Minimizing beta flecks in the Ti-17 alloy
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TRODUCTION
THE alloy Ti-17 is titanium-based with a nominal weight percent composition of 5Al-2Sn-2Zr-4Mo-4Cr0.1O. The high b stabilizer alloy content (4Mo 1 4Cr) provides deep hardenability for heat treatment section sizes to at least 16 cm (6.3 in.). The alloy Ti-17 was developed as a high strength, high toughness alloy for application in jet engine fan and compressor disk components for the temperature range up to 400 7C. Laboratory investigations and engine applications have demonstrated the Ti-17 alloy to be superior to other similar commercial b stabilized, a 1 b titanium alloys in terms of its combined strength, toughness, and low cycle fatigue capabilities. A melt process segregation issue called ‘‘b flecks’’ can be encountered with this alloy and must be addressed to achieve economically required product yields. The b flecks are recognized as localized volumes of large grain transformed b phase within the normal fine grain a 1 b microstructure of the in-process billet product. These b flecks are attributable to b stabilizing element segregation, which decreases the a 1 b to all b transus temperatures. The b flecks are readily detectable using the blue etch anodize (BEA) etch evaluation[1] of the a 1 b processed billet; the BEA evaluation is essential for b fleck detection in b forged disk components. The major segregating element for the Ti-17 alloy b flecks is Cr. However, Cr is required for the deep hardenability and weldability of this alloy. Proper melt process design for the Ti-17 alloy is essential to minimizing the formation of b flecks, but minor b flecks occasionally form even in the best melt processes. Because of those occasions, this article addresses the capability of using a high b phase field ingot, or billet, heat treatment for solid-state diffusion to minimize the extent of the b transus depression by Cr segregation.
it is partitioned toward the liquid at the solidification front based on normal phase diagram principles. The mechanism by which the enriched Cr liquid is locally entrapped, and how that knowledge can be used to develop a superior melt practice, will be left to those more astute in ingot solidification phenomena. It is noted, however, that the b flecks normally occur in the top third of the Ti-17 heats; that tendency has been associated with the solidification rate change at the initiation of the hot-top practice. Besides Cr, the b fleck segregation also involves partitioning to a lessor extent of the other Ti-17 alloying elements. A typical Ti-17 b fleck might show increases of 1.0 to 1.5 wt pct Cr and 0.5 wt pct Zr, along with decreases of about 0.5 wt pct Mo and about 0.2 wt pct Al. The Fe change will be an increase that ratios to about the same extent of partitioning as Cr, but Fe is only present to ,0.3 wt pct. Also, the O content likely decreases by about 0.01 wt pct, but this has not been confirmed. The direction of this partitioning is in agreement with that anticipated from the binary phase diagram solidus and liquidus profiles and with the published literature.[2] A
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