Insitu investigation of tensile deformation behaviour of cold-rolled interstitial-free high-strength steel in scanning electron microscope

Journal Pre-proof Insitu investigation of tensile deformation behaviour of cold-rolled interstitial-free highstrength steel in scanning electron microscope Shreya Mukherjee, Amrita Kundu, Partha Sarathi De, Jayanta Kumar Mahato, P.C. Chakraborti, M. Shome, D. Bhattacharjee PII:

S0921-5093(20)30118-0

DOI:

https://doi.org/10.1016/j.msea.2020.139029

Reference:

MSA 139029

To appear in:

Materials Science & Engineering A

Received Date: 10 October 2019 Revised Date:

25 January 2020

Accepted Date: 28 January 2020

Please cite this article as: S. Mukherjee, A. Kundu, P.S. De, J.K. Mahato, P.C. Chakraborti, M. Shome, D. Bhattacharjee, Insitu investigation of tensile deformation behaviour of cold-rolled interstitial-free highstrength steel in scanning electron microscope, Materials Science & Engineering A (2020), doi: https:// doi.org/10.1016/j.msea.2020.139029. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.

Credit author statement: Shreya Mukherjee: Performed Experiments, Amrita Kundu: Performed Microstructural analysis, Partha Sarathi De: Designing experiments, Jayanta Kumar Mahato: Void analysis, P.C.Chakraborti: Conceptualization, Methodology and supervision, M. Shome: Industrial collaboration, D.Bhattacharjee: Advisory role for the entire work

Insitu investigation of tensile deformation behaviour of cold-rolled interstitial-free high-strength steel in scanning electron microscope Shreya Mukherjee1a, Amrita Kundu1, Partha Sarathi De1bJayanta Kumar Mahato1c P.C.Chakraborti1d, M. Shome2 and D.Bhattacharjee3 1

Metallurgical and Material Engineering Department, Jadavpur University, Kolkata 700032, India 2 R&D Division, Tata Steel Ltd., Jamshedpur 831 007, India 3 New Materials Business, Tata Steel Ltd., Kolkata 700071, India a Presently at Metallurgical and Materials Engineering Department, Indian Institute of Technology Kharagpur 721302, West Bengal, India b Presently at Department of Engineering Design, Indian Institute of Technology Madras, Chennai 600036, India c Presently at Department Mechanical Engineering Department, Shobhit Deemed University, Meerut 250110, Uttar Pradesh, India d

Corresponding author, Tele: +913324146940, Fax: +913324137121, Email: [email protected]

ABSTRACT The progress of tensile deformation of cold rolled interstitial-free high-strength (IFHS) steel in the asreceived and 10 pct tensile pre-strained conditions has been studied through testing of miniature sized specimens in scanning electron microscope. Strength and ductility of the steel obtained through insitu tests have been compared with the macro tensile test results. The insitu test results show that the yield strength and tensile strength of miniature sized specimens are less as compared to that of macro tensile test results. But, uniform strain in case of insitu tests is higher than that obtained in case of macro tensile specimens. The insitu tests reveal that intergranular cracking of surface grains and formation of both intergranular and transgranular voids precede the final ductile fracture. The areal density of voids is found to increase with increase of Von Mises equivalent plastic strain. Under pre-strained condition void nucleation strain of the steel is found to be lower as compared to the as-received condition. Keywords: Interstitial-free steel; Insitu; Scanning electron microscope; Tensile; Void nucleation strain

1.0 Introduction Different grades of interstitial-free (IF) steels of less than 2 mm thickness find extensive applications for fabrication of various components in automotive industries primarily because of their good cold formability. Advances in steel making practice have led to the development of new generation of steels with exceptionally low levels of solute nitrogen and carbon content. The high formability of IF steels is achieved through lowering the amount of interstitial elements to a very low level. In this effort, addition of stabilising elements, e.g. Ti and/or Nb is done to eliminate the interstitial elements (C,N) which cannot be completely removed by steel making practice [1,2]. These elements act as a scavenger and remove the interstitial elements 1

