Development of α precipitates in metastable Ti-5Al-5Mo-5V-3Cr and similar alloys

doi:10.1016/j.matchar.2016.09.005

Materials Characterization

Volume 120, October 2016, Pages 220-228

https://doi.org/10.1016/j.matchar.2016.09.005 Get rights and content

Highlights

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The α phase precipitation has been investigated in four metastable β titanium alloys.
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The α phase in β matrix exhibits uniform distribution and clusters of α laths.
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Morphologies of α phase are thin film on β grain/sub boundaries and islands.
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The α phase with different morphologies transform to Widmanstatten structure.
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The development of the α phase introduces significant increase in hardness values.

Abstract

Present work describes the development of the α phase in β WQ specimens of Ti-5Al-5Mo-5V-3Cr (A1), Ti-5Al-3.5Mo-7.2V-3Cr (A2), Ti-5Al-5Mo-8.6V-1.5Cr (A3) and Ti-5Al-3.5Mo-5V-3.94Cr (A4) alloys as function of different ageing temperatures and time intervals. The α phase formed during ageing display different morphologies namely, thin film on prior β grain boundaries, sub boundary and islands of the α phase within the β matrix. Mechanism of the formation of these morphologies has been explained. The fine and even distribution of the α phase in β matrix exhibit two distinct features i.e. uniform distribution of α laths with two variants and clusters of parallel α laths. The α phase presents in all morphologies is transformed to Widmanstatten type morphology on further increase in ageing temperature and time. The development of the α phase in these alloys at 300 and 400 °C results in significant increase in hardness which is attributed to typical precipitation hardening.

Introduction

Titanium alloys in general can be categorized as α, near α, α + β, metastable β and β depending upon both the presence of β stabilizing elements as well as the volume fraction of the β phase. The phase transformations and associated physical and mechanical properties of these alloys therefore strongly dependent on alloy chemistry, thermomechanical processing and heat treatments. It is known that the thermomechanical processing and subsequent heat treatments in these alloys are employed to break the cast structure and tailor different volume fractions of the constituent phases [1], [2], [3], [4].

The metastable β titanium alloy Ti-5Al-5Mo-30V-3Cr (Ti-5553) is an imitative of the Russian VT-22 to replace the Ti-10V-2Fe-3Al (Ti-1023) in landing gear assemblies of aircrafts [5], [6], [7], [8], [9]. It has been reported that the alloy Ti-1023 is sensitive to both the temperature and strain rate fluctuations during thermomechanical processing. In addition, mechanical properties of this alloy deteriorate with increase in size of the components. This has been attributed to typical commercial heat treatment given to the alloy Ti-1023 i.e. solution treatment in α + β phase field followed by water quenching [10], [11], [12], [13], [14], [15], [16], [17], [18]. On the other hand, optimum properties of the alloy Ti-5553 has been achieved in air cooled condition after solution treatment in α + β phase field. The air cooling thus provides a wider processing window to produce larger sized components of the alloy Ti-5553 with better hardenability in comparison to that of the Ti-1023 [15].

The β → α transformation in metastable β titanium alloys is quite different than those in α + β. The precipitation of the α phase in former alloys occurs at lower ageing temperatures than the latter and therefore, the β → α transformation is more sluggish. The α phase precipitates in metastable β alloys after isothermal ageing of β phase below the β transus temperature. The α phase also appears during slow cooling from the β phase field. Such a heat treatment has strong bearing on the mechanical properties of these alloys which are function of the morphology, volume fraction, size, shape and distribution of the α phase [19]. For example, isothermal ageing of β phase results in fine distribution of α phase in the β matrix. This microstructure is beneficial for high cycle fatigue applications. While slow cooling display coarse α laths in β matrix that is good for creep and high fracture toughness applications.

