A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 2 2 2 8 154 MPa, the “breakthrough stress.” With continued deformation, the stress rose along the elastic line until plastic deformation began at about 276 MPa, rising into the Lüders region. The stress remained constant at around 294 MPa to the lower yield stress, until the test stopped at 0.00852 strain. Yielding in pure iron differs from that of hypoeutectoid plain carbon steels in that the grain boundary walls do not surround completely the grains, but exist in segments in the grain boundaries, (see sections 3.3 and 4.2 in Altshuler[3]). As a result, there is a smooth transition between the elastic deformation into Fig. 2 — Junction of three grain boundary walls. Horizontal distance of markers = 26.9 nm. Height from left grain = 31 nm. Fig. 3 — Polycrystalline iron, 14 ppmC. plastic deformation of iron, Fig. 6, yielding at 202.7 MPa at 0.2% offset. COTTRELL ATMOSPHERE PINNING OF DISLOCATIONS Compression tests were performed by Altshuler[6] and published by Altshuler and Christian[7] on single crystal iron, 44 ppm C and 0.005 ppm C. Neither iron had yield points at the elastic line. Furthermore, the 44 ppm C iron was saturated with solute carbon atoms, Hume-Rothery[8], while the 0.005 ppm C iron had a negligible amount of solute carbon atoms. The 0.2% yield offset for the 0.005 ppm C Fe is 15.47 MPa and 22.8 MPa for the 44 ppm C Fe without an upper and lower yield point. The stress difference between the upper and lower yield point was 18 MPa for polycrystalline iron, 10 ppm carbon. This is much less than the 85 MPa stress difference between the upper and lower yield point in AISI 1018 steel, (Fig. 5), and an average of 77.3 MPa for three tests. Takeda[9] says that solid solution strengthening by nitrogen and carbon had a negligibly small influence on the yield strength of ferritic iron. He states that the Cottrell atmosphere[1] is not supported by his experiments. If the upper and lower yield points in steel were primarily controlled by a Cottrell atmosphere, then the stress at the upper yield point would be independent of grain size. This contradicts considerable experimental evidence showing the dependence of the yield stress upon grain size resulting in the Hall-Petch equation[10]. The following discussion shows that the Cottrell atmosphere pinning of dislocations does not cause the upper yield point at the elastic line followed by a rapid drop in stress to the lower yield point in hypoeutectoid low carbon steels. Instead, it shows that grain boundary walls cause the upper yield point at the elastic line followed by a rapid drop in stress to the lower yield point in hypoeutectoid low car- bon steels. GRAIN BOUNDARY WALLS The concept that carbide grain boundary walls are present in steel was
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