G.B. Kale

Interdiffusion studies in titanium–304 stainless steel system

Abstract The interdiffusion characteristics of various elements in Ti–304SS system have been investigated in the temperature range between 976 and 1123 K by employing metallographic and electron probe microanalysis techniques. The diffusion zone of the couple has been found to be of solid solution type and the layer growth can be presented by two linear lines with change of slope lying in the region of Curie temperatures of the alloys. The layer growth kinetics (in the temperature range of 1023–1123 K) can be expressed by the following Arrhenius relationship: X=2.3×10 5 exp (−73.96 kJ/mol /RT)t 1/2 , m . The concentration and temperature dependence of effective interdiffusion coefficients are evaluated and the diffusion parameters are established. It is observed that the minimum activation energy value for interdiffusion of Ti (78.05 kJ/mol at 60 at.% Ti) and Fe (74.05 kJ/mol at 20 at.% Fe) and the activation energy for layer growth (73.96 kJ/mol) are in good agreement, suggesting that growth kinetics is controlled by the migration of the plane having the composition of ∼60% Ti, ∼20% Fe, ∼10% Ni and ∼10% Cr.

Reaction diffusion in the zirconium-iron system

Reaction diffusion in the zirconium-iron system has been investigated in the temperature range 973 to 1213 K using diffusion couples of pure zirconium and pure iron. Electron microprobe analysis and metallographic techniques have been employed to investigate the formation of compounds in the diffusion zone. The Boltzmann-Matano-Heumann method and Wagners method have been used to evaluate the interdiffusion coefficients. The temperature dependence of this diffusion coefficient has been established. The activation energy for interdiffusion in FeZr3 compound is found to be 120.0 kJ/mol. The formation of compounds and their stability in the diffusion zone have been discussed on the basis of existing thermodynamic and other physical properties.

Thermodynamic interdiffusion coefficient in binary systems with intermediate phases

Abstract A new form of diffusion coefficient, termed the thermodynamic interdiffusion coefficient has been introduced in this investigation; it is related to the conventional chemical interdiffusion coefficient and thermodynamic properties of the system. Three main advantages in using the thermodynamic interdiffusion coefficient are described by applying this alternative approach of diffusion analysis, as an example, to the Al–Ni system. These include (1) a better interpretation of the relative diffusivities in the different phases of the same system, (2) estimation of appropriate values of the activation energy of the diffusion process and (3) determination of the thermodynamic intrinsic diffusion coefficient (also known as the tracer diffusion coefficient) in an intermediate phase with narrow homogeneity range, which otherwise is not possible without tracer diffusion experiments.

Transition joints between Zircaloy-2 and stainless steel by diffusion bonding

Abstract The diffusion bonding between Zircaloy-2 and stainless steel (AISI 304L) using niobium, nickel and copper as intermediate layers has been investigated in the temperature range of 750 to 900°C. Bonding was carried out in a vacuum hot press, under compressive loading. Electron probe microanalysis and metallographic analysis showed a good metallurgical compatibility and also indicated the absence of discontunities, micropores and intermetallic compounds at various interfaces. The bond strength of the diffusion bonded assembly was found to be about 400 MPa for the couples bonded at 870°C for 2 h. The dimple structure on the fractured surface is indicative of the ductile mode of failure of the bonded assembly.

Solid-state diffusion reaction and formation of intermetallic compounds in the nickel-zirconium system

Chemical diffusion studies in the nickel-zirconium system are investigated in the temperature range of 1046 to 1213 K employing diffusion couples of pure nickel and pure zirconium. Electron microprobe and X-ray diffraction studies have been employed to investigate the formation of different compounds and to study their layer growth kinetics in the diffusion zone. It is observed that growth of each phase is controlled by the process of volume diffusion as the layer growth obeys the parabolic law. The activation energies for interdiffusion in NiZr and NiZr2, which are the dominant phases in the diffusion zone, are 119.0 ±13.4 and 103.0 ±25.0 kJ/ mole, respectively. The formation and stability of compounds over the temperature range have been discussed on the basis of existing thermodynamic and kinetic data.

Solid state bonding of Zircaloy-2 with stainless steel

Abstract Zircaloy-2 and stainless steel have been diffusion bonded together with titanium and iron as barriers. Electron microprobe and optical microscopy studies reveal that no intermetallic compound/intermediate phase forms at the interfaces involved. Layer growth kinetics of diffusion zones have also been studied and the absence of these intermediate phases have been discussed with respect to their nucleation and growth parameters.

