The evolution of hardness homogeneity in commercially pure Ti processed by ECAP

578 Vol.29 No.3 LIU Xiaoyan et al: The Evolution of Hardness Homogeneity in Commerci...

The Evolution of Hardness Homogeneity in Commercially Pure Ti Processed by ECAP

LIU Xiaoyan1, ZHAO Xicheng1, YANG Xirong1, JIA Jiangping2, QI Boli1 (1. School of Metallurgical Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China; 2. Xi’an Shaangu Power

Co., Ltd, Xi’an 710075, China)

Abstract: The evolution of hardness homogeneity in commercially pure titanium processed by equal channel angular pressing (ECAP) for up to 4 passes following route C at room temperature using a die of 90° was investigated by recording the microhardness on the cross-sectional and longitudinal planes of each billet. The results show that the hardness increases signifi cantly after the fi rst pass although there is a region of lower hardness on the cross-section running in a band near the bottom surface of the billet, and then increases by very small amounts in subsequent passes. With increasing numbers of passes, the lower hardness region near the bottom surface disappears and the microhardness values are distributed homogeneously throughout the cross-sectional and longitudinal planes after 4 passes of ECAP. The microhardness values in the central regions of the billet are slightly lower than those of the top and bottom surfaces. The results show that good homogeneity may be achieved throughout the billets after 4 passes of ECAP following route C.

Key words: commercially pure titanium; equal channel angular pressing; hardness; homogeneity

©Wuhan University of Technology and SpringerVerlag Berlin Heidelberg 2014(Received: Oct. 19, 2013; Accepted: Feb. 8, 2014)

LIU Xiaoyan(刘晓燕): Assoc. Prof.; Ph D candidate; E-mail: [email protected]

Funded partly by the National Natural Science Foundation of China (No. 5043430), Specialized Research Fund for the Doctoral Program of Higher Education of China (No.20116120110012) and the Natural Science Foundation of Shaanxi Province of China (No.2010JM6010)

DOI 10.1007/s11595-014-0960-1

1 Introduction

Severe plastic deformation (SPD) is recognized as an important technique to signifi cantly refi ne grains and achieve bulk ultrafi ne grained (UFG) materials[1-3]. Several different SPD procedures are available such as equal channel angular pressing (ECAP)[4], high pressure torsion (HPT)[5] and accumulative roll bonding (ARB)[3]. ECAP process has received lots of attention as the most prominent representative of the SPD method for using simple tools and producing relatively large billets of UFG materials[6,7]. Early work demonstrated the possibility of using ECAP to achieve high strength in a wide range of materials (e g, copper, aluminium and their alloys)[8-10]. However, processing by ECAP is

diffi cult in hexagonal close packed metals such as pure titanium, where the number of independent slip systems is limited. Semiatin et al[11] studied the workability of commercial-purity titanium during ECAP and found that it was not possible to process this material at room temperature with a channel angle of Ф=90°. These so-called “difficult-to-work” materials may be processed more easily by using an ECAP die having an increased channel angle based on the detailed finite element calculation[12]. Recently ECAP has been successfully applied for grain refi nement in commercially pure (CP) Ti at room temperature using a die with Ф=120° and Ф=135° [13-16]. Zhao et al[17] has successfully fabricated UFG CP Ti at room temperature using a die with Ф=90° by improving ECAP parameters based on the fi nite element simulation.

The success of the ECAP technique in achieving UFG microstructures and improved properties leads to interest in the evolution of homogeneity in materials processed by ECAP. In principle, the billet pressed through an ECAP die will undergo a homogeneous simple shear strain[18,19]. However in practice the imposed strain during the ECAP is not homogeneous. For example, there is a frictional force between the

Journal of Wuhan University of Technology-Mater. Sci. Ed. June 2014 579

billet and the die wall and this may change the strain distribution especially in the vicinity of the die walls[20]. In early studies about aluminium and its alloys, it was recognized that hardness may vary across any section of the pressed billet and detailed measurements were carried out to evaluate the evolution of homogeneity on the cross-sectional and longitudinal planes of billets processed by ECAP through a number of passes[21-23].

