2nd Sino-German Workshop on EPM Oct , 2005, Dresden, Germany PAN Mingxiang, ZHUANG Yan Xin, ZHAO Deqian, WANG Wei Hua Institute of Physics, Chinese Academy of Sciences, P.R. China Effect of Electric Field on Crystallization of Metallic Amorphous The financial supports: the National Natural Science Foundation of China (Grant No: ) and the Chinesisch– Deutsches Zentrum für Wissenschaftsförderung (Grant No: GZ032/7)

Contents 1.Motivation 2.Crystallization Methods of bulk metallic glasses (BMGs) 3.Experimental detail 4.Results and discussions 5.Summary and outlook

1. Motivation Phase transition: thermodynamics and kinetics of crystallization (nucleation and growth) New materials: Bulk metallic glass  bulk nanocrystals Application: Stability of BMGs Why do we study crystallization of BMGs?

2. Crystallization methods of BMGs High temperature (iso- or nonisothermal) High temperature and high pressure High temperature and external fields  Magnetic field  Pulse electric field  d.c. or a.c. electric field T x1 TgTg  Exoth. heat flow TlTl T x2 Undercooling liquid region  T xg Glass y state TmTm Crystallization region Temperature T p1 T p2 What is our aim of this work?

3. Experimental detail BMG sample composition: Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 (vit1) [Zr 48 Nb 8 Cu 14 Ni 12 Be 18 (vit4)] Sample shape: rod of 16 mm diamter  slices of 0.3 mm thick Schematic drawing of the electric field annealing apparatus heating furnace; 2 -- aluminum tube; 3 -- stainless steel electrodes; 4 -- insulation materials. ~V 5 samples mm 6 1 thermocouples Vaccum: 5X10 -3 Pa Temperature accuracy:  0.5 K Applied electric potential: 1.7 kV (a.c., 50 Hz) Heating rate: 10 K/min Held time: 1 h

Temperature-Time curve of two samples at two positions

Analysis methods for structures and phase transition: X-ray diffraction (XRD) Differential scanning calorimeter (DSC) Transmission electron microscope (TEM) Three samples for Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 BMG: a – as-cast b – annealing without electric field c – annealing with a.c. electric field Annealing temperature for vit1: 623, 643, 663, 683 and 693 K

4. Results and discussions Inset shows the total enthalpy of crystallization for the three samples: (a) as-prepared BMG, (b) annealed at 663 K for 1h; (c) annealed at 663 K for 1 h with the application of electric field. Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 BMG

DSC traces for sample b at different heating rate DSC traces for sample c at different heating rate

SampleT g (K)T x (K)T p1 (K)T p2 (K)E g (kJ/mol)E p1 (kJ/mol)E p2 (kJ/mol) Sample a Sample b Sample c Table: Values of T g, T x, T pi,  T and apparent activation energy of samples a, b and c a: as-cast; b: without electric field; c: with a.c. electric field.

Sample T g =A+B ln  T x =A+B ln  T p1 =A+B ln  T p2 =A+B ln  AgAg BgBg AxAx BxBx A p1 B p1 A p2 B p2 Sample a Sample b Sample c Values of A and B in Lasocka quation (  = A+Bln  ) for sample a, b and c

Bright-field TEM images and the corresponding selected area electron diffraction patterns for the samples b (left) and c (right) annealed at 663 K for 1 h.

V c = (  H ac -  H cc )/  H ac

Zr 48 Nb 8 Cu 14 Ni 12 Be 18 BMG

Bright-field TEM images and corresponding selected area electron diffraction patterns for the Zr 48 Nb 8 Cu 14 Ni 12 Be 18 alloys annealed at 673 K for 1 hour with (a) and without (b) the applied electric field.

Physical mechanism? Vekey and Majumdar: The effect of electric field on phase separation of glass. Nature 225 (1970) 172 Lai et al: Nanocrystallization of amorphous Fe–Si–B alloys using high current density electropulsing MSEa 287 (2000) 238 Holland et al: Crystallization of metallic glass under the influence of high density dc currents JAP 95 (2004) 2896 Electromigration force on the ions in electropulse annealing Enhanced diffusion rate due to a cooperative diffusion mechanism Stochastic resonance Suggested:

D 2 =(U C2 /U C1 ) 2 D 1 =(r 2 /r 1 ) 2 D 1 The growth rate of nucleus, U C, can be expressed as where t, a f and D are time, constant and diffusivity, respectively. Assuming that the U C is independent on the time, the diffusivity under the applied electric field, D 2, can be written as Subscript 1 – without applied electric field 2 -- with applied a.c. electric field r -- the crystallite size From experiment measurement, r 2  3 r 1,  D 2  9 D 1, i.e. D is markedly increased by the applied electric field. (W. Liu et al, J. Phys. D 30 (1997) 3366)

Generally, the formation of nucleus is determined by the driving force of phase transformation and atomic mobility. The critical nucleus size, r C, in the presence of electric field can be expressed as: E -- the electric field strength;  1,  2 -- the dielectric constant of amorphous and partially crystallized states respectively. The energy barrier of forming a critical nucleus size in the presence of electric field,  G*, can be expressed as:

Summary  The Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 BMG annealed in the presence of an electric field contain larger crystalline phase volume fraction and larger size of crystallites.  The nucleation rate and the growth rate of nucleus are markedly enhanced in the supercooled liquid region of the BMG by the applied electric field.  The electric-field-enhanced crystallization is due to the enhanced mobility of ionized diffusing components and the reduced energy barrier.  Both formation and growth of the nuclei in the BMG are affected by the applied electric field, although it is still not clear how the electric field affects formation of the nucleus.

Electromagnetic vibration process for producing bulk metallic glasses Nat. Mater. 4 (2005) 289 Effects of the Intensity and Frequency of Electromagnetic Vibrations on Glass- Forming Ability in Mg–Cu–Y Bulk Metallic Glasses Mater. Trans. 8 (2005) 1918 Recent report by T. TAMURA et al Outlook Effects and mechanism of electromagnetic vibration on crystallization of BMGs Improvement of metallic glassy formability and synthesis by applying electromagnetic vibration

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