Dynamic fragmentation and spheroidization of α phase grains ...

Maciej Motyka, Jan Sieniawski, Waldemar Ziaja, Malwina Mroczka, Mateusz Brylinski

Department of Materials Science, Faculty of Mechanical Engineering and Aeronautics, Rzeszow University of Technology,Rzeszow, Poland

Dynamic fragmentation and spheroidizationof a phase grains during hot deformationof Ti-6Al-4V alloyPaper presented at the \XIII Scientific Conference on Titanium and Titanium Alloys 2017",24–27 September 2017, Janów Podlaski, Poland

Development of mechanical properties of titanium alloys isusually achieved by the control of their microstructure(globular, lamellar, bimodal). Dynamic fragmentation andspheroidization of elongated a phase grains, formed frommartensite laths, in the microstructure of Ti-6Al-4V alloyare considered in the paper. The effects of a phase grainspheroidization was evaluated in tensile tests over a tem-perature range of 800–900 8C and strain rate range of10–3–10–2 s–1. Completed spheroidization of elongated aphase grains and subsequent growth of equiaxed grains forall deformation modes were found. A low value of apparentactivation energy for deformation was calculated – suggest-ing grain boundary sliding was the main deformation mech-anism. The results showed good correlation with the kineticrate equation based on the Zener–Hollomon parameter.

Keywords: Two-phase titanium alloys; Hot deformation;Microstructure; Fragmentation and spheroidization of aphase grains; Superplasticity

1. Introduction

Durability and reliability of structural elements made oftwo phase titanium alloys depend on their microstructure,

which can be developed over a wide range – from lamellarto equiaxed. The morphology of phase constituents is deter-mined by effects of phase transformations and dynamic re-covery and recrystallization processes – taking place duringheat treatment and hot working operations [1, 2]. It wasfound in previous investigations [3, 4] that a phase grains,highly elongated and deformed in thermomechanical pro-cessing can transform into equiaxed ones during a sub-sequent hot deformation operation. The morphology ofequiaxed a phase grains is then stable enough for superplas-tic deformation processing. Such knowledge enables reduc-tion of heat treatment operations – i. e. recrystallizationannealing, usually used for developing equiaxed micro-structure after deformation. The idea of the present researchwas to check that behavior over a wider temperature rangeand to evaluate microstructural evolution in Ti-6Al-4V al-loy.

Dynamic and static spheroidization processes of a grainsin titanium alloys were extensively researched and de-scribed by Semiatin and coauthors [5–7]. They describedthe evolution of microstructure during the thermomechani-cal processing of titanium alloys and quantified the mecha-nisms of spheroidization and other phenomena such ascoarsening, particle growth, and phase decomposition in ti-tanium alloys. Zherebtsov and others [8, 9] thoroughly de-

M. Motyka et al.: Dynamic fragmentation and spheroidization of a phase grains during hot deformation of Ti-6Al-4V

Int. J. Mater. Res. (formerly Z. Metallkd.) 109 (2018) 8 685

International Journal of

MATERIALS RESEARCH

Zeitschrift fur METALLKUNDE

OOriginal Contributions

scribed spheroidization in the lamellar microstructure of Ti-6Al-4V alloy due to warm deformation and annealing. Theyfound that deformed lamellae were prone to static spheroi-dization whereas during warm deformation they were sur-prisingly stable, especially in so-called kinked regions.

It is generally accepted that spheroidization of a lamellaeis realized through their fragmentation caused by plastic de-formation in the temperature range of a+b?b transforma-tion, or lower. It is assumed that stress caused by pile-up ofdislocations leads to shearing and rotation of grains – initialstage of fragmentation. Formation of low-angle and subse-quently high-angle boundaries in the smallest cross-sectionof a lamellae causes their partition and spheroidization(Fig. 1a). The fragmentation of a phase lamellae can becaused by shear bands forming during plastic deformation(Fig. 1b) [10, 11].

The latest results on a phase spheroidization [12] provedthat the size of fragmented pieces decreased with an in-crease of hot deformation strain. The effect of deformationmode was also established – the change of compressionaxes favored complete spheroidization.

