Influence of the high pressure torsion die geometry on the allotropic phase transformations in pure Zr

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Abstract

The effect of the press geometry in the α to ω + β transformation in pure Zr by high pressure torsion (HPT) was investigated. Specimens were processed in constrained and unconstrained setups using a wide range of applied pressures and 5 anvil turns. The resulting microstructures were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM and HRTEM) and electron backscatter diffraction (EBSD). Microhardness distributions were measured for each condition. The transformation kinetics and the stability of the fabricated nanostructures are enhanced when using an unconstrained setup. In the samples processed by constrained HPT the full transformation does not take place even at the highest pressures applied. Additionally, post-processing room temperature grain growth of some of the remaining nanocrystalline α-Zr grains with (0 0 0 1) orientations occurs, leading to a significant decrease in hardness. HRTEM allowed confirming the presence of pure β-Zr.

Introduction

High pressure torsion (HPT) is a severe plastic deformation processing technique that consists of applying simultaneously compression and shear strains to a disk [1]. Fig. 1 illustrates the two main HPT setups used in the present work, denominated unconstrained and constrained, respectively. In both cases the HPT press is formed by two anvils: the upper one is fixed and the lower one is free to rotate. In the unconstrained configuration (Fig. 1a) the surfaces of the two anvils are flat, and thus the material is free to flow outwards during torsional straining. As a result, disks processed by unconstrained HPT are thinned down to a thickness as small as about 0.2 mm. In the constrained die-set (Fig. 1b) the disk sample is fitted into a cavity in the lower anvil which does not allow as much outward flow of material during pressing. Thus, processing takes place in the presence of a back-pressure. Both setups can be utilized to apply severe plastic strains, and thus are used as tools to refine the grain size of materials below 100 nm [2]. It is well known that the applied pressure and the number of turns (i.e., the applied shear) influence the grain size, the hardness and the homogeneity of the resulting microstructure [1]. However, the effect of the HPT die geometry (constrained vs. unconstrained) is still widely unknown.

Although the principles of high pressure torsion were laid out as early as the 1950s [3], its widespread use by the scientific community to fabricate nanocrystalline materials took place mainly in the last decade [1]. Since then, HPT has been utilized successfully to nanostructure a vast number of materials: cubic metals such as Al and Al alloys [4], [5], [6], [7], [8], [9], Ni [10], [11], [12], Cu [13], [14], [15], steels [16], [17], [18], [19], W [20] and Pd [21] and, albeit to a lesser extent, hcp metals such as Mg [22], [23], [24], Ti [25], [26], [27], [28], [29], [30], [31], and Zr [32], [33], [34]. The versatility of HPT has increased over the years, as novel uses of this technique were gradually discovered. Among them, the fabrication of high strength materials and composites by the cold consolidation of powders [35], [36], [37], [38], [39], [40], [41], the controlled crystallization of amorphous materials [42] or the fabrication of metallic glasses [43]. Furthermore, it has been recently discovered that the combined application of pressure and shear by HPT also enhances the kinetics and the hysteresis of high pressure phase transformations, allowing high pressure phases to be retained at room temperature and ambient pressure [28], [30], [32], [33]. The application of HPT as a technique to induce phase transformations is still in its infancy.

Zirconium belongs to group IV transition metals and its alloys have traditionally found applications in the nuclear industry as fuel rod materials [44], [45]. Pure Zr has also a biocompatibility that is comparable to that of Ti and thus it has been proposed as an alternative biomaterial [46]. Pure Zr, as the other group IV transition metals, namely Ti and Hf, undergoes a wide range of phase transformations when subjected to high temperatures or high pressures [44], [45]. Under ambient conditions, the stable phase (α) has a hexagonal close packed structure. At temperatures higher than 1135 K it transforms into a bcc phase (β). When the α phase is subjected to hydrostatic pressure it transforms into a simple hexagonal structure (ω) at applied pressures ranging from 2 to 6 GPa and, finally, to the bcc β phase when the applied pressure exceeds 30 GPa. The variation of the lattice parameters of these three phases with temperature and pressure is given in [47]. A full α to ω transformation was only achieved when pressurizing pure Zr at elevated temperatures for long periods of time. Vohra and coworkers [48] synthesized bulk ω-Zr by applying a pressure of 4.5–6.5 GPa for 56 h and raising the temperature to 350 °C for 24 h of this period. The α to ω transformation was found to undergo some hysteresis, and thus the ω phase could be retained, at least partially, upon unloading. β-Zr is known to have a high superconducting temperature [49], [50] and may have also potential as a biomaterial [51]. However, stabilizing bcc (β) pure Zr under ambient conditions by conventional methods was not possible as the β to α or β to ω back transformations take place, respectively, when the temperature or the applied pressure decrease. So far the only possibility to fabricate bcc Zr at room temperature and atmospheric pressure was to alloy pure Zr with relatively large quantities of β-stabilizers [44].

