Effect of thermal annealing on the microstructure, mechanical properties and residual stress relaxation of pure titanium after deep rolling treatment

Accepted Manuscript Title: Effect of thermal annealing on the microstructure, mechanical properties and residual stress relaxation of pure titanium after deep rolling treatment Authors: Jie Huang, Kai-Ming Zhang, Yun-Fei Jia, Cheng-Cheng Zhang, Xian-Cheng Zhang, Xian-Feng Ma, Shan-Tung Tu PII: DOI: Reference:

S1005-0302(18)30260-3 https://doi.org/10.1016/j.jmst.2018.10.003 JMST 1370

To appear in: Received date: Revised date: Accepted date:

4-4-2018 7-5-2018 9-5-2018

Please cite this article as: Huang J, Zhang K-Ming, Jia Y-Fei, Zhang C-Cheng, Zhang X-Cheng, Ma X-Feng, Tu S-Tung, Effect of thermal annealing on the microstructure, mechanical properties and residual stress relaxation of pure titanium after deep rolling treatment, Journal of Materials Science and amp; Technology (2018), https://doi.org/10.1016/j.jmst.2018.10.003 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.

Effect of thermal

annealing on the microstructure,

mechanical properties and residual stress relaxation of pure titanium after deep rolling treatment Jie Huang 1, Kai-Ming Zhang 1, Yun-Fei Jia 1, Cheng-Cheng Zhang 3, Xian-Cheng Zhang 1, *, Xian-Feng Ma 2, *, Shan-Tung Tu 1 1

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Key Laboratory of Pressure Systems and Safety, Ministry of Education, School of

Mechanical and Power Engineering, East China University of Science and

2

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Technology, Shanghai 200237, P.R. China

Sino-French Institute of Nuclear Engineering and Technology, SunYat-Sen

University, Zhuhai 519082, Guangdong, P.R. China 3

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AECC Commercial Aircraft Engine Co. LTD, Shanghai Engineering Research

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Center for Commercial Aircraft Engine, Shanghai 201108, P.R. China

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[Received 4 April 2018; Received in revsied form 7 May 2018; Accepted 9 May

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2018]

Corresponding author. E-mail address: [email protected] (X.C. Zhang)

*

Corresponding author. E-mail address: [email protected] (X.F. Ma)

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*

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The aim of this paper was to investigate the effect of thermal annealing on the microstructure, mechanical properties, and residual stress relaxation of

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deep rolled pure titanium. The microstructure and mechanical properties of the surface modified layer were analyzed by metallographic microscopy,

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transmission electron microscope and in-situ tensile testing. The results showed that the annealed near-surface layer with fine recrystallized grains had increased ductility but decreased strength after annealing below the recrystallization temperature, where the tensile strength was still higher than that of the substrate. After annealing at the recrystallization temperature, the recrystallized near-surface layer had smaller grain size, similar tensile strength, and higher proportional limit, comparable to those

of the substrate. Moreover, the residual stress relaxation showed evidently different mechanisms at three different temperature regions: low temperature (T≤0.2Tm), medium temperature (T≈(0.2‒0.3)Tm), and high temperature (T≥0.3Tm). Furthermore, a prediction model was proposed in terms of modification of Zener-Wert-Avrami model, which showed promise in characterizing the residual stress relaxation in commercial pure

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Ti during deep rolling at elevated temperature.

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Keywords: Deep rolling; Ultra-fine grain; Tensile strength; Microstructure; Residual stress

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1. Introduction

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Failure of engineering components often occurs at surface or from surface due to

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high propensity of large stress and defects at surface. If compressive residual stress is

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introduced near surface, an improvement in both fatigue strength and abrasion resistance of components can be achieved. Hence, different mechanical surface [1]

methods have been developed to induce compressive residual

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enhancement (MSE)

stress and work-hardened surface layer. These methods include shot peening (SP),

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laser shock peening (LSP), low plasticity burnishing (LPB), and deep rolling (DR)

[2]

.