It should be pointed out that boundary splitting during thefragmentation is related to the thermal grooving process.The classical model of \grooving" – proposed by Mullins[13] – is based on grain growth of one phase (b) into inter-face (a/b) causing grain boundary formation (a/a) in adja-cent grain of the second phase (a) (Fig. 2).

Boundary splitting is generally considered to occur at theinitial stages of a phase grain spheroidization [14]. In thecase of static spheroidization (during annealing after warmrolling) dislocations of the same sign are first arranged toform low angle (h < 158) intra-lamellar a/a boundariesthrough which the adjacent b phase penetrates. It seems thisprocess is realized until a local equilibrium in surface ten-sion is set between the inter-lamellar a/b boundaries andthe intra-lamellar a/a boundary at the triple point. The ther-mal grooving process is related to the mass transport fromthe curved interfaces at the groove area to the flat interfacesof the same or adjacent a lamella. This leads to increasingthe dihedral angle, and dragging the triple point deeper untilthe equilibrium is re-established. As a consequence the in-tra-lamellar a/a boundary is being continuously eliminatedand new a/b can be formed [15].

Although fragmentation and spheroidization processes ofa phase grains in titanium alloys have been reported inmany papers [5–12, 16], their kinetics is strongly relatedto the parameters of initial microstructure – usually a lamel-lar one developed in heat treatment operations. The aim ofthis paper was to describe microstructural evolution in Ti-6Al-4V alloy with highly elongated a phase lamellae,formed from deformed martensite. The investigation wasbased on the results of tensile tests at elevated temperature:800–900 8C and strain rate of 10–3–10–2 s–1. It was in-tended to evaluate the degree of dynamic spheroidizationof elongated a phase grains and subsequent growth ofequiaxed grains. Deformation behavior was analyzed based

on the calculated apparent activation energy for deforma-tion and the correlation with the kinetic rate equation.

2. Research methodology

Martensitic two-phase a+b Ti-6Al-4V titanium alloy (Al –6.78, V – 4.38 Fe – 0.18, Si – 0.33 wt.% – Ti – balance –bars supplied by Teledyne Allvac) was examined. Cylindri-cal samples (1 = 22 mm, l = 70 mm) cut from the barswere thermomechanically processed (TMP):. preliminary heat treatment – water quenching from

1050 8C – stable b phase range (based on dilatometricresults [2]). The purpose was to achieved a martensiticmicrostructure in the examined alloy.

. plastic deformation in the a+b«b phase transforma-tion range (*900 8C) through open die forging in indus-try conditions using an MPM1000 double-acting powerhammer. Water quenched bars were heated up 980 8Cin the furnace and immediately locally necked downwith the area reduction of about 50%.

Tensile tests at elevated temperature were carried out usingan Instron 5982 testing machine. Round samples (d0 =6 mm, l0 = 8 mm) were deformed without protective atmo-sphere at temperature of 800, 825, 850, 875 or 900 8C andstrain rates 10–3, 5 · 10–3 or 10–2 s–1. Based on tensile curvesthe values of peak flow stress (rpm) and total elongation (A)were determined.

A light microscope (LM) Nikon Epiphot 300 equippedwith digital camera DeltaPix was used for microstructuralexamination. Metallographic specimens were etched usingKroll’s reagent. Evaluation of stereological parameters ofmicrostructural constituents was performed on longitudinaletched microsections using quantitative metallographicmethods and image analysis software Aphelion 3.2. Thegrain size of a phase was determined based on the area ofa rectangle circumscribed on the grain section – equivalentdiameter da for circle having the same area was calculatedand statistically analyzed.


686 Int. J. Mater. Res. (formerly Z. Metallkd.) 109 (2018) 8

Fig. 2. Schematic of b phase grain growth by \grooving" of a/b inter-phase [13].

Fig. 1. Fragmentation mechanism of a phaselamellae: (a) formation of dislocation sub-structure, (b) localized shear [11].

3. Results

The initial microstructure (Fig. 3) – after thermomechanicalprocessing – was composed of highly elongated a phasegrains in a matrix of b phase (similarly to that reported in[2–4]). The estimated thickness of a lamellae was*2 lm.

