HAL Id: jpa-00225136
https://hal.archives-ouvertes.fr/jpa-00225136
Submitted on 1 Jan 1985
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of
sci-entific research documents, whether they are
pub-lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
STRUCTURAL INFORMATIONS FROM
MACROSCOPIC PROPERTIES OF METALLIC
GLASSES
U. Köster
To cite this version:
J O U R N A L D E PHYSIQUE
Colloque C8, suppl6ment a u n012, Tome 46, d6cembre 1985 page C8-63
S T R U C T U R A L I N F O R M A T I O N S FROM M A C R O S C O P I C P R O P E R T I E S O F M E T A L L I C G L A S S E S
Dept. Chem. Eng., University Dortmund, 0-4600 Dortmund 50, F.R.G.
Resume - L'etude directe de la structure des materiaux non cristallins par les methodes diffractometriques conduit essentiellement aux fonctions de cor- relation de paires. Les approches plus macroscopiques sont non seulement utiles mais necessaires pour obtenir des informations plus detaillees sur l'ordre 2 courte distance, moyenne et longue distance.
Les proprietes electriques ou mecaniques, la cristallisation, la densite, la solubilite de l'hydrogene etc... sont influencees par la microstruture des materiaux non cristallins reels.
Abstract
-
Direct investigations on the structure of non-crystalline mate- rials using diffraction methods result mainly in pair correlation functions. Macroscopic approaches are not only welcome but necessary to reach further detail in short, medium or long range order.Electromagnetic properties, crystallization behavior, mechanical properties, density, hydrogen solubility, etc., are indeed affected by and can probe the microstructure of real non-crystalline materials.
INTRODUCTION
Whichever way it is prepared, an amorphous solid is not in configurational equi- librium, but is slowly relaxing by a homogeneous process toward an "ideal" meta- stable amorphous state. Relaxation toward this ideal amorphous state is distinct from heterogeneous crystallization which results in the stable crystalline state of the material.
Real amorphous solids can be assumed to include structural defects with a so far unknown structure and compositional fluctuations. Apart from the type, distributions and the concentrations of these defects, the structure of amorphous solids prepared by different techniques or conditions appear to be essentially similar. In another concept such a microscopic state of a real glass can be characterized by excess free volume or in more detail by the atomic short-range order, which includes both chemical and topological short-range order of the glass /I/. In a model proposed by Gaskell /2/ structural units similar to those building the related crystal structure are assumed; whereas topological as well as chemical short range order are built in, the rules which govern packing of such units in the glass are not known. In all concepts long range order is assumed to be controlled by micro-inhomogeneities, e.g. phase separation, nucleation sites.
Since the structure of amorphous materials is defined only by the non-existence of translational symmetry, there is no positive description of an ideal amorphous state and it may be difficult to describe the structure of a defect. Direct investigations of the structure in non-crystalline materials using diffraction methods result main- ly in pair correlation functions. Macroscopic approaches are not only welcome but necessary to reach further details in short or medium range order. An attempt to characterize "defect structures" in general is based on the analysis of structure sensitive properties.
C8-64 JOURNAL DE PHYSIQUE
HOW HOMOGENEOUS IS THE STRUCTURE OF A METALLIC GLASS
In rapidly solidified crystalline ribbons significant differences in microstructure have been observed depending on the distance from the contact surface of the ribbon. As shown in fig. 1 in Fe45Ni~5B10 ribbons an extremely fine-grained solid solution has been found at the contact side, whereas the structure near the free surface consists out of y-(Fe,Ni) and (Fe,NiI3B. Fig. 2 reveals the change in structure in a rapidly solidified A1-Cr ribbon in a schematic sketch: a thin amorphous layer at the contact side is followed by quasicrystals embedded in an aluminum matrix; near the free surface only the stable phases A1 and A17Cr have been found.
