Formation of Metastable Phases in Rapidly Quenched Binary Ti Alloys
- PDF / 1,401,428 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 24 Downloads / 231 Views
FORMATION OF METASTABLE PHASES IN RAPIDLY QUENCHED BINARY Ti ALLOYS S.H. WHANG AND C.S. CHI Department of Metallurgy & Materials Science Polytechnic University 333 Jay Street Brooklyn, New York 11201
ABSTRACT Rapid quenching of binary Ti alloys from the melt results in various metastable phases. A systematic study has been conducted in order to elucidate principles associated with the formation of metastable phases in binary Ti alloys resulting from rapid quenching. These metastable phases that include a', a" phases, metastable ý phase, and w phase are discussed with regard to their occurrence and the extension of a phase as a function of cooling rate. Effect of cooling rate and mechanical stress applied during cooling on metastable phase formation was investigated.
INTRODUCTION When metals and alloys are subjected to rapid quenching from the molten state, various metastable states are introduced in alloy phases and microstructures as well. These metastabilities in rapidly quenched alloys and materials have been well documented in recent years [1]. Metastable phases in binary Ti alloys known as a', a", metastable B phase and w phase have been studied in the past [2,3]. All these phases are competing each other in a sense and overlapped compositionally in a narrow range [4,5]. In particular, martensite transformation and occurrence of w phase can't be separable compositionally while w phase formation appears to be associated with electronic concentration per atom [6,7]. In recent years, Ti alloys have been routinely processed by rapid solidification techniques at the cooling range of i0ý-107 K/sec. The resulting alloy phases increase their meta -stability with increasing cooling rate. It is of interest to understand how further increased cooling rate affects solid solubility of solute, martensite transformation temperature and w phase formation.
EXPERIMENTS Splat foils (20um thick uniform) of binary Ti alloys were produced by the hammer-and-anvil technique. Solubility extension, occurrence of w phase and martensite transformation were studied by varying solute concentration. In addition, foils of different cooling rates and mechanical stress levels were prepared by varying 1) the hammer pressure; 2) the size of splat alloy piece while alloy composition was kept constant: Ti 9 7 . 2 Fe 2 . 8 . These foils were electropolished into thin films for TEM examinations. Alloy phase identification was carried out by bright field micrograph as well as by selected area electron diffraction patterns.
Mat. Res. Soc. Symp. Proc. Vol. 58.
1986 Materials Research Society
354
RESULTS AND DISCUSSION
a)
Solubility Extension of Solute
Titanium rich terminal compositions of binary Ti alloy systems that can be classified into four different types of phase diagram were investigated as to solid solubility extension in the a phase. The four types of phase diagrams are 1) ý-isomorphous; 2) eutectoid type (at ý-*ctransformation); 3) peritectoid type (at $-cttransformation); 4) monotectic - peritectic type. Figure 1 shows TEM microg
Data Loading...
