Quantitative microstructural characterization of a near beta Ti alloy, Ti-5553 under different processing conditions

MA TE RI A L S CH A R A CT ER IZ A TI O N 8 1 (2 0 1 3) 3 7– 4 8

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Quantitative microstructural characterization of a near beta Ti alloy, Ti-5553 under different processing conditions Sujoy Kumar Kar a,⁎, Atasi Ghosha , Nishant Fulzelea , Amit Bhattacharjeeb a

Indian Institute of Technology, Kharagpur 721302, India Defence Metallurgical Research Laboratory, Hyderabad 500 058, India

b

AR TIC LE D ATA

ABSTR ACT

Article history:

Ti-5553 (Ti–5Al–5Mo–5V–3Cr) is a near beta Ti alloy with potential applications in structural

Received 18 November 2012

components of aircrafts. In near beta Ti alloys, microstructural features can be varied over a

Received in revised form

wide range of length scales by changing different heat treatment parameters. Quantitative

20 February 2013

characterization of different microstructural features of each of those differently heat treated

Accepted 30 March 2013

material is of considerable importance to eventually develop a model relating quantitatively, processing, microstructure and mechanical properties. In the present work, quantitative

Keywords:

microstructure characterization, using stereological methods, has been performed on various

Ti-5553 alloy

microstructures developed through variations in separate heat treatment variables, as well as

Thermo-mechanical treatment

variation in each of the different individual variables of a set of sequential combined thermo-

Quantitative microstructure

mechanical processing steps to obtain beta annealed microstructure. Quantitative effect of

characterization

processing on microstructure is discussed and a TTT diagram is constructed for alpha

TTT diagram

precipitation in Ti-5553 alloy. © 2013 Elsevier Inc. All rights reserved.

Processing–microstructure correlation

1.

Introduction

Ti-5553 (Ti–5Al–5Mo–5V–3Cr) is a recently developed near beta Ti alloy that is gathering increasing interest in aircraft structural applications, especially in the landing gear components. It is replacing Ti–10V–2Fe–3Al alloy for several favorable reasons that include its wider processing window, higher hardenability and achievement of higher strength [1]. Because of its improved performance, this alloy is being planned for other applications also, for example in nacelles, fuselages, wings, and some corrosion prone areas that are difficult to inspect [1]. Based on the specific application, optimization of various properties like, strength, ductility, fracture toughness and fatigue is required to be carried out. As microstructural features individually affect each of these properties in a specific manner, quantitative relations between microstructure and individual properties would help the designer to find out the desired window of

length scales of various microstructural features to achieve optimum combination of those properties. In turn, processing parameters can be optimized to achieve the desired microstructure through knowledge of how different thermomechanical processing parameters affect individual microstructural features quantitatively. Towards this goal, the major requirement is to characterize the microstructural features of different samples obtained through variation in different heat treatment parameters. There have been a few studies on morphology evolution during aging treatments [2,3] and a few studies on alpha phase nucleation and elemental partitioning behavior [4,5]. However none of the published literatures reports quantitative characterization of various microstructural features. Processing route for beta annealed microstructure in near beta Ti alloys [6] is shown schematically in Fig. 1. These various steps should be followed sequentially to obtain desired beta

⁎ Corresponding author. Tel.: + 91 3222 283258, + 91 7602646515(mobile); fax: + 91 3222 282280e. E-mail address: [email protected] (S.K. Kar). 1044-5803/$ – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.matchar.2013.03.016

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MA TE RI A L S CH A R A CT ER IZ A TI O N 8 1 (2 0 1 3) 3 7– 4 8

Fig. 1 – Processing route for beta annealed microstructure in near beta Ti alloys. annealed microstructures. Variation in processing parameters like temperature and cooling rate (CR) at each of these different steps affects the length scales of different microstructural features. There have been some post recrystallization, direct aging treatments and corresponding quantitative microstructural characterization work carried out at the Ohio State University [7,8]. However, there exists need of quantitative characterization of microstructural features by varying processing parameters at different steps of the full beta annealing processing route as shown in Fig. 1. The present data would complement the previous researchers' characterization work to develop a bigger database to eventually develop models relating quantitatively, processing parameters, microstructure and properties. In the present study, a systematic variation has been carried out in the cooling rates (CRs) and aging temperature to produce a variety of microstructure and then the microstructural features have been measured using stereological methods. Stereological procedures to measure microstructural features of Ti alloys as developed in Hamish Fraser's group [9–11] have been used to measure various microstructural features here. Microstructural features that have been quantified include prior beta grain size, thickness and volume fraction (vf) of primary alpha laths, thickness of secondary alpha laths, and thickness of grain boundary alpha (henceforth will be designated as GB alpha). In the present work, four different types of thermal or thermomechanical treatments are applied. While the first type involves experiments to study the effect of a set of sequential thermo-mechanical steps to obtain beta annealed microstructures, each of the other three types is individual thermal treatment to study the effect of variation in a specific thermal parameter, namely the effect of recrystallization temperature, the effect of undercooling below beta transus and the effect of cooling rates from the solutionizing temperature. Under each of these four different types of thermal or thermo-mechanical treatments, quantitative effect on microstructure, of variation in different thermal treatment parameters is presented in the present paper. A TTT (Time Temperature Transformation) diagram is developed for alpha precipitation and is also presented in the present paper.

2.

Experimental Methods

2.1.