Single-phase diffusion study in β-Zr(Al)

Abstract The diffusion behaviour of Al and Zr was investigated in the β-Zr(Al) phase in the temperature range 1203–1323 K by employing single-phase diffusion couples of pure Zr/Zr–2.8 wt% Al. The interdiffusion coefficients show a small increase with increase in Al concentration and follow a quadratic compositional relation. The temperature dependence of the interdiffusion coefficients at various compositions was established. The activation energy for the interdiffusion coefficient decreases linearly with an increase in Al concentration. The intrinsic diffusivity of Zr is higher than that of Al in this phase field. The impurity diffusion coefficient of Al in β-Zr was determined by extrapolation of interdiffusion coefficients to limiting concentration of Al and it shows temperature dependence: D Al β-Zr (c Al =0) =5.567 −1.80 +2.65 ×10 −6 exp [(−220.08±3.33) kJ /RT] m 2 / s . A correlation between the impurity diffusion coefficients of various impurities in β-Zr and the atomic radii of the impurity atoms has been established and can be presented by the relation: log D imp β-Zr ( m 2 / s )=−14.57±1.22+ exp [(4.84±2.83)−(30.22±2.66)r ( nm )].

Chemical diffusion in ZrNb system

Abstract The temperature and concentration dependence of interdiffusion coefficients in zirconium-niobium system has been established in the temperature range between 1593 and 1993 K by employing an electron probe micro analyser. The interdiffusion coefficients are found to increase with increase in zirconium content in the alloy and can be described by the relation D (C) = A (T) e 5.6C Here, A(T) is the temperature dependent parameter and has the value of tracer diffusion coefficient at infinite dilution. C is the mole fraction of the component. The temperature dependence of interdiffusion coefficients can be represented by the Arrhenius expression of the type D (C) = D A 0 e Q /RT The activation energy ( Q c ) and frequency factor ( D 0 ) values in the composition range between 5 and 90% Zr lie in the range of 216 to 276 kJ/mol, and 10−7 to 10−5 m2/s, respectively. An attempt has also been made to evaluate the impurity diffusion data by using interdiffusion values.

Diffusion reaction between Zr–2.5 wt% Nb alloy and martensitic grade 403 stainless steel

Abstract Diffusion reaction between Zr–2.5 wt% Nb alloy and 403 stainless steel has been investigated by employing miniature type diffusion couples in the temperature range between 750°C and 940°C for 1–240 h. An electron probe microanalyser (EPMA) has been used to establish the concentration penetration profiles across the diffusion zone and a transmission electron microscope (TEM) has been used to identify various phases formed close to the interface. The microstructure of the bonded region on the 403 steel side is essentially a martensitic structure and remains nearly unchanged during annealing. However, the microstructure of the Zr–2.5 wt% Nb alloy changes substantially, leading to the formation of coarser α phase. The diffusion reaction is extremely sluggish. Localised melting occurs in the specimens annealed at and above 940°C. This is essentially due to the eutectic reaction between zirconium and iron. The experiments confirm that diffusion bonding of 403 steel to Zr–2.5 wt% Nb could be carried out at a pressure of 10 MPa at 900°C for 1 h. The formation of various phases in this multi-phase and multi-component system along with change in their composition with annealing temperature and the nature of reaction products is discussed.

Diffusion reactions in titanium/Inconel-600 system

Abstract The interdiffusion reactions in the Ti/Inconel-600 system in the temperature range 973–1153 K have been investigated by employing both the optical metallography and the electron probe microanalysis techniques. At 973 K the reaction zone showed the presence of solid solution phases like Ni-based (Fe, Cr, Ti) phase and titanium-based solid solution α-Ti. The two intermetallic phases, viz. TiNi(Fe, Cr) and Ti2Ni are also observed. At 1023 K the reaction zone shows the formation of the two intermetallic phases, viz. TiNi(Fe, Cr) and Ti2Ni along with solid solution phases such as Ni-based (Fe, Cr, Ti) phase and titanium-based solid solution β-Ti. At still higher temperatures (1073–1153 K), however, along with the solid solution phases such as Ni-based (Fe, Cr, Ti) phase and titanium-based solid solution β-Ti only one intermetallic phase, TiNi3 (Fe, Cr), is formed. The temperature dependence of the reaction constants, K, m/s1/2, derived from the layer growth kinetics, can well be expressed as for Ni-rich phase K=3.06×10 −5 exp (−262.16/RT), m/s 1/2 for TiNi 3 ( Fe , Cr ) K=3.49×10 −5 exp (−240.96/RT), m/s 1/2 for β-Ti K=1.25×10 −6 exp (−983.80/RT), m/s 1/2 . Here activation energy is expressed in kJ/mol. The interdiffusion parameters and the diffusion paths have also been established by employing these data.