Einfluss von Verunreinigungen in Magnesiumlegierungen auf die Texturentwickung und die mechanischen Eigenschaften
Final Report Abstract
The project had two major aims: (a) investigating the effect of high level of impurities (Cu and Ni) on the mechanical properties, texture and microstructure of experimental magnesium alloys based on the widely used systems AZ31 and AM50. (b) modifying an advanced cluster-type Taylor model to account for mechanical twinning and twin-induced hardening during simulation of the deformation texture and mechanical behavior in magnesium alloys. The main conclusions from part (a) of the project can be summarized as follows: • From a mechanical behavior point of view, increased amounts of Cu and Ni impurities are not critical but rather beneficial. • Compared to the benchmark AZ31 and AM50 alloys, higher amounts of impurities in the experimental alloys (300 ppm of each Cu and Ni) resulted in a noticeable improvement of strength (ΔσYS ~25 MPa) during compression at temperatures between RT and 400°C. • The experimental alloys containing 300 ppm of Cu exhibited higher flow stresses at all investigated temperatures than their counterpart alloys containing Ni. • The flow behavior (i.e. the shape of the flow curve) of the experimental alloys containing either Cu or Ni during deformation at room temperature and 400°C was quite similar (S-shaped hardening at RT and parabolic hardening at 400°C). At 200°C, the Cu-containing alloys demonstrated a parabolic hardening behavior typical for slip driven deformation, whereas the Ni-containing alloys showed a concaved or S-shaped hardening behavior that is typically associated with profuse extension twinning. • All investigated proportions of mixed AZ31 and AM50 alloying containing impurities of either Cu or Ni showed virtually identical flow curves at all deformation temperatures, except at 200°C where a composition of 25% AZ31 + 75% AM50 seemed to achieve the highest flow stress. • Addition of 300 ppm of each Cu and Ni to AM50 and AZ31 did not lead to considerable changes of the deformation textures seen in the reference alloys. At 200°C the sharp extrusion orientation < 11 2 0 > ED was replaced by two new orientations, < 10 1 0 > ED (resulting from double prismatic slip) and [0001]ED (resulting from {10 1 2} -extension twinning). At 400°C the development of a twinned basal orientation [0001]ED was suppressed and the deformation texture was rendered nearly random. • Cu impurities seemed to promote weaker deformation textures than Ni. At 200°C the basal poles in the Cu-containing specimens were tilted from the loading axis (unlike the samples with Ni) which suggests a reduced activity of twinning. At 400°C, Cu impurities seemed to produce a wider spread of orientations compared to Ni. • Addition of Cu and Ni impurities seemed to cause grain refinement relative to the reference materials. The main conclusions from part (b) of the project can be summarized as follows: • The modified GIA-TW model can correctly capture the orientation change, and thus texture evolution, caused by deformation twinning. Twinning was found to proceed more rapidly in the simulations than in experiment. • The orientation change by twinning was successfully slowed down by distinguishing twin growth from twin nucleation. • The importance of the contribution of twinning to deformation texture evolution decreases with increasing temperature or decreasing strain rate. At low temperatures, twinning plays an important role at low and intermediate strain ranges. At very high strains, without consideration of twinning the model can still reflect the experimental texture. • At very high temperatures, twinning must not be taken into account for deformation texture simulations of Mg alloys at all strain levels. • Contraction twinning does not affect the strain hardening behavior during deformation due to its low volume fraction. At elevated deformation temperatures (> 300°C) twinning induced hardening is not necessary for simulating the strain hardening behavior. Combined addition of Cu and Ni above the allowed concentration gave an improvement of the strength properties. In terms of corrosion resistance, these elements can be very critical, and a complementary corrosion study is necessary for the broader aspect of determining the impurity threshold of recycled magnesium alloys. With respect to texture modeling a future study should aim at investigating the relative importance of short-range vs. long-range stress interactions using a viscoplastic self-consistent (VPSC) model also. Such comparison would provide valuable insight into the possibilities of Taylor-type modeling of hexagonal metals. Some work can also be invested in modeling twinning nucleation which would improve the twinning prediction.
Publications
- Implementation of Mechanical Twinning in a Grain Interaction Model: Application to Magnesium Alloys. Adv. Eng. Mat. 12 (2010) 1008
S. Mu, T. Al-Samman, V. Mohles, G. Gottstein
