Thermal Expansion of Amorphous Iron-Chromium-Boron Alloys
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THERMAL EXPANSION OF AMORPHOUS IRON-CHROMIUM-BORON ALLOYS
G. HUNGER, H. W. BERGMANN AND B. L. MORDIKE Institut filr Werkstoffkunde und Werkstofftechnik, University of Clausthal, W. Germany ABSTRACT In amorphous iron-chromium alloys magnetic contraction is superimposed on the thermal expansion. Depending on the proportion of antiferromagnetic chromium the resultant coefficient of expansion is either positiv, negative or zero. The chromium content also changes the Curie temperature. Relaxation processes have a marked effect on the thermal expansion. Long term and short term expansion behaviour was compared and discussed. In addition the crystallization behaviour and mechanical properties of the alloys are reported. INTRODUCTION Crystalline materials with the exception of some Fe-Ni alloys (Invar) expand with increasing temperature. A zero coefficient of expansion has been observed in some metallic glasses'). There are two explanations for this. On the one hand quenqhed in defects lead to a contraction on heating (decrease in the free volume) 2 ). The structural relaxation effect is, however, irreversible3). Reversible contraction occurs, for example, when a magnetic contraction outweighs the normal thermal expansion. This has practical possibilities in eg. thermally controlled switching devices (Relays, Bimetalstrips). Magnetic contraction has been reported in Fe-B and Fe-B-Cr glassesI . In the present work the influence of Cr on the thermal expansion, crystallization and mechanical behaviour has been investigated. EXPERIMENTAL DETAILS Glass tapes 20 pm thick and 1 mm wide were prepared of (Fe100-xCrx) 8 3 B17 (0 5 x 5 25) by melt spinning in a helium atmosphere. The crystallization and relaxation behaviour was studied using a Perkin Elmer DSC 2 differential calorimeter. The crystalline phases were determined using X-rays5). A highly sensitive dilatometer was used to measure the thermal expansion (Perkin Elmer TMS 2) over the temperature range RT to 400 0 C. To establish the effect of relaxation the tapes were studied in the as quenched and various relaxed states (heated up to 100 times up to 3250C). RESULTS AND DISCUSSION Fig. 1 shows the dilatometer curves for the as quenched glasses, from which the Curie temperature, shown in Fig. 2, was determined. The Curie temperature decreases linearly with Cr content over the whole range investigated. For comparison the corresponding c ystallization temperatures for a holding time of 10 min are shown in Fig. 35). It can be seen from Fig. 2 that from 0-13% Cr the tapes are ferromagnetic at RT. In the as quenched state the thermal expansion is outweighed by the structural relaxation. Fig. 4 shows this effect in (Fe 9 0 Cr 1 0 ) 8 3 B17 -
232
It
x
SX
10"
x
0.
20
=1310
400
T/PC
-5.
-10
x= 3 x=O X=1
Fig.
1 Thermal expansion of as quenched (Fe Cr contents.
10 0
x Crx)
8 3 B17
glasses for various
T/K
L-
0,7. 0,6-
7Slope
C3
~7O
C 0,3 =7 E 0,2
GI
annealing time = 10min
800-
crystallization end
crystallization start
^'
1
3
5
10 at % Cr
13 15
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