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首页> 外文期刊>Scripta materialia >BONDING-INDUCED ATOMIC ORDERING IN THE ATHERMAL #OMEGA# PHASE: NEUTRON DIFFRACTION TEST AND CONSEQUENCES UPON MODELS OF THE BOND-LENGTH SYSTEMATICS IN ALLOYS
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BONDING-INDUCED ATOMIC ORDERING IN THE ATHERMAL #OMEGA# PHASE: NEUTRON DIFFRACTION TEST AND CONSEQUENCES UPON MODELS OF THE BOND-LENGTH SYSTEMATICS IN ALLOYS

机译：原子＃OMEGA＃相中键合诱导的原子有序：合金中键长体系的中子衍射测试及相关模型的结果

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摘要

The name omega (#OMEGA#) phase refers to an AlB_2-type structure which appears as a high-pressure modification in the elements of the Ti group, i.e., Ti. Zr, and Hf (1,2). In relatively dilute alloys of Ti. Zr and Hf with other transition metals, this phase can be formed by a diffusionless transformation and retained metastably at room temperature and atmospheric pressure by quenching the bcc structure, which is usually the stable high-T phase in these alloy systems (3). Recently, there has been a renewed interest in the characterisation of such quenched ("athermal") #OMEGA# phase in the Zr-Nb system (4.5,6.7.) and in the interpretation of the structural data in terms of theoretical (4.5,6.7.8) and phenomenological (9,10,11,12,13) pictures of the chemical bonding in this phase.The crystallography of the 2 phase and its relation with the bcc phase have been reviewed by Sikka et at (14) and in previous works by us (15,16,17). Here we shall only summarise the main points. The #OMEGA# and bcc phases can be described by an hexagonal lattice with three atoms per unit cell (18). The atomic positions are (0,0,0), (1/3,2/3,1/3 + z) and (2/3,1/3,2/3 - z) where z (0 <= z <= 1/6) is a parameter that allows a continuous description of the bcc percent->#OMEGA# transformation, z = 0 corresponding to the bcc phase percent (19). In this transformation two thirds of the (111)(bcc) layers collapse into double layers and the other third remains as single ones. If the collapse of the (111)_(bcc) layers is incomplete we have a trigonal symmetry, with 0 < z < 1/6, and the space group is P percent~- 3m1. When z = 1/6 we have the AlB_2-type, often called "ideal" #OMEGA# phase, with the inner atoms collapsed into the same plane (Fig. 1), which corresponds to an hexagonal structure, described by the space group P6/mmm. Two kinds of crystallographic sites can be distinguished in the #OMEGA# structure, with different sizes and coordination numbers. The site at (0,0,0) will be referred to as A-type, whereas those at (1/3,2/3,1/3 + z) and (2/3,1/3,2/3 - z) will be called B-type sites, as indicated in Fig. 1. The atomic environment of the A-type site in the #OMEGA# unit cell suggests (20) that this site is more suitable for a large atom, whereas the remaining B sites are more suitable for smaller atoms, as in the analogous AlB_2 (C32) type-structure adopted by metallic diborides (21,22).
机译：Ω相（＃OMEGA＃）是指AlB_2型结构，在Ti族元素即Ti中表现为高压变体。 Zr和Hf（1,2）。在相对稀薄的钛合金中。 Zr和Hf与其他过渡金属可通过无扩散转变形成，并通过淬灭bcc结构（在这些合金体系中通常是稳定的高T相）在室温和大气压下稳定地保留。最近，人们对Zr-Nb系统中这种淬灭的（“无热”）＃OMEGA＃相的表征（4.5,6.7。）以及从理论上解释结构数据（4.5， （6.7.8）和该相化学键合的现象学（9,10,11,12,13）图片.Sikka等人在（14）和（4）中回顾了2相的晶体学及其与bcc相的关系。在我们以前的作品中（15,16,17）。在这里，我们仅总结要点。 ＃OMEGA＃和bcc相可以由每个晶胞具有三个原子的六边形晶格描述（18）。原子位置是（0,0,0），（1 / 3,2 / 3,1 / 3 + z）和（2 / 3,1 / 3,2 / 3-z），其中z（0 <= z <= 1/6）是允许连续描述bcc百分比->＃OMEGA＃转换的参数，z = 0对应于bcc相位百分比（19）。在此转换中，（111）（bcc）层的三分之二折叠成双层，其余三分之一保留为单层。如果（111）_（bcc）层的塌陷不完全，则我们具有三角对称性，其中0

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