In amorphous materials it may be difficult to realize differences in the amorphous structure except in the case of glasses prone to amorphous phase separation. As shown in by Atzmon et al. /3/ the amount of phase separation increases with the distance from the contact side (fig. 3).
Fig. 1: Cross sections of rapidly solidified Feb5NikSB10 ribbons cast at differ- ent surface velocities of the quenching wheel: (a) 20 m/s; (b) 40 m/s.
free surface
quasicrystals
+
Al
I
amorphous
I
contact side
Fig. 3: '6
-
.- S$
-
. , h = l I- 1 3 % 'C --.- PA--- '" 0,-
C....
. h:6-8 --A-
-.---.- 25 35 45 55 65 75 85 95 105 115 28 (deg 1X-ray scattering intensity as a function of scattering angle for a rapidly cooled ribbon of (Au,Cu)glLag for different distances h from the cooling wheel (Atzmon et al. /3/).
INFLUENCE OF QUENCHING CONDITIONS
Ribbons produced by melt spinning exhibit smooth top surfaces (free surfaces), but the contact sides are characterized by very typical microtopological asperities (see fig. 4); e.g., large lift-off (non-contact) areas due to gas entrapment under the solidifying material. Reducing the pressure as well as the substrate velocity, both will lead to smoother contact surfaces reducing the non-contact areas and the amount of whirl streets along grooves. These whirl streets may be caused by liquid flow instabilities such as capillarity wave or Marangoni instability. Such surface asperities are assumed not only to influence mechanical properties /4/, but also a number of other properties, e.g. nucleation sites for crystallization or amount of amorphous phase separation.
JOURNAL DE PHYSIQUE
In addition, the duration of contact with the quenching wheel is of large importance and is assumed to be influenced by the wheel diameter; on loss of contact the cooling rate decreases catastrophically, depending on the thermal conductivity, pressure and relative velocity of the ambient atmosphere. The contact time will determine the state of quench, since a glass that leaves the wheel at a higher temperature will tend to undergo more relaxation during the low cooling rate after- wards.
INFLUENCE OF RELAXATION
On annealing metallic glasses many properties (e.g., density, ductility, internal friction, Curie temperature, superconducting transition temperature, etc.) have been observed to change significantly. There are two modes of relaxation which may take place nearly independent of each other /5/: Irreversible structural relaxation was explained in terms of the relaxation of the atomic level density fluctuations, which affects e.g. the density, and can be described by annealing out of free volume or topological short range ordering. The reversible relaxation below T has been attri- buted to the chemical short range ordering and could occur by a n o r cooperative atomic rearrangement.
TRANSPORT PROPERTIES
Electromagnetic properties, crystallization behavior, mechanical properties, den- sity, hydrogen solubility, etc., are indeed affected by and can probe the micro- structure of real non-crystalline materials:
Investigations of the magnetization curve /6/ have clearly shown that topological defects and their elastic stress fields determine all main properties of the hyster- esis loop. The Curie temperature which depends essentially on the nearest neighbor interactions is sensitive to the chemical short range order.
Superconductivity depends on the bulk phonon frequency as well as the short range order. The major decrease in T upon annealing is due to the "hardening" of phonon modes as the strains are relased / 7 / . Samples with transition widths of several millikelvins are characteristic for a homogeneous phase. Systematic studies are needed to correlate upper critical field behavior to the different forms of inhomo- geneities in metallic glasses.
In crystalline materials the electrical resistivity is known to be extremely struc- ture-sensitive and is used in many cases to follow annealing-out of defects as well as phase transformations. But since the electron mean free path is very short in metallic glasses, they may behave different. As yet very little is known on the influence of relaxation on the electrical resistivity; most work so far was done just with as-quenched material. In Be40Ti50Zr10 glasses an increase of resistivity of 1% has been observed after annealing about 50 K below the glass transition tem- perature and was associated due to structural relaxation / 8 / . Since a number of these ternary glasses are known to exhibit phase separation, one has to question this explanation.
