Elsevier

Materials Characterization

Volume 100, February 2015, Pages 104-107
Materials Characterization

A mixture of massive and feathery microstructures of Ti48Al2Cr2Nb alloy by high undercooled solidification

https://doi.org/10.1016/j.matchar.2014.09.016Get rights and content

Highlights

  • A mixture of massive and feathery microstructures was observed.

  • Massive γ formed during the solid-state transformations.

  • Defects and undercooling provide driving force for the nucleation of massive γ.

Abstract

A mixture of massive and feathery microstructures was observed in Ti48Al2Cr2Nb alloy subjected to the undercooled solidification rather than the heat treatments in most cases. Double recalescence events and primary β solidification confirmed that massive γ phase did not directly nucleate from the undercooled melt but formed during the solid-state transformations. It is believed that small white areas (aluminium-poor) along lamellar grain boundaries may be closely related to the formation of massive γ phase and feathery γ phase. High dislocation density and stacking faults were detected in massive γ phase by transmission electron microscopy. The high energy of defects and undercooling in the solid state phase transformation can provide sufficiently high driving force for the nucleation of massive γ phase.

Introduction

Massive transformation in γ-TiAl alloys was first observed by Wang et al. in 1992 through cooling from the α-phase field at different cooling rates [1] and has attracted much attention since then [2], [3], [4]. It is now generally admitted that proper composition, proper cooling rate and proper grain size are required to give rise to massive transformation [5]. Subsequently, lamellar or massive γ phase (γm) was found by Z. G. Liu et al. at matrix α2 grain boundaries in rapidly solidified Ti46Al2Cr2Nb alloy (a grain size of about 5–10 μm) with low cooling rate (9.9 × 105 K/s) and rapidly solidified Ti46Al2Cr2Nb1.0Y alloy (a grain size of about 0.5–3 μm) at high cooling rate (1.8 × 106 K/s), respectively [6]. Under non-equilibrium conditions, conventionally, such high cooling rate (105 K/s–106 K/s) is uncommon, but γm and feathery γ phase (γf) may occur at the edge of as-cast ingots or the sheet of TiAl based alloys because of the chilling by mould. The metastable microstructure has a noticeable influence on the quality of ingots or castings. γm was also discovered by O. Shuleshova et al. through substrate quenching of highly undercooled Ti50Al10Nb alloy and it was considered that γm was formed due to sluggish precipitation of the γ plates in the α matrix with increasing the Nb content [7]. It is well known that the effect of heavy alloying extended the massive transformation regime to lower cooling rate side in TiAl alloys [8], [9]. However, high undercooled solidification is also an effective way to realize rapid solidification of TiAl based alloys at a relativity low cooling rate, obtaining stable microstructure or metastable microstructure [10]. A bulk of investigations focused on the heat treatments of γm and γf. Little attention was paid to a mixture of γm and γf by undercooled solidification and the detailed massive transformation in undercooled TiAl alloys is still uncertain yet at present.

In this work, a mixture of γm and γf was observed in the high undercooled solidification. Furthermore, the objectives were to confirm the primary β solidification at high undercooling in Ti48Al2Cr2Nb alloy and to prove that γm is not the primary phase (L  γmetastable). In addition, defects such as dislocations and stacking faults besides undercooling in the solid state phase transformation can provide enough driving force for the nucleation of γm at a low cooling rate.

Section snippets

Experimental

The alloy with the nominal chemical composition Ti48Al2Cr2Nb was prepared from Ti, Al, Cr and Ti52.47Nb (wt %) with 99.99% purity (or better) in an electromagnetic cold-crucible melting facility under Ar atmosphere. As-cast ingots were homogenized in a vacuum heat treatment furnace at 1080 K for 5 h. Samples of about 1–2 g mass were cut from the master ingots, polished and processed in an electromagnetic levitation apparatus. The vacuum chamber was evacuated to 10 4 Pa and backfilled to 0.05 MPa

Results and discussion

The recalescence event at ΔT = 370 K for Ti48Al2Cr2Nb alloy is presented, revealed the different solidification behaviours and the relevant process stages are indicated in Fig. 1. The melting temperature is 1525 °C for Ti48Al2Cr2Nb alloy. The bulk undercooling (ΔT) is defined as the difference between the liquidus temperature (Tm) and the nucleation temperature (Tn) of the event [11]. The nucleation of primary β phase (~ 1155 °C) is represented by the steep recalescence (the melt undercooling below

Conclusions

A mixture of γm, γf and lamellar microstructures was detected in Ti48Al2Cr2Nb alloy at high undercooling levels. The double recalescence events, the BSE and the TEM analysis gave a direct evidence of primary β solidification at high undercooling levels in Ti48Al2Cr2Nb alloy. Hence, γm is not formed directly through solidified from the undercooled melt but via a series of solid-state transformations. It is obvious that the formation of γm and γf promotes the formation of small white areas

Acknowledgements

The author would like to acknowledge the financial support from National Basic Research Program of China (No 2011CB605503).

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This work was financially supported by National Basic Research Program of China (2011CB605503).

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