Elsevier

Intermetallics

Volume 18, Issue 4, April 2010, Pages 434-440
Intermetallics

Plastic deformation behavior of Ni3(Ti0.7Nb0.3) single crystals with D019 structure

https://doi.org/10.1016/j.intermet.2009.09.004Get rights and content

Abstract

We have investigated the plastic deformation behavior of Ni3(Ti0.7Nb0.3) crystals with the D019 structure in the temperature range of −196 to 1200 °C. The (0001)<21¯1¯0> basal slip, {11¯00}<112¯0> prism slip, and {2¯111}<2¯116¯> pyramidal slip are operative depending on the temperature and loading orientation. A strong yield stress anomaly (YSA) is caused by slip on the basal plane similar to that observed in other hcp-derivative geometrically close-packed (GCP) compounds such as D0a-Ni3Nb and D024-Ni3Ti, due to the Kear–Wilsdorf (K–W) locking of screw dislocations. The features of YSA by basal slip, such as the profile of the temperature vs. critical resolved shear stress (CRSS) curve are quite similar in those Ni3(Ti0.7Nb0.3), Ni3Nb, and Ni3Ti crystals; nevertheless, the profiles of the temperature vs. CRSS curves observed in the case of prism slip are significantly different for each crystal. The results indicate that in the case of these hcp-derivative GCP compounds, although the K–W locking, which is the origin of YSA, accompanies the thermally activated cross-slip of the screw partial dislocations from the (0001) primary slip plane onto the {011¯0} prism plane, the mobility of dislocations on the prism plane does not directly influence the YSA behavior of basal slip.

Introduction

Ni3X-type intermetallic compounds are known to crystallize in various geometrically close-packed (GCP) crystal structures depending on the type of the X atom. We found that the yield stress anomaly (YSA) appears not only in L12-Ni3Al [1] but also in other GCP compounds with hexagonal close-packed (hcp) structure derivative crystal structures such as D0a-Ni3Nb [2], D024-Ni3Ti [3], and D019-Ni3Sn [4]. Therefore, these GCP compounds also have a possibility as a strengthening phase in Ni-based superalloys. In addition, multiphase alloys composed of the abovementioned GCP compounds have recently been investigated as a new class of high-temperature structural materials [5], [6], [7], [8], [9], [10], [11], [12], [13]. Tomihisa et al. determined the pseudo-ternary phase diagram of Ni3Al–Ni3Nb–Ni3Ti [5]; this phase diagram revealed the existence of the D019-Ni3(Ti0.7Nb0.3) phase. Since the D019 structure is one of the simplest superstructures derived from the hcp lattice, it is expected that the Ni3(Ti0.7Nb0.3) phase plays an important role to carry the ductility in multiphase alloys. In order to verify this assumption, we attempted to identify the operative deformation modes in Ni3(Ti0.7Nb0.3) and examined the temperature dependence of the yield stress by using single crystals.

In this study, we also aim to elucidate the factors controlling the YSA behavior by slip on the close-packed plane in GCP compounds, which is caused by the Kear–Wilsdorf (K–W) locking of screw dislocations. In order to clarify this, we compared the plastic behavior of D019-Ni3(Ti0.7Nb0.3) with those of D0a-Ni3Nb and D024-Ni3Ti, which have analogous GCP crystal structures as shown in Fig. 1. The D019-Ni3(Ti0.7Nb0.3) and the D0a-Ni3Nb have the same two-fold stacking sequence of the close-packed plane (basal plane), but the arrangement of Ti(Nb) atoms on it is different each other; Ti(Nb) atoms show the triangle-type arrangement in the D019 and they show the rectangle-type arrangement in the D0a, respectively. On the other hand, D019-Ni3(Ti0.7Nb0.3) and D019-Ni3Ti have the same close-packed plane with triangle-type arrangement of Ti(Nb) atoms on it, but their stacking sequences are different; two-fold stacking in the D019 and four-fold stacking in the D024 (the so-called dhcp structure). Details on the features of their crystal structures should be referred to in our previous paper [14].

The 1/3<21¯1¯0> superdislocations operative on the (0001) basal plane in those compounds are, in general, dissociated into two 1/6<21¯1¯0> super-partial dislocations bound by an antiphase boundary (APB). We recently determined that the peak stress of YSA in those GCP compounds is controlled by the absolute value of the anisotropy of the APB energy between the slip plane and the cross-slip plane via the change in the locking probability of the screw dislocations [15]. Since the K–W locking process involves microscopic cross-slipping of the screw dislocations, the relation between the mobility of dislocation on the cross-slip plane and the YSA behavior by basal slip is of considerable interest. In order to elucidate this relationship, we compared the plastic behavior of D019-Ni3(Ti0.7Nb0.3) by the operation of basal and prism slips with those of D0a-Ni3Nb and D024-Ni3Ti which have analogous GCP crystal structures.

Section snippets

Experimental procedure

A master ingot composed of Ni3(Ti0.7Nb0.3) was prepared by arc melting under high-purity Ar gas. A single crystal was grown by the floating-zone method at a growth rate of 2.5 mm h−1 under an Ar gas flow. Rectangular specimens with dimensions of approximately 2 × 2 mm2 × 5 mm were prepared for compression tests from the as-grown single crystal. Three orientations, denoted by A, B, and C and depicted as a stereographic projection in Fig. 2, were chosen as loading orientations. The Schmid factors for

Yield stress of Ni3(Nb0.7Ti0.3) single crystal at three loading orientations

Fig. 2 shows the temperature dependence of yield stress (0.2% offset stress) in Ni3(Ti0.7Nb0.3) single crystals at three different loading orientations. An increase in the yield stress with rising temperature, termed YSA, was observed at all of the loading orientations. At A-orientation, the YSA was observed in two different temperature regions having the peak temperatures of 300 °C and 800 °C. The extent of YSA was weak in both temperature regions; the stress increment at both the peak

Discussion

Strong YSA was confirmed to occur by the basal slip, which was due to the exhaustion of mobile dislocations by the occurrence of K–W locking of screw dislocations, similar to those in D0a-Ni3Nb and D024-Ni3Ti with analogous GCP crystal structures. With regard to the controlling factors of the YSA behavior accompanying the K–W locking, we have previously reported that the absolute value of the anisotropy of the APB energy strongly affects the peak stress of the YSA [15]. Another potential

Summary and conclusions

  • (1)

    The (0001)<21¯1¯0> basal slip, {11¯00}<112¯0> prism slip, and {2¯111}<2¯116¯> pyramidal slip are operative in D019-Ni3(Ti0.7Nb0.3).

  • (2)

    Strong YSA appears by basal slip due to the K–W locking of the <21¯1¯0> screw dislocations involving the microscopic cross-slip from the (0001) primary slip plane onto the {011¯0} prism plane, similar to that observed in D0a-Ni3Nb and D024-Ni3Ti with analogous hcp-derivative GCP crystal structures. It was elucidated that the mobility of dislocations on the prism

Acknowledgements

This work was supported by the “Priority Assistance of the Formation of Worldwide Renowned Centers of Research – The 21st Century COE Program and Global COE Program (Project: Center of Excellence for Advanced Structural and Functional Materials Design)” and a Grant-in-Aid for Scientific Research and Development from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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