from ferrite through precipitation of carbides and/or carbonitrides. As a result, the deleterious effects of interstitial elements with regard to formability issues are eliminated. However, ordinary interstitial free steels exhibit low yield strength and tensile strength. So, the attention has been shifted towards developing IF steels with higher strength-formability combination, and therefore interstitial-free high-strength (IFHS) steel has been developed. Here, strength is achieved via solid solution strengthening route primarily by adding P, Mn, Si, Cr and Cu [2]. There have been detailed investigations to understand the role of steel chemistry, formation of precipitates, evolution and role of texture on strength-formability combination of different types of interstitial-free steels [2-5]. Tensile deformation behavior of any material is commonly studied through conventional tests of macro tensile specimens. Postmortem analysis of fracture surfaces is routinely done to understand the fracture characteristics of a material under tensile loading. Nowadays sophisticated experimental techniques, e.g. insitu deformation, electron back scatter diffraction, atomic force microscopy, digital image correlation etc. are getting popular for a much deeper understanding of plastic deformation behavior of different materials at the microstructural level. Over the past one decade or so in-situ mechanical tests in scanning electron microscope (SEM) are becoming increasingly popular for an in-depth knowledge about the mechanical behaviours vis-à-vis microstructure evolution of materials [6-18], and also for studying crack initiation and propagation behaviour in dissimilar metal welded joints [19,20]. With the help of atomic force microscope (AFM) Chandrasekaran and Nygårds [21] have investigated the deformation features which generate on the specimen surface during tensile deformation in an attempt to understand the plastic deformation behaviour of ultra-low carbon steel. It is known that IF steels fail in a ductile manner which is controlled by dislocation flow behaviour during deformation [22]. The complete process of such ductile fracture involves stages, like, void nucleation, void growth and their coalescence forming microcracks, the growth of which leads to final fracture. Hence, under tensile loading the total strain to fracture is considered to be composed of two parts: one responsible for void nucleation (εn) and the other for void growth and coalescence (εgc) [23]. Before void nucleation, plastic deformation results in strain hardening due to dislocation-dislocation and dislocation-precipitate/particle interactions. Voids are formed easily when the plastic flow in the matrix phase is quickly retarded by non-metallic inclusions and/or other second phase particles. In this situation, the local magnification of stress at the inclusion/particle-matrix interfaces leads to nucleation of 2

voids through decohesion of inclusions/second phase particles from the interfaces of the matrix phase and these interfaces are identified as void nucleation sites [24-26]. Quite naturally, the void nucleation strain depends upon the amount and size of inclusions or second phase particles and their distribution throughout the matrix phase [27]. Accordingly, in cases where the amount of inclusions is significantly more or size of the inclusions is large, as it is generally observed in commercial steels, or in cases where the microstructure of the material under investigation consists of a number of phases of widely different character εn becomes much less as compared to cleaner steels [24,25,27]. In single-phase materials with low inclusion contents the void nucleation strain would thus be expectedly higher. In case of IF steels with extremely low interstitial content voids mostly nucleate at the interfaces between the ferrite matrix and second phase particles, e.g. carbides, nitrides or carbonitrides, which are submicrosopic in size [25,27,28,30]. The influence of TiN precipitates on void nucleation during tensile deformation of IF steel has been studied in detail by García et al. [29]. Mazumdar and Ray [28] investigated the effect of tensile pre-strain on ductile fracture behaviour of 18 mm thick hot rolled IF steel. These investigators reported that void nucleation strain first increased with increase of pre-strain up to 6.7 pct and then decreased with higher amount of pre-strain. However, in automotive sectors interstitial free steels are mostly used in much thinner gauge and under cold rolled conditions. It is, therefore, necessary to study deformation and fracture behaviour of cold rolled interstitial-free steel sheets. Within the group of sheet steels used in automotive industries wide attention has been paid to understand the deformation and fracture behaviour of dual-phase steels because of the presence of phases (ferrite and martensite) having widely different character of the phase constituents (ferrite and martensite) present in dual-phase steels. But, similar attention has not been paid for interstitial free steels, possibly because of its cleanliness and its single-phase matrix character. In the present investigation an attempt has been made to study the progress of tensile deformation process of IFHS steel sheet in the as-received and tensile pre-strained conditions in two different specimen orientations - Rolling Direction (RD:Y direction) and Transverse Direction (TD:X direction) through testing of miniature sized specimens in SEM. Although plastic deformation is a 3-dimensional process, in the present investigation deformation behavior has been monitored on the 2-D specimen surface to understand the deformation and fracture mechanisms under tensile loading. The advantage of the present investigation is that such insitu tests make it possible to study the same set of grains after each deformation step, 3

