Surface nanocrystallization and gradient structure developed in the bulk TC4 alloy processed by shot peening

Accepted Manuscript Surface nanocrystallization and gradient structure developed in the bulk TC4 alloy processed by shot peening Y.G. Liu, M.Q. Li, H.J. Liu PII:

S0925-8388(16)31656-5

DOI:

10.1016/j.jallcom.2016.05.295

Reference:

JALCOM 37815

To appear in:

Journal of Alloys and Compounds

Received Date: 17 February 2016 Revised Date:

24 May 2016

Accepted Date: 27 May 2016

Please cite this article as: Y.G. Liu, M.Q. Li, H.J. Liu, Surface nanocrystallization and gradient structure developed in the bulk TC4 alloy processed by shot peening, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.05.295. 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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Graphical abstract

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Surface nanocrystallization and gradient structure developed in the bulk TC4 alloy processed by shot peening Y.G. Liu, M.Q. Li*, H.J. Liu

Xi’an 710072, PR China

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School of Materials Science and Engineering, Northwestern Polytechnical University,

*Corresponding author, Tel.: +86 29 88460328, Fax: +86 29 88492642.

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E-mail: [email protected]

Abstract

A gradient nanocrystalline structure with the grain size ranging from nanometer scale at the treated surface to micrometer scale in the matrix of the bulk

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coarse-grained TC4 alloy was fabricated by means of shot peening at an air pressure of 0.25 MPa and processing durations of 30~60 min. The microstructure characteristics including the constitution of deformation zones, grain size distribution

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and phase transformation had been systematically investigated. The experimental

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results showed that the thickness of deformation zones comprising equiaxed nanograins (EqN) layer, equiaxed ultrafine grains (EqU) layer, elongated ultrafine grains layer, the refined grains layer and low-strain matrix layer was up to 160 µm. Especially, the thickness of EqN layer was up to 40 µm and the average grain size of the treated surface was about 64.6 nm. Furthermore, a small amount of rare phase transformation of α phase from hexagonal close-packed crystal structure to face-centered cubic structure manifested in EqN layer and EqU layer. Microhardness 1

ACCEPTED MANUSCRIPT test showed that a gradient variation of the microhardness (HV0.025) with the depth from 486 at the treated surface to 315 in the matrix was obtained. Keywords:

Titanium;

Shot

peening;

Surface

nanocrystallization;

Phase

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transformation; Gradient structure

1. Introduction

Comparing with conventional coarse-grained materials, nanocrystalline materials

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have high hardness and strength, good tribological properties, enhanced electrical

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resistivity, higher thermal expansion coefficient, higher heat capacity and so on [1]. Surface nanocrystallization (SNC) [2, 3] on the strength of severe plastic deformation (SPD) is the most effective and promising method to induce nanocrystalline layer in bulk metallic materials and provides a very potential approach to improving surface

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and overall properties of metallic materials. Shot peening (SP) as a typical surface mechanical treatment method extensively used in industry can be applied to acquire a nanostructured surface layer [4-7]. The investigations on SNC by using SP have

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attracted an intense research interest during the last decades [8-14]. Liu et al. [8]

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obtained a nanocrystalline layer with an average grain size of 6.7 nm on the surface of TC17 alloy. Han et al. [9] showed that SNC on the surface layer of Ti-4Al-2V alloy processed by means of SP reduced the diffusion activation energy of Fe atom and promoted the diffusion coefficient of Fe atom during diffusion bonding with 0Cr18Ni9Ti stainless steel. Bagherifard et al. [10], Hassani-Gangaraj et al. [11] and Wen et al. [12] fabricated the nanostructured surface layers on metallic materials, and pointed out that the fatigue behaviors of the processed metallic materials were 2

ACCEPTED MANUSCRIPT significantly improved comparing with the untreated specimens. Jin et al. [13] predicted the strain rate distribution in commercially pure titanium (CP-Ti) processed by SP and investigated the effect of strain rate on SNC of CP-Ti. Hassani-Gangaraj et

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al. [14] developed a model linking finite element simulation of SP to dislocation density evolution to predict the grain size distribution in the surface layer of AISI 4340 steel. In addition, laser shock peening (LSP) [15] can drive a high pressure

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shock wave and also be used to achieve SNC of metallic materials owing to the

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continuing laser irradiation.

