Formation of a shock deformation induced ω phase in Zr 20 Nb alloy
Introduction
Phase transformation studies at high pressures have established that Group IV A metals such as Ti, Zr and Hf exhibit α (hcp) → ω → β (bcc) transitions with increasing hydrostatic pressure [1], [2]. The equilibrium transition pressures at 300 K for the α → ω and the ω → β transitions for Ti, Zr and Hf are indicated in Table 1 [1], [2]. The ω phase is also found at ambient pressure as a metastable phase in alloys of these metals with β stabilizing elements (such as V, Nb, Mo, Ta) in certain composition ranges. Rapid cooling from the β phase field or thermal aging of the metastable β phase can result in the formation of the ω phase. The thermally induced ω phase in the β matrix is invariably distributed as very fine particles in the β matrix with all possible orientation variants being present.
The formation of ω has been observed mainly under three conditions at ambient pressure: (i) rapid cooling from the β phase field leading to the formation of athermal ω [4]; (ii) isothermal aging in the temperature range of 473–773 K leading to the formation of isothermal ω [5]; and (iii) irradiation in the temperature range 303–573 K [6]. The mechanism of transformation is of displacive type in the first case, while in the other two cases diffusional atom movements are involved [7]. However, the lattice collapse mechanism of the β → ω transformation remains operative in all the cases resulting in the observed orientation relation between the β and the ω crystals. The morphology of the ω phase in all the aforementioned instances is of particulate type. In contrast, in the pressure induced α → ω transformation in pure zirconium under static condition the ω phase showed plate shaped morphology with a well-defined habit plane [1], [3], [8]. Microstructural investigations revealed that every plate corresponds to a single ω variant.
The choice of the habit plane in solid state phase transformations is governed by minimization of the interfacial energy and the strain energy associated with the transformation [9], [10]. While in the martensitic transformation, the strain energy contribution dominates and the invariant plane strain (IPS) condition is satisfied, the surface energy term dictates the selection of habit planes in several diffusional transformations. In the latter case the invariant line (IL) vector, lying along the habit plane, defines the primary growth direction [10].
The occurrence of a β to ω transformation has been observed for the first time in a Zr 20 Nb alloy on application of shock deformation and has been reported earlier [11]. The ω phase obtained by application of shock deformation was found to have a plate shaped morphology in this alloy, suggesting the operation of a shear mode. This is in contrast to the particulate ω observed in this alloy after thermal treatment [6]. The observation of the plate shaped ω phase in Zr 20 Nb, exposed to shock deformation, raises the following questions
- (i)
The equilibrium transformation sequence with the increasing pressure being α → ω → β, how does application of shock deformation lead to the β → ω transformation?
- (ii)
Athermal β → ω transformation on quenching invariably results in fine particle morphology with all possible crystallographic variants being present uniformly. Why does shock deformation produce ω plates each plate comprising single orientation variant?
- (iii)
Are ω plates forming under shock deformation resulting from a shear or a shuffle dominated displacive transformation?
The present paper makes an attempt to address these questions.
Section snippets
Experimental
The alloy was made by using high purity Zr and Nb. The elements were melted in an arc furnace. Repeated melting were carried out to ensure homogeneity of the alloy. The arc melted buttons were heat treated at 1173 K for 1 h, followed by a water quench to ensure the retention of the β phase in the alloy. Shock deformation was applied by a gas-gun device. The sample cut into a 5 mm diameter and 0.35 mm thick disc was fitted into a matching hole in a 2 mm thick SS304 circular disc. This disc along
Microstructure of the specimen before pressurization
The nature of the phases present in the β quenched specimens, before application of shock deformation, was investigated by XRD as well as TEM. XRD from the β quenched specimen showed the presence of the β phase alone. Fig. 1 shows a typical selected area diffraction (SAD) pattern taken from a β grain. This as well as other SAD patterns, obtained from different zones and the micrographs taken from the β grains, indicated the presence of the β phase only. Some of these SAD patterns, however,
Discussion
The β → ω transformation in Zr–Nb alloys have been studied in detail and well documented in literature [6], [7]. The key experimental observations of these studies are summarized in the following:
- 1.
The β → ω transformation occurs on rapid quenching in the composition range of 7–18% Nb in Zr–Nb alloys. The transformation cannot be suppressed by quenching. The resultant ω phase always remains finely divided – typically less than 5 nm in diameter. Generally the shape of these ω particles is ellipsoidal.
Conclusion
In pure zirconium, the occurrence of the transformation sequence, α → ω → β under hydrostatic stress is well known. This study shows that the beta phase in the Zr 20 Nb alloy can undergo a transformation to the omega phase when subjected to a shock deformation. Under hydrostatic pressure of up to 8 GPa, this transformation could not be introduced. Under shock deformation, all the variants of the ω phase do not grow. Only those which are favorably oriented with respect to the applied shear stress
Acknowledgements
It is pleasure to acknowledge the benefit of several discussions, which the authors had with Dr. P. Mukhopadhyaya, Dr. D. Srivastava and Dr. P.K. De.
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