Solidification Front Velocity of Ternary Titanium-Aluminides
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"* variation
of stochiometry (in binary and ternary alloys an Al concentration in the range of 45-50 at.% is considered to be best) [3] "* addition of a third element (to be effective in ductilization, the ternary additions have to be limited to a range of 1-3 at.%) [3] "* microstructural control via processing (e.g. rapid solidification yields a reduced ordering and may improve ductility). The aim of this paper is to study the correlation between the solidification velocity v and the undercooling AT prior to rapid solidification for the three ternary TiAl-alloys Ti 51A147Fe 2, Ti 5IAl 47Cr 2, and Ti 51A147Mn2 (the relationship between solidification velocity and microstructure as also seen in [4] will be discussed in a subsequent publication). TiAl in the corresponding concentration range solidifies through the peritectic reactions L+13--ct, L+ct-->, with (bcc) 13-Ti, (hcp) ct-Ti and 7,-TiAl. The experimental data will be compared with the LKT-theory (Lipton, Kurz, Trivedi; [I]) which provides a relation between v and AT for high undercoolings. Containerless processing leads to a severe reduction in heterogeneous nucleation sites and therefore the non-equilibrium state of the undercooled melt - in contrast to rapid quenching methods like melt spinning - becomes accessible for experimental observation. The high driving force for nucleation and growth of largely undercooled melts yields high growth rates also for low cooling rates prior to solidification. A very useful containerless processing method is the electromagnetic levitation technique that was applied in the present work: in-situ observation allows a time-resolved temperature measurement during undercooling and solidification. Direct 69 Mat. Res. Soc. Symp. Proc. Vol. 398 01996 Materials Research Society
investigation of the solidification front velocity v as a function of undercooling AT becomes feasible. In order to determine v various experimental devices have been produced by different authors: 1. photo diodes: arrays up to 34 diodes [4,5,6] were used, 2. pyrometer [7]: v is a function of the inverse rise-time of the pyrometer signal during recalescence, 3. capacitance proximity sensor with a photo diode [8]: the capacitance proximity sensor detects the moment of inoculation and the diode detects the arrival of the dendrites at the opposite side of the sample, 4. high speed video imaging [9]. In our work we use a high speed video device because it enables us to monitor the whole visible section of the sample. Observation of the morphology of the solidification front and the evaluation of the solidification-front velocity is possible. EXPERIMENT The alloys were supplied by the Gesellschaft ffir Elektrometallurgie (GfE) Ntirnberg with a purity of 99,8 %. Samples of approx. 1-2 g were cut from the rod shaped material and surface cleaned in a sand blast apparatus using A120 3-sand. The undercooling experiments were performed elctromagnetically in a UHV-device (fig. 1). This was evacuated to 10-6 Pa and refilled with high purity (99,9996 %) Ar or He, redu
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