Study of texture evolution in metastable β-Ti alloy as a function of strain path and its effect on α transformation texture

doi:10.1016/j.msea.2008.11.053

Materials Science and Engineering: A

Volume 504, Issues 1–2, 25 March 2009, Pages 24-35

https://doi.org/10.1016/j.msea.2008.11.053 Get rights and content

Abstract

Texture evolution in a low cost beta titanium alloy was studied for different modes of rolling and heat treatments. The alloy was cold rolled by unidirectional and multi-step cross rolling. The cold rolled material was either aged directly or recrystallized and then aged. The evolution of texture in α and β phases were studied. The rolling texture of β phase that is characterized by the gamma fiber is stronger for MSCR than UDR; while the trend is reversed on recrystallization. The mode of rolling affects α transformation texture on aging with smaller α lath size and stronger α texture in UDR than in MSCR. The defect structure in β phase influences the evolution of α texture on aging. A stronger defect structure in β phase leads to variant selection with the rolled samples showing fewer variants than the recrystallized samples.

Introduction

It is well known that alloying titanium with elements like Nb, Mo, Ta, V, Hf etc leads to stabilizing the β phase at room temperature, and hence the so-called metastable β-Ti alloy. Upto a certain amount of β stabilizer content, the β phase remains metastable, which upon aging at a suitable temperature leads to the formation of α phase. These metastable β-Ti alloys, characterized by high specific strength, sufficient ductility and fracture toughness along with excellent corrosion resistance, find extensive use in structural and biomedical applications. The most important feature of these alloys which makes them suitable for various applications is that a balance of various properties can be achieved by subjecting them to proper thermomechanical treatments. However, these alloys suffer from a serious limitation of high cost [1], due to titanium itself plus alloying elements like vanadium to stabilize the β phase at room temperature that renders them unsuitable for non-aerospace applications. The full potential of these alloys has not yet being exploited, particularly in the automobile industry, due to their high cost and hence there has been a motivation for developing low cost β titanium alloys. One such β alloy TIMETAL LCB^® has been developed by replacing the costly vanadium with cheap and easily available Fe–Mo and Fe–V masteralloys which are widely used in steel industry. Processing of metastable β-Ti alloys as needed to fabricate components, involves deformation plus thermal processing. Both these processes lead to modification in crystallographic texture.

It is well known that the β phase of metastable β-Ti alloys transform to (α + β) structure. During phase transformation from β to α phase, Burger's orientation relationship [2] is generally followed, i.e. ${0 0 0 1}_{α} ∥{〈1 0 1〉}_{β} and {1 1 \bar{2} 0}_{α}∥ {〈1 1 1〉}_{β}$ , however due to symmetry of the crystal, a single β orientation gives rise to 12 equivalent α orientations with equal probability [3]. Under certain thermal and thermomechanical treatments, it is possible to get higher probability for certain α orientation, rather than all orientations being equally probable. This phenomenon is known as Variant selection. This effect which comes into play at the crystallographic scale, also affects the morphology of the transformed α phase. Since texture plays an important role in determining mechanical properties of HCP structured (α) Ti and Ti alloys due to the inherent anisotropy of HCP crystal lattice, the knowledge of β to α transformation is essential for processing property optimization.

A number of comprehensive studies have been carried out on the evolution of texture during hot deformation or heat treatment in β phase of (α + β) [4], [5], [6], [7], [8], [9], [10], [11], [12], [13] or (α₂ + β) [14], [15], [16], [17], [18] titanium alloys and the evolution of corresponding α transformation texture. However, in (α + β) Ti alloy, the high temperature β phase cannot be retained at room temperature; therefore, the experimental techniques are not free from limitations. In the (α₂ + β) alloys although, the high temperature β phase can be retained, there is a limited scope in changing the texture of high temperature β phase. Therefore, various aspects of evolution of transformation texture are yet unclear. Particularly the effect of mode of prior β-deformation texture on the transformation texture has been little investigated. The present work is aimed at examining the response of different β textures as obtained by cold rolling and recrystallization on the texture evolved during subsequent operations. The focus of this study is to examine the influence of initial β texture as well as microstructure on the product α phase, isolating the effect of texture from the effect of microstructure keeping the defect structure approximately similar yet the crystallographic texture reasonably different. This could be achieved by following different modes of cold rolling, for example; unidirectional rolling, cross rolling etc. Cold rolling texture resulting from such varied rolling schedule has been studied for many close packed metals and alloys in the recent past [19], [20], [21], [22]. However, such studies are limited on BCC materials [23]. The present research program addresses four important aspects:

(i)
Texture evolution during different modes of cold rolling in a BCC material.
(ii)
Effect of different cold rolling BCC textures (unidirectional and cross rolling) and corresponding β to α (HCP) transformation texture.
(iii)
Effect of different defect structures (rolled and recrystallized) on β to α (HCP) transformation texture.
(iv)
Overall assessment of texture evolution during processing of β-Ti alloy.

The low cost β Titanium alloy (Ti–10V–4.5Fe–1.5Al) was chosen for this study because it gives cost advantage compared to other alloys.

Section snippets

As-received material

The low cost β Ti-alloy (hereafter referred to as Ti LCB) with nominal composition Ti–10V–4.5Fe–1.5Al was obtained from Defence Metallurgical Research Laboratory, Hyderabad, India. The composition of the alloy was re-examined using Energy Dispersive Spectrometry (EDS) which indicated the actual composition to be 84.15% Ti, 9.82% V, 4.22% Fe and 1.8% Al (in atomic %). The initial material was cast and hot forged in β phase field (900 °C) that finally resulted in an average grain size of about 175

As received

The (1 1 0) pole figure for the starting material (Texture Index TI = 1.567) is shown in Fig. 4. The pole figure shows spread of orientations along with a strong (1 1 0) $〈2 \bar{2} 1〉$ and (1 0 0) $〈0 1 \bar{4}〉$ component. Moreover, the intensities of ${\bar{1} \bar{1} 3}$ $〈\bar{1} 4 1〉$ and (1 0 1) $〈1 \bar{1} \bar{1}〉$ components were substantially high.

Deformation texture

Texture of as-rolled materials is shown in Fig. 5(a–d). It is to be mentioned here that texture of a BCC material is largely depicted through φ₁ = 0° and 90° sections as well as φ₂ = 45° sections. The

Discussion

Results of the present investigation show that prior deformation through different strain path has a profound effect on the evolution of texture in the β phase. The microstructure (substructure) has important consequence on recrystallization as well as on the subsequent transformation texture in α phase. Each of these aspects has been discussed in details in the following sub-sections.

Conclusions

1.
Cold rolling texture of β titanium alloy is characterized by the presence of a strong gamma fiber ranging from {1 1 1} $〈1 1 0〉$ to {1 1 1} $〈1 1 2〉$ . Texture is stronger in MSCR when compared to UDR and this is attributed to the inhomogeneous deformation taking place in the latter that contributes to weakening of the texture.
2.
The texture of recrystallized β phase qualitatively remains the same and in fact strengthens on recrystallization for UDR and MSCR sample. The strengthening is more in UDR and it

Acknowledgement

The authors express their sincere acknowledgement to Titanium Alloy Group, DMRL, Hyderabad, for providing the material. Thanks are due to Dr. Amit Bhattacharjee, Titanium Alloy Group, for his help at various levels. The microtexture studies were carried out using the SEM facility at Institute Nano-Science Initiative, Indian Institute of Science, Bangalore, for which we acknowledge the DST-FIST grant. Ashkar Ali A acknowledges the grant by Indian Institute of Science, Bangalore, under YEFP

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Study of texture evolution in metastable β-Ti alloy as a function of strain path and its effect on α transformation texture

Abstract

Introduction

Section snippets

As-received material

As received

Deformation texture

Discussion

Conclusions

Acknowledgement

Physica

Scripta Mater.

Scripta Mater.

Acta Mater.

Acta Mater.

Mater. Sci. Eng.

Acta Mater.

Mater. Sci. Eng.

Acta Mater.

Acta Mater.

Scripta Mater.

Mater. Sci. Eng.

Mater. Sci. Eng. A