Material and Processing

Ti–5Al–5Mo–5V–3Cr alloy was cast by a double vacuum arc remelting (VAR) process and homogenized at 1050 °C for 1.5 h. Table 1 shows the chemical analysis of the as cast material. Differential thermal analysis (DTA) and microstructure characterization on this material revealed the beta transus to be at around 845 °C. Four different types of thermal or thermo-mechanical treatments are applied. Those are elaborated in the following sections (2.1.1, 2.1.2.1, 2.1.2.2 and 2.1.2.3). Section 2.1.1 describes a set of sequential thermo-mechanical steps to obtain beta annealed microstructures as recommended in literature [6] and as indicated schematically in Fig. 1. Each of the other sections under 2.1.2 describes the individual thermal treatments to study the effect of a specific thermal processing. Section 2.1.2.1 describes the experiments to study the effect of recrystallization temperature, Section 2.1.2.2 describes the experiments done to study the effect of undercooling below beta transus, and Section 2.1.2.3 describes the experiments to study the effect of cooling rate from the solutionizing temperature.

2.1.1. Set of Thermo-mechanical Processing Steps to Obtain Beta-annealed Microstructure Samples of as cast Ti-5553 alloy were treated through a series of processing steps as shown schematically in Fig. 1 to obtain beta-annealed microstructure. Details of the different steps are illustrated here. In the deformation step, cast ingots were first forged to impart 55% reduction in beta phase field at 1050 °C after soaking at that temperature for 1.5 h. After this, the material was forged to give another 54% reduction in the (alpha + beta) phase field after soaking at 780 °C for 1.5 h, following which it was rolled to give a further reduction of 50% at the same temperature. Material from these (alpha + beta) finished plates was then recrystallized (recrystallization step of Fig. 1) at 950 °C for either of the following two time periods: 15 min or, 30 min. Samples were cooled to room temperature (RT) from recrystallization temperature at two different rates (furnace cool (henceforth will be designated as “fc”) and air cool (henceforth will be designated as “ac”)). Samples were then annealed at 700 °C (annealing step of Fig. 1) for either of the following two time periods: 30 min or, 2 h and subsequently cooled to RT at different CRs (fc, ac and covered air cool (henceforth will be designated as “c.a/c”)). Samples were then aged at any of the four different aging temperatures (400 °C, 450 °C, 500 °C and 600 °C) for 4 h (aging step of Fig. 1). Samples were air cooled after aging. Approximate values of CR for fc, ac and c.a/c are 10 °C/min, 60 °C/min and 15 °C/min respectively.

Table 1 – Compositional analysis of the as cast Ti-5553 alloy for the present work. 1st ingot Location

Al (wt.%)

V (wt.%)

Mo (wt.%)

Cr (wt.%)

O (ppm)

H (ppm)

N (ppm)

Top Bottom

5.48 5.17

5.10 5.33

5.06 5.34

3.22 3.10

500 1100

38 34

60 50

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2.1.2.

Variation in Separate Heat Treatment Variables

Apart from the above schedule, three separate sets of heat treatment schedules were carried out at DIL 805 A/D dilatometer (manufactured by BAEHR thermo-analysis GmbH) to study (a) the effect of beta recrystallization temperature on recrystallized beta grain size, (b) the effect of undercooling on sizes of GB alpha and intragranular alpha and volume fraction of alpha phase, and (c) the effect of CRs on length scale of intragranular alpha lath and GB alpha. While the effect of recrystallization temperature on recrystallized beta grain size was studied on as deformed (forged and rolled as described in Section 2.1.1) material, the effects of CR and undercooling below beta transus temperature have been studied on as cast material. Samples for heat treatments in dilatometer were cylinders of dimension 10 mm length by 4 mm diameter. Heat treatments using dilatometer ensure that heating rates, CRs and temperatures can be precisely controlled and measured and the treatments are done under controlled environment.

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2.1.2.3. Variation in Cooling Rates from Solutionizing Temperatures. Another set of as cast material was solutionized at 1000 °C for 15 min in the dilatometer and then cooled to RT at different cooling rates. These heat treatments were planned to study the effect of cooling rates on the thickness of primary alpha lath and the GB alpha. Fig. 4 shows the heat treatment schedule.

2.2.

Microstructure Characterization

Another set of dilatometer samples were solutionized at 1000 °C for 15 min and then step quenched to 750, 650 and 550 °C and aged at each temperature for 5 min and 30 min before those samples were quenched to RT. The heat treatment schedule is shown in Fig. 3. These experiments were planned to study the effect of undercooling below beta transus on the thickness of intragranular and GB alpha and on vf of total alpha and also to develop a TTT diagram for alpha precipitation.

Differently heat treated samples were polished using different grades of emery papers and finally polished in 0.05 μm colloidal silica medium. Samples were imaged at ZEISS EVO 60 SEM at 10 kV for characterization of primary alpha laths and GB alpha. TEM samples were prepared through ion milling using Gatan 691 Ion Miller operating at 5 keV. TEM imaging was carried out in FEI Tecnai G2 20S-TWIN TEM operating at 200 kV to characterize the secondary alpha laths. Samples were etched in Kroll's reagent before imaging optically in LEICA DM6000N microscope. For prior beta grain size characterization, a series of images covering the whole polished surface were taken and collaged to make sure that the image contains many number of grains. Stereological methods as described in [9–11], have been used to characterize different microstructural features. In essence, to measure alpha lath thickness and grain boundary alpha thickness, multiple sets of random lines were overlaid on the images. The length segments (λ) of the lines that fall on the particular microstructural feature are inverted and subsequently averaged to obtain (1/λ)mean. Thickness of the feature is obtained through the relation: (2/(3 × (1/λ)mean)). For measurement of prior beta grain size, a grid of cycloids is overlaid on low magnification collaged image containing multiple grains. Intersections of cycloids with grain boundaries are marked and counted. Twice the number of intersections per unit length of cycloids is taken as a measure of surface area per unit volume of grains, which, in this work, has been termed as prior beta grain factor that is indicative of grain size. The larger the value of this factor, the smaller is the grain size. According to Russ [12], a cycloid probe line forms the basis of an unbiased value of the surface area per unit volume, taking into account the 3-d to 2-d projections.

