JOURNAL OF MATERIALS SCIENCE 18 (1983) 2345-2353 Non-destructive testing and acoustic microscopy of diffusion bonds B. DERBY* Department of Metallurgy and Materials Science, University of Cambridge, Pembroke Street, Cambridge, UK G.A.D. BRIGGS Department of Metallurgy and Science of Materials, University of Oxford, Parks Road, Oxford, UK E. R. WALLACH Department of Metallurgy and Materials Science, University of Cambridge, Pembroke Street, Cambridge, UK Despite successful applications of diffusion bonding, there is a lack of good quality assessment methods. A survey of published results shows non-destructive testing tech- niques (N DT) developed for fusion welds to be ineffective in indentifying the much smaller defects found in diffusion bonds. An investigation into the possibilities of using electrical resistance measurements for NDT shows the requirement of equipment with a very high stability and sensitivity. Finally the use of very high frequency ultrasonic sound in an acoustic microscope is considered as a method of defect investigations. This acoustic microscopy proved to be a highly encouraging new line of investigation for the exami- nation of diffusion bonds. 1. Introduction Diffusion bonding is a high temperature solid- phase joining technique which has been proposed for industrial applications, especially in the field of aerospace. Before widespread use of the tech- nique can occur, large improvements will be needed in the field of quality assurance, by non- destructive testing (NDT). Conventional NDT techniques have been developed for fusion welding and so may not be applicable to solid phase welds. Most previous studies of diffusion bonds have used destructive techniques to estimate the degree of bonding. Early work assumed that the amount of bonding could be determined by measuring the tensile strength of a bonded sample as a fraction of that of the parent material. This fraction was taken as being the fractional area of the bond [ 1-3 ]. This technique has been examined critically [4] and was found to be inaccurate because a bond can display the tensile strength of the parent material when as little as 80% bonding occurs. However, such poorly bonded specimens may have poor fracture toughness properties and, therefore, tensile testing does not give a good guide to satis- factory bonding. The other destructive method used is of sectioning the bond the assessing the bonded area metallographically [5]. This seems to give good results, albeit with a certain degree of scatter. All destructive tests, although they will be useful in the development stage of the process, cannot be used to routinely examine finished components. For quality control in fusion welds, several non- destructive techniques are used. Some of these NDT techniques have been tried with diffusion bonds and an analysis of their potential is presented with respect to the defect size in diffusion bonds. As a development of ultrasonic NDT, a study has *Present address: Department of Engineering, University of Cambridge, Trumpington Street, Cambridge, UK. 0022--2461/83503.00+.12 9 Chapman and Hall Ltd. 2345