Au,Co)Ti3 and (Au,Co)2Ti, respectively, since the Co atoms are not expected to occupy the Ti-sites. Apart from the dark layer and bright particles, the region in the diffusion layer is believed to be a solid solution of (Co,Au)Ti and (Au,Co)Ti. Since all the apparent intermetallic phases have different crystal structures, the wavy interfaces must be obtained in this study.28)
3.2 The growth of the diffusion layerTo evaluate the growth of the diffusion layer, the DC of AuTi/CoTi was heat-treated at temperature of 1273 K and 1373 K for 5 h after the fabrication at 1573 K for 3 h. SEM images of the IDZs near the DC of AuTi/CoTi interface heat-treated at 1273 K and 1373 K for 5 h are shown in Figs. 3(a) and (b), respectively. It is seen that the (Au,Co)2Ti particles vanished in the DC heat-treated at 1373 K for 5 h. This evidence implied that the maximum solubility of this phase in the AuTi phase increases with increasing the heat-treatment temperature.
SEM images near the AuTi/CoTi interface of the specimen heat-treated at (a) 1273 K for 5 h and (b) 1373 K for 5 h.
The corresponding average concentration profiles across the IDZs of the DC of AuTi/CoTi in Fig. 3 are shown in Figs. 4(a) and (b). To emphasize the effect of heat-treatment temperature on the growth of the diffusion layer, the thickness of the diffusion layer in Figs. 2(b), 4(a) and 4(b) was summarized in Table 1. It is clearly seen that the thickness of the diffusion layer increases from 78 µm to 200 µm with increasing the heat-treatment temperature from 1173 K to 1373 K.
Concentration profiles of the inter-diffusion zone obtained by EDS when heat-treated at (a) 1273 K for 5 h and (b) 1373 K for 5 h.
To evaluate the kinetics of the growth layer, the diffusion coefficient was determined from the layer thickness and heat-treatment time, and it can be expressed as following;29–31)
| \begin{equation} X^{2} = 2Dt \end{equation} | (1) |
The diffusion coefficient as a function of the heat-treatment temperatures is plotted in Fig. 5(a). The apparent activation energy for the growth of the diffusion layer is calculated by the following equation;29,30)
| \begin{equation} D = D_{0}\exp(-Q/RT) \end{equation} | (2) |
(a) The parabolic growth of the diffusion coefficient evaluated at various heat-treatment temperatures, and (b) an Arrhenius-type plot of logarithm of diffusion coefficient and reciprocal temperature.
From eq. (2), the diffusion coefficient and the heat-treated temperatures are plotted in Fig. 5(b) as an Arrhenius-type plot. The apparent activation energy for the growth was calculated to be 127 kJ/mol. It should be noted that the activation energies reported for the growth of the diffusion layer in the AuTi/Fe,25) AuTi/Nb26) and AuTi/Co22) DCs were 110 kJ/mol, 110 kJ/mol and 168 kJ/mol, respectively. In the present study, the activation energy is comparable to those reported values.
3.3 Diffusion path near the AuTi/CoTi interfaceUsing the average concentration profile in the IDZ for the DC of AuTi/CoTi heat-treated at 1173 K for 5 h, a diffusion path is plotted in the Au–Ti–Co ternary compositional diagram at 1173 K and it is shown in Fig. 6. The diffusion path is indicated by the bold line. By considering the Au diffusion to the CoTi side, Au atoms diffused toward a solid solution of CoTi until the concentration reached to (Co20,Au30)Ti50 (mol%) then the intermetallic compound of (Au25,Co1)Ti74 (mol%) was formed. The composition of this intermetallic compound was altered from (Au25,Co1)Ti74 to (Au31,Co2)Ti67 (mol%). With the further diffusing, the solid solution of (Au45,Co5)Ti50 (mol%) was formed then the (Au64,Co4)Ti32 intermetallic compound was also existed, and eventually finished with the solid solution of AuTi containing Co. Thus, the diffusion path near the DC of AuTi/CoTi interface was expressed as CoTi ∼ (Co20,Au30)Ti50/(Au25,Co1)Ti74 ∼ (Au31,Co2)Ti67/(Au45,Co5)Ti50/(Au64,Co4)Ti32/(Au47,Co5)Ti48 ∼ AuTi. This evidence indicated that the diffusion path deviated from the single phase region of (Au,Co)Ti to the two-phase region of (Co,Au)Ti and (Au,Co)Ti3, followed by (Au,Co)Ti and (Au,Co)2Ti region, although the DC of AuTi/CoTi is expected to form a continuous solid solution of (Au,Co)Ti. This diffusion path is similar to the diffusion path with outward horn.32) The outward horn refers to the deviation from the zig-zag path due to the concentration dependent diffusivity in the two-phase DC. Interestingly, the formation of the single phase layer near the interface of the DCs has been frequently reported in the DCs containing the diffusion path with outward horn.17,32–36) This formation is explained by this diffusion path could be intersected with the single phase area in the phase diagram. Therefore, the existence of the (Au,Co)Ti3 layer near the DC of AuTi/CoTi interface is related with the diffusion path with outward horn.
Diffusion path obtained from the concentration profiles when heat-treated at 1173 K for 5 h.
The diffusion paths for the DC of AuTi/CoTi heat-treated at 1273 K and 1373 K for 5 h are plotted in the isothermal section at 1273 K and 1373 K as shown in Figs. 7(a) and (b), respectively. By considering the diffusion from the CoTi to AuTi side, the diffusion path was expressed as CoTi ∼ (Co18,Au30)Ti52/(Au25,Co1)Ti74 ∼ (Au37,Co3)Ti60/(Au46,Co4)Ti50 ∼ (Au48,Co4)Ti48/(Au64,Co4)Ti32/(Au49,Co3)Ti48 ∼ AuTi near the interface of the DC of AuTi/CoTi heat-treated at 1273 K. With increasing the temperature to 1373 K, the diffusion path was CoTi ∼ (Co16,Au33)Ti51/(Au25,Co1)Ti74 ∼ (Au30,Co4)Ti66/(Au40,Co10)Ti50 ∼ AuTi. The formation of the diffusion paths was also explained by the above reasons.
Diffusion path obtained from the concentration profiles when heat-treated at (a) 1273 K for 5 h and (b) 1373 K for 5 h.
The diffusion couple of the stoichiometric AuTi and CoTi intermetallic compounds was prepared to obtain the phase reaction and diffusion behavior at various heat-treatment temperatures. The results obtained are summarized as follows;
This work was supported by Grant-in-Aid of Scientific Research (Kiban S 26220907) from the Japan Society for the Promotion of Science.