Although the free-standing nanocrystalline materials exhibit a very high strength, they frequently suffer from a great decrease in ductility [16]. It is widely accepted that a gradient naocrystalline structure (GNS) with the grain size increasing from

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nanometer scale to initial grain size on the surface of bulk metallic materials is an effective approach to simultaneously improving the strength and the ductility [17]. Wang et al. [18] obtained a GNS layer with the grain size ranging from 10 mn to 200

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nm on the surface of T10 steel via dry sliding friction. Cai et al. [19] fabricated a GNS

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surface layer of Cu-Zn alloys via surface mechanical attrition treatment (SMAT) and illustrated the superior strength-ductility synergy. Yang et al. [20] and Huang et al. [21] showed that the GNS surface layer of pure Cu and AISI 316L stainless steel processed by SMAT promoted fatigue resistance. Titanium and its alloys have attracted great attention due to their excellent properties and comprehensive applications [22, 23]. Consequently, it is very meaningful and attractive to obtain a GNS layer on the surface of titanium and its alloys for further improving their properties so as to 3

ACCEPTED MANUSCRIPT vanquish the severe service-environments. However, the investigations on GNS of titanium and its alloys are just underway that the preoccupation with the GNS has been barely given to titanium and its alloys up to now [24].

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In this paper, SP is used to fabricate a GNS layer on the surface of a bulk coarse-grained TC4 titanium alloy (approximately corresponding to ASTM Ti-6Al-4V). The aim of this study is to present a systematical investigation on the

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GNS of TC4 alloy via X-ray diffraction (XRD), scanning electron microscope (SEM),

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transmission electron microscopy (TEM) and microhardness tester. In particular, the microstructure characteristics including the constitution of deformation zones, grain size distribution and phase transformation of the GNS layer are analyzed in depth.

2. Experimental procedure

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The chemical composition (wt. %) of as-received TC4 alloy bar with a diameter of 40.0 mm is composed of 6.41Al, 4.19V, 0.02Fe, 0.006C, 0.001N, 0.16O, 0.002H and balance Ti. The annealing treatment of TC4 alloy was performed at the heating

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temperature of 873 K for 1 h and cooling in the air to room temperature. The TC4

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alloy specimens with a dimension of 70 mm×19 mm×4 mm were manufactured from the as-annealed TC4 alloy and then the surface of TC4 alloy specimens were grinded with silicon carbide paper to grade 600. SP was carried out on an air blast machine MP6000PT, in which the peening

nozzle with a diameter of 10 mm, the standard cast steel shots S230 with a diameter of 0.6 mm and the mass flow rate of about 10 kg/min were adopted. The distance between the peening nozzle and the treated surface of TC4 alloy specimens was about 4

ACCEPTED MANUSCRIPT 300 mm. In addition, in order to ensure the occurrence of significant grain refinement and eventual SNC, SP was conducted at an air pressure of 0.25 MPa and the far longer processing durations of 30~60 min.

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XRD analysis of TC4 alloy before and after SP was carried out on a X’Pert Pro MPD X-ray diffraction instrument with a Cu Kα radiation. The average crystallite sizes were calculated by analyzing diffraction peaks via Scherrer-Wilson method [25].

