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Tensile Strength


Tensile stress versus strain data is shown in Figure 7 for the two alloys in both the solution treated and aged conditions. In general, the yield and tensile strengths increase and the elongation to fracture decreases with increasing time of age hardening. Greater strengths and lower elongations to failure are observed with the 1.4% silicon alloy. Tensile test results are summarized in Table 2. Young’s Modulus was constant for all alloys and measured 188 GPa. A strong work harden- ing behavior was observed for the solution treated materials with a work hardening exponent of 0.32 and 0.23 for the 1.0% Si and 1.4% Si alloys. The age hardened materials had work hardening exponents that were an order of magnitude


lower (see Table 2). Optical microscopy was performed on the gage sections of each tensile bar to judge the deformation character. Deformation twinning was not apparent and slip bands were predominately planar in nature. Figure 8 shows slip in the 1 and 1.4% steels aged for 10 hours. The slip is easily seen at the grain boundaries.


Fracture surfaces were examined using a scanning electron microscope. The solution treated material failed by trans- granular microvoid coalescence. Upon ageing, the fracture transitioned to transgranular cleavage and followed the den- dritic structure. Figures 9(a) and 9(b) shows transgranular dimple rupture of both solution treated materials. Figure 9(c) and 9(d) shows transgranular cleavage.


Table 1. Chemical Composition of Cast Steels (weight percent)


(a)


(b)


(c)


(d)


(e)


(f)


Figure 5. Chemical mapping of non-metallic inclusions for 1 and 1.4% silicon modified Fe-Mn-Al-C alloys. The 1% sili- con modified alloy ternaries show nitrides (a) oxides (b) and sulfides (c) are primarily manganese and silicon based. The 1.4% silicon alloy also shows that nitrides (d) oxides (e) and sulfides (f) are also predominantly manganese and silicon rich. Visual inspection and comparison reveals fewer oxides than nitrides or sulfides.


International Journal of Metalcasting/Winter 10 11


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