So far, the evolution of microhardness homog-eneity of CP Ti processed by ECAP has not yet been investigated. The present work is focused on the first systematical investigation on the evolution of microhardness homogeneity on cross-sectional and longitudinal planes of CP Ti submitted to ECAP-Route C for up to 4 passes at room temperature using a die of 90°.

2 ExperimentalCommercially pure Ti of grade I was received

in the form of plate and its chemical composition (in wt%) was 0.012 Fe, 0.022 C, 0.06 O, 0.003 N, 0.001 H and balance Ti. In the as-received condition, the average grain size of CP Ti was about 26 μm. The pure Ti plate was cut into billets with cross-section of 18×18 mm2 and length of 85 mm along the rolling direction of plate.

The processing by ECAP was conducted at room temperature using a die with channel angle of 90° and an external arc of curvature of 20°, where this geometry gave an imposed equivalent strain of about 1 on each separate pass through the die[24]. Billets were processed for up to four passes giving a maximum total strain of about 4. The pressing was performed using a pressing speed of 3.5 mm s1 and processing route C in which the billet was rotated 180° clockwise along its longitudinal axis between adjacent passes[25]. Each billet was coated with a lubricant containing graphite powder and MoS2 before pressing.

Based on the orthogonal notation used in many earlier studies of ECAP[26], the X or cross-sectional plane is perpendicular to the pressing direction, the Y or flow plane is parallel to the side face of the billet at the point of exit from the die and Z is the longitudinal plane parallel to the top surface at the point of exit from the die. Two groups of billets were selected after pressing through each of 1, 2, 3 and 4 passes, respectively. The billets from one group were cut perpendicular to their longitudinal axis. All of the microhardness measurements for this group were taken

on the cross-sectional (X) planes of each billet. The billets from another group were cut along the central longitudinal lines perpendicular to the top surface. All of the microhardness measurements for this group were taken on the vertical longitudinal (Y) planes. The surfaces were carefully polished to a mirror-like fi nish.

Microhardness measurements were taken along a series of well-defined traverses on each polished surface using HX-1000TM microhardness tester equipped with a Vickers indenter using a load of 200 gf and a dwell time of 10 s. These separate traverses are depicted schematically in Fig.1. The microhardness measurements were taken on the cross-sectional (X) planes and 16 separate traverses that were parallel to Y axis were made by incremental spacing of 1.0 mm. These measurements were also recorded at interval of 1.0 mm giving total numbers of 16 datum points for each line. For the second group on the longitudinal (Y) planes, seven longitudinal traverses were made at distances of 1.0, 3.0, 6.0, 9.0 (center line), 12.0, 15.0, 17.0 mm from the top surface. All measurements were taken over rectangular sections in the central regions having widths of 18 mm and overall lengths of 40 mm. The individual measurements were taken along each linear traverse at interval of 3.0 mm giving total numbers of 14 datum points for each line. At each selected position, a cluster of four equally spaced indentations were made around the selected point with the indentations separated from these positions by a distance of 0.5 mm as illustrated in Fig. 1. Each datum point was calculated by averaging these four values and the error bars were calculated using the 95% confi dence value.

The individual microhardness measurements were plotted against the position measured on the cross-sectional plane for the first group and the position measured longitudinally for the second group. For both groups, the individual measurements were plotted in the form of color-code contour maps to provide a direct


visual representation of the data. The microhardness of the CP Ti under the as-received unpressed condition was also measured to provide a comparison with the as-pressed billets.

The ECAP billets processed through 1, 2, 3 and 4 passes were selected for microstructural observation near the center using transmission electron microscopy (TEM). TEM analysis was performed in a JEM-3010 FEG electron microscope operated at 300 kV.

