Phase-Separations of Coherent Precipitates of Ordered Phases in Elastically Constrained Alloys
- PDF / 353,863 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 110 Downloads / 197 Views
0980-II05-26
Phase-Separations of Coherent Precipitates of Ordered Phases in Elastically Constrained Alloys Minoru Doi, Hiroshi Kumagai, Kanako Nakashima, and Takao Kozakai Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, 466-8555, Japan
ABSTRACT Phase-separations of coherent precipitates of ordered phases were investigated by means of transmission electron microscopy (TEM) and the theoretical analysis from a thermodynamic point of view. When the two-phase microstructures of A1(γ)+L12(γ’) in Ni-Al-Ti and Ni-Si-Fe alloys are isothermally heated inside the two-phase region of A1+L12 , coherent L12 precipitate particles sometimes exhibit a phase-separation and A1 phase newly appears and grows in L12 particles. Phase-separations of the same type as the above are also observed in coherent twophase microstructures of A2+D03 and A2+B2 in Fe-Si-V and Fe-Al-Ni alloys respectively: coherent D03 or B2 precipitates exhibit a phase-separation and A2 phase newly appears and grows in both precipitates. Such phase-separation is realized under the influence of chemical free energy and elastic energies. INTRODUCTION Many of the properties of materials are originated from the microstructure. In particular, the useful properties of phase-separated alloys are closely related to the morphology of coherent second-phase particles. Such two-phase alloys are, in general, elastically constrained to some extent due to the lattice mismatch between two phases. A number of researchers have pointed out the importance of the effects of elastic energies on the morphology of particles [1-4]. The present authors’ group has studied the effects of elastic energies originated from the lattice mismatch on the two-phase microstructures: e.g. γ(A1)/γ’(L12) in Ni-base alloys, A2/D03 in Fe-base alloys, αTi(A3)/Ti3Al(D019) and TiAl(L10)/Ti3Al5(P4/mbm) in Ti-Al system, the modulated structure formed by spinodal decomposition in ZrO2-Y2O3 alloy, etc. [2,5-7] (in this paper, the expression of M/P is used to differentiate the second (particle) phase (P) from the matrix phase (M) in a two-phase microstructure of M+P). Among the interesting phenomena characteristic of elastically constrained systems, seemingly most strange phenomena are the split of a single particle into a doublet or an octet of particles [8,9] and the decelerated coarsening of particles [10], which can never be understood until the elastic energies are considered as part of the driving force. In the present studies, the microstructure changes due to heat-treatments of coherent twophase microstructures of A1+L12 , A2+B2 and A2+D03 in elastically constrained Ni-base and Fe-base ternary alloy systems were observed with transmission electron microscopy (TEM) and theoretically interpreted from the thermodynamic point of view.
EXPERIMENTAL DETAILS Ni-8.5Al-5.4Ti, Ni-13Si-3.1Fe, Fe-16.2Si-13.3V and Fe-15Al-10Ni alloys (each numerical value indicates mol%) were prepared with arc melting. Alloy sheets or disks were homogenized at 1473 K foll
Data Loading...
