Materials Letters 232 (2018) 142–145

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Materials Letters

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Shear localization and recrystallization in high strain rate deformation inTi-5Al-5Mo-5V-1Cr-1Fe alloy

https://doi.org/10.1016/j.matlet.2018.08.0950167-577X/� 2018 Published by Elsevier B.V.

⇑ Corresponding author.E-mail address: pwchen@bit.edu.cn (P. Chen).

Chun Ran, Pengwan Chen ⇑State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, PR China

a r t i c l e i n f o

Article history:Received 2 May 2018Received in revised form 17 August 2018Accepted 18 August 2018Available online 20 August 2018

Keywords:Ti-5Al-5Mo-5V-1Cr-1Fe alloyMicrostructureRecrystallizationDeformation and fractureAdiabatic shear band

a b s t r a c t

To study the dynamic behavior and microstructural evolution in high strain rate deformation of Ti-5Al-5Mo-5V-1Cr-1Fe (Ti-55511) alloy, a series of forced shear tests of hat shaped specimens have been con-ducted using a split Hopkinson pressure bar combined with ‘‘strain frozen” technique. Localized shearband is induced in these tests. This paper indicates that the flow stress of Ti-55511 alloy is independenton the punching depth, and thermal softening has a minor effect on the onset of adiabatic shear band anddynamic recrystallization formation. The concept of ‘‘adhesive fracture” can be identified as the dynamicfailure mechanism for Ti-55511 alloy based on the crack propagation path.

� 2018 Published by Elsevier B.V.

1. Introduction

The term ‘‘Adiabatic shear band” (‘‘ASB”) has been widelyaccepted by researchers since it was first proposed by Zener andHollomon [1] and it is a well-known failure mechanism and occursquite frequently in a variety of materials in dynamic loading situ-ations [2]. Ti-5Al-5Mo-5V-1Cr-1Fe (Ti-55511), a typical near b typetitanium alloy, is superior as an aircraft structural material due toits 15–20% weight loss as compared to Ti-6Al-4V alloy [3]. By con-trast with a vast body of literature dedicated to the dynamicbehavior and microstructural evolution of titanium alloys [4–7],there are few experimental attempts to investigate on Ti-55511alloy of the same problem [3]. Although the main mechanism foradiabatic shear is the competition between hardening effect (strainand strain rate) and thermal softening effect, the whole process isvery complex and involves high strain rates, high local tempera-ture, large plastic deformation and so forth [2]. However, themechanical behavior and microstructural evolution in Ti-55511alloy in dynamic deformation process are still not well understood.In addition, although Timothy and Hutchings [8] and Xue et al. [4]have pointed out that the main stage of void evolution within shearbands in Ti-6Al-4V alloy are nucleation, growth and coalescence,the crack propagation path is still not totally clear. Therefore, thepurpose of this work is initiated to bring experimental evidenceon microstructural evolution within ASB.

2. Material and methods

The Ti-55511 alloy used in the present investigation was in theform of forged bar with a diameter of 155 mm from AECC BeijingInstitute of Aeronautical Materials, PR China. A series of forcedshear tests were carried out at 293 K by means of split Hopkinsonpressure bar technique using hat-shaped (HS) specimens, details ofthe testing were described previously [3]. This specially designedspecimen configuration allows the creation of a well-controlledlocalized shear band during deformation and has been successfullyused in the studies of large strain, high strain rate deformation ofmetals in conditions of forced localized shear [9–13]. High-strength steel stopper rings were used to ensure a prescribed dis-placement in the principal plastic deformation region, differentshear strains were obtained by varying the thickness of the stopperring. It should be pointed out that we just focus on the microstruc-tural evolution of Ti-55511 alloy after ASB formed in this work andthree HS specimens were used for each loading condition.

The samples for microstructural observation were cut parallelto the deformation direction by electrical discharge machiningand metallographic specimens were prepared by standardmechanical polishing and etched in the Kroll’s reagent. Transmis-sion electron microscopy (TEM) observation was focused on thecrack tip and its adjacent region, and the TEM samples were pre-pared by focused ion beam (FIB) technique, detail informationwas described by Schaffer et al. [14]. Optical microscopy (OM)and TEM observations were performed on LEICA DMI 3000 M andFEI Tecnai G2-F30, respectively.

C. Ran, P. Chen /Materials Letters 232 (2018) 142–145 143

3. Results and discussion

Typical shear stress versus punching depth curves are shown inFig. 1. One of the prominent features in these curves is that, for dif-ferent punching depths, the shear stress shows a plateau and thena sharp increase when the steel stopper ring is contacted. It isinteresting to note that the flow stresses are almost a constantfor different punching depths and equal to approximately600 MPa, indicating that the flow stress of Ti-55511 alloy is inde-

0.0 0.5 1.0 1.50

200

400

600

800

1000

1200

Shea

r Stre

ss (M

Pa)

Punching depth (mm)

Without stopper ring Pd=1.1 Pd=1 Pd=0.9 Pd=0.8 Pd=0.7

Fig. 1. Shear stress vs. different prescribed punching depths.

