Characterization of Intermetallic Precipitates in Ni-Base Alloys by Non-destructive Techniques

Abstract

The present industrial scenario requires all engineering structure to be designed considering stability of several parameters at the operating conditions (e.g. Temper‐ ature, pressure, resistance to mechanical and surface degradation). Choice of materials for any engineering component should be such that it operates safely for reliable function, without failure during in-service, giving optimum component life. Due to scarcity of various resources and cost of manufacturing, regular maintenance and evaluation of structural integrity at every stage of production is necessary. Non-destructive techniques (NDT), along with modern computational facility help in non-intrusive investigation of the component at regular intervals of the operating stages for many critical applications. This will result in increment of designed component life and also help in maximizing utilization of natural resources. For long, Ultrasonic has been associated with defect detection, but with the recent advances in electronics in combination with computational capabilities Ultrasonic velocity measurements have also been attempted for characterization of solutionising and precipitation behavior in various alloy systems such as aluminium alloys, ferritic steel, maraging steel, nickel base alloys and titanium alloys. As the speed of sound in a homogeneous medium is directly related to both elastic modulus and density, any changes in elastic property with varying degree of inhomogeneities will affect in pulse transit time through a sample of given thickness. Due to variation in elastic modulus of the matrix in the alloy resulting from the various precipitates; it has been attributed towards the change in ultrasonic velocity of the alloy and thereby resulting in popularization of Ultrasonic testing for online monitoring of the component. The precipitation hardening has been believed to arise from the formation of very small solute clusters which uses significant scattering of the conduction electron cause during the rearrangement, while the formation of the precipitates; resulting in variation of electrical resistivity of the aged alloy. In the present chapter, a few non-destructive techniques used to characterize different microstructural features and the precipitation behavior, evolved through various heat treatments using Direct Current Potential Drop (DCPD) technique for measuring electrical resistivity and Ultrasonic Testing for measurements of ultrasonic parameters are presented. Further, validation of the observed results on microstructural features is also presented through hardness and microscopy studies.Thus, this study in effect can be used for non-destructive evaluation of the microstructures.