Obtaining of a fine near-lamellar microstructure in TiAl alloys by Spark Plasma Sintering

doi:10.1016/j.intermet.2016.01.003

Intermetallics

Volume 71, April 2016, Pages 88-97

https://doi.org/10.1016/j.intermet.2016.01.003 Get rights and content

Highlights

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A fine lamellar microstructure is obtained by Spark Plasma Sintering with the IRIS alloy (Ti_49,92Al₄₈W₂B_0,08).
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The control of the grain growth is due to the formation of β borders at the periphery of grains at high temperature and to the incorporation of Boron.
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The formation mechanism involves the precipitation of γ lamellae in α grains and that of γ grains and secondary β₀ precipitates in borders.

Abstract

This work presents a study of the Spark Plasma Sintering of a boron and tungsten containing alloy (Ti_49,92Al₄₈W₂B_0,08, called IRIS) as a function of the sintering temperature. Microstructures of sintered alloys are analyzed by scanning and transmission electron microscopies. Investigations mainly focus on a fine near-lamellar microstructure. Attention is paid to both characteristic dimensions of this microstructure and orientation relationships between various phases.

The fine near-lamellar microstructure is formed by lamellar grains surrounded by extended γ zones containing β₀ precipitates. The size of lamellar grains ranges from 35 to 45 μm while the width of the borders remains between 5 and 10 μm. Effects of both boron addition and sintering temperature are studied. Orientation relationships between γ lamellae and γ grains in the borders, as well as between the γ matrix and the β₀ precipitates are investigated. As a result of these investigations, a formation mechanism of this microstructure is proposed and discussed. The origin of the grain growth limitation during the SPS processing is particularly analyzed.

Graphical abstract

Fine near-lamellar microstructure (a) obtained by Spark Plasma Sintering with the Ti49,92Al48W2B0,08 alloy and its formation mechanism (b to e).

Introduction

The aim of the present work is to use the Spark Plasma Sintering (SPS), a powder metallurgy route, to obtain a TiAl alloy exhibiting balanced mechanical properties; i.e., an acceptable ductility at room temperature and a high strength at high temperatures. As powder metallurgy provides homogeneous and non-textured microstructures [1], the option we have chosen was to obtain a fine and resistant lamellar microstructure. “Fine” grains, which refers to grains smaller than 50 μm, are indeed identified as promising for achieving a good balance between ductility and creep resistance. Improvement of creep properties is anticipated by the predominance of a lamellar structure and by the addition of heavy elements. The formation of the lamellar structure in TiAl alloys occurs in α grains during cooling and thus requires an incursion within (or nearby) the α-field. Consequently, the challenge lies in the limitation of α grain burst, which usually quickly occurs after crossing the α-transus.

In this context, the Ti_49,92Al₄₈W₂B_0,08 chemical composition [2], named IRIS, was selected on the basis of preliminary studies. First, a boron incorporation of 0.6 at % has been found to limit the grain growth in as-SPSed TiAl alloys and to stabilize the microstructure over a wide range of temperature [3]. However, such an amount of boron also increases the temperature of γ lamellae precipitation that results in their thickening, detrimental to the strength. Therefore, only a small amount of boron was incorporated. Second, it has been demonstrated that the addition of elements as W or Re enhances the mechanical properties of as-SPSed alloys, provided that they spread homogeneously in the γ matrix [4].

In a previous work, the densification by SPS of the TNM alloy (Ti_50,15Al_43.9Nb₄Mo_0.95B_0,1), has been studied [5]. Due to the addition of heavy elements such as Nb, Mo and W, both TNM and IRIS alloys distinguish from classical peritectic TiAl alloys by solidifying through a β phase field. In the case of TNM, the phase diagram shows the existence of a single β phase field for the investigated composition [6]. Heating the TNM alloy in this β field is assumed to lead to a final refinement of microstructure upon cooling thanks to the precipitation of α grains within β grains with up to 12 different orientation variants [7]. Concerning the IRIS alloy, a high amount of aluminum has been introduced to allow enough γ phase in lamellar grains to transmit the deformation, with the aim of improving the ductility. Indeed, the TNM alloy sintered by SPS exhibits a limited ductility, interpreted as a result of a too high volume fraction of α₂ phase in the lamellar zones, correlated to its low aluminum content [5].