and thus giving the opportunity to follow the deformation behaviour on a local scale. The results of the insitu tensile tests have been compared with that obtained by testing macro tensile specimens in order to know the differences between bulk properties and that measured using miniature sized specimens. Another objective of the present investigation is to assess the influence of specimen orientation and tensile pre-strain on void nucleation strain in IFHS steel sheet under tensile loading. 2.0. Experimental 2.1. Material and metallography Cold-rolled interstitial-free high-strength (IFHS) steel sheet of 1 mm nominal thickness received from TATA Steel, Jamshedpur, India has been used in the present investigation. The chemistry of the steel in wt pct is: C-0.0029, Mn-0.39, Si-0.004, S-0.007, P-0.05, Cr-0.018, Al-0.044, Cu-0.005, Nb-0.001, Ti-0.042, N-0.0018 and balance Fe. The large surface (X-Y plane) of a small coupon of the steel sawed from the steel sheet was metallographically polished and then etched with Marshall’s reagent for 15 seconds. Microstructure of the steel was observed in an optical microscope, Leica DM2500, integrated with Leica digital camera DFC550. Grain size analysis of the steel was done using an automatic image analysing software, Leica Application Suite. 2.2. Tensile pre-straining of specimen blanks It is quite familiar that 10 pct pre-strain is a commonly used industrial practice in case of sheet steel [30]. However, such pre-straining is reported to be either beneficial or detrimental to fatigue strength of sheet steel [31]. The present investigation was, therefore, restricted to 10 pct tensile pre-straining both in RD and TD orientations. Tensile pre-straining of specimen blanks was done before fabrication of miniature sized tensile specimens for insitu testing in SEM and also for conventional tensile tests. In doing so, specimen blanks of 170 mm long 50 mm width were machined from the as-received steel sheet keeping long axes of the blanks parallel and perpendicular to the rolling direction of the steel sheet, i.e., along RD and TD orientations of the rolled sheet respectively. These blanks were deformed by 10 pct under tensile loading under strain-control mode at a strain rate of 10-3s-1 in a servohydraulic universal testing machine, Instron 8500R, of ± 100 kN capacity. Miniature sized tensile specimens and macro tensile specimens were fabricated from the central part of the uniformly deformed region of the specimen blanks. 4

2.3. Tensile tests Tensile specimens of rectangular cross-section were fabricated from the as-received and tensile pre-strained specimen blanks keeping the long axis of the specimens parallel to RD and TD orientation of the steel sheet. Figure 1 shows the geometry of the specimen used in the present study. Strain-control tensile tests have been done at a strain rate of 10-3s-1 in the same servohydraulic universal testing machine of ±100 kN capacity, Instron 8500R, using Instron Bluehill tensile testing software.

Fig. 1. Geometry of tensile specimen.

2.4. Insitu tensile tests in scanning electron microscope Miniature sized tensile specimens with RD and TD orientations were fabricated from the asreceived steel sheet and from tensile pre-strained specimen blanks using wire electrical discharge machining (EDM) process. Figure 2 shows the geometry of the miniaturized specimen used in the present study. Before testing, one surface of the specimen was metallographically polished and etched as before. The insitu tensile deformation experiments were performed in the vacuum chamber of a field emission gun scanning electron microscope (FEGSEM), FEI Quanta 450. The loading device is a screw driven GATAN, UK make tensile deformation stage, where the load is transmitted to the specimen through pin-loading arrangement. From trial experiments it was observed that during the initial loading phase the displacement of the grips was mostly accommodated in securing the specimen tightly with the pin. As a result, the actual specimen deformation during initial phase of loading became extremely less. 5

