Precipitation Reactions in Al-Zr-Alloys
- PDF / 1,625,522 Bytes
- 5 Pages / 415.8 x 637.2 pts Page_size
- 34 Downloads / 229 Views
PRECIPITATION REACTIONS IN AI-Zr-ALLOYS
T. Haugan, E. Nes, and N. Ryum The University of Trondheim, The Norwegian Institute of Technology, Division of Physical Metallurgy, 7034 Trondheim-NTH
ABSTRACT A new mode of precipitation of the metastable AI 3 Zr in Al-Zr-alloys has been investigated. The salient aspects of this precipitation reaction are: (i) The formation of precipitate configurations of a well defined macroscopic morphology, (ii) Each individual precipitate within the colonies grows in crystallographic directions, and (iii) The absence of a high angle grain boundary as a reaction front.
INTRODUCTION During the last decade there has been a great interest in the effect of small additions of Zr on the properties of aluminium alloys. Improvements in strength and ductility, in fatigue properties, resistance to recrystallization and stress-corrosion [1,2,3j have been observed in the heat-treatable Al-Mg-Zn- and Al-Mg-Cu-alloys and in the none-heat-treatable Al-Mg-alloys, while no improvements have been found in the Al-Mg-Si-alloys. The beneficial effects can be attributed to the precipitation in the solid state of finely dispersed Zr-rich particles with high termal stability [4]. A prerequisite for the effect is that a sufficient high supersaturation of Zr can be obtained in the alloys. The equilibrium solubility of Zr in pure Al is fairly low, being 0.28 wt% at the peritectic temperature, where the equilibrium phase is the tetragonal AI 3 Zr (8). The solubility of Zr in the alloys mentioned
above have not been measured systematically.
It
has
been known for
a long time [5] that the solid solubility of the transition elements in Al can easily be extended by a rapid solidification rate. Recently it has been observed that even though the equilibrium 8-phase may be effectively suppressed by rapid solidification, another coarse and Zr-rich phase may form that may reduce or eliminate the effect of Zr [6,7,81. Very similar particles have also been found in Al-Hf-alloys [9]. The particles have, due to their peculiar forms been referred to as sponge-particles or petal-like constituents and their inner structure and morphology have been described by several authours [8,9]. Under what conditions and by what mechaNes and Billdal [a] nisms they form have not been studied in much detail. analyzed some of their results according to a model developed by Kerr et. al. [I0" for similar reactions in Al-Ti-alloys and concluded that the spongeparticles most probably are formed in the liquid. Their investigation was mainly a detailed structural description of the particles, and their results were not conclusive as regards the way in which the particles are formed. The present investigation was undertaken to establish more firmly under what conditions and in what way the sponge-particles are formed.
16]
[71,
EXPERIMENTAL AND RESULTS 0
An AI-1.4 wt% Zr-alloy was cast from 1000 C into a wedge-shaped Cumould. The temperature was measured by small thermocouples placed along the center-line of the mould, as shown on Fig. 1 a).
Data Loading...