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The Scherrer-Wilson equation was written as: FW × cosθ = K × λ/d + 4 × ε × sinθ,

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where FW was the broadening of diffraction peaks, K was the shape factor of the lattice constant (1.89), λ was the wavelength of the X-ray radiation (λ=0.1542 nm), d was the average crystallite size, ε was the microstrain and θ was the Bragg angle. The cross-sections of TC4 alloy specimens for SEM observation were mechanically

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polished and then chemically etched in a solution of 1 ml HNO3, 3 ml HF, 7 ml H2O2 and 20 ml H2O. SEM observation was performed on a SUPRA 55 field emission scanning electron microscope operated at an acceleration voltage of 15 kV. The

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cross-sectional thin foils of TC4 alloy specimens for TEM observation were prepared

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by means of cutting, grinding and ion thinning with proper incident angles at low temperature. TEM observation was performed on a Tecnai G2 F30 field emission transmission electron microscope operated at an acceleration voltage of 300 kV. In order to determine the microstructure characteristics at different depths, TEM observation was conducted successively from the treated surface to the matrix. The microhardness measurement was conducted on a Shimadzu HMV-2T microhardness tester at a load of 25 g and dwell time of 10 s. The lengths of perpendiculars between 5

ACCEPTED MANUSCRIPT the indentation points and the treated surface were measured by using light microscope.

3. Results and discussion

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3.1 The microstructure prior to SP The microstructure of TC4 alloy prior to SP is shown in Fig. 1(a) and consists of α phase with a total volume fraction of about 85.7% and a complementary small

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amount of β phase as confirmed by the XRD pattern at the upper right corner of Fig.

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1(a). Meanwhile, the primary α phase is mainly featured by its equiaxed morphology. As seen from Fig. 1(b), the typical bright field (BF) TEM image of primary α phase indicates that only few dislocation lines (DLs) are in the coarse-grained α phase of TC4 alloy at the annealed condition. The corresponding selected area electron

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diffraction (SAED) pattern demonstrates that there is no misorientation in the grain and the grain should be approximately perfect crystalline. Fig. 1(c) shows the grain size distribution of primary α phase. It can be seen from Fig. 1(c) that the average

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grain size of primary α phase is about 9.37 µm.

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3.2 Post-processed structure characteristics 3.2.1 Deformation zone Fig. 2(a) displays the XRD patterns of the surface layer in TC4 alloy processed at

different processing durations. As shown in Fig. 2(a), it is clear that a dramatic broadening of Bragg diffraction peaks occurs at the processing durations of 30 min and 60 min, which is mostly contributed from grain refinement and the presence of high-level microstrain [26-28]. The significant peak broadening leads to a serious 6

ACCEPTED MANUSCRIPT overlap among the adjacent peaks such as the peaks of α (002), β (110) and α (101). The average crystallite sizes with respect to the processing durations of 30 min and 60 min have been determined to be about 51 nm and 23 nm respectively by calculating

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from full width at half maximum (FWHM) of Bragg diffraction peaks of α (100), α (102), α (110) and α (103). The result demonstrates that the grains in the surface layer of SP processed TC4 alloy have been effectively refined into nanometer scale and the

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nanocrystallization degree notably increases with the increasing of the processing

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duration. Fig. 2(b) shows the XRD patterns at different depths below the treated surface of TC4 alloy processed at a processing duration of 60 min. As seen from Fig. 2(b), the evident broadening of Bragg diffraction peaks occurs as well from the treated surface to about 100 µm below the treated surface. Meanwhile, FWHM of

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diffraction peaks of α (102), α (110) and α (103) increase with a decrease in the depth as illustrated in Fig. 2(c). The above-mentioned result indicates that grain refinement of α phase distinctly intensifies as the depth decreases.