During the tensile tests complete spheroidization of agrains took place – for all temperatures and strain rates ap-plied (Figs. 4–8). No elongated a grains were found in thedeformed zone of samples. Histograms of a phase graindiameter da drawn after all deformation modes exhibitedgood agreement of values distribution with normal (Gaus-sian) one (Fig. 9). The smallest da = 1.9 lmwas determinedfor the alloy after the tensile test carried out at 900 8C withstrain rate of 10–3 s–1, the largest – da = 9.97 lm – for the800 8C/10–2 s–1 variant (Table 1).

Tensile tests at elevated temperature were realized with-out the measurement of sample diameter. Therefore,stress–strain curves (Fig. 10) were drawn based on initialsample diameter. It was assumed that for small strain (A <



(a)

(b)

Fig. 4. Microstructure of thermomechanically processed Ti-6Al-4Valloy after tensile test at 800 8C and strain rate: (a) 10–3, and(b) 10–2 s–1.

(a)

(b)

Fig. 5. Microstructure of thermomechanically processed Ti-6Al-4V al-loy after tensile test at 825 8C and strain rate: (a) 10–3, and (b) 10–2 s–1.

Fig. 3. Highly elongated grains of a phase in microstructure of ther-momechanically processed Ti-6Al-4V alloy (water quenching fromthe temperature of 1050 8C and subsequent hot forging at 900 8C).

15%) necking would be quite small and calculated peakstress (rpm) values (Table 2) are slightly lower than trueones. However, the softening effect showed by the curves(Fig. 10) was higher than the one expressed in terms of truestress. Full stress–strain curves are presented for showingdifferences in achieved elongation. Results of elongationwere contained in the range of 317–581% (Table 3).

4. Discussion

The hot deformation behavior of two-phase a+b titaniumalloys is a problem most frequently investigated in Ti-6Al-4V alloy [10, 11, 16]. It is generally accepted that for strainrates lower than 10–1 s–1 at temperatures within a+b«bregion as well as in the b phase field, the stress–straincurves are steady-state type [16]. They indicate that themechanisms of softening are sufficiently fast to balancethe rate of work hardening. Suggested mechanisms are dy-namic recrystallization (DRX), superplasticity or dynamicrecovery (at high strain rates).

Determination of deformation kinetics requires analysisof the flow stress as a function of temperature and strainrate [16]. That behavior is generally expressed in terms ofa kinetic rate equation [17]:

_e ¼ Arne �QR

Tð Þ ð1Þ

where: _e – strain rate, r – flow stress, A – frequency factor,Q – apparent activation energy, R – gas constant, T – tem-perature (in Kelvin), n – stress exponent.

The stress exponent n is an inverse of strain-rate sensitiv-ity index m expressed by [18]:

m ¼d log r

d log _eð2Þ

Based on variation of flow stress with strain rate on a log–log scale (Fig. 11) and Eq. (2) the strain rate sensitivity in-dex m was determined. The calculated value – about 2.5 –is smaller than reported in literature (3.4 [16]) due to thequite high value of strain-rate sensitivity index m *0.4 –typical for fine-structure superplasticity (FSS) [18].



(a)

(b)

Fig. 7. Microstructure (DIC) of thermomechanically processed Ti-6Al-4V alloy after tensile test at 875 8 C and strain rate: (a) 10–3, and(b) 10–2 s–1.

(a)

(b)

Fig. 6. Microstructure (DIC) of thermomechanically processed Ti-6Al-4V alloy after tensile test at 850 8C and strain rate: (a) 10–3, and(b) 10–2 s–1.

The Arrhenius plot for estimating the apparent activationenergy for hot deformation, in the a+b region, is drawn inFig. 12. Quite good correlation was found, especially forstrain rates of 10–3 and 10–2 s–1. Estimated apparent activa-tion energy was about 140 kJ · mol–1. The calculated value

was about two times lower than reported in the literaturefor Ti-6Al-4V alloy – *330 kJ · mol–1 [16, 19, 20]. It isworth adding, that these values are related to different typesof microstructure – an equiaxed one and a phase grain size*8 lm. Therefore, it was assumed in present work that



(a)

(b)

Fig. 9. Histograms of a grain diameter in Ti-6Al-4V alloy after tensiletest at (a) 800 8C, and (b) 825 8C.

