Hardness measurements of accumulative roll-bonded Mo foils

Abstract

The hardness of accumulative roll bonding (ARB)-processed multilayers has been measured in the past with microindentation studies. In some cases the lateral indent size of microindenters, however, is too large. For example, if multilayers are ARB-processed that consist of arrays of different elemental foils the hardness ratio of the layers has been reported as a key factor for the development of diffuse necking. To predict diffuse necking during ARB processing, flow stress data are usually taken from tensile tests. Flow stresses can be estimated of individual layers based on hardness measurements. In this work results are compared between hardness measurements on ARB-processed Mo multilayers with micro- and nanoindentation. Two sets of Mo multilayers were examined, one with as-received Mo foils and a second set with annealed Mo foils as starting material. The microhardness measurements show an increase in hardness with the first two passes before the hardness reaches a near plateau for the annealed Mo foils. The as-received foils that appear to be cold-worked yield a constant hardness for the first five passes that approaches the hardness of the annealed samples after four passes. The nanoindentation measurements were conducted at the center of Mo grains. If the indentation size effect (ISE) is taken into account the nanoindentation-based hardness measurements of the annealed Mo foil multilayers agree with the microhardness measurements. For the multilayer comprised of as-received Mo foils, agreement is obtained with the microhardness measurements even without taking into account an ISE. These observations are discussed with respect to the changes in microstructural length scales during ARB processing and the indent sizes. A main conclusion is that nanoindentation measurements yield reliable hardness data if the ISE is considered.