Compared with other methods, DR can generate deeper work-hardened layer and

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larger compressive residual stress

[3]

. During a DR process, the roller pins keep in

contact with the workpiece surface. When the load is applied on the roller pins, a pressure will be generated at the contact zone. When the contact stress at surface is

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higher than the elastic limit of the material, plastic deformation will occur at the surface, which leads to grain refinement and compressive residual stress near the surface, as shown in Fig. 1(a). The DR processing can introduce a residual stress distribution similar to SP near the surface. However, the surface roughness after DR treatment is obviously lower than that after SP [4]. To assess the effectiveness of MSE for applications that require high temperature

service, the thermal stability of microstructure and residual stress induced by MSE have to be considered. Most studies on the thermal stability of DRed and LSPed metallic materials focused on the microstructure and considered the issue of [1,5,6]

mechanical properties and residual stress relaxation induced by MSE

. The

microstructure evolution is known to depend significantly on temperature and aging time, for example

[7,8]

. Specifically, near-surface nanoscale microstructure tends to be [7,9]

.

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more thermally stable and still effective than compressive residual stress

However, there is only scarce experimental data on DRed pure Ti. We need to provide

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more studies about the thermal stability of DRed pure Ti.

On the other hand, metallic materials treated by MSE have higher yield strength and tensile strength at elevated temperature than the original [10], in spite of associated

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ductility decrease. To account for this cold-work hardening effect, either phase

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transition with grain refinement [11] or annealing [12] have been tried.

considered the effect of aging time

[15]

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Most studies on the residual stress relaxation focused on temperature and initial cold deformation

[13,14]

and

[16]

. Presently,

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Zener-Wert-Avrami formula is the most widely used model on analysis of residual [17,18]

. However, Ren et al.

[19]

found the main

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stress relaxation induced by MSE

mechanism of stress relaxation in 6061-T651 aluminum alloy was different in two temperature regions, which showed that the Zener-Wert-Avrami formula ignored the

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thermal activation energy related to each temperature and aging time in special material. Thus, it is the aim of this work to further discern the uncertainties. Details on

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the prediction model of residual stress relaxation are discussed in later sections. In the present work, after DR being conducted on pure titanium (Ti) sheet,

various factors involved in the thermal stability were investigated. Firstly, the thermal

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stability of microstructure and mechanical properties of pure Ti after deep rolling at elevated temperature were investigated. Then, the relaxation mechanism of residual stress in plastic deformed layer (PDL) induced by DR was discussed. Lastly, a modification of the Zener-Wert-Avrami model was made to predict the residual stress relaxation.

2. Experimental procedures 2.1 Materials Pure titanium sheets with 6 mm thickness were used in the present study. The chemical composition is shown in Table 1. The multi-overlap deep rolling was carried out with a rolling tool. The main

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feature of the process is that the rolling tool repetitively moves with the horizontal and rotational feed along the specified route and with a constant vertical feed of 0.02 mm

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each passes, until a total accumulated vertical feed of 0.4 mm. During the

multi-overlap deep rolling, there were overlap regions where double rollings would be carried out in one pass. Hence, the plastic deformation accumulation was induced into

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the near-surface layer. All the as-received sheets were firstly treated by multi-overlap deep rolling with the route schematically shown in Fig. 1(b) and with the parameters

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shown in Table 2.

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The subsequent isothermal annealing treatments were carried out in an electric

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resistance furnace (SX2-4-10), followed by air cooling. The matrix of annealing treatments for the specimens for OM observation and mechanical testing is shown in

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Table 3. Moreover, the specimens for TEM characterization were annealed at 250 °C, 400 °C and 600 °C for 20 min. The specimens used to measure residual stress were

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512 min.

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annealed at the temperature range from 200 °C to 500 °C for duration from 4 min to

2.2 Microstructure observation and mechanical test The cross-sectional optical micrographs of deep rolled samples were obtained

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using an inverted Zeiss optical microscope. The samples for OM observation with a size of 10 mm× 10 mm× 6 mm were electrolytic polished with an electrolyte of 5% HClO4 and 95% CH3COOH at a voltage of 30 V for 60 s and etched in Kroll solution (hydrofluoric acid, nitric acid and water solution with a ratio of 1:2:17) at room temperature. Transmission electron microscopy (TEM) was performed using a JEM-2100

microscope. The TEM foils were prepared by the following steps: (i) mechanically polishing until a thickness of 30~60 μm; (ii) twin-jet electro-polishing with an electrolyte of 10 mL HClO4, 90 mL CH3(CH2)3OH and 150 mL CH3OH at a voltage of 50 V at -42 to -45 °C. Tensile tests of micro specimens were carried out in the strain rate of 2.08310-4 s-1 at room temperature by using EVOMA15 scanning electron microscope (SEM)

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and in-situ tensile table to obtain the tensile stress-strain curves of annealed surface layer of deep rolled pure Ti. To further research the tensile properties of the

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deep-rolled structure, the corresponding fracture surfaces were examined with SEM. The micro specimens were cut from the fine-grain surface layer (0‒200 μm distance from surface) and from the substrate of subsequently annealed pure Ti with a gauge

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dimension of 8 mm× 2.5 mm× 0.2 mm.

