G.J. Cuello

Structural properties and stability of the bcc and omega phases in the ZrNb system. I. Neutron diffraction study of a quenched and aged Zr-10 wt% Nb alloy

Abstract This paper deals with the composition dependence of the lattice parameters and other structural and stability properties of the bcc and omega phases in the Zrue5f8Nb system. Our approach combines a neutron diffraction measurement on a Zr-10 wt% Nb alloy with the assessment of previously published experimental data. Measurements are reported for the alloy in both the as-quenched condition and after an aging treatment at 673 K. The composition changes in bcc and omega occurring during the aging are examined. In addition, a critical analysis is presented of the existing estimation method for composition which makes use of information about the lattice parameters. The new experimental data obtained in this work are also used in comparisons with previous studies of bcc and omega based on X-ray diffraction methods. The existence of some significant discrepancies on the structural parameters of the omega phase is discussed.

Structural properties of metastable phases in Zr–Nb alloys: I. Neutron diffraction study and analysis of lattice parameters

Abstract This paper reports the results of a neutron diffraction study of a series of Zr–Nb alloys quenched from high temperatures, where bcc (β) is the stable phase. Upon quenching, two additional structures are formed and retained metastably, viz. the hcp (α′) phase, which forms through a martensitic transformation, and the omega (ω) phase, which forms by a displacive transformation involving the collapse of (111) bcc planes. The lattice parameters (LPs) of the β, α′ and ω phases are determined as functions of the Nb content in the composition interval 4≤at.% Nb≤31, which represents a significant expansion of the experimental database for these various metastable structures. With the present data, reliable trends in the composition dependence of the LPs are established. In general, a smooth, essentially linear dependence upon the Nb content is detected, corresponding to Vegard’s law. However, a striking deviation from this simple behaviour is detected in the a ω parameter for Zr-rich alloys, i.e. in the region where this LP is directly related to the shortest interatomic distance (ID) in the ω phase. The need for a new analysis of the composition dependence of the various IDs in the ω phase is emphasized.

Atomic ordering and systematics of bonding lengths in the Ti–V omega phase: a neutron diffraction study

Abstract A new model describing the structural and bonding properties of the omega (Ω) phase in Zr–Nb alloys has recently been presented [12] . This model, which was aimed at explaining the composition dependence of the bonding lengths, predicts that the Ω phase is ordered, i.e., that some crystallographic sites are preferentially occupied by Zr atoms. Such feature, which should in principle be observed in other, related Ω phases, has not yet been tested against direct measurements. This problem has now been studied in the Ti–V system, which is the analogue of Zr–Nb in the 3d-transition series. Neutron diffraction measurements have been performed in quenched Ti–V alloys with V contents between 14 and 17 at.%. The diffraction spectra have been analysed using the Rietveld method, and a systematic analysis is reported here of the possibility of deviations from the random occupation of the two sublattices which are distinguished in the Ω structure. In addition, these new diffraction data are used in an evaluation of the shortest interatomic distances which are relevant for a comparison with the predictions of the model of Grad et al.

Systematics of lattice parameters and bonding distances of the omega phase in ZrNb alloys

Abstract Neutron-diffraction measurements and experimental data from the literature are combined in a study of characteristic changes in the structural parameters a ω , c ω and z ω , of the omega phase in the Zr Nb system, and in systematizing relations between the omega and the parent BCC phases. The interatomic distances in the omega phase reveal remarkable regularities in their composition dependence, which point to peculiarities in the bonding characteristics of this phase.

Crystal Structure of Cu-Sn-In Alloys Around the η-Phase Field Studied by Neutron Diffraction

Study of the Cu-Sn-In ternary system has become quite important in recent years, due to new environmental regulations increasingly restricting use of Pb for bonding technologies in electronic devices. A key relevant issue concerns the intermetallic phases which grow in the bonding zone and strongly affect its quality and performance. In this work, we focus on the η-phase (Cu2In or Cu6Sn5) that exists in both end binaries and as a ternary phase. We present a neutron diffraction study of the constitution and crystallography of a series of alloys around the 60xa0at.% Cu composition, and with In contents ranging from 0xa0at.% to 25xa0at.%, quenched from 300°C. The alloys were characterized by scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and high-resolution neutron diffraction (ND). Rietveld refinement of ND data allowed improvement of the currently available model for site occupancies in the hexagonal η-phase in the binary Cu-Sn as well as in ternary alloys. For the first time, structural data are reported for the ternary Cu-Sn-In η-phase as a function of composition, information that is of fundamental technological importance as well as valuable for ongoing modeling of the ternary phase diagram.

Structural properties of metastable phases in Zr-Nb alloys. III. The athermal Ω phase

Abstract By means of high-resolution neutron diffraction experiments, a new study of the lattice parameters and other structural properties as well as the mass fraction of the Ω phase formed by quenching Zr–Nb alloys, has been made. With the present results and those from the recent neutron diffraction work by Benites and co-workers (J. Alloys Comp. 299 (2000) 183; 302 (2000) 192) reported on papers I and II of this series, an analysis is performed of the composition dependence of various key aspects of the diffusionless (‘athermal’) Ω phase transition in the Zr–Nb system. In particular, we discuss the misfit between the Ω particles and the parent bcc phase, the hexagonal to trigonal symmetry change occurring in Ω and the effect of the latter upon the shortest interatomic distance in this phase. The picture emerging from the present study is also used to investigate the interatomic distance correlation previously reported by Grad et al. (Z. Metallkd. 87 (1996) 726).

Analysis of multiple scattering and multiphonon contributions in inelastic neutron scattering experiments

Abstract We present a method of analysis of inelastic neutron scattering (INS) experiments aiming at obtaining the density of phonon states in an absolute scale, as well as a reliable value of the mean-square displacement of the atoms. This method requires the measurement of the neutron total cross section of the sample as a function of energy, which provides a normalization condition for the INS experiment, as well as a value of the mean-square displacement. The method is applied in the case of an incoherent neutron scattering system, viz. the Ti–52wt.% Zr alloy. The applicability of this method to the study of metal alloys and other systems is discussed.

Structural Properties and Stability of Metastable Phases in the Zr-Nb System

The lattice parameters of the bcc (b) and omega (W) phases occurring metastably in a series of Zr-rich Zr-Nb alloys have been determined at and above room temperature (TR) using neutron diffraction techniques. In the first place, the effect of temperature changes upon the lattice parameters of the b and W phases in alloys with 10 and 18 at.% Nb was monitored using neutron thermodiffraction. A method of analysis is applied to the data which involves a confrontation between the observed structural properties and an idealised -or ``reference- behaviour (RB) which admits a simple mathematical description. A generalised form of Vegards law is adopted as RB for the b phase, whereas a specific RB is proposed for the W structure. The experimental data are well accounted for by this interpretation scheme, leading to a picture of the isothermal reactions occurring at high temperature which involves the transfer of Nb from the W to the b phase. Finally, the neutron diffraction data on the W phase are combined with an electron microscopy study for the alloy with 10 at.% Nb aged at 773 K, which provides information on the composition of this phase and its evolution towards thermodynamic equilibrium.