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Fig. 3(a) displays the typical SEM micrograph on the cross-section of TC4 alloy

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processed at a processing duration of 60 min. As seen from Fig. 3(a), the microstructure difference between the deformation zones and the matrix is significantly distinct and can be obviously observed in the SP processed TC4 alloy. The total thickness of the deformation zones is about 160 µm as shown in Fig. 3(a). Meanwhile, there is no sharp boundary between the deformation zones and matrix. Furthermore, it is worth noting that the thickness of the SPD layer with the indistinguishable grain boundaries is up to 90 µm, which is much bigger than that of 7

ACCEPTED MANUSCRIPT TC4 alloy processed by LSP [29] or sliding wear [30]. To investigate the microstructure evolution, high magnification SEM and TEM observations at different deformation zones, marked I~V in Fig. 3(a), are conducted. It

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can be determined that the region V should correspond to the initial stage of plastic deformation as seen from the local magnification SEM image at a depth of 130 µm below the treated surface illustrated in Fig. 3(b). The α grains are slightly elongated,

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and their grain sizes are close to the initial grain size of α phase. In order to determine

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the fine-structure characteristic at this region, TEM observation is performed at the contiguous depth of about 130 µm below the treated surface. As seen from Fig. 3(c), _

the high-density DLs, dislocation tangles (DTs) and {1012} twin are simultaneously developed at this depth, i.e., the dislocation slip and deformation twinning

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concurrently occur to accommodate the plastic strain of α phase in TC4 alloy during SP making the deformation continuously proceed. In terms of the Von-Mises criterion, the homogeneous plastic deformation of the metals and alloys at least needs five

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independent slip systems. In the case of the hexagonal close-packed (hcp) α phase, the

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prismatic and basal slip systems provide only four independent slip systems. The deformation of α phase consequently needs twinning to accommodate plastic strain in addition to dislocation slip. As a result, both twinning and dislocation slip play major roles during SP of TC4 alloy. The high magnification SEM image at a depth of 90 µm below the treated surface is shown in Fig. 3(d). As seen from Fig. 3(d), the grains are severely elongated and the long axis of the elongated grains tends to be parallel to the treated surface. 8

ACCEPTED MANUSCRIPT The sizes of short axis of the elongated grains are mostly in the range of 1~3 µm, indicating the grains are at the refined grain scale. Fig. 3(e) displays the BF TEM image taken from the contiguous depth of about 90 µm below the treated surface.

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Higher-density dislocations are found in the grain/subgrains interior, responsible for the strongly work-hardening during SP. Besides, it can be found that the depths which the elongated grains are at the refined grain scale are in the range of 83~112 µm by

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SEM observation.

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Then, the TEM observation is performed at a depth of about 75 µm below the treated surface. As seen from the BF TEM image illustrated in Fig. 3(f), the prominent grain refinement occurs at such a depth that the very fine and elongated grains almost exist everywhere. The corresponding SAED pattern shows discontinuous rings pattern

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suggesting coexistence of both grains and subgrains. The sizes of short axis of the elongated grains are smaller than 1 µm, indicating that the grains are at the ultrafine grain (UFG) scale. Meanwhile, TEM observations corresponding to the region III

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show that the grain morphology is still elongated and the grain size is smaller than 1

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µm. Fig. 3(g) and (h) show the typical TEM images at a depth of about 50 µm below the treated surface. As seen from Fig. 3(g) and (h), the grain morphology differs considerably from that observed at the region III and the microstructure is mainly composed of 100~300 nm equiaxed grains. And, the equiaxed UFGs can maintain their morphology and grain scale until the depth decreases to about 40 µm. To confirm the presence of the nanograins, TEM observation of the treated surface is realized as seen from Fig. 4(a) and (b). The extremely fine and equiaxed 9

ACCEPTED MANUSCRIPT grains are observed in dark field (DF) image shown in Fig. 4(b). The corresponding SAED pattern illustrated in Fig. 4(a) exhibits nearly complete rings, demonstrating unambiguously that the nanograins are random in crystallographic orientation. In

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addition, as seen from the inset of Fig. 4(b), the average grain size at the treated surface via statistical analysis method is about 64.6 nm, which is larger than the XRD result. Generally speaking, the crystallite size obtained by XRD analysis is defined as

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the size of the coherently scattering domains, which should correspond to the

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defect-free crystal, consequently XRD can distinguish the subgrains with small misorientations and give the average size of subgrains [31]. However, the conventional TEM DF image provides the average size of the grains with high angle grain boundaries.