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2.3 Measurements of residual stress

Residual stress was determined by using a Proto-iXRD MG40P FS STD residual

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stress tester and X-ray diffraction (XRD) with sin2Ψ method. Lattice strain measurements were performed using Cu-Kα radiation source at the {213} lattice

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planes of the α-Ti, using specimens with dimension of 7 mm× 7 mm× 6 mm. To measure the residual stress distribution in depth on the deep-rolled near-surface layer,

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electrolytic polishing was used to remove the excess material with a solution of 5%

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HClO4 and 95% CH3COOH at a voltage of 30 V.

3. Results and discussion

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3.1 Effect of annealing on deep rolled near-surface microstructure of pure titanium and its properties 3.1.1 Microstructure Fig. 2 shows the cross-sectional metallographs of pure Ti before DR and after DR. Typical fiber microstructure and significantly refined microstructure near the

surface subjected to DR can be observed from Fig. 2, compared to the original microstructure with equally distributed coarse grains, as a result of shear stress and accumulated plastic strain induced by deep rolling. Fig. 3 shows the TEM observations of the three near-surface deformation microstructures of pure Ti after DR (with increasing distance from surface as follows): (b) severe plastic deformed layer (SPDL) with ~200 nm grain size, (c) medium plastic deformed layer (MPDL), and (d)

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light plastic deformed layer (LPDL). It could be identified from Fig. 3 that elongated

lamellae with a mean boundary interval of several hundred nm and dense dislocations

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are distributed in the MPDL, and twin lamellas could be seen in the LPDL.

It is evident in Figs. 4 and 5 that fine-grained near-surface microstructure exhibiting obvious transition between the three plastic deformed layers, which

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remained stable during annealing at 250 °C and at 400 °C. However, when the aging

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temperature increased to 600 °C (close to the common recrystallization temperature of pure Ti ranging from 550 to 690 °C), the size of fine-grains increased significantly

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(grain size of ~10 μm in surface for aging 10 min) and the transition of the three PDL

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had evolved, as shown in Fig. 6.

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tend to disappear, suggesting that the ultrafine-grained near-surface microstructure

To further investigate the thermal stability of the ultrafine-grained near-surface microstructures induced by DR, TEM characterizations were performed at ~15‒30 μm

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depth from the surface of specimens after annealing for 20 min, as shown in Fig. 7. It could be observed from the TEM microstructures that there is neither obvious grain

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growth nor decrease of dislocation density during annealing below the recrystallization temperature, exhibiting good thermal stability of microstructure induced by DR up to 400 °C. Noticeable grain growth occurred during annealing at

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the recrystallization temperature. Moreover, it was noted from corresponding selected area electron diffraction (SEAD) that the patterns arranged in circularity at 200 °C and 400 °C but in matrix at 600 °C, indicating that remarkable grain growth occurred only above ~550 °C. It indicates a typical discontinuous recrystallization process, which is similar to the ARB processed CP-Ti [6]. As the recrystallization temperature of pure Ti ranges from 550 to 690 °C, this

phenomenon was believed to be attributed to the fact that high strain rate induced by DR results in severe lattice distortions and highly disorientated subgrain boundaries in near-surface layer

[20]

, which act as driving force for recrystallization of fine-grained

microstructure during annealing. Due to high stored energy of the system, fine grains would be devoured by coarse-grain boundaries, and therefore the thickness of

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fine-grained region will constantly decrease with increasing annealing time.

3.1.2 Mechanical properties

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The tensile stress‒strain curves of micro specimens of as-rolled substrate, as-rolled surface layer (corresponding to 0 min), and subsequently annealed surface layer are shown in Fig. 8. The tensile strength and total elongation determined from

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the curves were plotted in Fig. 9 as a function of annealing duration. It is evident from

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Fig. 9 that the elongation of the fine-grained surface layer was significantly increased

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with either the annealing temperature or increasing duration after the tensile strength

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was decreased. Moreover, it is noted from Fig. 8 that although the tensile strength was decreased due to annealing at 200 °C and 400 °C, it is still higher than that of the

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substrate. Specially, with the annealing temperature rising to the recrystallization temperature of pure Ti (600 °C), the tensile strength of the surface material was

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reduced approaching to that of the substrate until duration for 40 min, but the proportional limit of the surface material was still higher than that of the substrate.

