Relative Influence of Different Factors on Young’s Modulus in Nickel and Zirconium base Alloy Systems upon Precipitation of Intermetallics

The influence of precipitation of intermetallics on changes in Young’s modulus of nickel and zirconium base alloys has been discussed in this paper. This study indicates that the change in the modulus due to thermal ageing in these alloys is essentially governed by the change in the modulus of the matrix due to removal of precipitation forming elements. 1.0 Introduction Mechanical properties in various precipitation hardening alloys, such as aluminum base, nickel base and iron base (ferritic and maraging steels) alloys, are derived through controlled precipitation of different intermetallic phases. Ultrasonic measurements have been used for characterization of precipitation behaviour in these alloy systems [1-4]. In most of the alloy systems, ultrasonic velocity has been found to increase with the precipitation of various intermetallic phases, irrespective of the type of the precipitates. The increase in the ultrasonic velocity has been attributed essentially to increase in the Young’s modulus of the alloy. In the presence of precipitates, the material can be considered as a composite of three components, i.e. matrix, matrix-precipitate interface and precipitate. Since, ultrasonic velocity depends on the Young’s modulus and density of the material [5], it is dependent upon the Young’s modulus and density of these three components [4]. The Young’s modulus of a precipitated alloy is governed by the changes in the modulus of (1) matrix due to changed composition, (2) intermetallic precipitates and (3) precipitate-matrix interface. Even though several studies have been carried out to understand the influence of precipitation on the average elastic moduli as reflected in ultrasonic velocity, very few attempts have been made to probe into the individual influences of the abovementioned three components. In the present study, ultrasonic velocity measurements have been carried out for studying the variations in Young’s modulus of two nickel base superalloys (Inconel 625 and Nimonic PE 16) and one zirconium base alloy (Zircoloy-2) upon precipitation of different intermetallic phases. The study indicates that the change in the modulus due to precipitation of intermetallic phases in nickel and zirconium base alloys is essentially governed by the change in the modulus of the matrix due to removal of precipitation forming elements. ECNDT 2006 Th.2.2.1