Fig. 2. Miniaturised tensile specimen geometry. To obviate the above difficulty in measuring specimen strain an alternative, but less critical, method has been used. In this method, after polishing and etching of one specimen surface two microindentation marks were placed along the centre line of the uniform section of the specimens using 100 gm load for 10 seconds. The distance between the two microindentations was ~2000 µm. These two indentation marks were considered as gauge markings for measurement of actual deformation of the specimens. It is worthy to mention here that because of the limited load and displacement capacity of the tensile deformation stage and the available vacuum chamber size of the SEM, the specimens used for insitu tensile deformation experiments were very small in size and do not conform to any test standard. Further, in this type of tests specimen strains have been measured on the surface which is completely in the plane stress condition. In the present study, the specimen was loaded in steps at a displacement rate of 1 mm/ min. After each loading step secondary scanning electron (SE) images were captured keeping the field of view almost unaltered. Further, after each loading step the SE images were also taken at low magnification in order to record the location of the two microindentation marks. The separation between these two microindentation marks was measured after each loading step and then loading was again resumed. These steps were followed until complete fracture of the 6

specimens. Thus a series of SE images of the specimens were obtained from the surface of the tensile deformed specimen. These images have been carefully examined to understand the deformation behaviour of the investigated steel under tensile loading. Fracture surfaces of the miniature sized tensile specimens were examined in the same FEGSEM to know the tensile failure characteristics of the investigated steel. A representative load-displacement plot obtained directly from the GATAN, UK make insitu tensile deformation stage is shown in Fig.3.

Fig. 3. Representative load-extension plot (part) obtained during testing of miniature sized specimen in the SEM.

2.5. Investigation of voids and estimation of void nucleation strain After completion of insitu tensile tests, one half of the specimens were cut through thickness of the specimens along the specimen axis in a Struers SecoTom10 machine keeping cutting wheel speed (rpm) and feed rate constant at 300 and 0.01 mm s-1 respectively. After through thickness sectioning one half (RD: Y-Z plane, TD: X-Z pane) was mounted using conducting Bakelite, and then polished in an automatic polishing machine, Struers TegraPol 21, using diamond suspension of successively finer diamond grits of sizes of 9, 6, 3 and 1µm respectively. Figure 4 shows one of such polished Bakelite mounted specimens. 7

Fig. 4. Bakelite mounted polished specimen used for imaging of the voids.

The polished specimens were observed in the unetched condition in FEGSEM under SE imaging mode at regular intervals along the length of specimens leaving 50 microns from the fracture end. At each interval seven different fields were selected along the specimen thickness to observe the voids. In each case, the voids were observed at the mid thickness of the fractured specimens and at three different fields (very close to mid thickness) on both sides of the mid thickness of the longitudinally sectioned specimens. The voids observed in the images were analysed by using Java based Image J image analysing software. In this study the areal density of voids has been considered.

3.0. Results and Discussion: 3.1. Microstructure Optical microscopy reveals that the microstructure of the steel consists of polygonal grains of ferrite as shown in Fig.5. The average two-dimensional ferrite grain size as determined by linear intercept method is found as approximately ASTM 10. Hardness of the steel at 1 kg load is found as 135 VPN with a standard deviation of 2.35.

8

Fig. 5. Microstructure of the investigated IFHS steel. 3.2 Macro tensile behaviour The engineering tensile stress-strain curves of the steel in RD and TD directions, both in the as-received and tensile pre-strained conditions are shown in Fig. 6(a-b). The corresponding tensile properties have been summarised in Table 1. Results shown in this Table 1 are closest to the mean values of three tests, the standard deviation of the experimental results being in the range of 2.08 to 3.6 for yield strength and tensile strength and 0.50 to 1.15 for uniform and total elongation. It is found that while with 10 pct tensile pre-strain 0.2 pct offset yield strength is increased by about 80 pct, the corresponding increase of tensile strength is only 8 pct. This shows that under pre-strained condition the strain hardening capacity of the steel is drastically reduced. The present results showing very large increase of yield strength under 10 pct tensile prestrained condition is in conformity with the variation of tensile yield strength with the amount of pre-strain as reported by Mazumdar and Ray [28] for hot-rolled interstitial free steel and also the work carried out on 2.3 mm thick cold rolled IF steel by De et al. [32]. In both investigations it has been observed that up to about 7.5 - 8 pct tensile pre-strain yield strength is increased very rapidly. The strain hardening behaviour of IF steels has been studied by other investigators and it has been found that with about 5-8 pct deformation either by rolling or by tensile loading dislocation cell structures are formed [33,34]. The present results showing very steep rise of tensile yield strength of the investigated IFHS steel with 10 pct tensile pre-strain is thus attributed to the increase of mobile dislocation density and formation of dislocation cell structure. It is also observed that concomitant with the increase of yield strength and tensile 9

strength, uniform elongation of the steel is reduced by 50 pct. Interestingly, uniform elongation of the pre-strained specimens when added with the amount of pre-strain becomes equal to the uniform strain of the specimens without pre-strain irrespective of specimen orientation (RD and TD). Total elongation, however, is not reduced to that extent as it occurs in case of uniform ductility. In previous investigations [28,32] similar variation of uniform and total elongation with tensile pre-strain has also been observed in both hot and cold rolled IF steels.

