b) mechanical fatigue at room temperature
Phase 1 yielded the following results: Only the Mg-free and the Mg 0.10% alloys have been characterized through the LCF domain (270 – 220 - 170 MPa) down to that of HCF (140 – 120 MPa). The Mg 0.05% has been limited to LCF (Figure 10).
At 270 - 220 MPa, the number of cycles to rupture is in- creased tenfold by the addition of Mg 0.10%, whereas an addition of 0.05% brings absolutely no improvement. The non- linearity observed for TYS and UTS is striking- ly confirmed here. As no notable gain was consequently expected from a further addition in the 0.10% – 0.20% “plateau” domain, no further experiments were done.
The HCF performance of the two variants is similar and very good: 130 – 140 MPa, which is in good agreement with the previous values for the Mg-free alloy.1
C) Creep Strength: influences of V and mg
Phase 1 confirmed the remarkable improvement of the 300C (572F) creep strength afforded by an addition of V 0.21% when compared to the V less alloy at identical Mg levels. This had already been observed by a different method (determination of σ 0.1/100) in past R&D work3 which had later led to the Mg-less alloy described in the first paper, but is here confirmed by the present method.
Note that 4% is the maximum possible deformation on the machines employed and the test is interrupted when this value is reached either by continuous deformation or rupture of the sample.
In addition to the major effect of V, Mg also has a strong positive effect: Compare the two blue curves in Figure 11.
Phase 2 showed that increasing the Mg content above 0.10% to 0.20% did not improve the creep strength in the least way. Here again, this non-linear effect of Mg is observed with a sharp improvement between 0.00 and 0.10%.
Phase 3 was entirely devoted to the exploration of lower V levels; 0.17%, 0.11% and 0.05% were tested. As V 0.17% had already been characterized in phase 1, this offered a useful estimation of the reproducibility of the creep test at some two years’ time interval. The results appear in Figure 12. • The reproducibility of the test at V 0.17% ap- pears excellent.
• Decreasing V from 0.21% to 0.17% has absolute- ly no impact and decreasing further to 0.11% and 0.05% only very moderately decreases the 300C (572F) creep strength as measured by this test.
22
Figure 8. 250C Tensile Properties “NLV” (after 100 hrs. at 250C [482F]).
International Journal of Metalcasting/Summer 2011
Figure 7. Effect of Cu between 3.5% and 4.0% on the room temperature properties at Mg 0.07% and 0.10% - 0.20% (“plateau”).
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