It was recently discovered that high pressure torsion allowed to stabilize nanocrystalline ω + β pure Zr under ambient conditions [32], [33]. The shear strain imposed enhances the transformation kinetics, as first pointed out by Zilbershtein [52], reducing the transition applied pressure and the time required for the full transformation to occur. For example, a full α to ω + β transformation was observed after applying 6 GPa and 5 anvil turns at a speed of 1 revolution per minute (i.e., the processing lasted 5 min) at room temperature in an unconstrained press [33]. The volume fraction of transformed phase was found to increase with increasing applied pressure and strain [34]. Alpha to ω shear-induced transformations by HPT were observed also recently in Ti [28], [30], [31]. The microscopic mechanisms responsible for these shear-induced phase transformations and for the stability of the high pressure phases under ambient conditions, as well as the influence of the parameters of the HPT processing on the resulting microstructures, are still not clear.

This work aims to investigate the influence of the HPT die geometry (constrained vs. unconstrained) in the microstructure and in the hardness distribution of pure Zr. This material is processed at room temperature using both setups and a wide range of applied pressures. The resulting microstructures are examined by X-ray diffraction, conventional and high resolution transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). The micromechanisms responsible for the α to ω + β allotropic transformation in pure Zr during HPT processing will be critically examined on the light of these findings.

Section snippets

Experimental procedure

The material studied here is commercially pure (99.8%) alpha-Zr (Fe: 330 ppm; Mn: 27 ppm; Hf: 452 ppm; S: <550 ppm; Nd: <500 ppm). Several (5.2 mm × 140 mm × 200 mm) slabs of this material, with an equiaxed grain size of 13 μm, were purchased from Haines and Maassen (Bonn, Germany). Disks of 10 mm diameter and 2 mm in thickness were cut out of the as-received slabs and processed by high pressure torsion (HPT) in both constrained and unconstrained die-sets (Fig. 1) at room temperature, using applied pressures

X-ray analysis of the α to ω + β transformation by HPT in pure zirconium

Fig. 2 shows the X-ray diffractograms corresponding to the disks processed by HPT at room temperature using applied pressures of 1, 2, 3, and 6 GPa and 5 anvil turns in both unconstrained (Fig. 2a) and constrained (Fig. 2b) die-sets. All samples were kept for 6 months at room temperature before characterization. Selected peaks belonging to the α, ω and β phases are indicated. For the sake of clarity not all the peaks have been indexed, but all have been identified as belonging to the three

Discussion

The present results demonstrate that the geometry of the HPT setup affects significantly the resulting microstructure. Fig. 8 is a schematic that summarizes the microstructural evolution of pure Zr processed using unconstrained and constrained setups at the pressure range investigated here. White hexagons denote alpha grains and gray ones represent omega/beta grains. The starting material is coarse grained alpha-Zr (Fig. 8a). We observe, firstly, that the α to ω + β transformation takes place at

Conclusions

It is known that pure α-Zr transforms into ω + β-Zr when processed by HPT under certain conditions of applied pressure and shear. Here, pure alpha-Zr has been processed by constrained and unconstrained high pressure torsion using applied pressures ranging from 1 to 6 GPa and, in all cases, 5 anvil turns with the aim of studying the effect of the HPT die geometry on the allotropic phase transformation. The microstructure of the processed samples has been examined by X-ray diffraction, transmission

Acknowledgments

Funding from CICYT under projects MAT 2006-11202, CCG07-CSIC/MAT-2270, MAT2009-07078-E, and MAT 2009-14547-C02-01 is acknowledged. APZ thanks the Spanish Ministry of Science and Innovation for an R&C contract. The authors are grateful to the CAI Difracción de Rayos X and the “Luis Brú” National Center for Microscopy at the UCM. Assistance from María José Vera and Bruno Romero is greatly appreciated. This paper was written in the city of Hamburg and it is dedicated to Martin Huebner.

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