Thermal stability of microstructures induced by laser shock peening in TC17 titanium alloy

Accepted Manuscript Thermal stability of microstructures induced by laser shock peening in TC17 titanium alloy Yang Yang, Kai Zhou, Hua Zhang, Haibo Hu, Hongchao Qiao PII:

S0925-8388(18)32137-6

DOI:

10.1016/j.jallcom.2018.06.030

Reference:

JALCOM 46366

To appear in:

Journal of Alloys and Compounds

Received Date: 6 December 2017 Revised Date:

24 April 2018

Accepted Date: 3 June 2018

Please cite this article as: Y. Yang, K. Zhou, H. Zhang, H. Hu, H. Qiao, Thermal stability of microstructures induced by laser shock peening in TC17 titanium alloy, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.06.030. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Thermal stability of microstructures induced by laser shock peening in TC17 titanium alloy Yang Yang,a–d* Kai Zhou,a Hua Zhang,a Haibo Hu,c Hongchao Qiaod

b

School of Material Science and Engineering, Central South University, Changsha, 410083, China

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a

Key Laboratory of Ministry of Education for Nonferrous Metal Materials Science and Engineering, Central South University, Changsha,410083, China

Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang, 621900, China d

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c

Shenyang Institute of Automation, Chinese Academy of Science, Shenyang, 110016, China

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* Corresponding author: School of Materials Science and Engineering, Central South University, Changsha, 410083, China. E-mail address: [email protected]

Abstract

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The thermal stability of microstructures induced by laser shock peening (LSP) in TC17 titanium alloy (Ti-5Al-2Sn-2Zr-4Mo-4Cr) was investigated by comparing the microstructural

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characteristics and mechanical properties, which were characterized by transmission electron microscopy (TEM) and Vickers hardness tests of LSPed and subsequently annealed samples.

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The LSPed TC17 titanium alloy samples were annealed at 573, 623, 673, and 723K for 1 h. Comparison of the gradient microstructures studied at different depths of the LSPed and LSP+573K/1h and LSP+673K/1h annealed specimens indicated that the dislocation density markedly decreased and the dislocation cells became clearer in the 573K/1h annealed specimen while there were no great changes in deformation twin density and the average grain size at the surface did not change greatly. After annealing at 673K for 1 h, the dislocation density and deformation twins density both decreased greatly at different depths 1

ACCEPTED MANUSCRIPT and the average grain size at the surface increased compared to the LSPed and 573K/1h annealed specimens. The average grain sizes in the top surface layers of the LSPed, 573K/1h, 623K /1h, 673K/1h and 723K/1h annealed samples were 396 nm, 422 nm, 493 nm, 1.04 µm, and 2.46 µm, respectively. The critical temperature at which the microstructures of the LSPed

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TC17 titanium alloy changed significantly was 673K; the fine grains at the top surface started abnormal growth when the annealing temperature exceeded that value. The hardness values

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decreased significantly when the annealing temperature reached 673K. It can be concluded that 673K was the critical temperature below which the microstructures induced by LSP were

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thermally stable in the TC17 titanium alloy.

Keywords: Laser shock peening (LSP); Microstructure; Thermal stability; TC17 titanium

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alloy

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ACCEPTED MANUSCRIPT 1. Introduction Plastic deformation can induce a large number of crystal defects such as dislocations and deformation twins in the metals [1, 2] and can form strengthened microstructures with refined grains [3–5], which are thermodynamically metastable [6]. The crystal defects in deformed

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metals will be rearranged and annihilated under certain conditions, such as high temperature, and the refined grains will coarsen at high annealing temperature and/or long annealing times

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[7], thereby weakening the macroscopic properties. Laser shock peening (LSP), a kind of surface treatment process with an ultrahigh strain rate, can induce a large number of crystal

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defects such as dislocations and deformation twins and refine the grains in surface layers immediately [8, 9]. Dekhtyar and Chen found that the shear band plays a significant role in the grain refinement of two-phase titanium alloy during severe plastic deformation [10, 11]. The microstructural characteristics and formation mechanism of TC17 titanium alloy induced

