Deformation and its Structure in Ll o Ti-Al-Nb Compound Alloys
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DEFORMATION AND ITS STRUCTURE IN Llo Ti-Al-Nb COMPOUND ALLOYS Yoo-Dong Hahn and Sung H. Whang Metallurgy & Materials Science Department Polytechnic University 333 Jay Street, Brooklyn, NY11201 ABSTRACT The ternary TiAl-Nb (Llo) alloys stabilized at 10000C for a week were prepared into miniature specimens for compressive deformation tests. The specimens were deformed in uniaxial compression at room temperature as well as various high temperatures. The yield stress and fracture strain were determined with respect to Nb concentration, and as a function of temperature to investigate positive temperature dependence. Dislocation structures and other deformation structures of the deformed alloys were studied by TEM. INTRODUCTION Although binary Ti-Al (Llo) compounds have an attractive high temperature mechanical and thermal properties, they are intrinsically brittle from room temperature up to 700oC in both single [1] and polycrystalline forms [2]. The deformation of these compounds over this temperature range largely is attributed to the [110](111) type slip system and (111)[112] type twins (Fig. 1). Improvement of ductility and high temperature strength has been carried out primarily by adding a third element [3-7]. Both V and Nb have a large solubility in TiAl (Llo) and the addition of Nb is expected to show a solution strengthening upon alloying as was the case in the V addition [4].
On the other hand, the addition of Nb on high temperature strength and positive temperature denpendence would be of interest since such a positive temperature dependence in single crystal TiAl (Llo) [8] and Ni3AI (L12) [9, 10] are not associated with an order-disorder transformation and, therefore, may be affected by alloying elements. This paper basically deals with Z the alloying effect on mechanical property at RT as well as high temperatures and positive temperature dependence behavior in TiAl-Nb (Llo) alloys. 0
EXPERIMENTAL PROCEDURES 0
Alloys and specimen preparations
of Ti-Al-Nb (LO) alloys were described elsewhere in the proceedings [4]. The ---
specimens were deformed in uniaxial compression at 400oC, 600oC and 800oC
'Y
in a vacuum at the initial strain rate of 10- 3 S-1. X RESULTS AND DISCUSSIONS Llo phase in binary Ti-Al system is stabilized over Al rich off-stoichiometric TiAl compositions for some Mat. Res. Soc. Symp. Proc. Vol. 133. 41989 Materials Research Society
Fig. 1:
0
Ti
*
Al atom
atom
vector for inpossBurgers ible dislocations (Lio) TiAl
386
Llo TisoxAIso1x
Llo (TilxNbx)45AI55
400 300
MPa
MPa 300 250
200 200
1
0
x
Fig. 2:
2
4
3
, at % Excess
5
0
Al
Relationship between yield stress and fracture strain with excess Al in TiAl (Llo) compounds
0.1 X ,
Fig. 3:
at
0.2
%
Nb
Nb addition on yield stress and fracture strain in TiAlNb (Llo) alloys
reason. Within the Llo phase boundary, compositional variation on yield stress and fracture strain were examUNIAXIAL COMPRESSION ined and the results are plotted 40 (ROOM TEMP.) against excess Al concentration as shown in Fig. 2. The yield str
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