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Furthermore, the grains are still mostly equiaxed at a depth of about 20 µm below the treated surface as shown in Fig. 5(a) and (b). Meanwhile, the corresponding SAED pattern shows less continuous rings than that at the treated surface, suggesting

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the increase of the grain size. The histogram in the inset of Fig. 5(b) shows that most

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of the grains are in the size range of 30~120 nm with an average grain size of about 70.4 nm and a biggest grain size of about 304.9 nm, which are bigger than that at the treated surface. Additionally, the thickness of the equiaxed nanograins layer via TEM observation is determined to be about 40 µm for TC4 alloy subjected to SP treatment during 60 min. In contrast to the aforementioned results, the microstructure characteristics of TC4 alloy subjected to SP treatment during 30 min have also been investigated as a 10

ACCEPTED MANUSCRIPT function of the depth via TEM as shown in Fig. 6. As seen from Fig. 6(a1) and (a2), the grains at the treated surface have also been refined to the nanograins. By observation of the deformed microstructure at different depths, it is found that the

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microstructure characteristics at a processing duration of 30 min are similar with that at a processing duration of 60 min. The grains at the depths of 0~17 µm below the treated surface are equiaxed nanograins, the grains at the depths of 17~30 µm below

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the treated surface are equiaxed UFGs, the grains at the depths of 30~51 µm below the

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treated surface are elongated UFGs, the microstructure at a depth of 51 µm below the treated surface is mainly composed of extensive DTs and the region at a depth of 51 µm below the treated surface should correspond to the refined grains region and low-strain matrix. Based on the above-mentioned results, the deformation zones of the

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SP processed layer in TC4 alloy perpendicular to the treated surface are in sequence of equiaxed nanograins (EqN) layer, equiaxed UFGs (EqU) layer, elongated UFGs (ElU) layer, refined grains (RG) layer and low-strain matrix (LS) layer.

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3.2.2 Grain size

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The high-magnification SEM observation of TC4 alloy subjected to SP treatment during 60 min has been conducted so as to further analyze the variation of α grain size with the depth. It must be pointed out that the statistical grain sizes at the depths of 0~60 µm below the treated surface are not included for the indistinguishability of grain boundaries in such a depth range. In the meantime, the sizes of the elongated grains are referred to the sizes of short axis in this work. The variation of the measured grain size with the depth is shown in Fig. 7. As seen from Fig. 7, the grain 11

ACCEPTED MANUSCRIPT sizes at the depths of 60~70 µm below the treated surface are less than 470 nm. As the depth increases to 83 µm, the grain sizes increase from hundreds nanometers to about 1 µm. As the depth increases from 83 µm to 112 µm, the grain sizes increase from 1

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µm to 3 µm. At the depth larger than 112 µm, the primary α grain sizes gradually increase to the initial grain size. Therefore, combined with the previous analysis

layer has been induced in TC4 alloy by means of SP.

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3.2.3 Phase transformation

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results illustrated in Fig. 4 and Fig. 5, it is reasonable to confirm that a GNS surface

In this study, it is interesting to find a rare phase transformation of α phase in EqN layer and EqU layer of TC4 alloy subjected to SP treatment during 60 min. High-resolution TEM (HRTEM) observation is conducted on a nanograin shown in

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the lower right corner of Fig. 8(a). The fast-Fourier-filtered (FFT) image and inverse FFT (IFFT) image corresponding to the region B are shown in Fig. 8(b) and (c), respectively. The crystal structure of the nanograin is likely to the body-centered cubic

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(bcc) structure due to the fact that the zone axis (ZA) of the FFT image is [001] as

