Elsevier

Intermetallics

Volume 9, Issues 10–11, October–November 2001, Pages 915-922
Intermetallics

Relationship between tensile and primary creep properties of near γ-TiAl intermetallics

https://doi.org/10.1016/S0966-9795(01)00090-5Get rights and content

Abstract

This paper presents the tensile and primary creep behaviour of fully lamellar microstructures for three near γ-TiAl compositions: binary TiAl, ternary TiAl+W and TiAl+NbMnWMoSi. Reducing the lamellar interface spacing or aging to precipitate β along interfaces increases the 760 °C tensile strength. Primary creep at stresses from 140 to 276 MPa and 760 °C is characterized in terms of the instantaneous strain occurring during creep loading and a subsequent primary transient. The presence of interface β precipitates reduces the instantaneous strain and increases the time to 0.5% creep strain. Reducing lamellar interface spacing does not influence the instantaneous strain, but decreases the strain rate during the primary transient. Correlating the creep response with deformation structures after primary creep suggests that the instantaneous strain depends on the mobility of interface dislocations and the primary transient depends on dislocation mobility in both the lamellar interfaces and γ lamellae.

Introduction

Many envisaged aero-engine applications of near γ-titanium aluminide intermetallics involve temperature/stress combinations that will inevitably cause creep deformation. Consequently, creep of near γ-TiAl has been extensively studied and it is well established that the α2+γ fully lamellar microstructure exhibits the lowest creep strain rate [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], primarily because lamellar interfaces hinder dislocation motion. However, many potential applications limit the allowable lifetime creep strain to 0.5 or 1%, making primary creep response of paramount importance. Early concerns regarding fully lamellar microstructures exhibiting larger primary creep strain than other microstructural states [3] have been allayed by demonstrating that in terms of time to strain, the fully lamellar condition exhibits improved primary creep resistance [13]. Nevertheless, primary creep makes up a large portion of the first 1% of creep strain.

Primary creep consists of two major components: an instantaneous strain that occurs virtually simultaneously with load application, and a primary transient of decreasing strain rate. Previous investigations of the primary creep of γ-TiAl have concentrated on the interactions between lamellar interfaces and dislocations occurring during the primary transient. At lamellar interfaces the local internal coherency shear stress may be as high as 220 MPa [14], [15], [16], causing interfaces to act as dislocation sources during creep deformation [14], [15], [16], [17], [18], [19], [20]. The influence of these dislocations on primary creep is uncertain, but it has been suggested that creep strain is accommodated by mobile interface and matrix dislocations [21], and that interface coherency stresses cause dislocation emission from the lamellar interfaces and consequently a high creep rate [13], [18]. The latter suggestion is supported by numerous observations of dislocation loops emanating from lamellar interfaces [13], [15], [16], [17], [18], [19], [22].

Many near γ-TiAl compositions contain β stabilising elements including W, Mo, Nb, Cr or combinations of these elements. Of these elements, W additions cause the greatest improvement in creep resistance [23]. It has been concluded that the formation of W enriched β particles, both along lamellar interfaces and lamellar grain boundaries, reduces the steady state (or minimum) creep strain rate compared to the retention of W in solid solution [24], [25], [26]. However, the role of β phase during primary creep has not been specifically investigated.

In this paper a first attempt is made to determine the influence of lamellar interface spacing and β particles on the instantaneous strain due to creep loading and the primary transient. To this end, the instantaneous strain and primary transient of three near γ-TiAl intermetallics is examined and correlated with high temperature tensile properties.

Section snippets

Materials and experimental procedure

The three near γ-TiAl intermetallics investigated are summarized in Table 1. During heat treatment samples were wrapped in Ta foil and encapsulated in quartz tubes evacuated and backfilled to 1/3 atm with Ar. The solution heat treatments were specifically designed to produce fully lamellar microstructures without massive γ or Widmanstätten γ structures that may degrade creep resistance [10]. The microstructural response of the three compositions to the solution heat treatments of Table 1 has

Heat treated microstructures

Examples of the fully lamellar microstructure of each of the three compositions after the solution treatment of Table 1 are illustrated in Fig. 1, with the important microstructural attributes summarized in Table 2. As expected air cooling TiAl after solutionizing [heat treatment (2)] results in narrower lamellae compared to furnace cooling [heat treatment (1)]. The combination of furnace cooling to 1300 °C followed by air cooling [heat treatment (3), Fig. 1a] results in a range of lamellar

Discussion

Table 3 and Fig. 3 clearly demonstrate two important results: decreasing the lamellar interface spacing and β precipitation along lamellar interfaces both increase the 760 °C tensile strength. The former effect is evident from the increased tensile properties of furnace cooled versus air cooled TiAl [heat treatment (1) versus (2)], while the latter effect is evident by comparing the tensile properties of the unaged and aged states of TiAl+W and TiAl+WMoSi [heat treatments (4) and (6) versus

Summary and conclusions

In this paper the tensile and creep properties of three near γ-TiAl compositions with fully lamellar microstructures are analysed to determine the microstructural features influencing the instantaneous creep strain and the primary creep transient. The major conclusions are:

  • 1.

    Both refinement of the lamellar interface spacing and the precipitation of β along lamellar interfaces increase the 760 °C tensile strength.

  • 2.

    Lamellar conditions with precipitated interface β exhibit lower instantaneous creep

Acknowledgments

The Natural Sciences and Engineering Research Council of Canada and the Department of National Defence (Canada) have financially supported this research. The authors thank Tak Terada for assistance with tensile testing.

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