3 Results

3.1 Microstructure after ECAPThe typical TEM images of the microstructure

developed during the different ECAP passes are shown in Fig.2. The elongated parallel shear bands with width of about 300 nm were observed after the first pass in Fig.2(a). Within these parallel bands some areas had high dislocation density and deformation twin marked by black arrow. The width of the elongated parallel shear bands decreased and some equiaxed grains/subgrains were found after the second pass in Fig.2(b). It can be seen that the microstructure was characterized by fairly equiaxed ultrafine grains after 3 and 4 passes in Fig.2(c) and 2(d), respectively. The grain size analysis was carried out after 4 passes of ECAP corresponding to the maximum strain of about 4. For these conditions, an average equiaxed grain size of about 170 nm was determined.3.2 Vickers microhardness after ECAP

Individual measurements of Vickers microhar-dness on the cross-sectional planes were taken along 16 horizontal traverses separated by spacing of 1.0 mm. For simplicity, the results in Fig.3 show only the points

recorded along the central traverse and at points 1.0 mm from the top and bottom surfaces after (a) 1 pass, (b) 2 passes, (c) 3 passes and (d) 4 passes, respectively. The lower dashed lines in Fig. 3 represent the value of the microhardness under the as-received condition. It is apparent from Fig.3 that the microhardness of CP Ti increased significantly after a single pass from an initial value of about 155 HV to values for all traverses within the range of about 170-220 HV. The increase in microhardness HV continued to 4 passes when almost all values of HV were larger than 220.

The careful analysis of the data after a single pass shown in Fig.3(a) indicates that the increase in the


microhardness was not homogeneous for these three traverses and in practice the average microhardness value (about 182 HV) adjacent to bottom surface was markedly lower than in the center with an average value of about 207 HV. The difference between the microhardness values at the center traverse and the traverse adjacent to the bottom surface decreased and the average microhardness value (about 225 HV) of center was slightly lower than that (about 229 HV) of the bottom surface after 4 passes. All individual points for each traverse lie within a relatively narrow range of not more than ±10 and ±5 HV after 1 pass and 4 passes, respectively. The values (center and at 1.0 mm from the top and bottom surfaces) for each vertical position lay within a range of not more than ±20, ±15, ±10, ±9 HV after 1 pass, 2 passes, 3 passes and 4 passes, respectively.

In order to provide a direct and simple visual presentation, Fig.4 shows color-coded contour maps of the cross-sectional planes after (a) 1 pass, (b) 2 passes, (c) 3 passes and (d) 4 passes, respectively. The significance of the colors is shown by the color scale given on the right. In these plots, the microhardness values are plotted on the cross-sectional planes with the Y direction lying along the bottom axis, the Z direction lying along the vertical axis. It is readily apparent after 1 pass in Fig.4(a) that the region of lower hardness is clearly evident adjacent to the bottom surface of the billet. This inhomogeneous region is confined to within a width of about 2 mm from the bottom surface

of the billet. This result is reasonably consistent with several predictions derived from finite element modeling[12,27-29]. Possible sources of these inhomogen-eities include the frictional forces between the billet and the die walls[30] and the presence of a “dead zone” or corner gap, which occurs if the billet loses contact with die wall at outer corner as it passes through the shear zone[31,32]. Furthermore, the microhardness values continue to increase over the entire cross-section but this region of lower hardness is gradually removed after 2 passes and it is fully removed after 4 passes. The microhardness values remain almost the same and the homogeneity of microhardness distribution is increased with the increasing number of passes.


Microhardness measurements were recorded on the longitudinal planes along seven separate traverses giving a total of 98 datum points for each sample. For simplicity, only three traverses are plotted in Fig.5 for the longitudinal planes after (a) 1 pass, (b) 2 passes, (c) 3 passes and (d) 4 passes, respectively: for all plots, the front edges of the billets lie on the left and the rear edges of the billets lie on the right. These traverses are located along the center line and at 1.0 mm from the top and bottom surfaces of the billets, where again the lower dashed line shows the hardness under the as-received condition. The results demonstrate that there is a signifi cant increase in the microhardness after 1 pass of ECAP, to about 210-225 HV. However, the hardness values closer to the bottom surface are quite close to the other traverses, which are not similar to those plotted in Fig.3(a). The increase in microhardness HV continues to 4 passes when almost all values of HV are larger than 240. As shown in Fig. 5, it is observed that there is a high level of homogeneity in the microhardness distributions on the longitudinal plane of the billets. It is consistent with earlier experimental investigation of CP Ti[15].

