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 3 0 grain boundary wall segments as in pure iron. After the upper yield point, there is a rapid multiplication of dislocations resulting in stress drop to the “final breakthrough stress.” Then stress increases with elastic deformation into the Lüders region and to the lower yield stress, Fig. 5. The polycrystalline pure iron grain walls had a thickness of about 70 µm and the grain diameter was 127 µm. From Equation 1, the carbon in the grain boundaries needed to surround completely all the grains was 118.4 ppm. Because the iron had 14.4 ppm C, Table 1, the walls do not surround completely all the grains which is consistent with Fig. 3. CONCLUSIONS For annealed ferritic hypoeutectoid steels, the dominant factor that creates an upper yield point with elastic deformation, followed by a sharp drop in stress, is the existence of hard grain boundary walls enclosing the grains. If the grain boundary walls surround the grains, that will cause a pileup of dislocations which then must be fractured for propagation between grains. The dislocations must then fracture the walls for plastic deformation to continue. This is the primary cause for the upper point of hypoeutectoid plain car- bon steels. Due to the hardness of grain boundary walls, a considerable amount of potential energy is pent up until the walls are broken. This causes a rapid drop in stress below the Luders stress region to a minimum stress, identified as the “final breakthrough stress.” After the final breakthrough stress, the stress first rises elastically and then plastically into the Luders region and the lower yield stress. Yielding in pure iron differs from that of steels since the grain boundary walls do not surround completely each grain. Grain boundary walls that do not completely surround each grain form segments caused by insufficient carbon in the iron. Yielding occurs when dislocations break free of solute atoms, the Cottrell atmosphere, then pass around disordered atoms at the grain boundary, and finally move in a different direction from one grain into the next one. ~AM&P For more information: Thomas L. Altshuler, 819 Freedom Plaza Circle, Apt. 205, Sun City Center, FL 33573, thomaslaltshuler@gmail.com. References 1. A.H. Cottrell, B.A. Bilby, Proceed- ings of the Physical Society, 62(1), p 49–62, 1949, https://doi.org/10.1088/ 0370-1298/62/1/308. 2. H. Sun, et al., Effect of Stress Concentration on Upper Yield Point in Mild Steel, Materials Transactions, Vol. 47, No. 1, p 96-100, 2006. 3. T. Altshuler, Yield Stress in Ferritic Steels Influenced by Grain Boundary Walls, 2021 AISTech Conference Proceedings, PR-382-092, p 955-948, June 29, 2021. 4. T.L. Altshuler, Atomic-Scale Materials Characterization, Advanced Materials and Processes, Vol. 130, No. 3: p 18-23, 1991. 5. T.L. Altshuler, Examination of Plain Carbon Steels using an Atomic Force Microscope, Atomic Force Microscopy/ Scanning Tunneling Microscopy, edited by Samuel H. Cohen, Springer Science + Business Media LLC, p 167-180, 1994. 6. T.L. Altshuler, Deformation Processes in Body Centered Cubic Materials, Doctor of Philosophy Thesis, Oxford Fig. 5 — AISI 1018 steel, tensile test B. Fig. 6 — Polycrystal iron, 14 ppmC, Specimen C, tensile test.
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