Fig. 2. Typical OM micrographs of Ti-55511 alloy: a) Undeformed

pendent on the punching depth. Apparently, the correspondingnormal stress of Ti-55511 alloy can be taken as 1200 MPa, whichis almost 300 MPa lower than that in cylindrical specimens’ [15].The observed discrepancy may be attributed to the geometricalimperfection of HS specimens.

Typical OM micrographs of Ti-55511 alloy are shown in Fig. 2.The initial microstructure and the well-developed localized shearbands of Ti-55511 alloy are depicted in Fig. 2a and b, respectively.Fig. 2c and d are the higher magnification of region ‘‘A”. As shownin Fig. 2, a phases adjacent to the ASB are elongated along the sheardirection due to the strong shear deformation, and a very distinc-tive boundary separates the shear band from the surroundingdeformed structures. It should be noted that though the cracksextend into the shear band to some extent (e.g. Fig. 2d), almostall cracks occurred at the shear band/matrix interface. This is ingood accordance with the aforementioned experimentak observa-tions [3,16]. Combined with our previous work [3], the sequence ofthe microstructural evolution within an ASB for Ti-55511 alloy canbe summarized, and Fig. 3 is the schematic representation. Asshown in Fig. 3a, an ASB forms due to the occurrence of severestrain concentration. Then, microcracks are nucleated at the shearband/matrix interface, and adjoining microcracks coalesce to a big-ger crack (Fig. 3b). With further deformation, the crack propagatesalong two ways. One is that the crack propagates along the shearband/matrix interface up to failure or fracture (Fig. 3d). The otherone is that the crack extends into the ASB and propagates alongthe shear band/matrix interface (Fig. 3c) up to failure or fracture(Fig. 3e). It is interesting to note that the features of ASB evolution

, b) Without stopper ring, c) Pd = 1.0 mm and d) Pd = 0.9 mm.

Fig. 4. Typical TEM micrographs of Ti-55511 alloy: a) The interface between the ASB and matrix and b) Higher magnification of crack tip.

Fig. 3. Schematic representation of the microstructural evolution within ASB for Ti-55511 alloy.

144 C. Ran, P. Chen /Materials Letters 232 (2018) 142–145

in Ti-55511 alloy are similar to those of adhesive fracture. Hence,‘‘adhesive fracture” can be identified as the dynamic failure mech-anism for Ti-55511 alloy. However, further investigation is stillneed to clarify the mechanism.

Typical TEM micrographs of the interface region between theASB and the matrix are shown in Fig. 4a. Apparently, themicrostructure shows a crack with a length of ca 2 lm, and thecrack propagates along the interface. Fig. 4b shows the typicalmicrostructure of the ASB closed to the crack tip. Obviously, closeto the matrix region, the microstructure shows dislocation pile-upgroups due to serve shear deformation. In addition, some nanograins, dynamic recrystallization (DRX), with a typical size of ca6 nm are observed in the ASB region. Apparently, the DRX grainsare essentially free of dislocations. The corresponding selected areadiffraction pattern (SADP) shows a ring pattern, which is typical ofnano-grained polycrystalline materials. This is in accordance withthe result reported by Wang et al. [16].

T ¼ DT þ T0 ¼ bRsdcqC

þ T0 ð1Þ

where q is the mass density, C is the specific heat, T0 is the ambienttemperature and b is the fraction of plastic work converted to heat,which is taken as 0.9 in this work [2]. For Ti-55511 alloy, q and C are4625 kg/m3 and 523 J/(kg K), respectively. Here, T0 = 293 K.

The estimated maximum temperature in our tests is nearly573 K. Such a temperature is much lower than those of a?b phasetransformation (about 1163 K) and DRX (0.4Tm [17], practically

760 K). Hence, this observation indicates that thermal softeninghas a very minor effect on the onset of ASB and DRX formation.Similar findings have also been reported by Rittel et al. [5].

4. Conclusion

1. The shear stress of Ti-55511 alloy in HS specimen is indepen-dent on punching depth.

2. Thermal softening has a minor effect on the onset of ASB andDRX formation.

3. The features of microstructural evolution in Ti-55511 alloy inhigh strain rate loading situation are similar to those of adhe-sive fracture, and the concept of ‘‘adhesive fracture” is proposedas the dynamic failure mechanism for Ti-55511 alloy.

Acknowledgements

This work was supported by the National Natural Science Foun-dation of China (grant number 11472054).

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