The associated color-coded maps for the longitudinal planes are shown in Fig.6 for (a) 1 pass, (b) 2 passes, (c) 3 passes and (d) 4 passes, respectively. The results indicate that the billets processed to 1 and 2 passes of ECAP exhibit a high degree of homogeneity while the billets processed by 3 passes exhibits some heterogeneity, but after 4 passes there is relatively good homogeneity. In terms of the longitudinal distributions of hardness values after 3 passes it is apparent from

Fig.6(c) that the average hardness values are slightly lower at the rear of the billet and higher at the front but this variation is no longer present after 4 passes in Fig.6(d). This variation along the billet length is probably a consequence of the inherent back-pressure introduced when using a solid die.

4 Discussion

The number of individual microhardness measurements recorded in this study was exceptionally large, with 256 individual measurements on each cross-sectional plane and 98 individual measurements on each vertical longitudinal plane. This number of datum points and each datum point calculated by averaging four adjacent points within a distance of 0.5 mm were suffi cient to minimize the errors in the results.

The results in the present experiments related to hardness measurements taken on two different sectional planes in billets processed by route C. Fig.7 shows a plots of the average Vickers microhardness values for the two different orientations after processing by ECAP for up to 4 passes, where these average values of HV were determined by calculating all the measurements points on each plane as used in the construction of Figs.4 and 6. It is apparent from Fig.7 that the microhardness of both X and Y planes increases significantly after the first pass of ECAP. There is a small increase in each additional pass for the Y plane to a maximum value of about 254 HV after 4 passes whereas for X plane there is a saturation value after the third pass at a maximum value of about 228 HV and then the hardness remains the same in the fourth pass. In this case, the average microhardness of Y plane is higher than that of X plane, which is due to the texture that exists after 4 passes. The Y plane has strong prismatic texture[33]. This orientation of the prismatic planes is not conductive to type a slip on the basal and prismatic planes, which is usually the first mode to


occur, thus other deformation modes such as type a+c slip on the pyramidal plane are activated, resulting in a higher fl ow stress and microhardness.

The trend may be attributed to the differences in the deformation mechanisms between the initial and later stages during the ECAP. Early studies indicated that the superior properties of CP Ti attained after only a single pass at room temperature refl ect the increased densities of dislocation and twins introduced at this lower temperature and the consequent increase in the rate of strain hardening[13,15], which were also found in this work as shown in Fig.2(a). Therefore, the hardening mechanism which is visible in Fig.7 after a single pass is due to the increase in the dislocation density, the introduction of twinning and the associated strain hardening. Pure Ti has high stacking fault energy and a rapid recovery rate and continuous dynamic recrystallization (CDRX) occurs easily in pure Ti[34]. The CDRX process is a recovery dominated process and proceeds by continuous absorption of dislocations in subgrain boundaries (LAGBs), which eventually results in the formation of ultrafi ne recrystallized grains with HAGBs as shown in Fig.2(d). The CDRX was observed in CP Ti during shear deformation[35]. HAGBs may soften materials in the steady state of deformation by enhancing the annihilation of dislocations. So there is a small microhardness increase in each additional pass.

It can be seen from the above results that the cross-sectional and longitudinal planes of CP Ti are capable of achieving a reasonable level of homogeneity after 4 passes in ECAP. It is important to note that the extent of microstructural homogeneity attainted during ECAP depends, at least in part, on the hardening characteristics of the material[20]. Earlier studies demonstrated that there is a good correlation between the microhardness values and the internal microstructures within the material[21]. Therefore good microstructural homogeneity may be achieved throughout the cross-sectional and longitudinal planes of CP Ti after pressing in ECAP for a total of at least 4 passes.

5 Conclusions

The evolution of homogeneity in commercially pure Ti was investigated by taking microhardness measurements on the cross-sectional and longitudinal planes after processing by ECAP through 1, 2, 3 and 4 passes.

The average microhardness increases signifi cantly after 1 pass although there is a region of lower hardness on the cross-sectional plane at the areas adjacent to the bottom surfaces of the billets. There are additional but smaller increases in microhardness in subsequent passes and the area of lower hardness close to the bottom surface disappears after 4 passes.

Commercially pure Ti achieves an essentially homogenous microhardness distribution on both the cross-sectional and longitudinal planes after 4 passes of ECAP.

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The evolution of hardness homogeneity in commercially pure Ti processed by ECAP