Fundamental investigation of the mechanisms of deformation and recrystallisation of cold deformable Mg alloys micro-alloyed with are earth elements and microstructure optimization for the development of a new class of Mg-alloys.

• Applicants: Professor Dr.-Ing. Sangbong Yi; Privatdozent Dr.-Ing. Stefan Zaefferer
• Subject Area: Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: from 2008 to 2013
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 58078348

• Final Report Year: 2013

Final Report Abstract

Main aim of the project was the understanding of the origin of the significantly improved ductility of rare-earth alloyed Mg compared to pure Mg. We followed two main hypotheses for ductility improvement, i.e. a change of stacking fault energy (SFE) and a related change of slip system activity and/or a change of crystallographic texture which would reduce the strong plastic anisotropy of sheet material. In order to test these hypotheses experimental and theoretical investigations were carried out. On the experimental side the microstructure and texture was studied at different stages of the deformation and recrystallization process by scanning electron microscopy, electron backscatter diffraction, x-ray and synchrotron diffraction and, very importantly, by transmission electron microscopy. The theoretical investigations were carried out by first principle calculations using a density functional theory approach, molecular dynamics calculation with a newly developed interatomic potential and by texture simulations using a visco-plastic self-consistent model. The project started with production of binary Mg-RE alloys, containing different amounts of Ce, Nd and Y, and ternary alloys containing Y and Zn. The alloys were hot rolled and subsequently cold rolled in order to minimize the effect of dynamic recrystallization, which unavoidably occurs during hot deformation of Mg alloys. Using these highly deformed materials, an experimental analysis of deformed structure, deformation mechanisms as well as recrystallization kinetics was be successfully conducted. As reference pure Mg was tested as well. It turned out that the Y-containing alloys showed about 5 times higher ductility than pure Mg, based on the higher activation of non-basal deformation modes such as compression / secondary twinning and pyramidal slip, and homogeneous deformation. The experimental and theoretical investigations on the SFE change showed that indeed the SFE of all -type slip systems is reduced by solid solution alloying of yttrium and various rare earth (RE) elements. This leads to a more frequent appearance of I1 and I2 basal plane stacking faults. The SFE of the slip systems is, in contrast, suggested to rise slightly. Nevertheless, slip is significantly more frequently observed in the alloyed Mg than in pure Mg. From these observations we proposed that the higher ductility of MgY and other MgRE alloys is not due to a changed critical resolved shear stress and therefore improved mobility of dislocations but due to an improved nucleation rate of these dislocations on I2 stacking faults in conjunction with increased hindrance of basal slip by these immobile stacking faults. Besides the change in stacking fault energy and the related inherent ductilization of the material we also showed that deformation and recrystallization textures of the RE-alloyed magnesium develop significantly different from those of pure Mg. Particularly after recrystallization the textures are weaker and show a larger spread of the basal poles towards the rolling direction. Our investigations showed that the development of recrystallization texture is due to the homogeneous deformation of shear bands as nucleation sites, the more homogeneous distribution of stored energy as driving force and the higher grain boundary pinning due to solute drag as a grain size-limiting factor. The observed texture change and small recrystallized grain size contribute extrinsically to the ductilization of the alloy. The exact fraction of extrinsic and intrinsic contributions to ductility could not be determined in detail.

Publications

• „On the role of nonbasal deformation mechanisms for the ductility of Mg and Mg-Y alloys“, Acta Materialia, Vol. 59, 2011, 429-439
  S. Sandlöbes, S. Zaefferer, I. Schestakow, S. Yi, R. Gonzalez-Martinez
  
• On the origin of shear bands in cold rolled Mg-3Y”, th In: W.J. Pool, K.U. Kainer: Proceedings of the 9th Intern. Conference on Magnesium alloys and their applications, (2012) 553-558
  J. Chen, S. Zaefferer, P. Konijnenberg
  
• “A comparison of mechanical properties and annealing behaviour of hot rolled MgY and MgYZn alloys”, Mg2012, Vancouver, 2012
  S. Zaefferer, Y. Wang
  
• “A study on origin and nature of shear bands in cold rolled Mg-3Y alloy using 3D EBSD” , Mg2012, Vancouver, 2012
  S. Zaefferer, J. Chen
  
• “Microstructural evolution during recrystallisation of magnesium alloys“, Materials Science Forum, Vol. 706 – 709, 2012, 1291-1296
  S. Yi, J. Bohlen, S. Sandlöbes, S. Zaefferer, D. Letzig, K.U. Kainer
  
• „Complementary TEM and ab initio study on the ductilizing effect of Y in solid solution Mg-Y alloys“, Magnesium alloys and their Applications 2012, Vancouver, 2012, 467-472
  S. Sandlöbes, M. Friák, A. Dick, S. Zaefferer, S. Yi, D. Letzig, Z. Pei, L.-F. Zhu, J. Neugebauer, D. Raabe
  
• „Influence of rare earth addition on texture development during static recrystallization and mechanical behaviour of magnesium alloy sheets“, Materials Science Forum, Vol. 702-703, 2012, 651-654
  S. Yi, L. Rayas, S. Sandlöbes, S. Zaefferer, D. Letzig, K.U. Kainer
  
• „The relation between ductility and stacking fault energies in Mg and Mg-Y alloys“, Acta Materialia, Vol. 60, 2012, 3011-3021
  S. Sandlöbes, M. Friák, S. Zaefferer, A. Dick, S. Yi, D. Letzig, Z. Pei, L.-F. Zhu, J. Neugebauer, D. Raabe
  
• „Ab initio and atomistic study of generalized stacking fault energies in Mg and Mg–Y alloys”, New Journal of Physics 15 (2013), 043020
  Z. Pei, L-F. Zhu, M. Friák, S. Sandlöbes, J. von Pezold, H. W. Sheng, C. P. Race, S. Zaefferer, B. Svendsen, D. Raabe, J. Neugebauer
  
• „Basal and non-basal dislocation slip in Mg–Y”, Mat. Sci. Eng. A 576 (2013), 61-68
  S. Sandlöbes, M. Friák, J. Neugebauer, D. Raabe
  
• „Deformation behaviour of wrought Mg alloys: influence of rare earth elements addtion (AZ vs MN)“, Magnesium Workshop 2013, Madrid, Spain, May 21. -24. 2013
  S. Yi, J. Victoria-Hernandez, J. Bohlen, H.G. Brokmeier, D. Letzig, N. Schell, A. Schreyer, K.U. Kainer
  
• „Static recrystallisation behaviour of cold rolled Mg-Y(-Zn) magnesium alloys“, Magnesium Workshop 2013, Madrid, Spain, May 21. -24. 2013
  Y.M. Kim, S. Yi, S. Sandlöbes, S. Zaefferer, D. Letzig