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It is significant to note from the SEM micrographs (Fig. 8(d)) that the tensile fracture surface show more evidence of ductility improvement with increasing annealing temperature, as revealed by the dimples on fracture surface that are far

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thicker at 600 °C as compared to that of 200 °C and 400 °C, suggesting lower ductility of surface material. Moreover, it could be also seen from Fig. 8(d) that the elongation increase could be primarily correlated with the enlarged dimples due to the extended annealing duration at 600 °C. It is also consistent with the macroscopic fracture that necking is more obvious for the samples with longer annealing. The tensile strength and total elongation of the micro specimens of deep rolled

surface layer after annealing for 20 and 40 min are plotted in Fig. 10 as a function of annealing temperature. Total elongation increased at the cost of tensile strength decrease. It is apparent that both of tensile strength and total elongation depended linearly on temperature but with opposite trends. Moreover, the variations of the tensile strength and total elongation of all specimens annealed for 20 min were smaller than that for 40 min. Specially, those properties kept nearly a constant value at

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200 °C, which can be attributed to nonevent recovery for annealing at 200 °C for 20 min, as supported by the TEM observation in Fig. 7.

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This study indicates that DR improves the tensile strength of near-surface layer as a result of grain refinement and high density of dislocation induced by plastic

deformation accumulation, but DR reduces the ductility. In the contrary, annealing

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obviously changes both the tensile strength and total elongation. To analyze the

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balance relationship between the tensile strength and the ductility, a diagram is plotted

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with the tensile strength as a function of the total elongation of DR-Ti, ARB-Al, ARB-IF steel, and ARB-Ti in Fig. 11. The tensile strength of the annealed materials in

with other materials

[6,21,22]

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Fig. 11 was normalized by the tensile strength of as-rolled materials

[6]

. Compared

, evident linear relationship between strength and ductility

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could be noted in DR pure Ti, similar to the ARB processed CP-Ti [6]. It is interesting that this relationship is quite different from the studies on the steel and aluminum

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processed by ARB [5,6], where the elongation of steel and aluminum began to improve noticeably only after the strength was reduced to half of the as-processed materials.

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In conclusion of the above experimental observations, the evolution of

ultrafine-grained near-surface microstructure induced by DR in pure Ti during annealing could be supposed as follows. At lower temperature, the annihilation and

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rearrangement of lattice defects can partly recover the mechanical properties variation induced by DR. Then, due to high density of dislocations in sub-grain boundary and lamellae boundary, migration of these boundaries and formation of high angle grain boundary will occur at elevated annealing temperature, which reduces the dislocation density. However, as dislocation tangle induced by plastic deformation could not be completely eliminated, the strength of the surface layer subjected to DR and

subsequently annealing at 600 °C remained higher than that of original material. After annealing, fine-grained surface layer exhibits a linear strength-plasticity balance.

3.2 Effect of annealing on residual stress relaxation of deep rolled pure titanium The residual stress distributions of DR pure Ti with depth during annealing treatment are shown in Fig. 12. In comparison to the as-rolled state (corresponding to

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zero aging time in Fig. 12), both the maximum value and compressive residual stress

affected depths decreased with either aging time or the aging temperature. It is evident

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in Fig. 12(b) and (d) that higher aging temperature led to higher release rate of

compressive residual stress in the initial stage of aging, and then the release rate was progressively decreasing with increasing aging time. For more details, the maximum

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compressive residual stress reduced by release ratio of 48.7% and 65.4% after

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exposing at 250 °C and 400 °C for 256 min, respectively. Specifically, the

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near-surface compressive residual stress essentially annealed out after exposing at 500 °C for 4 min, and the surface residual stress changed from compressive to tensile.