First, this paper presents various microstructures obtained with the IRIS alloy as a function of the SPS cycle. In a second step, a particular attention will be paid to the characterization of a fine near-lamellar microstructure as well as its formation mechanism. The influence of the β phase on microstructure formation mechanisms specifically activated during the SPS densification cycle will also be examined and discussed. In the discussion section, due to the similarities between TNM and IRIS compositions, the present results will be largely compared to those obtained with the TNM alloy [5]. Mechanical properties of the as-SPSed IRIS alloy will be presented in a forthcoming paper.

Section snippets

Experimental

The densification by Spark Plasma Sintering has been conducted in the SYNTEX 2080 machine of the PNF2 (Plateforme Nationale de Frittage Flash/CNRS in Toulouse, France). Diameter and height of samples were either 36 mm and 8 mm, or 8 mm and 6 mm, respectively. The conventional SPS cycle described in details in Ref. [8] was used. A load corresponding to 100 MPa was reached in about 2 min. The heating rate was initially 100 K/min and then reduced to 25 K/min during 3 min to avoid an overshooting

Evolution of the microstructure with the dwell temperature

Fig. 1 depicts microstructures of Ø8 samples obtained for dwell temperatures ranging between 1200 °C and 1350 °C. At 1200 °C, dendritic cores containing β₀ bright precipitates and interdendritic channels free of precipitates with a darker grey level can be clearly distinguished. The formation of β₀ precipitates results from the supersaturation in W of the γ phase present in dendritic arms [9].

At 1250 °C, two major phases can be identified thanks to SEM micrographs: the γ phase with a dark-grey

Microstructures of β solidifying alloys densified by SPS

In the present study, γ + α₂ double-phased, duplex and near-lamellar microstructures were successively obtained by increasing the dwell temperatures during the sintering of the Ti_49,92Al₄₈W₂B_0,08 IRIS alloy, as classically observed with TiAl based alloys [11], [18]. The microstructure is influenced by the former dendritic microstructure of powder particles up to 1300 °C. The near-lamellar microstructure is made of small lamellar grains surrounded by γ borders containing β₀ precipitates. The

Conclusion

During this work, the IRIS alloy with the Ti_49,92Al₄₈W₂B_0,08 chemical composition was densified at various temperatures by Spark Plasma Sintering. γ + α₂ double-phased, duplex and near-lamellar microstructures were obtained by increasing temperature.

A fine near-lamellar microstructure was reached in a temperature range of nearly 80 °C. This microstructure is constituted of lamellar grains surrounded by γ borders containing β₀ precipitates. The size of lamellar grains is 35–45 μm. The width of

Acknowledgments

This study has been conducted in the framework of the cooperative project “IRIS-ANR-09-MAPR-0018-06” supported by the French Agence Nationale de la Recherche (ANR) (IRIS-ANR-09-MAPR-0018-06), which is acknowledged. The CEMES group thanks the PNF2 for providing SPS facilities (Plateforme Nationale de Frittage Flash/CNRS in Toulouse, France).

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Citation Excerpt :
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During the last decade, it has been shown that the W-containing IRIS-TiAl alloy displays promising properties for structural applications at high temperatures. The manufacturing process of this alloy is divided into three successive steps: electrode production, powder atomisation and spark plasma sintering (SPS) densification. However, an IRIS alloy densified by using pre-alloyed powders atomized by EIGA process (Electrode Induction melting Gas Atomisation) has recently been found to exhibit chemical heterogeneities. The aim of the present work is to look for the origin of such heterogeneities all along this manufacturing process. The microstructures and the chemical compositions of the material obtained after these different steps are thus investigated at intermediate and local scales by using various experimental tools. A particular attention is paid to the distribution of tungsten atoms in correlation to the constituent phases. Effects of these heterogeneities on mechanical properties are measured by performing tensile tests at room and high temperatures. It will be demonstrated that these heterogeneities are issued from tungsten segregation occurring during the first stage of the initial solidification of the electrode, thus prior to atomisation.

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Obtaining of a fine near-lamellar microstructure in TiAl alloys by Spark Plasma Sintering

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental

Evolution of the microstructure with the dwell temperature

Microstructures of β solidifying alloys densified by SPS

Conclusion

Acknowledgments

Intermetallics

Intermetallics

Intermetallics

Acta Mat.

Intermetallics

Scr. Mat.

J. Mater. Process. Technol.

J. alloys Compd.

Intermetallics

Acta metall. mater

Acta metall. mater

Acta metall. mater