Predicting flow curves of two-phase steels from spherical nanoindentation data of constituent phases: Isostrain method vs. non-isostrain method

A procedure is suggested to predict the flow curves of two-phase steels using data from nanoindentation experiments performed with two spherical indenters having different radii. The procedure incorporates two steps: First, the “macroscopic” (or size effect corrected) stress–strain relations of each constituent phase are estimated based on the concepts of indentation stress/strain and indentation size effect. Then, the “overall” (or composite) flow curve of two-phase steel is extracted in two different ways; an isostrain method (ISM) and a non-isostrain method (NISM). The appropriateness of the proposed procedure was examined by performing a series of spherical nanoindentation tests on various two-phase steels (consisting of ferrite–pearlite or ferrite–bainite). Reasonable accuracy of the prediction was validated by comparing the predicted curves to the tensile curves obtained from standard tests of bulky samples. In addition, interestingly, the predictions made by the simple ISM were almost identical to those by the more sophisticated NISM, though the NISM used more realistic assumptions.