Final Activity Report Summary - DUCTILE BMG COMPOSITES (Ductilisation of Bulk Metallic Glasses (BMGs) by Length-scale Control in BMGs Composites and Applications)
At the start of this network research in 2004 bulk metallic glasses (BMGs), which have no crystalline lattice for dislocation glide, were known for their high elastic limit of 2 % strain which was of interest for many structural applications, as noted by Cambridge partner two, Ashby and Greer, in Scripta Mater 54 (2006) 321. However, when solicited beyond this elastic limit, BMGs fractured with insignificant macroscopic plasticity. The network research discovered new methods for ductilisation of BMGs:
1. ductilisation by elemental additions. In an article entitled in Physical Review Letters 94 (2005) 205501, the Network Ph.D. student J. Das from IFW, Dresden, showed how the addition of a few atomic percent Al to Cu-Zr glasses led to several percent plastic deformation prior to fracture.
2. ductilisation by nanocrystallisation. In Network publications such as the one in the Journal of Non-Crystalline Solids 353 (2007) 327, Mr K. Hajlaoui, a network Ph.D. student from INP, Grenoble, using controlled nanocrystallisation as detected by electron microscopy and synchrotron radiation obtained several percent plastic deformation prior to fracture.
3. ductilisation by control of surface stresses. In the joint network publication in Philosophical Magazine Letters, which was in press by the time of the project completion, F.O. Mear, a network post-doc from Cambridge, optimised the state of surface stresses by shot-peening to several percent plastic deformation prior to fracture.
At the start of this Network, some expected shear bands to melt due to release of elastic energy while others did not expect significant heating. Lewandowski and Greer of the Cambridge team, using a fusible coating method, established that the release of elastic energy in the form of heat resulted in a sharp rise in the temperature, of the order of 1000 K, of the deformed material and strong shear-softening that made deformation heterogeneous, as mentioned in Nature Materials 5 (2006) 15. Using electron microscopy for observations down to the nanometer regime, in the joint network publication in Applied Physics Letters 93, 2008, 031907, the network postdoc, K. Georgarakis from INP, Grenoble, directly confirmed that atoms melt up to 100 nm from the center of shear bands.
Joint work at the European synchrotron radiation facility (ESRF) by network post-docs allowed for the atomic structure of BMGs to be studied by Fourier transformation methods that converted experimental diffraction data from reciprocal space to atomic radial distribution functions in real space. See for example J. Antonowicz et al, Journal of Alloys and Compounds, which was in press in 2008 and was also available online. In these experiments, the structures of BMGs were examined before and after heavy deformation and during annealing. The modification of the atomic structure in the shear band atoms was found to be mainly at the level of the second-nearest neighbour atomic shell and linked to the interconnectivity of the basic atomic structural units.
In the network simulation work, performed in Ioannina, Partner 11, and DTU, Partner 4, the atomistic mechanisms for tensile deformation accommodation in a microscopic CuZr metallic glass were examined by molecular dynamics. 23 % of the atoms were found to belong to tiny Cu-centered icosahedral clusters (ICO) and approximately 41 % to Zr-centered slightly larger ICO-like clusters. Under deformation, the number of Cu-ICOs remained dynamically constant until yielding through a continuous cluster destruction-recreation process. Plastic deformation was found to be homogeneous in microscopic computer glasses and this explained recent experimental results demonstrating fundamental differences of plastic deformation in bulk metallic and microscopic ones (refer to Ch. E. Lekka et al, Applied Physics Letters 91 (2007) 214103). The structure of metallic glasses thus consisted of quasi-chemical clusters of some 10 atoms which were efficiently packed together to form super-clusters of some 70 or 80 atoms, leading to strong medium-range order on a scale of 1.5 to 2 nm, as summarised by the Coordinator A.R. Yavari in Nature, 439 (2006) 405.
1. ductilisation by elemental additions. In an article entitled in Physical Review Letters 94 (2005) 205501, the Network Ph.D. student J. Das from IFW, Dresden, showed how the addition of a few atomic percent Al to Cu-Zr glasses led to several percent plastic deformation prior to fracture.
2. ductilisation by nanocrystallisation. In Network publications such as the one in the Journal of Non-Crystalline Solids 353 (2007) 327, Mr K. Hajlaoui, a network Ph.D. student from INP, Grenoble, using controlled nanocrystallisation as detected by electron microscopy and synchrotron radiation obtained several percent plastic deformation prior to fracture.
3. ductilisation by control of surface stresses. In the joint network publication in Philosophical Magazine Letters, which was in press by the time of the project completion, F.O. Mear, a network post-doc from Cambridge, optimised the state of surface stresses by shot-peening to several percent plastic deformation prior to fracture.
At the start of this Network, some expected shear bands to melt due to release of elastic energy while others did not expect significant heating. Lewandowski and Greer of the Cambridge team, using a fusible coating method, established that the release of elastic energy in the form of heat resulted in a sharp rise in the temperature, of the order of 1000 K, of the deformed material and strong shear-softening that made deformation heterogeneous, as mentioned in Nature Materials 5 (2006) 15. Using electron microscopy for observations down to the nanometer regime, in the joint network publication in Applied Physics Letters 93, 2008, 031907, the network postdoc, K. Georgarakis from INP, Grenoble, directly confirmed that atoms melt up to 100 nm from the center of shear bands.
Joint work at the European synchrotron radiation facility (ESRF) by network post-docs allowed for the atomic structure of BMGs to be studied by Fourier transformation methods that converted experimental diffraction data from reciprocal space to atomic radial distribution functions in real space. See for example J. Antonowicz et al, Journal of Alloys and Compounds, which was in press in 2008 and was also available online. In these experiments, the structures of BMGs were examined before and after heavy deformation and during annealing. The modification of the atomic structure in the shear band atoms was found to be mainly at the level of the second-nearest neighbour atomic shell and linked to the interconnectivity of the basic atomic structural units.
In the network simulation work, performed in Ioannina, Partner 11, and DTU, Partner 4, the atomistic mechanisms for tensile deformation accommodation in a microscopic CuZr metallic glass were examined by molecular dynamics. 23 % of the atoms were found to belong to tiny Cu-centered icosahedral clusters (ICO) and approximately 41 % to Zr-centered slightly larger ICO-like clusters. Under deformation, the number of Cu-ICOs remained dynamically constant until yielding through a continuous cluster destruction-recreation process. Plastic deformation was found to be homogeneous in microscopic computer glasses and this explained recent experimental results demonstrating fundamental differences of plastic deformation in bulk metallic and microscopic ones (refer to Ch. E. Lekka et al, Applied Physics Letters 91 (2007) 214103). The structure of metallic glasses thus consisted of quasi-chemical clusters of some 10 atoms which were efficiently packed together to form super-clusters of some 70 or 80 atoms, leading to strong medium-range order on a scale of 1.5 to 2 nm, as summarised by the Coordinator A.R. Yavari in Nature, 439 (2006) 405.