Table 1. Macro and insitu tensile test results Properties

As-received RD(0)

Pre-strained RD(10)

As-received TD(0)

Pre-strained TD(10)

0.2% offset yield strength (MPa)

231 (IS: 215)

414 (IS: 350)

219 (IS: 182)

403 (IS: 384)

Tensile strength (MPa)

413 (IS: 375)

445 (IS: 412)

403 (IS: 369)

437 (IS: 409)

% Uniform elongation

20 (IS: 25)

9 (IS: 17)

20 (IS: 26)

10 (IS: 17)

% Total Elongation

39 (IS: 68)

25 (IS: 52)

33 (IS: 75)

26 (IS: 67)

Strain hardening Exponent (n)

0.20 (IS: 0.22)

0.052 (IS: 0.053)

0.20 (IS: 0.24)

0.056 (IS: 0.039)

Strength coefficient (k), MPa

705 (IS: 627)

546 (IS: 532)

695 (IS: 674)

542 (IS: 507)

# Numbers in the parentheses used to identify specimens indicate the amount of pre-strain. ## IS: insitu

10

a

b

Fig. 6(a-b). Engineering stress-strain curves of the investigated IFHS steel in (a) RD and (b) TD orientations with and without tensile pre-strain 3.3 Insitu tensile deformation behaviour Results of insitu tensile tests are included Table 1. Figure 7(a-d} compares the tensile properties obtained from testing of miniature sized specimen with the corresponding results obtained from testing of macro tensile specimens in the form of bar diagram. It has been observed that the nature of insitu tensile stress-strain curves of the four types of specimens, viz. RD(0), RD(10), TD(0) and TD(10), are similar to that shown in Fig. 6(a-b) for macro tensile specimens. As it has been observed in case of macro tensile tests, the tensile properties of miniature sized tensile specimens shown in Table 2 also reveals similar trend in the variation of strength and ductility of different specimens. It is, however, important to note that tensile elongation (total) of as received and 10 pct tensile prestrained specimens measured from the distance of separation between two microindentation marks which were placed on the specimen surface before the commencement of the tests is 27 to 52 pct more than what is found in cases when testing is done using macro tensile specimens and strain is measured with the help of an extensometer. Large elongation in case of miniature sized specimen is due to the very small gauge length (~ 2000 microns) considered in the present investigation. On the other hand, it is found that in case of mini specimen the yield and tensile strength is always less as compared to macro specimen results. Moreover, it is observed that the difference in tensile strength between two types of tests is less as compared to yield strength. On the other side, the uniform strain values obtained from both types of tests are more or less comparable although for miniature 11

sized specimens this value is always on the higher side, the percentage difference being in the range of 5 to 8. The reason for higher uniform tensile ductility in case of mini specimens as compared to macro tensile tests is believed to be due to the fact that deformation of the surface grains is less restricted for the prevalence of complete plane stress state of the specimen surface. The present results clearly show that the bulk tensile properties and tensile properties evaluated from measurement on the specimen surfaces are different. Though the yield and tensile strength are obtained considering the actual specimen cross-sectional area in the specimen gauge length region, the strain has been measured over ~2000 microns on the surface of the specimens where deformation is less restricted. Hence, about 10 pct decrease in yield and tensile strength in case of insitu tests is not unreasonable.

(a)

(b)

(c)

(d)

Fig. 7(a-d). Bar diagrams compares tensile properties obtained from insitu and macro tensile tests.

