Influence of oxygen on phase transformations in a Ti-48 At. pct Al alloy
- PDF / 911,253 Bytes
- 9 Pages / 606.24 x 786 pts Page_size
- 56 Downloads / 207 Views
11/9/03
3:37 PM
Page 2067
Influence of Oxygen on Phase Transformations in a Ti-48 At. Pct Al Alloy WILLIAMS LEFEBVRE, ANNICK LOISEAU, and ALAIN MENAND The composition and structure of Ti-48 at. pct Al alloys with various oxygen contents, quenched from a homogeneous state, have been studied by means of one-dimensional atom-probe (1DAP) and transmission electron microscopy (TEM) analysis. Two regimes are observed. The change from one regime to the other depends on the global oxygen content. If the oxygen content is lower than 1.2 at. pct, the :m massive transformation is involved during the quench. The alloys, hence, exhibit massive m-structure regions and regions having a two-phase (2 ) ultrafine lamellar structure. Very thin 2 plates, saturated with oxygen, are observed in m regions. The precipitation of these 2 plates is promoted by excess oxygen in the m structure. Within ultrafine lamellar-structure regions, oxygen is concentrated in 2 lamellae (not saturated with oxygen) and is found to be responsible for the high volume fraction of 2 phase. When the oxygen content is larger than 1.2 at. pct, the massive transformation is suppressed and the ultrafine lamellar structure is only observed in quenched samples. Analysis of the : 2 chemical ordering in the classical lamellar structure, formed within the ( ) dual-phase field, shows that high oxygen contents favor the chemical-ordering reaction of phase at high temperatures (e.g., 1423 and 1523 K). It has, hence, been inferred that, above 1.2 at. pct O, the : m massive transformation is suppressed and replaced by the : 2 : transformation paths.
I. INTRODUCTION
DEPENDING on the cooling rate from the single -phase field ( is an hcp disordered phase observed at high temperatures), several kinds of phase transformations can be involved in the Ti-48 Al alloy (concentrations are in at. pct unless otherwise mentioned).[1–5] Some of them are described by the continuous-cooling-transformation (CCT) diagram shown in Figure 1. For a slow cooling rate from (curve A in Figure 1), the Ti-48 Al alloy displays a fully (2 ) lamellar structure (designated subsequently as a classical lamellar structure), where 2 is a DO19-ordered phase and g is an L10-ordered phase.[4] For much higher cooling rates (e.g., the oil quench represented by curve B in Figure 1), three phase transformations are involved successively. The first one is the : m massive transformation. Insofar as many studies have been performed on the subject during the last decade, it is now possible to give a schematic description of this transformation. In a first stage, precipitates having a plate shape[10] appear along grain boundaries at an average temperature of 1370 K.[2,11–16] Then, these precipitates rapidly grow in the adjacent grain (i.e., the grain on the other side of the grain boundary) according to the migration of the prior- grain boundaries.[10,17–19] The velocity of m growth WILLIAMS LEFEBVRE, Senior Lecturer, and ALAIN MENAND, Professor, are with t
Data Loading...
