Effect of rapid solidification and alloying addition on lattice distortion and atomic ordering in L1 0 tial alloys and t
- PDF / 1,733,830 Bytes
- 11 Pages / 594 x 774 pts Page_size
- 18 Downloads / 265 Views
I.
INTRODUCTION
MUCHeffort has been put into the investigation of ordered intermetallic alloys and particularly of the intermetallic titanium aluminides, TiAI, Ti3A1. II-7] These ordered alloys offer advantages over conventional, disordered alloys for high temperature structural applications.t8-11] Many such ordered alloys show an increase in the yield strength with increasing temperature m-17] while the restricted atomic mobility in ordered lattices results in slower diffusion processes and better creep resistance. In spite of a combination of a high melting point with good oxidation resistance, a favorable strength-to-weight ratio, and a resistance to softening at elevated temperatures, the development of ordered L10TiAI alloys for high temperature applications has met with only limited success.t~s] The major problem is their brittle fracture and low ductility at low temperatures caused by a limited number of available slip systems. {18J The titanium aluminum system (Figure 1)119] contains three intermetallic compounds based on the compositions of Ti3A1, TiA1, and TiA13. Of these ordered structures, TiA1 is of interest as it has high strength and modulus values, especially when normalized by its relatively low density, and creep and oxidation properties at elevated temperatures which are much greater than the values for conventional solid solution type titanium alloys.[2~ The structure of TiA1, also known as % is Ll0, which is face-centered tetragonal. The crystal structure (Figure 2) shows a layered arrangement of Ti and A1 atoms on successive (002) planes. At the equiatomic composition, stoichiometric TiA1, the structure is isomorphous with CuAu, an ordered face-centered tetragonal. [21'n]
In general, the Llo structure constructed from two types of atoms has an axial ratio of less than unity if the atomic size is the main parameter determining it. This is true of most alloys in this group, i.e., PtCo, AuCu, NiZn, NiMn, etc. The ,/-phase has an axial ratio of 1.02, indicating that the structure may be determined by parameters other than the atomic size--for example, the electronic structure. The L10 structure approaches face-centered cubic if it becomes disordered, and thereby, increased ductility by reducing planar fault energies. However, the structure remains highly ordered to its melting point. This highly ordered structure is a source
THE S Y S T E M
TITANIUM-ALUMINUM
1800 1670
~
1600
L
o(.~1400
,,';:
-
TiAI
~ 1200-
///
,~ I000-
I--
l
665~,,
600I~"TIAI2,~TiAI..i
~b
2b
Ti
3b Atomic
TI
4b
50
Percent
6b
?O
Rapidly
Quenched
80
90
Aluminum
TisAI TIsAI+TJAI TIAI
T 3A
METALLURGICALTRANSACTIONSA
~TIAI 3
800-
Ti
DINKO VUJIC and ZHIXIANLI, Graduate Students, and SUNG H. WHANG, AssociateProfessor, are with PolytechnicUniversity,333 Jay Street, Brooklyn,NY 11201. ManuscriptsubmittedJuly 6, 1987.
/ ;',--~i ,' , '~._',L
TIA{2 + TiAI 3
I00 AI
TIAI3 +(AI)
Phases
Fig. 1--Rapidly quenchedphases in binaryTi-A1system. VOLUME19A,OCTOBER1988--2445
TiAt(Llo) p4/mmm
.C)---Ti
l
I
Data Loading...