12

3.4. Microscopic deformation behaviour It is well established that in polycrystalline materials microscopic behaviour of plastic deformation is inhomogeneous in nature. This is also true even in case of single phase material. The reason for such inhomogeneous plastic deformation is attributed to the differences in crystallographic orientation of individual grain with the loading axis. Apart from grain-to-grain variation, inhomogeneous deformation also occurs even in a single grain. Raabe et al. [35] has elaborately discussed the occurrence of deformation heterogeneity within individual grain in coarse grained aluminium specimen. Chandrasekaran and Nygårds [21] studied the heterogeneity of plastic deformation in fine and coarse grained ultra-low carbon steel using atomic force microscopy and electron back-scatter diffraction technique. They reported that inhomogeneous plastic deformation features are more predominant in large grains. Using EBSD technique, Allain-Bonassoa et al. [36] reported the heterogeneity of plastic deformation in case of IF steel. Banerjee et al. [37] also observed the heterogeneity of plastic deformation during tensile loading of IFHS steel even at the grain level. In this case, miniaturised tensile specimen was deformed in FEGSEM and secondary electron images captured during deformation has been analysed with the help of an in-house developed programme for studying grain scale deformation behaviour. In the context of present investigation the progress of plastic deformation under tensile loading of miniaturised RD(0), RD(10) and TD(10) specimens has been presented in Figs. 8 through 10 respectively. With the experimental conditions adopted in this study the slip lines (marked by red circles) are first observed at 300 MPa in few grains in case of RD(0) specimen. Even though at 300 MPa slip lines are not prominent enough undulation of the specimen surface is visible in the SE image (Fig. 8b). It is only above 360 MPa prominent slip lines are observed and all the grains deformed by slip. Further, slip lines are not very straight, but rather wavy in nature, and there is hardly any continuity of the slip lines in adjacent grains. Such wavy nature of slip lines arises because of cross slip which is very common in case of body centred cubic (BCC) iron and other metals. The absence of continuity of slip lines in adjacent grains indicates the orientation difference of the grains with respect to the loading axis. Interestingly, at the microscopic level undulation of the specimen surface becomes very prominent with progress of deformation because of movement of the grains out of the plane. It 13

is also visually clear that the grains are elongated in the loading direction. Using atomic force microscopy Chandrasekaran and Nygårds [21] measured the roughness of the specimen surface due to such out of plane movement of grains to find the heterogeneity of plastic deformation within the grains and at the grain boundaries of ultra-low carbon steel. Intragranular misorientation measurements using EBSD technique led these investigators to comment that increased surface roughness with increase of plastic strain is due to significant rotation within the grains. Following the discussion of several investigators [21,38-41] on the origin of surface roughness due to plastic deformation it is understood that the grains on the surface do not undergo unconstrained plastic deformation normal to the specimen surface. Such microscopic deformation inhomogeneity is attributed to the difference in inhomogeneous deformation caused by the difference in crystal orientations of the grains at the surface layer. Due to different orientation of slip systems in adjacent grains mismatch strain is developed at grain boundaries [39]. This leads to rotation of grains with respect to each other in order to maintain the strain incompabilities. The resulting out-of-plane component of the relative rotation contributes to surface roughening at the free surface. The surface undulation observed in the present investigation though is the result of inhomogeneous plastic deformation normal to the specimen surface; it is actually influenced by the orientation of the surrounding surface and subsurface grains. Another noteworthy feature observed in this investigation is the formation of intergranular cracks (marked by red dotted circle) on continued deformation of RD(0) specimen. Although integranular cracking during cyclic deformation of different IF steels and BCC iron is well documented in literature [30, 42-44], report on intergranular cracking under tensile loading of IF steel is hardly available. Besides intergranular cracking, voids are also found to be nucleated at the grain boundaries. It is thus summarised that plastic deformation under tensile loading of the investigated IFHS steel in RD(0) orientation initiates in some favourable grains and subsequently spreads over all the grains. Although there was sporadic formation of intergranular cracks of surface grains, these did not link to give rise to final failure. Evidence of tearing of grains in few locations (marked by green circle) was also revealed in this investigation. Microvoids (marked with yellow circle) were also identified on the specimen surface at a later stage of deformation and final failure occurred following through thickness shear with formation of large number of voids in the fracture zone. 14

(a) Unstressed

(b) 300 MPa

(c) 359 MPa

(d) 373 MPa

(e) 354 MPa

(f) Inner surface of Failed specimen (fracture zone)