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shown in Fig. 8(b). Nonetheless, if so, the interplanar distance of the {110} plane is determined to be 0.2073 nm as indicated in Fig. 8(c), which shows a clear deviation from the normal value of 0.2337 nm. It should be noted that deformation-induced phase transformation from hcp structure to face-centered cubic (fcc) structure has been found to be operative in α phase [32]. If the crystal structure of the nanograin is fcc structure, the interplanar distance of {200} plane is 0.2073 nm, suggesting that the lattice parameter of the fcc phase is calculated to be 0.4146 nm. The lattice parameter 12

ACCEPTED MANUSCRIPT is consistent with the previous results which are mostly in the range of 0.41~0.43 nm [32-36]. Therefore, the crystal structure of the nanograin is indeed fcc structure. In addition, the deformation twinning is found to be induced in α phase of TC4 alloy

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subjected to SP treatment during 60 min as seen from Fig. 9. As shown in Fig. 9(b) and (c), the twinning plane is identified as (111) plane. It is generally understood that the {111} type twins are only possible to appear in fcc structures. Therefore, it further

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verifies that the phase transformation can occur in α phase of TC4 alloy.

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Although no fcc structure has been detected by XRD as the concentration is probably less than the XRD detection limit, the above-mentioned results prove that the rare phase transformation from hcp structure to fcc structure indeed occurs in α phase of TC4 alloy during SP. The phase transformation is believed to be a

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deformation-induced process and markedly influenced by the strain rate and grain size [32, 36]. It is generally understood that the strain rate dramatically increases with the decreasing of the depth [14], and the grains near the treated surface have been

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markedly refined, which result in the occurrence of the rare phase transformation.

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3.2.4 Microhardness

Fig. 10 shows the variation of microhardness with the depth from the treated

surface to the matrix of TC4 alloy subjected to SP treatment during 60 min. As seen from Fig. 10, the total thickness of hardening layer due to SP is about 300 µm. Meanwhile, as expected, there is also a gradient change in microhardness of SP processed TC4 alloy. The average microhardness (HV0.025) of TC4 alloy before SP is about 315, microhardness at a depth of about 10 µm below the treated surface of TC4 13

ACCEPTED MANUSCRIPT alloy is about 486, increased by 54.3%, and microhardness gradually decreases with the increasing of depth from the topmost surface to the matrix of SP processed TC4 alloy and tends to be a steady value. Hardness is a basic mechanical property of

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material which is usually defined as the resistance offered by the material to indentation. In terms of the Hall-Petch relationship, an increase in microhardness can be attributed to grain refinement. Meanwhile, microhardness is in a positive

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correlation to dislocation density. During the SP process, SPD with high strain and

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strain rate causes the dislocations to generate and multiply, and then the work-hardening and grain refinement occur and lead to an increase in microhardness. Consequently, the gradient variation of microhardness with the depth in the SP processed TC4 alloy can be obtained.

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4. Conclusions

In this work, a nanostructured surface layer has been fabricated in the bulk coarse-grained TC4 alloy by means of SP at an air pressure of 0.25 MPa and the

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processing durations of 30~60 min. The microstructure characteristics at different

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depths from the topmost surface to the matrix of SP processed TC4 alloy are analyzed via XRD, SEM, TEM and microhardness tester. The main conclusions can be drawn as follows:

(1) The deformation zones with a thickness of 160 µm comprise EqN layer, EqU

layer, ElU layer, RG layer and LS matrix layer. The thickness of EqN layer with an average grain size at the treated surface of 64.6 nm is about 40 µm. Based on the evolution of the grain morphology and gradient variation of the grain size with the 14

ACCEPTED MANUSCRIPT depth, it is confirmed that the GNS has been successfully obtained in TC4 alloy. (2) A small amount of rare phase transformation of α phase from hcp crystal structure to fcc crystal structure with the lattice parameter of 0.4146 nm occurs in

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EqN layer and EqU layer. (3) The gradient variation of microhardness (HV0.025) from 486 to 315 with the

grain refinement and high dislocation density.

