Room temperature deformation and mechanisms of slip transmission in oriented single-colony crystals of an α/β titanium alloy

doi:10.1016/S1359-6454(98)00364-4

Acta Materialia

Volume 47, Issue 3, 5 February 1999, Pages 1019-1034

https://doi.org/10.1016/S1359-6454(98)00364-4 Get rights and content

Abstract

Primary creep at low homologous temperatures and low stresses has been widely reported in α/β Ti alloys. Creep in these alloys is dependent on microstructure, with the colony microstructure showing the least creep resistance. There exists a Burgers orientation relationship between the α and the β phase, which has been assumed to allow for easy slip transmission across the α/β interfaces. Constant strain rate and creep tests were performed on single-colony crystals of a near-α alloy oriented for slip along different prismatic slip systems in the α phase. A distinct anisotropy in the deformation behavior of different colony crystals is found. The origin of this anisotropy is due to the relative misalignment of the slip systems between the α and the β phases which results in the formation of residual dislocations during slip transmission. A simple model is presented based on the accumulation of residual dislocations at both the α/β interface and the α matrix, which provides insight into the mechanism of slip transmission, strain hardening and primary creep of these colony structures.

Introduction

Two-phase α/β titanium alloys are used in a wide variety of aerospace, energy and biomedical applications. These alloys exhibit high specific strength, elastic modulus and fracture toughness. In spite of the high strength and high melting temperature of these alloys, it has been widely reported in the literature1, 2, 3, 4, 5, 6, 7, 8, 9that these alloys creep at low homologous temperatures (T/T_m<0.2) and at fractions of the yield strength (∼60% of yield strength). Odegard and Thompson[6]have suggested that a large strain rate sensitivity of these alloys can explain the room temperature creep behavior of these alloys. Another possible explanation for this extensive low-temperature creep is a time-dependent sliding of various interfaces present in these alloys. Ankem and Margolin[10]have shown that α/β interface sliding occurs at room temperature during constant strain rate deformation. Interface sliding along α/β interfaces, grain boundaries and colony boundaries has also been reported during creep deformation by Chakrabarti and Nichols[7]and Miller et al.[8].

A variety of microstructures can be produced in these alloys by using different cooling rates from above the β transus, and low-temperature creep is strongly dependent on the microstructure[8], with the colony microstructure showing less creep resistance than other microstructures that can be obtained through heat treatment. A Burgers orientation relationship (OR) between the α and the β phases has been observed11, 12, 13. The Burgers OR is presumed to allow easy slip transmission between the two phases, making the colony size an important microstructural dimension8, 14, 15. It has been found that slip frequently traverses the length of the colonies, and creep resistance tends to increase as the colony size decreases[8]. Thus, the low-temperature creep strength tends to increase with the following progression of microstructures: colony, Widmanstatten and basketweave7, 8and this has been attributed to the differences in the slip lengths. However, there has neither been a consideration of the actual process of slip transmission in these α/β alloys, nor has its effect on the macroscopic mechanical behavior been fully understood.

The present work focuses on the deformation behavior and microstructural characterization of single-colony crystals of a near-α Ti–5%Al–2.5%Sn–0.5%Fe (Ti-5-2.5-0.5) oriented for slip on different prismatic slip systems in the α phase. An anisotropy in the deformation behavior of individual α/β colonies has been observed. The microscopic features of deformation in these colonies have been characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The various factors contributing to the different interface resistance to slip transmission in the single-colony crystals are analyzed and discussed, and models for the slip transmission across the α/β interface are presented. Finally, a correlation between the macroscopic mechanical properties and the microstructural features is presented.

Section snippets

Experimental procedures

Single-colony crystals of a near-α Ti-5-2.5-0.5 alloy were grown using a float zone technique in a Crystallox furnace using 10 mm diameter rods with typical transverse zoning rates of 1–2 mm/h. These crystals containing a single variant of α in a single crystal of β was then oriented using a Phillips XRG 3000 back-reflection Laue camera operating at 30 kV and 20 mA. Slices of single colonies were cut such that the cut face was close to (0001)_α. These slices were then oriented using Laue

Phase morphology in the single colony crystals

Fig. 1 shows an SEM composite micrograph of a single colony. Since the composition used in the present investigation is a near-α Ti alloy, the majority phase is α with thin laths of β running across the face of the sample. However, the β lath morphology is not plate-like but is rather in the shape of an irregular honeycomb, as seen in the two additional views normal to the (0001)_α face seen in Fig. 1. Fig. 2(a) is a bright field (BF) TEM image of a single β lath taken along [0001]_α beam

Possible sources of anisotropy and strength of colony crystals

The possible sources to explain the anistropy and the strength of the α/β interfaces will be analyzed and discussed in this section. We will attempt to show that based on our experimental results, and present knowledge of the physical properties of the α and β phases, that it is the magnitude and direction of the residual dislocations which accumulate near the α/β interface that are likely to be responsible for the observed anisotropy.

There are reports in the literature[10]that the β phase can

Slip transmission model

In the present study, we have concentrated our efforts on the interaction of the edge segments of dislocation loops with the broad face of the β laths. The interaction of the screw dislocations with the side faces will be considered elsewhere[24]. In the present modeling efforts, it is assumed that [0001]_α is exactly parallel to [101]_β. Thus, we ignore the 0.76° misorientation between these two directions. Clearly this secondary misorientation is present for both OA and OB, and thus is not

Microstructure–mechanical properties correlations

In this section, an attempt is made to correlate the constant strain rate and creep deformation characteristics of the two-colony crystals with the observed and modeled microstructural features. The major differences in the constant strain rate deformation behavior of the two-colony crystals are:

1.
a higher yield strength and a larger strain hardening exponent for colony OB than colony OA (see Fig. 4). In constant load deformation, the differences in the strain hardening for the two-colony

Summary

Single-colony crystals of a near-α Ti alloy were successfully grown using a float-zone technique. The deformation behavior of these colonies when oriented for slip along different prismatic slip systems showed a remarkable anisotropy in properties in compression tests, including yield strength and strain hardening exponent, as well as significantly different creep resistances. No interface sliding was detected in either orientation, although there exists a significant resolved shear stress on

Acknowledgements

This work is supported by the Air Force Office of Scientific Research under the Grant No. F49620-95-1-0153. The authors would also like to acknowledge J.M. Scott at WPAFB, Dayton, Ohio, U.S.A. for help with the growth of the single-colony crystals.

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¹: Present address: Micron Technology Inc., 800 S. Federal Way, Boise, ID 83707, U.S.A.

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Room temperature deformation and mechanisms of slip transmission in oriented single-colony crystals of an α/β titanium alloy

Abstract

Introduction

Section snippets

Experimental procedures

Phase morphology in the single colony crystals

Possible sources of anisotropy and strength of colony crystals

Slip transmission model

Microstructure–mechanical properties correlations

Summary

Acknowledgements

Acta metall.

Metallography

Acta metall.

Acta metall.

Acta metall.

Metal Prog.

J. Mater.

J. Mater.

J. Mater.

Metall. Trans.

Metall. Trans.

Metall. Trans.