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by laser shock processing were investigated in our previous work [12]. The dislocation configurations induced by dynamic deformation in LSP are unstable due to the limited

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movement time for dislocations in the shock wave front during extremely short laser pulse durations, which are prone to rearranging and evolving into stable structures, while the

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ultrafine grains will coarsen at high temperature. Therefore, the microstructures and corresponding mechanical properties are unstable in LSPed metals [13, 14], and it is of great importance to investigate the thermal stability of microstructural characteristics for the strengthening effect in practical applications. Luo [15] found that the top surface nanograins induced by LSP grew when the annealing temperature was higher than the dynamic recrystallization temperature, which is about 0.36 Tm in Ni-based superalloy, proving that a higher temperature is necessary for grains to grow 3

ACCEPTED MANUSCRIPT in nanostructured material. Altenberger [16] investigated the thermal stability of near-surface microstructures induced by deep rolling and LSP in AISI 304 stainless steel and Ti-6Al-4V and found that the surface nanograins induced by deep rolling in AISI 304 stainless steel remained stable when the annealing temperature was lower than 0.5 Tm, which was 0.6 Tm

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for the highly tangled dislocations and dense dislocation substructures in the LSPed sample; the nanostructures in deep-rolled Ti-6Al-4V remained stable when the annealing temperature (0.2–0.5Tm) while the microstructures did not change obviously in the

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was lower than 650

LSPed one when the annealing temperature was lower than 900

. Xu [17] focused on the

the grain size in 700

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thermal stability of microstructures in IN718 alloy subjected to laser peening and found that /300min and 800

the laser peened and 600

/300min annealed samples were larger than those of

/300min annealed ones; the thermal stability of laser peened

microstructures was fairly good when the annealing temperature was lower than 600

and

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the release of residual stress was correlated with the dislocations and grain size. Most of the present investigations of thermal stability of laser shock processed metals

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only concern the microstructures at the top surface and there are few discussions about the evolution of gradient microstructures induced by LSP. In this paper, the evolution of the

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gradient microstructures in annealed LSPed TC17 titanium alloy was investigated by transmission electron microscopy (TEM) and the effect of the microstructural evolution on a mechanical property was analysed by Vickers hardness tests. The critical annealing temperature of LSPed TC17 titanium was deduced by comparing the surface microstructures at different depths and the corresponding hardness test results of different annealed specimens. 2. Material and methods 4

ACCEPTED MANUSCRIPT The chemical composition of two-phase TC17 titanium alloy for LSP is shown in Table 1 and a rectangular specimen with dimensions of 30 mm × 15 mm × 5 mm was prepared for the next LSP experiment. The samples was rolled and recrystallization annealed before LSP; the

Table 1 Chemical composition of TC17 titanium alloy (wt %) Al 4.5–5.5

Mo

Cr

Sn

Zr

3.5–4.5

3.5–4.5

1.6–2.4

1.6–2.4

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equiaxed coarse grains before LSP possessed an average grain size of about 43 µm.

Fe

Ti

0.30

Bal.

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In the LSP experiment, aluminum foil with a thickness of 0.1 mm was taken as the thermo-protective layer, and flowing water with a thickness of 1 mm was taken as the

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confining layer. Other processing parameters used during LSP are shown in Table 2. Table 2 Processing parameters used in LSP Type

Value

Beam divergence of output (mrad)

≤2

Spot diameter (mm)

2.5

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Pulse energy (J)

7 15

Laser wavelength (nm)

1064

Energy stability (%)

< 1.5

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Pulse width (ns)

Overlapping ratio (%)