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Acknowledgements

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increasing of the depth is obtained. An increase in microhardness is attributed to both

We acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 51475375) and Fund of the State Key Laboratory of Solidification Processing in NWPU (Grant No. KP201305).

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[26] L.F. Hou, Y.H. Wei, X.F. Shu, B.S. Xu, Nanocrystalline structure of magnesium alloys subjected to high energy shot peening, J. Alloys Compd. 492 (2010) 347-350. [27] R. Huang, Y. Han, Structure evolution and thermal stability of SMAT-derived

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ACCEPTED MANUSCRIPT layer of Ti-6Al-4V alloy after sliding wear in vacuum, Mater. Charact. 50 (2003) 275-279. [31] M. Samadi Khoshkhoo, S. Scudino, J. Thomas, T. Gemming, H. Wendrock, J.

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Eckert, Size evaluation of nanostructured materials, Mater. Lett. 108 (2013) 343-345. [32] I. Manna, P. Chattopadhyay, P. Nandi, F. Banhart, H.-J. Fecht, Formation of face-centered-cubic titanium by mechanical attrition, J. Appl. Phys. 93 (2003)

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[33] P. Chatterjee, S.S. Gupta, An X-ray diffraction study of strain localization and anisotropic dislocation contrast in nanocrystalline titanium, Philos. Mag. A 81 (2001) 49-60.

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quenching, Mater. Lett. 132 (2014) 157-161. [36] M. Phasha, A. Bolokang, P. Ngoepe, Solid-state transformation in nanocrystalline Ti induced by ball milling, Mater. Lett. 64 (2010) 1215-1218.

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Fig. 1 (a) The original microstructure of TC4 alloy before SP and the inset showing the corresponding XRD pattern; (b) The BF TEM image of the primary α phase and the inset showing

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the corresponding SAED pattern; (c) The grain size distribution of primary α phase in TC4 alloy.

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Fig. 2 XRD analysis of TC4 alloy after SP (a) at different processing durations and (b) at a

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processing duration of 60 min and different depths; (c) the variation of the FWHM with the depth

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at a processing duration of 60 min.

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Fig. 3 (a) Typical SEM image on the cross-section of TC4 alloy processed at a processing duration of 60 min; (b) and (d) are the enlarged SEM images at the depths of 130 µm and 90 µm below the treated surface respectively; (c) and (e) are the TEM images at the depths of 130 µm and 90 µm

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below the treated surface respectively; (f) TEM image at a depth of 75 µm below the treated

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surface; (g) and (h) are the BF and DF images at a depth of 50 µm below the treated surface.

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Fig. 4 TEM images at the treated surface: (a) BF image and the inset showing the corresponding

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SAED pattern; (b) DF image and the inset showing the distribution of grain size.

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Fig. 5 TEM images at about 20 µm below the treated surface: (a) BF image and the inset showing the corresponding SAED pattern; (b) DF image and the inset showing the distribution of grain

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size.

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Fig. 6 Typical TEM images showing the microstructure characteristics of TC4 alloy subjected to SP treatment during 30 min.

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Fig. 7 Variation of the measured grain size with the depth via high-power SEM observation.

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Fig. 8 (a) The HRTEM image from the black framed region presented in the inset image at the

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Fig. 9 (a) HRTEM observation of α phase at the treated surface, with a FFT image corresponding to the white framed region in the inset; (b) schematic illustration of FFT image in (a); (c) the IFFT

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image corresponding to the white framed region in (a).

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Fig. 10 Variation of the microhardness with the depth in the SP processed layer of TC4 alloy.

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Highlights: ·A gradient nanocrystalline structure was induced in surface layer of Ti-6Al-4V.

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·The constitution of deformation zones with a thickness of 160 µm was discussed.

·The thickness of nanograins layer with an average grain size of 64.6

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peening.

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·A rare α phase with face-centered cubic structure appears due to shot

·The gradient variation of microhardness with the increasing of the

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depth is obtained.