Dendrite arm climb by temperature gradient zone melting during solidification of a high-speed tool steel
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R. RIEDL, Department Head, is with the PVD Department, ETA S.A. Fabrique d'Ebauches, CH-2540 Grenchen, Switzerland. H.F. FISCHMEISTER is Professor of Metallurgy, University of Stuttgart, and Director, Max-Planck-Institut fiir Metallforschung, D-7000 Stuttgart l, Federal Republic of Germany. Manuscript submitted January 24, 1989. 264-- VOLUME 21A, JANUARY 1990
Fig. 1--111ustration of the mechanism of dendrite arm climb (from left to right) by TGZM.
Fig. 2--Longitudinal section through a directionally solidified specimen of AISI TI (Fe-0.76 pct C, 17.6 pct W, 4.1 pct Cr, 1.1 pct V (wt pct)) passed through a temperature gradient of 4.4 K / m m at 0.021 mm/s. Scanning electron micrograph, atomic number contrast (unetched).
the gradient there. The steep gradient at the trailing edge accelerates the leveling of the profile by solid-state diffusion. Published micrographs of solidification structures 17'8l occasionally show evidence of asymmetrical concentration profiles similar to Figure 3, although this evidence has not been interpreted in terms of TGZM. In the solidification structure of AISI M2 which had been solidified slowly enough to develop the delta-eutectoid METALLURGICAL TRANSACTIONS A
(a)
it m E
growth ,.
S2
direction
j
I I t
liquid
I
5 4 3--
0
I solid I 100
I 200
I , 300
distance,
[~m]
(b) Fig. 3 - - A s Fig. 2, (a) alloy F e - I . I C , 6.4W, 4.8Mo, 4.1Cr, 0.8V, 0.8Nb (wt pct), quenched from above the onset of eutectic crystallization, etched in Oberhoffer's reagent; (b) microprobe trace for Cr across side arms, along growth direction.
the dendrite tips, i . e . , the isotherm velocity, R, for two values of the average temperature gradient through the solidification interval, (7 = (TL - T s ) / ( Z L - Z s ) . The gradient was derived from measurements with thermocouples embedded in representative specimens. The treatment by Allen and Hunt [u predicts that the climb distance should be inversely proportional to R, while the influence of (7 should be modest, because with constant climb rate and isotherm velocity R, an increase in (7 (which would increase the driving force for TGZM) will shorten the time available for climb. These trends are verified by the data in Figure 4. The maximum climb distances shown in Figure 4 correspond to about one-third of the final dendrite spacing. Typically, there were about 100 secondary arms between the tip and the freezing front of the eutectic, and the interval between the peritectic temperature and the end of solidification accounts for approximately 0.23 of the total solidification time. Assuming constant climb velocity throughout the solidification interval for the sake of simplicity, this would put the total contribution of T G Z M climb to the coarsening of the dendrite structure at somewhere around 1.4 pct for the solidification conditions of this experiment, which are in the range of technical ingot solidification. In Allen and Hunt's unidirectional freezing experiments with transparent organics, the coarsening by T G Z M was about 4 pct. Let us r
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