DIFFUSION
HYDROGEN IN METALLIC GLASSES
Hydrogen can be used as a probe to investigate structure and properties of metallic glasses in some detail. Diffusion as well as partial molar volume of hydrogen in metallic glasses /12/ have been found to depend very sensitive on structural differ- ences and can be used to obtain informations on the energy distribution of the interstitials in disordered materials (see fig. 5a). Hydrogen diffusivity increases as the deepest traps are occupied. Very recently measurements of the hydrogen dif- fusion coefficient in liquid-quenched and vapor-deposited PdgOSipO and its depen- dence upon temperature and hydrogen concentration have been found to be in excellent agreement with this model if a Gaussian distribution of energies is assumed /13/. The width of the energy distribution of the interstitials depends on the method of preparation.
The compositional dependence of hydrogen solubility after cathodic charging of Ni-Zr glasses is shown in fig. 5b. In these glasses hydrogen is known to be located pre- ferentially in distorted Zr-tetrahedra. Therefore the solubility increases as the number of Zr-tetrahedra with the Zr content. In fig. 5c & d the relative narrow energy distribution of zr4-tetrahedra and mixed Z~~TM-tetrahedra are combined with the much broader distribution for sites in the quenched-in free volume /14/. The mean energy in particular of the mixed tetrahedra depend on the size and electronic structure of the late transition metal TM. Such a model can be used to explain the increase of hydrogen solubility of zirconium-base glasses in the sequence Fe + Ni + + Co + Pd. In Pd-Zr glasses hydrogen atoms are assumed to occupy not only the zr4- tetrahedra but also mixed or even pure palladium tetrahedra, thus resulting in the rather high solubility observed in these glasses. In addition, the higher ductility of these glasses even at relatively high hydrogen contents can be understood, since most sites of the excess free volume are still free.
E excess
free
volumeZr3Ni
-
tetrahedra'28-68 JOURNAL DE PHYSIQUE
CRYSTALLIZATION
Crystallization of metallic glasses have been found to proceed by nucleation and growth /15/ of crystals into an unchanged amorphous matrix as shown for example in fig. 6. Such behavior is in sharp contrast with grain growth in a microcrystalline material. Only crystallization of Mg70Zngg glasses exhibits some similarity to grain growth in a microcrystalline material: even after annealing at relatively low temperatures the whole specimen has been found to be fully crystallized with a mean grain size of about 3 nm; no crystallization front could be detected so far. This unusual behavior might be due to the very low glass transition temperature leading to an extremely high nucleation rate.
Fig. 6: Polymorphic crystallization in Fe74Cr2B24 (20 s annealed at 456OC).
Information on kind, number and distribution of nucleation sites can be revealed from crystallization kinetics and statistics. As shown in fig. 7 for Co70B30 very often a crystal-free layer has been observed at that side of the ribbon which was in contact with the qnenching wheel, i.e. the side exhibited to higher quenching rates; this observation indicates that the nucleation sites or pre-existing nuclei are formed during the rapid solidification process of the ribbon. Higher quenching rates usually result in a reduced number of quenched-in nucleation sites.
The structure of metastable phases formed first during crystallization is supposed to exhibit some similarity with that of the amorphous matrix, e.g. the metastable tetragonal Fe3B phase which exhibits the very same structural units as Fe-B glasses. Heat of crystallization, activation energies of crystal growth as well as mechanical properties are related to the strength of atomic bonds and can be used for further insight into the atomic structure of the metallic glass. All these parameters, however, are only comparable if crystallization proceeds by the very same reaction. The so-called "crystallization temperature T
"
depends not only nucleation and growth rate, but also on the number of penche&in nucleation sites.INFLUENCE OF METALLOID CONCENTRATION
From the variation of several properties (e.g., crystallization behavior, micro- hardness, density, electrical resistivity) with metalloid concentration in numerous metal-metalloid glasses a strong variation of short and medium range order can be concluded /16/. These results can be explained along with an "excess volume" model, where the (structural) excess volume is progressively reduced as the metalloid content in the glass approaches that of a crystalline phase.