8

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The thermal stability of LSPed microstructures in TC17 titanium alloy was investigated by comparing the microstructures at different depths of 573K/1h and 673K/1h annealed LSPed specimens with those of the deformed LSPed specimen, and the critical annealing temperature at which the microstructures clearly changed was found by comparing the top surface microstructures of the 573K/1h, 623K/1h, 673K/1h, and 723K/1h annealed specimens with that of the LSPed one. A salt bath furnace was used in the annealing experiment to achieve a uniform heating result and avoid any influence of a surface oxide layer on the

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ACCEPTED MANUSCRIPT results of the hardness test. Ivasishin found that the effect of heating rate on microstructural evolution during heating to peak temperature has a strong effect on subsequent heat treatment behaviour at the peak temperature [18, 19] and should thus be taken into account in the design of industrial heat treatment practices; therefore, the sample in this study was put into the

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furnace when it reached the peak temperature.

The thermal stability of microstructures was characterized by TEM, and the TEM

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specimens were prepared by adhering two top surfaces of LSPed TC17 titanium alloy face to face with Gatan G1 glue and then cutting perpendicular to the top surface into slices with a

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thickness of about 0.8 mm. The slices were firstly thinned by mechanical milling to about 30 µm and then ion thinned with thinning parameters of 5 kV and 7°to perforate and 3 kV and 2°to enlarge the thin areas.

Vickers hardness tests were performed to analyse the changes of the mechanical

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properties of annealed specimens after LSP. The load was 1 Kg with a load duration of 10 s. The hardness value in this study was the cross-sectional hardness at different depths below the

3. Results

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top shocked surface, and at each depth, the hardness value was the mean of three test results.

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3.1 Thermal stability of gradient microstructures The TEM images of LSPed, LSP+573K/1h annealed, and LSP+673K/1h annealed

specimens at different depths are shown in Fig. 1. Figs.1(a–c) show the microstructures about 100, 50, and 30 µm below the top surface of the LSPed specimens, respectively; Figs. 1(d–f) and Figs. 1(g–i) show those of the LSPed+573K/1h and LSP+673K/1h ones. The microstructures 100 µm below the top surface of the LSPed specimen featured more deformation twins with dense dislocation tangles (DTs), as seen in Fig. 1(a). 6

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Figure 1 Microstructures at different depths of LSPed and annealed samples. (a–c) LSPed;

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(d–f) LSPed+573K/1h; (g–i) LSPed+673K/1h. (a, d, g) 100 µm below the top surface; (b, e, h) 50 µm below the top surface; (c, f, i) 30 µm below the top surface.

The density of dislocations decreased after 573K/1h annealing, as seen in Fig. 1(d). The

dislocation density decreased significantly and there were few deformation twins in the LSP+673K/1h annealed specimen, as seen in Fig. 1(g). Figs. 1(b), 1(e), and 1(h) show the microstructures 50 µm below the top surface. The LSPed specimen showed fewer deformation twins compared to the 100-µm depth, while the dislocation density clearly increased and the dense DTs evolved into dislocation cells (DCs), 7

ACCEPTED MANUSCRIPT as seen in Fig. 1(b). There were fewer dislocations and clearer DCs in the microstructures of the LSPed+573K/1h annealed sample, as seen in Fig. 1(e). After 673K/1h annealing, the degree of distortion and dislocation density dropped considerably, there were nearly no deformation twins in the microstructures, as shown in Fig. 1(h), and the DCs were clearer and

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of larger size than those in the LSPed and LSP+573K/1h annealed specimens.

The microstructures at 30 µm depth from the top surface are shown in Figs. 1(c), 1(f),

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and 1(i). The microstructures of the LSPed sample featured subgrains with sizes in the range of 200–500 nm which evolved from DCs, as seen in Fig. 1(c). The subgrains became clearer

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with fewer dislocations in the LSP+573K/1h annealed specimens, and there were no obvious changes in subgrain size, as seen in Fig. 1(f). The dislocation density decreased significantly and the subgrains became larger, with sizes in the range of 500–1100 nm, in the LSP+673K/1h annealed specimen.

