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Bulk Amorphous Alloys

#3349


Investigation of Methods of Manufacture and Deforming Bulk Amorphous Alloys

Tech Area / Field

  • MAT-ALL/High Performance Metals and Alloys/Materials
  • MAT-SYN/Materials Synthesis and Processing/Materials
  • PHY-SSP/Solid State Physics/Physics

Status
3 Approved without Funding

Registration date
09.08.2005

Leading Institute
Baykov Metallurgy and Materials Institute, Russia, Moscow

Supporting institutes

  • VNIIKhT (Chemical Technology), Russia, Moscow\nMISIS (Steel and Alloys), Russia, Moscow

Collaborators

  • Tohoku University / Institute for Materials Research (IMR), Japan, Sendai

Project summary

About 30 years ago, a new kind of glass-forming systems was discovered. They form so called metallic glasses. These materials are known to have excellent mechanical, electromagnetic and anticorrosive properties. They are produced by extremely fast cooling of melt up to 107 grades per second. The only negative feature of these materials is their drastic brittleness and instability of amorphous state during heating. Intensive studies on new class of metallic glasses, so called bulk amorphous alloys are carrying out last decade [1,2,3]. Their basic features are, firstly, their ability for easy amorphization i.e. they are solidified in amorphous state at low cooling rates and, secondly, glass transition temperature of these alloys is significantly lower than their crystallization temperature. In the interval between these temperatures bulk amorphous alloys exist in the supercooled liquid state, they behave like viscous materials [1, 4, 5]. Such materials can be produced in the form of bulk amorphous bars. They exhibit in supercooled liquid state very low viscosity and excellent formability. In this sense, bulk amorphous preforms may be considered as new kind of structural and functional materials.

Two essential features of amorphous alloys should be taken into account while defining potentials of using them as preforms for metal forming i.e. glass-forming ability and amorphous state stability [7]. The former can be estimated by the critical cooling rate of the melt (dT/dτ)c and the relative glass transition temperature Trg = Tg/Tm where Tg is the absolute glass transition temperature and Tm is the melting point. The latter can be estimated by means of the complex criterion: Hr =(Tx-Tg)/(Tm-Tx) where Tx is the crystallization temperature.

Recent developments have resulted in the creation of metallic glasses with high glass-forming ability, wide supercooled liquid range (Tx-Tg) and high stability of amorphous state. These parameters approach sometimes those of regular non-metallic glasses [6].

Metallic glasses in supercooled liquid state (Tg< Tsls<Tx) exhibit Newtonian-viscous flow with strain-rate-sensitivity coefficient m=1.0 in wide range of strain rates. It should lead to extremely high formability of these materials, but up to now such information is limited by the results obtained by A.Inoue, T.Masumoto and colleagues [1, 2, 3]. Elongation up to about 20,000 % was reached by them during tension of La55Al25Ni20 at 473 K with super high strain rate of 5*105 s-1. Special rheological state results in decreasing apparent viscosity of this material down to 200...10 Pa*s. Such a low viscosity shown by this alloy looks very promising as a new group of metal forming processes.

Thermal properties of amorphous materials (metallic glasses) can be attributed either to the processes in amorphous state or to the processes of crystallization. The metallic glass at the temperatures over the glass transition temperature Tg is considered to be the supercooled liquid, which is equilibrated intrinsically but metastable as regards to crystallization. At the temperatures below Tg it is metastable nonequilibrium material. So, Tg cannot be considered as the material constant because it depends on conditions of the experiment: the lower the cooling rate the lower Tg value. Vitrification is thus a pure kinetic phenomenon. During heating, the metallic glass relaxes towards the configuration of lower energy. In the vicinity of Tg, it softens abruptly so its viscosity decreases some orders in magnitude in a very short temperature range. Further heating leads to crystallization which results in increasing viscosity proportional to the volume fraction of solid crystalline particles.

Temperature dependence of shear viscosity of the supercooled liquid η(T) is its essential feature which can be successfully used in analysis of mass-transfer mechanism as well as in developing technological regimes of forming processes [4,5]. At the temperatures below Tg the viscosity decreases faintly with increasing temperature resulting in a very low apparent activation energy, which is typical for isoconfigurational viscosity. Transition from the glassy state to the supercooled liquid equilibrium state looks in the η(1/T) diagram as narrow temperature range of sharp increase in the slope of the curve related to increased apparent activation energy. Isothermal heating of nonequilibrium amorphous material in glassy state results in its relaxation towards equilibrium state.

Equilibrium viscosity in the supercooled liquid region drops rapidly with temperature increase up to the melting point for nonmetallic glasses. As to metallic glasses, the viscosity drops with increasing temperature up to crystallization onset where the viscosity starts to increase sharply and reaches the values similar to that of fine structure polycrystalline materials. So, the minimum in viscosity appears very evidently on the η(1/T) diagram at a certain temperature Tvs inside supercooled liquid temperature range signing the onset of steady state Newtonian-viscous flow [4, 5].

The criteria of bulk amorphous alloys’ structure formation are at present time not enough clear. Rheological properties of bulk amorphous alloys are practically not investigated. Without that it is not possible to develop optimal regimes of their forming. This project is aimed to studying these problems.

Scientists and specialists that represent participant institutions in the project have long-term experience in creation of new materials of various composition and destination. Their works is well known in Russia and abroad. It proves high scientific level of project executives’ team and vouches for its successful realization.

References

  1. T.Masumoto, Mater. Sci. Eng., A 179/ A 180 (1994), p. 8.
  2. A.Inoue and T.Zhang, Mater. Sci. Forum, 243-245 (1997), p. 197.
  3. A.Inoue, T.Zhang and A.Takeuchi, Mater. Sci. Forum, 269-272 (1998), p. 885.
  4. O.M. Smirnov, The Japan Soc. for Res. on Superplasticity, (1991), p. 813.
  5. O.M.Smirnov, Mater. Sci. Forum, 245-245 (1997), p. 443
  6. H Rawson, Inorganic Glass - Forming Systems, Academic Press, 1967
  7. S.Surinach, E.Illekova, G.Zhang, M.Poulain and M.D. Baro, in Rapidly Quenched Metastable Materials, Supplement (1997), p. 298.


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