Fig. 8 shows the change of microhardness and tensile strength vs. boron content for Co-B glasses. Whereas microhardness is not very sensitive against relaxation and may be a good measure for bond strength in the glass, tensile strength depends strongly on the quality of the surface as well as on the amount of quenched-in free volume necessary for localized ductility.
Fig. 8: Microhardness and tensile strength vs. boron content in Co-B glasses.
PHASE TRANSFORMATION IN Ni-Zr-B GLASSES
Structural studies on intertransition metal glasses indicate some differences between the structure of metal-metalloid glasses and intertransition metal glasses. By adding boron, zirconium-rich Ni-Zr glasses are supposed to undergo a phase trans- formation into a structure typical for a metal-metalloid glass as the boron concen- tration increases /17,18/.
C8-70 JOURNAL
DE
PHYSIQUEFig. 9: Influence of boron content on macroscopic properties of Ni-Zr-B glasses: (a) density and (b) electrical resistivity /17/; (c) microhardness and (d) hydrogen solubility /18/.
In Ni-Zr glasses hydrogen is known to be located preferentially in distorted Zr- tetrahedra large enough for enclosing hydrogen. If additional boron fills up the larger Bernal holes, this should not influence the hydrogen solubility very much and cannot explain the remarkable decrease observed in density or resistivity. There- fore, we believe that even small amounts of boron can destabilize the formation of Zr-tetrahedra leading towards a glass with a structure more typical for metal- metalloid glasses (trigonal prismatic packing). Silicon addition instead of boron does not exhibit similar dramatic effects /14/, but silicon is known to form C16 phases with zirconium (Zr3Si and Zr2Si), i.e. phases with same structure as NiZr2.
REFERENCES
/1/ T.Egami, "Atomic Short-Range Ordering in Amorphous Metal Alloys", in: "Amorphous Metallic Alloys", ed. F.E.Luborsky, Butterworths, London 1983, p.100 /2/ P.H.Gaskel1, J.Non-Cryst.Solids
32
(1979), 207/3/ M-Atzmon, W.L.Johnson, CALTECH-Report CALT-822-138 (Pasadena, 1982) /4/ U-Kbster, U.Herold, H.-G.Hillenbrand, Script.Met.
17
(19831, 867 /5/ A.L.Greer, J.Non-Cryst.So1.61&62
(19841, 737/6/ H.Kronmiiller, N.Moser, "Magnetic After-Effects and the Hysteresis Loop",
/7/ S.J.Poon, "Superconducting Properties of Amorphous Metallic Alloys",
in: "Amorphous Metallic Alloys", ed. F.E.Luborsky, Butterworths, London 1983, p.432
/8/ K.V.Rao, "Electrical Transport Properties", in: "Amorphous Metallic Alloys", ed. F.E.Luborsky, Butterworths, London 1983, p. 401
1
/9/ U.Koster, U.Herold, Proc. 4th Int. Conf. on Rapidly Quenched Metals, ed. T.Masumoto, K.Suzuki, Sendai 1982, p.587
/lo/ B.Cantor, R.W.Cahn, "Atomic Diffusion in Amorphous Alloys", in: "Amorphous Metallic Alloys", ed. F.E.Luborsky, Butterworths, London 1983, p.487 /11/ J.HorvZth, K.Pfahler, W.Ulfert, W.Frank, H.Mehrer, this conference
/14/ H.-W.Schroeder, "Wasserstoffldslichkeit und -versprodung in metallischen Glasern", Fortschritt-Berichte VDI: Reihe 5, Nr. 94, Diisseldorf 1985 /15/ U-Kdster, Z.Metallkde.
2
(1984), 691/16/ H-Warlimont, Z.Metallkde.