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3.2 Thermal stability of top surface microstructures The microstructures at the top surface of the LSPed TC17 titanium alloy after annealing were

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characterized by fewer dislocations and larger grains, as seen in Fig. 2. The dislocations and deformation twins in deformed metals were annihilated in the annealing process and the

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grains evolved from DCs. The top-surface grain-size distribution histograms with statistics from 15 different observation fields of the LSPed specimen and all annealed specimens are shown in Fig. 3.

Most of the grain sizes at the top surface of the LSPed specimen were in the range of 300–600 nm and the average grain size was 396 nm, as seen in Fig. 3(a). The grain size distribution histogram did not change obviously after 573K/1h annealing, and the average grain size was 422 nm, as shown in Fig. 3(b). The grains coarsened slightly in the 8

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Figure 2 Top surface microstructures of LSPed and annealed samples. (a) LSPed; (b)

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LSPed+573K/1h; (c) LSPed+623K/1h; (d) LSPed+673K/1h; (e) LSPed+723K/1h

Figure 3 Grain-size distribution diagrams of the LSPed sample and all annealed samples: (a) LSPed; (b) LSPed+573K/1h; (c) LSPed+623K/1h; (d)LSPed+673K/1h; (e) LSPed+723K/1h 9

ACCEPTED MANUSCRIPT LSPed+623K/1h annealed specimen, and the average grain size was 493 nm (Fig. 3(c)), which was 24% larger than that of the LSPed specimen. The top grains obviously coarsened in the LSPed+673K/1h annealed specimen, and most grain sizes were distributed in the range of 0.6–1.4μm with an average grain size of 1.04 µm, which was 2.6 times bigger than that of

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the LSPed specimen, as seen in Fig. 3(d). After 723K/1h annealing, the grain size increased substantially and abnormal growth occurred in which the grain size distribution was

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characterized by the polarization phenomenon. Part of the grains had sizes distributed in the range of 1.0–1.5 µm, while the other part was in the range of 2.5–3.5 µm; the average grain

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size was 2.46 µm, as seen in Fig. 3(e), which was 6.2 times bigger than that of the LSPed specimen. The critical annealing temperature was 673K, above which the microstructures of the LSPed TC17 titanium alloy changed significantly. 3.3 Hardness test results

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The hardness distributions along the depth direction of the LSPed sample and all annealed samples are shown in Fig. 4. All the hardness values decreased after annealing treatments at

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different temperatures. The extent of the decrease was small in the 573K/1h and 623K/1h annealed specimens, while it became larger when the annealing temperature was higher than

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673K. The hardness value of the top surface was enhanced by 24% in the LSPed titanium alloy compared with the substrate and by 21, 20, 15, and 12% after annealing at 573, 623, 673, and 723K for 1 h, respectively. The critical temperature when the hardness values of the LSPed top surface obviously changed was 673K. The hardness test results conformed with the microstructural characteristics in the TEM images, which proved that 673K was the critical temperature above which the microstructures changed significantly. The decrease of hardness values should be related to the decrease of the 10

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density of crystal defects and the growth of refined grains near the surface.

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Figure 4 Hardness distributions of LSPed and annealed specimens at different depths 4. Discussion

As seen from the TEM images at different depths of the LSPed TC17 titanium alloy, the density of deformation twins decreased closer to the top surface and there were nearly no

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deformation twins in the microstructures at 30 µm depth and the top surface. In the LSP, the shock wave attenuated gradually when propagating into the target material, so the strains and strain rates were higher closer to the top surface. It is generally believed that higher strain

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rates are conducive to the generation of deformation twins, while the experiments presented in

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this paper showed the opposite result, which may ascribed to the ultrahigh strain rate of 107 s-1 scale [20] near the top surface, which meant that there was not enough time for the twin atoms to fulfill twin deformation [21, 22]. When the LSPed TC17 titanium alloy was annealed at lower temperatures, the crystal

defects such as dislocations and deformation twins were partly annihilated but the remaining defects still acted as strong obstacles to the grain boundary migration so the grain size increased slowly. The decrease of crystal defects in the microstructures showed clearer dislocation configurations than those observed in the LSPed sample. The top surface grains 11