(a)

(b)

Fig. 8. Microstructure (DIC) of thermomechanically processed Ti-6Al-4V alloy after tensile test at 900 8C and strain rate: (a) 10–3, and(b) 10–2 s–1.

Table 1. Grain diameter of a grains determined for Ti-6Al-4V titanium alloy deformed in tensile tests under various conditions.

Equivalent a phase grain diameter da (lm) (standard deviation)

Deformation

temperature

(8C)

Strain rate (s–1)

800 825 850 875 900

10–3 3.44 (0.96) 3.76 (1.03) 6.14 (1.44) 6.84 (1.62) 9.97 (2.30)

5 · 10–3 2.19 (0.53) 2.74 (0.68) 4.15 (0.96) 4.29 (0.97) 6.42 (1.34)

10–2 1.90 (0.51) 2.31 (0.57) 3.27 (0.75) 4.31 (1.08) 5.64 (1.28)



(a) (b)

(c) (d)

(e)

Fig. 10. Tensile curves of thermomechanically processed Ti-6Al-4V alloy deformed at: (a) 800, (b) 825, (c) 850, (d) 875, and (e) 900 8C.

the deformation mechanism is controlled by diffusionalprocesses (energy of self-diffusion in Tia Q = 150 kJ ·mol–1 [16]). It was assumed, due to spheroidal shape of aphase grains after hot deformation (Figs. 4–8), that themost probable deformation mechanism was grain boundarysliding (GBS). The activation energy for grain boundarysliding in Ti-6Al-4V alloy is 174 kJ · mol–1 and it is inagreement with the values of grain boundary diffusion [21,22]. Once again the data refer to equiaxed microstructure

and a phase grain size *6 lm. In the present work the aphase grain size (Table 1) depended on deformation condi-tions – temperature and strain rate. The highest diameterswere obtained for the lowest strain rates and it was relatedto grain growth – which will be discussed later. Based onquantitative metallography results (Table 1) it was assumedthat the diameter of newly formed spheroidal a grainsshould be similar to the width of a lamella from which theywere formed (Fig. 3) – *2 lm. Grain refinement favors



Table 2. Peak stress rpm during tensile tests at elevated temperature of Ti-6Al-4V titanium alloy.

Peak stress rpm (MPa)

Deformation

temperature

(8C)

Strain rate (s–1)

800 825 850 875 900

10–3 48.7 40.3 34.8 31.6 27.8

5 · 10–3 91.3 67.0 68.0 60.3 49.1

10–2 119.1 109,8 91.4 85.0 69.0

Table 3. Elongation A in tensile tests at elevated temperature of Ti-6Al-4V titanium alloy.

Elongation A (%)

Deformation

temperature

(8C)

Strain rate (s–1)

800 825 850 875 900

10–3 530 409 415 325 459

5 · 10–3 468 524 460 317 581

10–2 569 442 389 365 341

Fig. 12. Dependence of stress rpm on deformation temperature in thea+b regime.

Fig. 11. Stress (rpm) – strain rate relationship for applied deformationtemperatures.

GBS during hot deformation [18]. It has been reported forcommercially pure titanium (Grade 2) that grain refinementto the submicrocrystalline level (da *0.8 lm) reduced thevalue of activation energy of GBS from 140 kJ · mol–1 to80 kJ · mol–1 [23].

On the basis of calculated apparent activation energy it ispossible to extend the analysis of deformation kinetics tak-ing into consideration the temperature compensated strainrate parameter Z (Zener–Hollomon parameter), given by:

Z ¼ _eeQR

Tð Þ ð3Þ

The relationship rpm – Z exhibits a good linear fit (Fig. 13)and confirms that the kinetic rate equation (Eq. (3)) isobeyed in the limits of temperature and strain rate beingconsidered.

