Structural properties and stability of metastable phases in the Zr-Nb system: Part I. Systematics of quenching-and-aging
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THE structural properties and the relative stability of the phases formed by alloying the elements of Group IV of the Periodic Table (i.e., Ti, Zr, and Hf) with other transition metals (TMs) is a matter of continuous theoretical and practical interest. These elements present a high-temperature bcc () phase, which transforms upon cooling into an hcp (␣) phase.[1] In their alloys with other TMs, the bcc → hcp transformation occurs upon quenching in a martensitic way, when the content of the alloying element is relatively low. In more concentrated alloys, an alternative diffusionless transformation of the  phase occurs, which originates the so-called “athermal” ⍀ phase. This phase has been the subject of various experimental investigations in the past.[2] More recently, a systematic study of the ⍀ phase and other metastable phases has been carried out, which includes neutron-diffraction (ND) experiments in the prototype systems Zr-Nb and Ti-V,[3–10] as well as calculations of the electronic structure in TMs and ordered alloys.[11,12,13] In particular, in Zr-Nb alloys quenched from high temperatures, the lattice parameters (LPs) of the metastable phases, ␣, , and ⍀ have been determined as functions of composition.[7,8] In this way, a database has been developed, using high-quality ND measurements performed at the Institut Laue–Langevin in Grenoble and structure refinements based on the Rietveld method.[14] The general theme of the present article is the effect of an isothermal heat treatment, often called “aging,” upon those metastable phases that form by quenching. Regarding the ⍀ phase, it has long been accepted that this phase can also be formed isothermally in alloys of Ti, Zr, and Hf.[2] However, the properties and behavior of such “isothermal” ⍀ phases have not yet been established as a function of composition as accurately as those of the athermal ⍀ phase. ´ NDEZ GUILLERMET, G. AURELIO, Graduate Student, and A. FERNA Professor, are with the Centro Atomico Bariloche and CONICET, Bariloche (RN), 8400, Argentina. G.J. CUELLO, Scientist, is with the Institut LaueLangevin, F-38042 Grenoble, Cedex 9, France. J. CAMPO, Researcher, is with the Instituto de Ciencia de Materiales de Aragon, CSIC-Universidad de Zaragoza, 50009 Zaragoza, Spain. Manuscript submitted October 2, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
A key problem is that the ⍀ regions formed upon aging are too small to be studied by standard metallographic techniques. As a consequence, the composition changes that are expected to occur upon aging (Section V) have previously been studied by relying on, e.g., estimates based on LP data from X-ray diffraction experiments.[15–18] On the basis of this indirect information, the possibility of a metastable equilibrium between two phases, viz.,  and ⍀ , has been speculated upon by various authors.[16,19] However, in order to develop a more detailed picture of the actual processes, additional information is needed, in particular, on the reactions also involving the ␣ phase. Motivated by such a lack of i
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