ACCEPTED MANUSCRIPT grew quickly due to the weak block effect of crystal defects on grain boundary migration when the annealing temperatures were higher. Grains at the top surface of the 573K/1h, 623K/1h, and 673K/1h annealed LSPed titanium alloy grew uniformly in a homogeneous growth process of recrystallized grains that occurred under the driving force of the decrease in

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the total grain boundaries in the annealing process. There were two maxima in the grain size distribution diagram of the 723K/1h annealed specimen, and some grains became

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significantly coarser than the others, showing the typical features of abnormal grain growth after the normal grain growth of refined grains. This may be a result of the concentration of

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different crystal defects in the surface recrystallized grains of LSPed TC17 titanium alloy: the grains with higher defect density or some favourable orientation would grow preferentially at a high annealing temperature by annexing the surrounding grains to form abnormally large grains in the subsequent annealing process. It can be seen from the experimental results that

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the critical annealing temperature above which the microstructures of LSPed TC17 titanium alloy changed significantly was 673K.

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The increased hardness of the LSPed TC17 titanium alloy was the result of the strengthening effects of crystal defects such as dislocations and deformation twins and the

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grain refinement strengthening (via the Hall-Petch relation) at the top surface. When the annealing temperatures were lower than 673K, there was a gradual decrease of the crystal defects’ densities in the LSPed TC17 titanium alloy with increasing annealing temperatures, while the top surface showed a moderate grain growth, so the sectional gradient hardness values dropped insignificantly. The hardness values decreased significantly when the annealing temperatures were high up to 723K, at which the density of crystal defects would have decreased greatly with obvious coarsened surface grains, so the two kinds of 12

ACCEPTED MANUSCRIPT strengthening effects had both been weakened. 5. Conclusions The thermal stability of the microstructures and mechanical properties of LSPed TC17 titanium alloy were investigated by comparing the gradient microstructures and hardness

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values of 573K/1h, 623K/1h, 673K/1h, and 723K/1h annealed LSPed titanium alloy with those of the original LSPed one. The conclusions drawn from the experimental results were as

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follows:

(1) The LSPed+573K/1h annealed specimen showed clearer cell structures at different

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depths and there were no obvious changes of grains sizes at the surface compared to the LSPed one. The density of crystal defects such as dislocations and deformation twins in the LSPed+673K/1h annealed specimen was lower than that of the LSPed and LSPed+573K/1h

coarsened significantly.

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annealed specimens, and the DCs, subgrains near to the surface, and top surface grains had all

(2) The TEM bright-field images and grain size distribution diagrams of the LSPed and LSPed+623K/1h,

LSPed+673K/1h,

and

LSPed+723K/1h

annealed

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LSPed+573K/1h,

specimens showed that the average grain sizes at the top surface were 396 nm, 422 nm, 493

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nm, 1.04 µm, and 2.46 µm, respectively. (3) The gradient hardness values in all annealed specimens decreased in comparison with

that of the LSPed one and the HV values dropped greatly when the annealing temperature was up to 673K. The decrease in the density of crystal defects and the growth of the grains near to the surface were the main reasons for this. It can be concluded that 673K was the critical temperature below which the microstructures induced by LSP were thermally stable for TC17 titanium alloy, and the 13

ACCEPTED MANUSCRIPT service temperature of the LSPed TC17 titanium alloy should not be higher than that.

Acknowledgements

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51574290 and 51274245) and NSAF (No. U1330126).

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This work was supported by the National Natural Science Foundation of China (Nos.

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ACCEPTED MANUSCRIPT Highlights 1. Deformation twins become less prevalent at ultrahigh strain rates (107/s). 2. Thermal stability of microstructures at different depth induced by laser shock

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processing at different annealing temperatures. 3. Thermal stability of mechanical property characterized by gradient Vickers hardness test.

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4. Bimodal grain structure formed during annealing at 723 K.