Fig. 2 – Schematic of recrystallization treatments at different temperatures.

Fig. 3 – Heat treatment schedule to study the effect of undercooling below beta transus.

2.1.2.1. Variation in Recrystallization Temperature.

A set of as deformed samples were heated to three different temperatures (920, 950 and 1000 °C) above beta transus at a heating rate of 120 °C/min and held at each of the temperatures for 30 min under vacuum of 5 × 10−4 mbar and cooled to RT at 60 °C/min in argon atmosphere in dilatometer. This CR is similar to that experienced during air cooling. And that's why this CR has been chosen to simulate the usual commercial practice of air cooling. Fig. 2 shows a schematic of these heat treatment schedules. These experiments were planned to study the effect of beta recrystallization on recrystallized beta grain size.

2.1.2.2. Variation in Undercooling below Beta Transus.

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MA TE RI A L S CH A R A CT ER IZ A TI O N 8 1 (2 0 1 3) 3 7– 4 8

factor (PBGF), as measured by stereological procedure, was found to be 1.3 mm 2/mm 3 along the rolling direction (RD) and 1.38 mm 2/mm 3 perpendicular to the RD. These are very big grains of average approximate size of 4.4 mm perpendicular to RD and much longer along the RD.

3.1.

Fig. 4 – Heat treatment schedule to study the effect of cooling rates. To measure, volume fraction (vf) of primary alpha laths, a rectangular grid of points is overlaid on the high magnification SEM image and vf is calculated by the number fractions of those points lying on primary alpha laths.

3.

Effect of Variation in Recrystallization Temperature

Single step recrystallization treatment is given on as deformed (forged and rolled) material as described in Section 2.1.2.1. Recrystallization temperature was varied as per schematic thermal treatment schedule of Fig. 2. Upon heating at three different temperatures above beta transus, at 920 °C, 950 °C and 1000 °C and holding at each of those temperatures for 30 min and subsequently cooling to RT at 60 °C/min, new recrystallized beta grains formed. This single step recrystallization treatment carried out on cylindrical samples of diameter 4 mm ensured complete recrystallization at each of the three different solutionizing temperatures (920, 950 and 1000 °C). Criteria used for determining complete recrystallization have been the presence of uniform small new grains over the whole cross section. Table 2 shows the beta grain factors (recrystallized beta grain size) as measured using stereological procedure at different temperatures. Fig. 6 shows one such recrystallized microstructure recrystallized at 920 °C.

Results

In this section, to begin with, separate effects of controlled variations in solutionizing temperature (Section 3.1), undercooling below beta transus temperature (Section 3.2) and cooling rate from solutionizing temperature (Section 3.3) on quantified values of size and volume fraction of microstructural features in Ti-5553 will be presented. After that, in Section 3.4, combined effects of variation in thermal treatment parameters at various steps of a set of sequential thermo-mechanical steps for beta annealing microstructure for optimized properties will be presented. For study of the effect of variation in single step solutionizing temperature (Section 3.1), as well as for study of the combined effect of a sequential thermo-mechanical processing steps (Section 3.4), the starting material was in as deformed (forged and rolled at beta and alpha-beta phase fields as mentioned in Section 2.1.1) condition. As deformed microstructure is shown in Fig. 5. While Fig. 5(a) shows the structure parallel to the rolling direction (RD), Fig. 5(b) shows the structure perpendicular to the RD. Prior beta grain

3.2. Effect of Variation in Undercooling Temperature below Beta Transus To study the effect of under-cooling below beta transus (~ 845 °C) on microstructure, as cast samples were given heat treatment as per schedule given in Fig. 3, which has been described in Section 2.1.2.2. Fig. 7 shows the SEM micrographs of the step quenched samples. Micrographs of (a) and (b) correspond to step quench (SQ) temperature of 750 °C for hold time of 5 min and 30 min respectively. Micrographs of (c) and (d) correspond to SQ temperature of 650 °C for hold time of 5 min and 30 min respectively. Fig. 8 shows the TEM micrographs of samples step quenched to 550 °C and held for 5 min and 30 min. In the sample step-quenched to 550 °C and held for 5 min, no alpha phase was visible in the SEM micrographs. Hence, thickness of grain boundary alpha could not be obtained for this sample. However TEM image (Fig. 8a) reveals very few fine intra-granular alpha precipitates of size ~ 25 nm for these samples. Intra-granular alpha lath

Fig. 5 – As deformed grain structure on collaged image: a) perpendicular to RD and b) parallel to RD; RD is marked by arrow on the figure.

MA TE RI A L S CH A R A CT ER IZ A TI O N 8 1 (2 0 1 3) 3 7– 4 8

Table 2 – Effect of variation in recrystallization temperature on prior beta grain size. Recrystallization temperature

Hold time

Prior beta grain factor

Approximate grain diameter

(°C)

(min)

(mm2/mm3)

(μm)

920 950 950 1000

30 15 30 30

34.3 34.2 26.2 19.4

174 175 228 310

thickness and vf of alpha for this sample were measured on TEM micrographs. Thickness of intragranular and GB alpha, as well as vf of total alpha have been quantified and tabulated in Table 3 for step quenched samples. The plot of grain boundary alpha thickness and intragranular alpha thickness vs. degree of undercooling (Fig. 9) shows a monotonic decrease in thickness in both types of alpha precipitates (intragranular and GB alpha). A sharp decrease in thickness in both GB alpha and intra-granular alpha is observed for initial 200 °C undercooling. Vf data for different isothermal holding conditions have been used to develop a TTT diagram for alpha precipitation; this is illustrated in the discussion (Section 4.5).