The β → α precipitation has been investigated in alloy Ti-5553 during isothermal ageing as function of time and temperature [10], [11], [19], [20]. However, the effect of different amounts of the alloying elements on β → α precipitation in this alloy has not been studied. The compositions of the three alloys have been designed in present study based on same Mo equivalent (8.15) of the Ti-5553 [21]. Present work is thus concerned with the ageing characteristics of these alloys (Ti-5553 and its derivatives) as function of time and temperature. An attempt has been made to correlate the kinetics of β → α precipitation in these alloys based on microstructural evolution and hardness values.

Section snippets

Experimental

The four experimental alloys with nominal compositions (A1: Ti-5Al-5Mo-5V-3Cr, A2: Ti-5Al-3.5Mo-7.2V-3Cr, A3: Ti-5Al-5Mo-8.6V-1.5Cr and A4: Ti-5Al-3.5Mo-5V-3.94Cr) were melted in the form of 600 g pancake using non-consumable vacuum arc melting. The details of melting procedure, analyzed chemical compositions and corresponding β transus temperatures are given elsewhere [21].

The as-cast alloys were solution treated (ST) in β phase field (900 °C for 30 min) and then water quenched (WQ). The β ST WQ

Results

The optical and BSE microstructures of the β ST WQ samples exhibit the presence of single phase. The representative microstructures of the alloy A1 are given in Fig. 1(a and b). The XRD and SAD patterns of the alloy A1 in β ST WQ condition is shown in Fig. 1c and d. The XRD patterns of these alloys also confirm the presence of the β phase. The SADP displays streaks which indicate the presence of fine athermal ω phase in β matrix.

The XRD patterns of all the four β ST WQ samples aged at 300 °C for

Discussion

Although the present alloys exhibit the presence of typical β phase in β ST WQ condition, a fine distribution of the athermal ω phase is also present in the β matrix. The presence of athermal ω phase is quite common in metastable β titanium alloys and this is treated as instability in microstructure. The ω phase cannot be detected by XRD and SEM techniques due to its very fine size and low volume fraction. The instability in microstructure of β ST WQ specimens in metastable β titanium alloys

Conclusions

1.
Development of the α precipitates has been investigated in β ST WQ specimens of Ti-5Al-5Mo-5V-3Cr (A1), Ti-5Al-3.5Mo-7.2V-3Cr (A2), Ti-5Al-5Mo-8.6V-1.5Cr (A3) and Ti-5Al-3.5Mo-5V-3.94Cr (A4) alloys as function of different ageing temperatures and time intervals.
2.
The α precipitates formed during ageing display different morphologies namely, thin film on prior β grain boundaries, sub boundary and islands of the α phase within the β matrix. Mechanisms of the formation of these morphologies have

Acknowledgements

The authors wish to acknowledge Defence Research and Development Organization for financial support. We are grateful to Dr. Samir V Kamat, Director, Defence Metallurgical Research Laboratory for his kind encouragement. Authors thank Electron Microscopy, Titanium Alloy, Structure and Failure Analysis Groups of DMRL for their kind help.

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Citation Excerpt :
However, a strong texture component is not distinctively developed. Aging at 600 °C is the most preferred condition for controlled lamellar α growth reported in the literature for conventionally manufactured Ti alloys [7,24,30,39,40]. Fig. 6 presents the microstructural changes and their corresponding EBSD grain orientation maps and pole figures of LPBF processed and heat-treated samples.
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Development of α precipitates in metastable Ti-5Al-5Mo-5V-3Cr and similar alloys

Highlights

Abstract

Introduction

Section snippets

Experimental

Results

Discussion

Conclusions

Acknowledgements

Acta Mater.

Acta Mater.

J. Mater. Process. Technol.

Mater Charact

Mater. Sci. Eng.

Mater. Des.

Mater. Sci. Eng.

Mater. Sci. Eng.

Scr Mater

Acta Metall

Acta Metall.

Acta Metall

Acta Metall

Mater. Sci. Eng.

Scr Mater

Mater. Sci. Eng.