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To further analyze the residual stress relaxation mechanism of DR pure Ti, DR

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pure Ti was annealed at 200, 250, 300, 400, and 500 °C. Fig. 13 shows the surface residual stress relaxation with aging time. Note that residual stress reduced quickly at the initial stage of aging time. For more details, the surface residual stress released by

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59 MPa compared to the as-rolled state after exposing 4 min at 400 °C, i.e. 42.4% of total relaxation. Similarly, the surface residual stress relaxation after exposing 4 min

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at 500 °C accounted for 69.6% of total relaxation. Then the release rate of surface residual stress gradually decreases with time, and a transition between the stages of the initial fast release and the subsequent stable release can be identified, as shown in

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Fig. 13. With the aging temperature increasing, the transition region tended to disappear. The relationship between residual stress and aging time at different temperatures can be usually analyzed in terms of the Zener-Wert-Avrami formula [23]:

  exp[( At )m ] 0

(1)

where 𝜎 is the residual stress after the aging time of t, 𝜎0 is the initial value of the residual stress, m is a numerical parameter dependent on the corresponding relaxation mechanism, and A is a function dependent on material and temperature according to Eq. (2): A  B exp(

H ) RT

(2)

(3)

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relaxation, R is the Boltzmann constant and T is the aging temperature.  lg[ln( 0 )]  m lg t  m lg A 

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where B is a material constant, ∆𝐻 is the activation enthalpy for residual stress

Eq. (1) can be converted into Eq. (3). According to Eq. (3) and the experimental relaxation curves (Fig. 13), the value of numerical parameter m could be determined

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by the diagrams of lg [ln (𝜎0 /𝜎)] as a function of lg t, as shown in Fig. 14. It is noted from Fig. 14 that the measured data were divided into two regions in terms of

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temperature: a region of low temperatures at 200 °C and 250 °C (below 0.2Tm), and

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the other region of medium temperatures between 300 and 500 °C (~(0.2‒0.3)Tm),

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where Tm is the melting temperature of the pure Ti (~1668 °C). At medium temperature region, the data at each fixed temperature were well fitted by straight

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lines (the average value of m was 0.079), which indicates that the relaxation mechanism may be unified at the medium temperature. However, the relaxation

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process could be divided into two stages at low temperature where the data were fitted by straight lines with significantly different slop m (the average value of m for stage I

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and II were 2.01, 0.144, respectively), indicating that the relaxation mechanism of DR pure Ti may depend on the exposing duration at low temperature. The activation enthalpy ΔH could be obtained through the regression analysis in terms of Eqs. (1)

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and (2), and ΔH of stages I and II were 0.220 and 2.646 eV, respectively. The relaxation mechanism parameters of deep rolled pure Ti are shown in Table 4. This observation is similar with previous studies on the residual stress relaxation in 1Cr12 [24] and IN718 [25], where the data were divided into two parts, which showed noticeable effect of temperature on the value of m. It is noted from this result that the dominant relaxation mechanism is likely temperature-dependent in DR pure Ti.

However, it is not necessarily a general trend for all materials, e.g., the value of m for AA6110 aluminum alloy between 50 and 300 °C were not noticeably different at initial stage [23]. Specially, Ren et al. [19] revealed further that the main mechanisms of stress relaxation in 6061-T651 aluminum alloy were different between two temperature regions, i.e. dislocation slip and climb from 200 to 300 °C, and dynamic recovery between 400 and 500 °C, respectively.

fault energy

[26]

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Compared with previous research on the metallic material with low stacking , it is reasonable to propose the relaxation mechanism as follows. At

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the initial stage, thermal relaxation is dominated, which may be attributed to annihilation and rearrangement of crystalline defects induced by deep rolling

[27]

.

Then, with increasing exposing time (the increase of temperature will reduce the

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required time), the relaxation mechanism is more likely to be dominated by thermally [26]

due to the annihilation of

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activated gliding and rearrangement of dislocations

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crystalline defects. The thermal activation enthalpy (2.646 eV) of surface residual stress relaxation at medium temperature region is slightly larger than the self-diffusion

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activation energy of α-Ti (2.125 eV), suggesting that the dominant relaxation

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mechanism at the stage II is most likely the thermally activated gliding of dislocations. According to the above analysis, it is reasonable to conclude that the relaxation process of surface residual stress in deep rolled pure Ti is controlled by two

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mechanisms that depend on both of the exposing time and temperature range.

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4. Conclusions

The effects of annealing on the microstructure, mechanical properties and

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residual stress relaxation of deep rolled pure Ti have been experimentally studied. The following conclusions have been obtained: (1) The fine-grained near-surface microstructure and mechanical properties of deep rolled pure Ti strongly depended on the aging temperature during annealing process. The fine-grained near-surface microstructure of deep rolled pure Ti occurred via discontinuous recrystallization and the size of the near-surface grain increased

from 200 nm to 10 μm with the duration increasing during annealing at the recrystallization temperature. The recrystallized microstructure exhibited a similar tensile strength with that of the substrate, but had a higher proportional limit. (2) Below the recrystallization temperature, the size of the near-surface fine-grain was stable during annealing. The ductility of deep rolled pure Ti linearly increased with the tensile strength decreasing after annealing, but the tensile strength

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of the annealed post-DR pure Ti was still higher than that of the substrate.