3.3. Effect of Variation in Cooling Rates from Solutionizing Temperatures To study the effect of cooling rate from solutionizing temperature on thickness of intragranular and GB alpha and vf of total alpha, as cast samples were solutionized at 1000 °C for 15 min followed by cooling to RT at different cooling rates using dilatometer as explained in Section 2.1.2.3 (as per the schedule given in Fig. 4). It resulted in variation in size of intragranular and GB alpha as well as vf of total alpha. Stereologically measured size and volume fraction data of microstructural features are tabulated in Table 4.

GB alpha thickness, as well as presence and continuity of GB alpha vary over different grain boundaries. Average GB alpha thickness measured only on grain boundaries containing GB alpha has been reported here. Fig. 10 shows the variation of thickness of intragranular and GB alpha as a function of cooling rate. Variation in volume fraction of alpha phase as a function of cooling rate (data of Table 4 along with a fitted trend line) is plotted in Fig. 11.

3.4. Effects of Variation in Cooling Rates and Temperatures at Different Steps of a Combined Set of Sequential Thermo-mechanical Processing Steps Leading to Beta Annealed Microstructure A combined set of steps designated as “homogenization”, “deformation”, “recrystallization”, “annealing” and “aging” as shown in Fig. 1 is given to obtain beta annealed microstructure with a notion to achieve optimum set of properties for in service performance. Thermal treatment parameters, like temperatures and cooling rates at different steps of “recrystallization”, “annealing” and “aging” have been varied as described in Section 2.1.1, and applied to a series of different samples BA1 through BA14. Size and vf of GB alpha, intragranular primary alpha and intragranular secondary alpha vary in these different samples according to variation in thermal treatment parameter in the complete set of steps. Using stereological procedures described in Section 2.3, recrystallized beta grain size as well as all other microstructural features, namely grain boundary alpha, primary alpha, and secondary alpha have been characterized in terms of size and/or volume fraction. Characterization of grain boundary alpha and primary alpha has been carried out on SEM micrographs, while, characterization of secondary alpha has been carried out on TEM micrographs. Size and vf of primary alpha have been measured on high magnification SEM BSE images, where only intragranular alpha laths were present. So, the vf data presented here do not provide the relative amount of intragranular alpha to GB alpha. GB alpha layer thickness has been measured by random line construction and subsequent measurements on the intersected line segments using stereological procedure as mentioned in Section 2.3. For average GB alpha thickness, only grain boundaries that are decorated with alpha layers have been considered. Table 5 tabulates the quantified values of size and vf of alpha precipitates as measured using stereological methods, along with values of thermal treatment variables of “recrystallization”, “annealing” and “aging” steps (refer to Fig. 1) for samples BA1 through BA14. Data of this table along with that in literature [7,8] can constitute a processing–microstructure database which can be used for developing a data-based model (Neural Network model) to relate processing to microstructure in this alloy. The effects of individual processing steps on microstructural features as observed in this table are discussed in Section 4.4.

3.4.1. Fig. 6 – Microstructure of sample recrystallized at 920 °C, 30 min hold.

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Recrystalized Beta Grain Size

Upon recrystallization at 950 °C for 15 min and 30 min in regular furnace, the average grain sizes obtained were 408 μm and 1.11 mm respectively.

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Fig. 7 – SEM backscattered micrographs of samples step quenched to different temperatures below beta transus and held for different times.

3.4.2.

Grain Boundary Alpha

Example of variation in grain boundary alpha is presented in Fig. 12. While (a) and (b) compare the grain boundary alpha thickness of two samples (samples BA9 and BA13 respectively) cooled at different rates (fc and ac) from recrystallization temperature, (c) (sample BA2) and (d) (sample BA14) show examples of grain boundaries with discontinuous alpha layer and no alpha layer respectively. Although most of the grain boundary alpha layers are continuous, however, some grain boundaries are observed with discontinuous or no grain boundary alpha layer. Even on single boundaries, some parts are decorated by continuous alpha layer, while the other parts don't have any alpha on them. The reason for some grain boundaries to have continuous grain boundary alpha

layer, while other grain boundaries do not to have either continuous or any alpha layer is under the scope of ongoing research at the Ohio State University and is thought to be an interactive effect of misorientation between the grains, orientation of the grain boundary and the variant selected by the grain boundary alpha [reference: personal communication with Mr. Vikas Dixit, a graduate student with Prof. Hamish Fraser in the Ohio State University, based on his unpublished results as of June 2012]. In the present work, various grain boundaries with alpha layer on them in each sample have been characterized to report the GB alpha thickness. Comparing the thickness of GB alpha layer (see Table 6) of samples that have undergone the same heat treatment schedule except the cooling rate from recrystallization

Fig. 8 – TEM images of samples step quenched to 550 °C and aged for a) 5 min and b) 30 min.

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Table 3 – Quantitative values of microstructural features developed through step-quenching and holding at different temperatures below beta transus temperature.

Table 4 – Effect of variation in cooling rate from solutionizing temperature on thickness of grain boundary alpha and intragranular alpha and also on volume fraction of alpha.

Step Aging quenching time temperature

Cooling rate

Grain boundary alpha thickness

Intragranular alpha lath thickness

Volume fraction alpha

(°C/min)

(μm)

(μm)

(%)

2.5 10 20 50 100

0.94 0.53 0.26 0.09 0.07

0.36 0.06 0.07 0.05 0.06

57 38 27 22 7

Grain boundary alpha thickness

Intragranular Volume alpha lath fraction thickness alpha

(°C)

(min)

(μm)

(μm)

(%)

750

5 30 5 30 5 30

0.45 0.57 0.10 0.18 – 0.07

0.39 0.47 0.09 0.15 0.03 0.03

10 25.5 21 37.5 13 32.7

650 550

temperature (variation in CR of step III of Fig. 1), it has been found that air cooling resulted in thinner GB alpha compared to furnace cooling in each case of the different sets of heat treatment conditions. Air cooling resulted in around 271–444 nm thick GB alpha, while furnace cooling resulted in 409–571 nm thick GB alpha. It should be noted that these values are much larger than that obtained through single step cooling from solutionizing temperature as presented in Table 4 (Section 3.3).

3.4.3.

Primary Alpha Lath

3.4.3.1. Vf of Primary Alpha. Referring to data of Table 5, annealing at 700 °C for 30 min (in case of samples BA4, BA5 and BA6) resulted in 40 to 47 vol.% of alpha, whereas annealing at the same temperature for longer time (120 min) (in case of samples BA8, 9, 10, 12, 13, and 14) resulted in a higher volume fraction of alpha (51–66%). Vf data of samples BA7 through BA14, all recrystallized and annealed at the same temperature and hold time conditions, are regrouped in Table 7; comments on corresponding aging temperatures and secondary alpha formation are also provided. 3.4.3.2. Thickness of Primary Alpha.

Comparison of average thickness of primary alpha laths between individual sample data for which all other processing steps (steps I, II, III and IVb of Fig. 1) are the same except the cooling rate from the annealing

Fig. 9 – Variation in grain boundary alpha thickness and intragranular alpha thickness with degree of undercooling at 30 min aging.

temperature (step IVa of Fig. 1) is presented in Table 8. In each of such comparison, slower cooling (fc) results in thicker primary alpha laths compared to faster cooling (c.a/c).

3.4.4.

Secondary Alpha

Most of the samples did not show secondary alpha even after aging at 600 and 500 °C for 4 h. Secondary alpha laths of thickness < 45 nm are observed only in a few samples aged at 500 °C, where primary alpha fractions are less than near equilibrium for the corresponding aging temperature. One such example of secondary alpha is shown in Fig. 13 (TEM micrograph of sample BA2).

4.

Discussion

Results of individual single step heat treatments will be discussed first in Sections 4.1, 4.2 and 4.3, which correspond to thermal treatment schedules of Figs. 2, 3 and 4 respectively. In Section 4.4, combined effects of a set of sequential thermo-mechanical steps as indicated in Fig. 1 will be discussed.

4.1. Effect of Variation in Recrystallization Temperature in the Single Step Recrystallization Treatment In Section 3.1, it has been stated that complete recrystallization is observed in 4 mm diameter samples upon single step

Fig. 10 – Plot of grain boundary alpha thickness and intragranular alpha thickness vs. cooling rate.

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energy of 95 kJ/mol obtained for beta phase of Ti–6Al4V alloy [13].

4.2. Effect of Variation in Undercooling Temperature below Beta Transus

Fig. 11 – Plot of volume fraction of alpha vs. cooling rate.

recrystallization performed in dilatometer, at all the recrystallization temperatures used in the present study. However, percentage recrystallization depends not only on amount of prior deformation, recrystallization temperature and hold time, but also on sample size and cooling rate from recrystallization temperature. A systematic study of the effects of these various factors on recrystallization behavior of this alloy is under the scope of ongoing research and will be reported in future paper. The data of Table 2 presented in Section 3.1 show the variation in recrystallized beta grain size with temperature. When natural logarithm of recrystallized beta grain size is plotted against inverse of absolute temperature, a linear plot is obtained, suggesting an Arrhenius relation with activation energy of 90.1 kJ/mol. This value compares with the activation

As described in Section 2.1.2.2, as cast samples were solutionized 150 °C above beta transus aiming for complete homogenization. Post solutionizing, step quenching and holding experiments at different temperatures below beta transus for different times were carried out (refer to Fig. 3) to study the evolution of microstructure under different undercooling conditions below beta transus. The observation of sharp decrease in GB and lath alpha thickness through initial 200 °C undercooling (refer to Fig. 9, Section 3.2) suggests rapid slowing down of alpha growth kinetics with decreasing temperature. So, to achieve fine alpha (both intragranular and GB alpha) precipitates, cooling rate through the first 200 °C below beta transus should be maintained high. The vf data at 750 °C, 30 min holding (25.5%) compares with the data obtained from the projected plot of the beta approach curve reported in literature [14] for this alloy. From that beta approach curve, which was drawn based on data of 1 h holding, projected value of alpha vf is found to be around 27% at 750 °C.

4.3. Effect of Variation in Cooling Rate from Solutionizing Temperature As described in Section 2.1.2.3, as cast samples were homogenized 150 °C above beta transus to get rid of any segregation before cooling to RT at different precisely controlled rates (refer to Fig. 4). As observed in Fig. 10, only through very slow cooling at a rate of 2.5 °C/min, very thick intragranular alpha forms, all other cooling rates invariably produce very thin (<70 nm thick)

Table 5 – Quantified microstructural data with corresponding thermal treatment parameters. Sample Rx temp name and time

CR from Rx temp

(°C, min) (°C/min).

Annealing CR from Aging CR from temp and ann. temp and aging time temp time temp

Primary α thickness

Vf of Secondary primary α α thickness

(°C, min)

(°C/min)

(°C, min)

(°C/min)

(μm)

(%)

0.13 (+/−0.04) 0.16 (+/−0.01)

47 42

BA1 BA2

950, 30 950, 30

67 67

750, 30 750, 30

67 67

400, 60 500, 60

60 60

BA3 BA4 BA5 BA6 BA7

950, 950, 950, 950, 950,

30 30 30 30 15

67 67 67 67 10

750, 30 700, 30 700, 30 700, 30 700, 120

67 15 15 15 15

450, 60 500, 60 450, 60 400, 60 500, 240

60 60 60 60 60

0.13 0.13 0.13 0.100 0.16

(+/−0.02) (+/−0.03) (+/−0.03) (+/−0.01) (+/−0.03)

41 43 40 47 35

BA8 BA9 BA10 BA11

950, 950, 950, 950,

15 15 15 15

10 10 10 67

700, 700, 700, 700,

120 120 120 120

15 10 10 15

600, 500, 600, 500,

240 240 240 240

60 60 60 60

0.15 0.18 0.18 0.14

(+/−0.036) (+/−0.03) (+/−0.035) (+/−0.02)

52 62 53 47

BA12 BA13 BA14

950, 15 950, 15 950, 15

67 67 67

700, 120 700, 120 700, 120

15 10 10

600, 240 500, 240 600, 240

60 60 60

0.19 (+/−0.016) 0.15 (+/−0.030) 0.270 (+/−0.19)

55 66 55

GB α thickness

(μm) – 0.01 (+/−0.001) – – – – 0.040 (+/−0.001) – – – 0.040 (+/−0.001) – – –

(μm) 0.14 (+/−0.04) 0.22 (+/−0.06) 0.24 0.21 0.20 0.15 0.200

(+/−0.08) (+/−0.09) (+/−0.09) (+/−0.09) (+/−0.03)

0.410 0.57 0.54 0.33

(+/−0.14) (+/−0.39) (+/−0.25) (+/−0.03)

0.34 (+/−0.02) 0.27 (+/−0.2) 0.44 (+/−0.140)

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Fig. 12 – SEM BSE micrographs with grain boundaries: a) of sample BA9 showing thicker grain boundary alpha layer; b) of sample BA13 showing thinner grain boundary alpha layers; c) of sample BA2 showing discontinuous grain boundary layer; and d) of sample BA14 showing no grain boundary alpha layer.

intragranular alpha laths. However these laths can grow under subsequent annealing and aging treatments as will be discussed in Section 4.4. GB alpha thickness variation plot shows that it decreases monotonically with increasing cooling rate. Higher cooling rate (> 20 °C/min) is necessary to obtain thinner GB alpha. Air cooling (at approximate rate of 60 °C/min) would result in, on average, thin (< 70 nm) GB alpha. These trends of variation in thickness of intra-granular and GB alpha due to the variation in cooling rates from the solutionizing temperature should be similar both in as cast and as processed samples. Hence these results would give a quantitative effect of cooling rate on thickness of these alpha precipitates in general. Plot of variation of vf with cooling rate (Fig. 11) reveals that very slow cooling at 2.5 °C/min cooling rate results in alpha fraction near equilibrium (~ 60%). Higher cooling rates (>10 °C/min) result in lower fraction of alpha phase. Subsequent annealing and aging treatments at alpha + beta phase field are necessary to attain near-equilibrium fraction of alpha. The combined effects of various thermo-mechanical processing steps are described next in Section 4.4.

4.4. Effects of Variation in Cooling Rates and Temperatures at Different Steps of a Combined Set of Sequential Thermo-mechanical Processing Steps Leading to Beta Annealing Microstructure In this section, combined effects of a set of sequential processing steps (deformation, recrystallization, annealing, aging) as explained in Section 2.1.1 on various microstructural features (prior beta grain size and size and vf of GB alpha, primary intragranular alpha and secondary intragranular alpha) of beta annealed microstructure will be discussed with reference to data for samples BA1 through BA14 as given in Table 5 and Section 3.4.

4.4.1.

Recrystallized Beta Grain Size

Samples BA1 through BA14 have been recrystallized in regular furnace. Recrystallized grain sizes for these samples, as given in Section 3.4, are much larger compared to the recrystallized grain sizes of single step recrystallization study done at dilatometer on smaller sized samples as explained in Sections 3.1 and 4.1. This indicates that recrystallization carried out on larger samples

Table 6 – Effect of variation in CR from Rx temperature on grain boundary alpha layer thickness. Sample name

BA8 BA12 BA9 BA13 BA10 BA14

CR from ann. Aging temp and CR from aging temp time temp

Avg. GB α thickness

Rx temp and time

CR from Rx temp

Annealing temp and time

°C, min

°C/min

°C, min

°C/min

°C, min

°C/min

(μm)

950, 15 950, 15 950, 15 950, 15 950, 15 950, 15

Fc Ac Fc Ac Fc Ac

700, 120 700, 120 700, 120 700, 120 700, 120 700, 120

c.a/c c.a/c fc fc fc fc

600, 240 600, 240 500, 240 500, 240 600, 240 600, 240

ac ac ac ac ac ac

0.41 0.34 0.57 0.27 0.54 0.44

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MA TE RI A L S CH A R A CT ER IZ A TI O N 8 1 (2 0 1 3) 3 7– 4 8

Table 7 – Summary of data on vf of alpha for samples BA7 through BA14.

4.4.3.

Sample name

Volume fraction of primary α (%)

BA7 BA8 BA9 BA10 BA11 BA12 BA13 BA14

35 52 62 53 47 55 66 55

4.4.3.1. Volume Fraction of Primary Alpha. Low vf of alpha for 30 min annealing time, as described in Section 3.4.3.1, indicates to very slow precipitation kinetics in this alloy, and as a result it demands the annealing hold time should at least be for 120 min to obtain larger vol.% of primary alpha. This point will be illustrated in greater detail through the TTT plot for alpha phase in this alloy discussed later in Section 4.5. Table 7 presented in Section 3.4.3.1 indicates that vf of total intragranular alpha corresponding to aging temperature of 500 °C (aging time 4 h) is 62–66% (BA9, BA13), whereas the same corresponding to the aging temperature of 600 °C (aging time 4 h) is 52–55% (BA8, 10, 12, 14). These vf of intragranular alpha are indicative of near equilibrium vf of alpha at the temperatures of 500 °C and 600 °C respectively, which will be used in TTT diagram construction as described in Section 4.5.

Comment Aging temperature: Aging temperature: Aging temperature: Aging temperature: Aging temperature: Aging temperature: Aging temperature: Aging temperature:

500 600 500 600 500 600 500 600

°C. °C °C °C °C. °C °C °C

resulted in much larger recrystallized beta grain size compared to precise heat treatment done on 4 mm diameter samples in dilatometer (refer to Section 3.1, and 4.1 and Table 2). This deviation may be attributed to difference in sample size, as well as, difference in preciseness in temperature control between the two cases. Larger sample size for the samples BA1 through 14 as compared to the dilatometer samples (used for single step recrystallization study) not only causes uninhibited grain growth (no sample size restriction), but also ensures much slower cooling rate causing further grain growth during cooling through the higher temperature range.

4.4.2.

Grain Boundary Alpha

As described in Section 3.4.2, the GB alpha thickness for the samples BA1 through 14, which underwent the full set of sequential processing steps, is much larger as compared to that of samples that underwent single step cooling from the solutionizing temperature. This is because of subsequent growth of GB alpha during subsequent steps after the step they had been formed. To obtain thinner grain boundary alpha layer, which may be desirable for some mechanical properties, a faster cooling rate (ac or fan ac) from recrystallization temperature should be performed. Table 5 shows that, on average, the samples BA1 through BA3, all air cooled from recrystallization temperature have thinner GB alpha of 138–239 nm thickness compared to the samples BA11 through BA14 with thicker GB alpha of 271–444 nm, which also underwent air-cooling at recrystallization step (step III of Fig. 1). The latter samples (BA11 through BA14) have undergone much more hold time in subsequent heat treatment steps (annealing (step IVa) and aging (step IVb) steps of Fig. 1) compared to the former samples (BA1–BA3), however, these latter samples were held at recrystallization temperature (recrystallization step III of Fig. 1) for less time and also for these latter samples the annealing temperature (annealing step IVa of Fig. 1) was lower. Although smaller hold time at the recrystallization temperature (step III) causing smaller beta grains and more grain boundaries and also lower annealing temperature (step IVa) should have caused the latter samples to have thinner GB alpha, the larger hold times at subsequent annealing (step IVa) and aging (step IVb) steps for these samples (BA11 through BA14) have allowed further GB alpha growth in thickness.

Primary Alpha Lath

4.4.3.2. Thickness of Primary Alpha. Comparisons of thickness of alpha laths in Table 8, as presented in Section 3.4.3.2, show that slower cooling rate from the annealing temperature at step IVa of Fig. 1 results in coarser intragranular primary alpha. Therefore, it is concluded that the cooling rate from annealing temperature majorly affects the intragranular primary alpha lath thickness. 4.4.4.

Secondary Alpha

As mentioned in Section 3.4.4, secondary alpha is not observed in most of the samples even after aging for 4 h. Only in a couple of samples aged at 500 °C, where vf of primary alpha was less than near equilibrium for the aging temperature, finer secondary alpha laths of thickness <45 nm are observed. This indicates that during aging (step V of Fig. 1), instead of nucleation of secondary alpha in the solute (beta-stabilizer) enriched beta matrix, in most cases, further growth of already precipitated primary alpha was favored. It is the relative kinetics between nucleation of secondary alpha and growth of already precipitated primary alpha during aging, which would dictate the formation of secondary alpha in this alloy.

4.5.

Construction of TTT Diagram

From literature, it is found that the upon solutionizing followed by step quenching and holding for longer time at 750 °C and 650 °C, Ti-5553 alloy results in around 40 and 48% of alpha respectively [8]. Considering these data as near equilibrium vf of alpha at corresponding temperatures, these data as well as corresponding near equilibrium vf of alpha at 500 and 600 °C as discussed in Section 4.4.3.1 are plotted in Fig. 14. From this plot, the near equilibrium vf of alpha at 550 °C has been interpolated to be around 60%. Volume fraction data of step-quenched samples at 5 min and 30 min at three different step-quenching temperatures (presented and discussed in Sections 3.2 and 4.2) and the above mentioned long term aging data (as equilibrium vf of alpha for corresponding temperatures) have been used to construct a plot of fraction transformed vs. log (time), as shown in Fig. 15(a). Fraction transformed reaches the value of 100% at equilibrium alpha fractions for corresponding temperature; these are shown by dashed and dotted horizontal lines in Fig. 15(a).

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Table 8 – Effect of variation in cooling rate from annealing temperature on average thickness of primary alpha laths. Sample name BA8 BA10 BA11 BA13 BA12 BA14

Rx temp and CR from Rx time temp 950 950 950 950 950 950

°C, 15 °C, 15 °C, 15 °C, 15 °C, 15 °C, 15

min min min min min min

fc fc ac ac ac ac

Annealing temp and time 700 700 700 700 700 700

°C, 120 °C, 120 °C, 120 °C, 120 °C, 120 °C, 120

CR from ann. temp

min min min min min min

c.a/c fc c.a/c fc c.a/c fc

Aging temp and time 600 600 500 500 600 600

°C, 240 °C, 240 °C, 240 °C, 240 °C, 240 °C, 240

min min min min min min

CR from aging temp

Avg. prim. α lath thickness (μm)

ac ac ac ac ac ac

0.15 0.18 0.14 0.15 0.19 0.27

Recrystallization kinetics demands plots of Fig. 15(a) to be sigmoidal curves. Vf data of 5 min and 30 min fall in the straight line portion of the sigmoidal curves. The curves taper down at the start (0% transformation) and at the completion (100% transformation) of alpha transformation at any temperature. The straight line portion of the curves in Fig. 15(a) joining the vf data of 5 min and 30 min is extended beyond these points and shown in dotted line till the 0% and 100% transformation horizontal lines, where they are tapered down to meet the horizontal lines. From this plot, a time–temperature transformation (TTT) diagram has been constructed as shown in Fig. 15(b). Near start (~1%) and near end (~99%) of transformation are projected in the plot of temperature vs. log time. As a result “C” shaped curves for near start and near completion of transformation for alpha phase are obtained as shown in Fig. 15(b). The ‘C’ curves show the nose at a temperature close to 700 °C, where the alpha precipitation kinetics would be the fastest, and below this temperature slow kinetics demands longer hold time at lower temperatures, for example, more than 5.5 h hold is necessary at 500 °C to reach near equilibrium vf of intragranular alpha in a beta matrix where beta matrix is not enriched with extra solute content through alpha precipitation at any prior heat treatment step. In the beta matrix, which is enriched with extra solute content through alpha precipitation at

prior heat treatment steps, holding time at the aging temperature should be much larger to obtain fine secondary alpha platelets.

Fig. 13 – TEM micrograph of secondary alpha laths in sample BA2.

Fig. 14 – Variation of near equilibrium vf of alpha as a function of temperature.

5.

Conclusions

• A detailed stereology based quantitative characterization has been carried out on variety of microstructures of Ti-5553 alloy obtained through variation in processing parameters. Effect of these processing parameters on quantitative values of size and volume fraction of microstructural features has been presented here. These stereologically quantified microstructure data under different processing conditions will provide a modeler with a rich dataset to develop models relating processing, microstructure and mechanical properties in this commercially important near beta Ti alloy. • Recrystallized beta grain size follows an Arrhenius type relation with temperature with apparent activation energy of ~90 kJ/mol. • To obtain thinner grain boundary alpha layer, which may be desirable for some mechanical properties, a faster cooling rate (air cooling or fan air cooling) from recrystallization temperature should be performed. • The cooling rate from annealing temperature majorly affects the intragranular primary alpha lath thickness. • A TTT diagram for alpha phase formation has been constructed for the alloy which can be a guideline for various heat treatments. The ‘C’ curves show the nose at a temperature close to 700 °C, where the alpha precipitation kinetics would be the fastest, and below this temperature slow kinetics

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MA TE RI A L S CH A R A CT ER IZ A TI O N 8 1 (2 0 1 3) 3 7– 4 8

the project under ER & IPR scheme& (ER & IPR No. ERIPR/ER/ 1100397/M/01/1382). The authors would like to acknowledge Prof. Dipankar Banerjee of IISC Bangalore for his technical inputs and guidance throughout the course of this work.

REFERENCES

Fig. 15 – TTT diagram for intragranular alpha phase formation in Ti-5553 alloy.

demands longer hold time at lower temperatures, for example, more than 5.5 h hold is necessary at 500 °C to reach near equilibrium vf of intragranular alpha in a beta matrix where beta matrix is not enriched with extra solute content through alpha precipitation at any prior heat treatment step. • In the beta matrix, which is enriched with extra solute content through alpha precipitation at prior heat treatment steps, precipitation of secondary alpha precipitates demand a much larger hold time at the aging temperature than the time predicted by the TTT diagram. During the last step aging, the growth of already precipitated alpha is favored in comparison with the nucleation of secondary alpha.

Acknowledgment We are grateful to Director DMRL, Dr G. Malakondaiah, DS for allowing us to publish the results and also for providing the material. We also thank DRDO HQ, New Delhi for granting

[1] Boyer RR, Briggs RD. The use of beta titanium alloys in the aerospace industry. J Mater Eng Perform 2005;14:681–5. [2] Dehghan-Manshadi A, Dippenaar RJ. Development of α-phase morphologies during low temperature isothermal heat treatment of a Ti–5Al–5Mo–5V–3Cr alloy. Mater Sci Eng A 2011;528:1833–9. [3] Jones NG, Dashwood RJ, Jackson M, Dye D. Development of chevron-shaped a precipitates in Ti–5Al–5Mo–5V–3Cr. Scr Mater 2009;60:571–3. [4] Nag S, Banerjee R, Hwang JY, Harper M, Fraser HL. Elemental partitioning between α and β phases in the Ti–5Al–5Mo–5V–3Cr–0.5Fe (Ti-5553) alloy. Philos Mag 2009;89: 535–52. [5] Nag S, Banerjee R, Srinivasan R, Hwang JY, Harper M, Fraser HL. Omega-assisted nucleation and growth of alpha precipitates in the Ti–5Al–5Mo–5V–3Cr–0.5Fe beta titanium alloy. Acta Mater 2009;57:2136–47. [6] Lütjering G, Williams JC. Titanium. . 2nd ed.New York: Springer; 2007. [7] Welk BA. M.S thesis. Microstructural and property relationships in titanium alloy Ti-5553. Columbus: The Ohio State University; 2010. [8] Foltz JW. PhD dissertation. The relationships between microstructure, tensile properties and fatigue life in Ti–5Al–5V–5Mo–3Cr–0.4Fe (Ti-5553). Columbus: The Ohio State University; 2010. [9] Kar SK, Searles T, Lee E, Viswanathan GB, Tiley J, Banerjee R, et al. Modeling the tensile properties in β-processed α/β Ti alloys. Metall Mater Trans A 2006;37A:559–66. [10] Tiley J, Searles T, Lee E, Kar SK, Banerjee R, Russ JC, et al. Quantification of microstructural features in α/β titanium alloys. Mater Sci Eng A 2004;372:191–8. [11] Searles T, Tiley J, Tanner A, Williams R, Rollins B, Lee E, et al. Rapid characterization of titanium microstructural features for specific modelling of mechanical properties. Meas Sci Technol 2005;16:60–9. [12] Russ J, Dehoff R. Practical stereology. . 2nd ed.New York: Plenum Publishers; 2000. [13] Gil FX, Rodriguez D, Planell JA. Grain growth kinetics of pure titanium. Scr Mater 1995;33:1361–6. [14] Jones NG, Dashwood RJ, Dye D, Jackson M. Thermomechanical processing of Ti–5Al–5Mo–5V–3Cr. Mater Sci Eng A 2008;490:369–77.