Hot deformation behavior of Ti-6Al-4V alloy is deter-mined by its starting microstructure and phase constituentsevolution during deformation. It was found that tensile de-formation over the temperature range 800–900 8C andstrain rate range 10–2–10–3 s–1 led to complete spheroidiza-tion (Figs. 4–8) of highly elongated grains of a phase(Fig. 3). The resulting microstructure is composed ofequiaxed a grains and intergranular b phase. The exceptionis the microstructure after deformation at 900 8C (Fig. 8).Based on calorimetric measurement of the investigated al-loy [2] that value is in the range of a+b?b phase transfor-mation. Equiaxed grains of transformed b phase – espe-cially formed after deformation at the strain rate of 10–3 s–1

– were found in the microstructure (Fig. 8a). The effects ofphase transformation obviously influence plastic deforma-tion – this aspect was not considered in similar works [10,11, 16].

Results of microscopic observations (Figs. 4–8) and quan-titative analysis (Table 1) showed that the a phase grains growduring hot deformation. Analysis indicated that process wasfavored by increasing deformation temperature and reducingstrain rate (Fig. 14). It is worth noting that a grains preservedtheir spheroidal shape up to rupture. This is typical for super-plastic deformation where the GBS is a main deformationmechanism [3, 18]. Fine-structure superplasticity seems alsoto be confirmed by high values of elongation (Table 3).

Usually helpful for determination of deformation mecha-nism is the relationship grain size – Zener–Hollomon pa-rameter (Fig. 15). It is generally accepted that a linear fitof that data confirms DRX during deformation [17]. The re-sults are significantly scattered (Fig. 15) so their unequivo-cal interpretation is difficult. However a linear fit could beconsidered, particularly the grain growth a phase was found– usually connected with recrystallization.

5. Conclusions

Understanding the effect of microstructure on hot deforma-tion behavior of Ti-6Al-4V alloy also requires analysis ofphase constituents’ evolution during the deformation. Espe-cially a phase grains elongated in thermomechanical pro-cess are not stable during subsequent hot deformation op-erations. They are prone to dynamic fragmentation andspheroidization. Based on analysis of this behavior in ten-sile tests over the temperature range 800–900 8C (a+b re-gion) and strain rate range of 10–3–10–2 s–1 the followingconclusions can be drawn:



Fig. 14. Dependence of a phase grain size on deformation temperatureand strain rate.

Fig. 15. Variation of a phase grain size with Z parameter in the a+b re-gion of Ti-6Al-4V.

Fig. 13. Flow stress rpm – Z parameter relationship in the a+b range inTi-6Al-4V alloy.

1. Elongated a phase grains in the initial microstructure ofTi-6Al-4V alloy were completely spheroidized for alldeformation modes used.

2. The material exhibits fine-structure superplasticity –equiaxed a phase grains preserved their shape until rup-ture at high values of elongation.

3. Grain boundary sliding was found to be the main defor-mation mechanism – based on strain-rate sensitivity in-dex m = 0.4 and low value of apparent activation energyQ = 140 kJ · mol–1.

4. Despite the significant change of a grain morphologycaused by deformation, the relationship peak stress rpm– Z exhibits good correlation with the kinetic rate equa-tion based on the Zener–Hollomon parameter.

5. Equiaxed a phase grains, formed by dynamic fragmen-tation and spheroidization, are prone to significant graingrowth – especially for higher deformation temperatureand lower strain rates.

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(Received October 3, 2017; accepted March 2, 2018; onlinesince June 20, 2018)

Correspondence address

Associate Professor Maciej Motyka, D.Sc.Department of Materials ScienceFaculty of Mechanical Engineering and AeronauticsRzeszow University of TechnologyAl. Powstancow Warszawy 12Rzeszow 35-959PolandTel.: +48178651123Fax: +48177432414E-mail: [email protected]

BibliographyDOI 10.3139/146.111660Int. J. Mater. Res. (formerly Z. Metallkd.)109 (2018) 8; page 685–693# Carl Hanser Verlag GmbH & Co. KGISSN 1862-5282



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