far greater effect as it would segregate to the centre of the casting, thereby contributing to shrinkage porosity. What is most likely, however, is simply that hydrogen porosity is, at least over this specific range of tensile results and elongations, a defect of lesser importance. That is, either much larger defects contribute to the failures, or the effect of hydrogen on the defect clusters present, is negligible. In summary, at this level of quality, hydrogen has little or no effect on the measured mechanical properties. Examina- tion of the values of µ -3σ (for the engineering stress and strain), and those shown on the flow curve of Fig. 13, (for the true stress and strain) suggests there may be a change in the relative spread or distribution of results. It may also be expected that the value of Weibull modulus, m, will also show differences because this value is a direct indicator of the flaw size distribution. Weibull plots for the tensile strength are presented in Fig. 15(a), and for elongation to
failure in Fig. 15(b). Note that the Weibull plot for Ef similar in form to that for the tensile strength; for the un-
was
of 329 MPa for the untreated material. The value of m was 44.7 for the treated alloy, with a σo
value of 332 MPa. For
both stress and elongation data, the position parameter is almost the same, but the values of Weibull modulus are different, which suggests a change in flaw size distribution between the two conditions. These differences in Weibull modulus are actually similar to those between the melt ve- locities for Alloy 1, shown in Table 5a.
During the tensile testing to generate the results used in Figs. 13-15, samples displaying oxide defects on the frac-
3.89. From the engineering tensile stress data for these sets of tests, the Weibull modulus, m, was 25 with a σo
value
treated material m was 5.6 and the position parameter Efo was 3.83. For the treated material, m was 7.9 and Efo
was
Figure 13. True stress-true strain data overlaid on the model flow curve derived from the Ludwik-Holloman equation, showing the relative spread of data between the materials which was a) recycled and untreated, or b) subsequently treated by degassing with high purity Ar.
(a)
Figure 14. True stress-true strain plots for untreated and treated samples that correspond to Fig. 13.
(b)
Figure 15. Weibull plots for Alloy 4 with either no melt treatment, or with a melt treatment, where the procedure conducted was rotary degassing with high purity Ar. Plot (a) is for the tensile strength, and (b) is for the elongation at failure, Ef
. International Journal of Metalcasting/Summer 2011 51
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