Study of cooperative grain boundary sliding by using macroscopic marker lines

Abstract

Grain boundary sliding (GBS) is an important mechanism of high-temperature deformation, in particular for superplastic flow (SP). EI,21 Important information on GBS development has been gained ~3.41 by using a technique of marker lines 151 inscribed at the prepolished surface before deformation. Usually, tiny marker lines are used in order to increase accuracy of measurements of marker line offsets at grain boundaries, caused by GBS during deformation. Recently, r6-91 cooperative manner of GBS during SP deformation has been reported. Here, the results of a cooperative GBS (CGBS) study by using macroscopic marker lines are presented. The idea of these experiments was based on the assumption that rough marker lines will be offset only at boundaries that had slid a significant amount; the absence of numerous small offsets, which take place when tiny marker lines are used, will not obscure the general trend in grain displacement. Experiments were performed on Pb-62 pct Sn eutectic alloy, which has a well-documented superplastic behavior I1~ and in which GBS has been studied thoroughly by using tiny marker lines. 131 Tensile specimens 12 x 3 x 2 mm 3 were cut by using an electrical discharge machine from rolled strips with grain size 3 /xm and were mechanically polished with final polishing on 0.5-/zm alumina. Macroscopical marker lines were inscribed on the prepolished surface with silicon carbide paper (number 600) perpendicular, parallel, and at +45 deg with respect to the tensile axis (in the last case, marker lines were inscribed on both sides of the prepolished specimen). A network of marker lines was drawn using a diamond needle, and transverse marker lines were inscribed by using diamond paste with 0.5-/~m size of particles at the prepolished surface of a specimen in order to compare offsets of marker lines of a different size. Specimens were deformed in a MTS machine with strain rate /~ = 10 -4 S I at T = 300 K, which is close to the optimal SP conditions. IwI The deformed surface was studied in a scanning electron microscope (SEM). The experimental results revealed the following: