TiAlNb-alloy with a modulated B19 containing constituent produced by powder metallurgy
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TiAlNb-alloy with a modulated B19 containing constituent produced by powder metallurgy Heike Gabrisch, Uwe Lorenz, Michael Oehring, Jonathan Paul, Florian Pyczak, Marcus Rackel, Frank-Peter Schimansky, Andreas Stark Helmholtz-Zentrum Geesthacht, Max-Planck Str.1, 21502 Geesthacht, Germany ABSTRACT Intermetallic TiAl alloys are of interest to the aero engine industry because of their light weight, corrosion resistance and excellent high temperature strength. This justifies the continued effort to improve properties and processing of these alloys. A critical parameter that limits the practical implementation of Ti aluminides is their low ductility at room temperature. Recently, a new class of TiAl alloys based on a modulated lath structure has been introduced that exhibit an excellent combination of ductility and strength. A key component in this alloy is the orthorhombic phase B19 that is attributed to alloying with high amounts of niobium. The driving forces and mechanisms that lead to the observed modulated structures involving the B19 phase are not fully understood yet. As a first step to a better understanding we present a study of the thermal stability range of the phases involved. INTRODUCTION Structural applications in aero-engines require good high-temperature strength in combination with low density. These demands are met by intermetallic TiAl alloys that are considered as replacement for Ti alloys and Ni base superalloys in aero-engines and in the automotive industry [1-3]. The microstructure of TiAl alloys varies with chemical composition and processing parameters [4]. While the microstructure can be adjusted for optimized strength, a general drawback of all TiAl alloys is their low ductility at room temperature [2]. The best room temperature ductility is provided by near J or duplex microstructures, whereas lamellar D2 + J microstructures have better high-temperature strength [5]. For practical applications alloys are needed that possess sufficient room temperature ductility as well as excellent strength at elevated temperatures. Such a promising combination of mechanical strength and ductility has been reported for a recently developed TiAl alloy with high amounts of Nb addition [6]. Its microstructure consists of lamellar (D2 + J) colonies and pearlite-like regions of B19/Eo and J laths. The typical feature of this alloy is the closely spaced co-existence of Eo and B19 phases within B19/Eo laths. In the TEM this structure is recognized from contrast modulations within the laths that are caused by the elastic strain between the co-existing Eo and B19 phases. While these nano-laminates are thought to improve the mechanical properties of the alloy, their origin is not known yet. First principles calculations indicate that the B19 phase may form through decomposition of E/Eo phase [7]. On the other hand, according to experimental findings, B19 is reported to form in D2 phase after fast cooling from the D regime [8].
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The present study is motivated by the expected improvement of the mechanical propertie
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