(3) The thermal stability of the residual stress in near-surface layer of DR pure Ti

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was highly affected by temperature. Due to different residual stress relaxation mechanisms, the relaxation process below the recrystallization temperature could be divided into two regions, i.e. low temperature (T≤0.2Tm) and medium temperature

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(T≈(0.2‒0.3)Tm). The relaxation process at low temperature was further divided into

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fast-relaxation stage (stage I) and stable-relaxation stage (stage II).

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(4) A modified Zener-Wert-Avrami model was proposed for the surface residual stress relaxation of DR pure Ti at elevated temperature, which provided consistent

Acknowledgments

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prediction with the experimental result.

This work was financially supported by the National Natural Science Foundation

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of China (Nos. 51725503 and 51575183) and the “111 Project”. The author, Xian-Cheng Zhang is also grateful for the support by the Shanghai Pujiang Program,

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Young Scholar of the Yangtze River Scholars Program, and Shanghai Technology Innovation Program of SHEITC (No. CXY-2015-001).

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M

A

N

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(1994) 947-954.

Table List Table 1 Chemical composition of pure Ti (wt%) Elements

Ti

Fe

C

N

H

O

Content

Bal.

≤0.20

≤0.08

≤0.03

≤0.015

≤0.18

Total

vertical Single

vertical Number of pass Horizontal

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Table 2 Parameters of multi-overlap deep rolling feed

Feed (mm)

(time)

speed (mm/min)

0.4

0.02

20

18

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Feed (mm)

Table 3 The matrix of annealing treatments for deep rolled pure Ti

10

400

10

600

10

200

-

40

80

20

40

80

20

40

-

20

40

80

10

20

40

-

10

20

40

-

N

20

400 600

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properties test

250

U

Mechanical

Duration (min)

A

OM

Temperature (°C)

M

Item

T (°C)

200

250

300

400

500

1.995 (I)

2.025 (I)

0.075

0.097

0.064

0.192 (II)

0.096 (II)

2.778

2.399

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m

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Table 4 Surface residual stress relaxation mechanism parameters of deep rolled pure Ti

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ΔH (eV)

0.220 (I) 2.760 (II)

Figure List

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Fig. 1. Schematic of deep rolling treatment (a) and the route for multi-overlap deep

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rolling (b)

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Fig. 2. Cross-sectional metallographies of pure Ti before and after deep rolling

Fig. 3. TEM micrographs of the near-surface regions of deep rolled pure Ti

IP T SC R U N

A

Fig. 4. Cross-sectional metallography of deep rolled pure Ti after annealing at 250 °C

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CC E

PT

ED

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for: (a) 10 min; (b) 20 min; (c) 40 min; (d) 80 min

Fig. 5. Cross-sectional metallography of deep rolled pure Ti after annealing at

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400 °C for: (a) 10 min; (b) 20 min; (c) 40 min; (d) 80 min

Fig. 6. Cross-sectional metallography of deep rolled pure Ti after annealing at

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PT

ED

M

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N

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600 °C for: (a) 10 min; (b) 20 min; (c) 40 min

Fig. 7. Effect of annealing temperature on the grain size of deep rolled pure Ti

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ED

Fig. 8. Tensile stress-strain curves of deep rolled pure Ti and subsequently annealed pure Ti at different temperatures: (a) 200 °C; (b) 400 °C; (c) 600 °C. (d) SEM

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micrographics of fracture surface

Fig. 9. Evolution of mechanical properties of deep rolled pure Ti during annealing

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Fig. 10. Effect of annealing temperature on the tensile strength and total elongation of

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N

U

SC R

deep rolled pure Ti

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Fig. 11. Relationship between the tensile strength and total elongation of different

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PT

ED

materials

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Fig. 12. Residual stress distributions of deep rolled pure Ti with depth after annealing at:

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(a) 200 °C, (b) 250 °C, (c) 300 °C, (d) 400 °C, (e) 500 °C

Fig. 13. Relationship between surface residual stress and annealing time

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PT

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IP T

SC R

Fig. 14. Surface residual stress relaxation curves of deep rolled pure Ti