Loading direction Fig. 8 (a-f). Sequential secondary electron images of the deformed RD(0) specimen surface. On the other hand, in case of RD(10) specimen slip lines are not visible below 360 MPa (Fig.9b). This happens because of hardening associated with pre-straining. It should be noted here that 0.2 pct offset yield strength in 10 pct pre-strained, RD(10), specimen was found as 350 MPa (Table 2). Hence, the first observation of slip lines at 360 MPa does not accurately correlate with yield strength data of miniature sized specimen. But, in contrast to RD(0) specimen intergranular cracking is not observed in case of pre-strained specimen. Instead of intergranular cracking of surface grains formation of microvoids are very prominent (marked 15

with yellow circle). These voids were eventually linked and formed long crack on the specimen surface. In case of TD(10) specimen with yield strength of 384 MPa (insitu test) the present experimental condition could not capture deformation features before reaching to tensile strength. In this specimen both intergranular and intragranular voids were formed. The intragranular voids are formed within the slip bands. The impingement of slip band with the grain boundary creates inergranular voids. Final failure, however, occurred in a ductile manner.

(a) Unstressed

(b) 361 MPa

(c) 398 MPa

(d) 415MPa (UTS)

(e) 338 MPa

(f) Failed

Loading direction Fig. 9 (a-f). Sequential secondary electron images of the deformed RD (10) specimen surface.

16

(a) 410MPa (UTS)

(b) 407 MPa

(c) 395 MPa

(d) 380 MPa

(e) 321 MPa

(f) failed specimen surface

Loading direction Fig. 10 (a-f). Sequential secondary electron images of the deformed TD(10) specimen surface.

3.5. Void nucleation strain In this study, we have considered the variation of areal density of voids with Von Mises equivalent plastic strain. Assuming plastic incompressibility and plane strain condition at the mid thickness and in regions very close to the mid thickness of specimen, ignoring elastic strain and assuming the existence of isochoric plastic deformation the Von Mises equivalent plastic strain is obtained as [45]

(1) where, to is the specimen thickness and t is the thickness in the observed area of interests of the failed specimens.

17

Typical void morphology and distribution in RD(10) fractured tensile specimen are illustrated in Fig. 11(a-d) which represent a set of images recorded at varying distances from the fracture end. These images are just for a single field of view at different distances from fracture end. It is observed that the voids are not always isolated ones and void size gradually decreases with increase of distance from fracture end. The large voids observed in these images are believed to nucleate from second phase particles / inclusions. The nature of the variation of areal density of voids formed during tensile deformation as a function of equivalent strain is shown in Fig. 12 (a-d) for all the four types of specimens, i.e., RD(0), RD(10), TD(0) and TD(10) respectively. It is observed that the void density gradually decreases with decreasing the equivalent plastic strain, i.e., with increase of distance from the fracture end. It is also observed that the areal density of voids does not increase with increase of equivalent plastic strain at the same rate. At lower strain the void areal density increases slowly and above some critical strain this void density increases rapidly. Accordingly, the nature of variation of void density curves with strain has been separated into two stages; one where void density increases slowly (Stage-I) and the other where void density increases rapidly (Stage-II). In this work, the void density data with Von Mises equivalent plastic strain is found to follow a third order polynomial function with correlation coefficient varying between 0.96 to 0.99. It is also found that the void density does not become zero but, approaches to a low value. It needs to be noted here that the polynomial curve fitting of void density data does not bear any physical meaning with void nucleation/growth mechanism.

(a) 200 microns

(b) 500 microns

18

(b) 750 microns

(d) 1000 microns

Fig. 11 (a-d). Typical void morphology and variation of void density at different distances from the fracture end of RD(10) specimen. The results of Mazumadar and Ray [28] on number density of voids with radial strain in different tensile pre-strained specimens of hot rolled IF steel show similar pattern as that observed in the present study. They demarcated two regions of their number density of voids against radial strain curves as that one due to decohesion and fracture of inclusions at lower strain (Stage-I), and decohesion of precipitate particles from the matrix at higher strain (StageII). The changeover strain of these two mechanisms was considered as void nucleation strain. In the present investigation too, the strain corresponding to the changeover in the pattern of void density curve with equivalent plastic strain from Stage-I to Stage-II has been considered as void nucleation strain (ԑn) which has been indicated in Figure 12(a-d). It is found that in case of 10 pct tensile pre-strained specimen the void nucleation strain is decreased by 15 to 17 pct as compared to as-received conditions. Moreover, ԑn for specimen with TD orientation is found higher than RD oriented specimen. However, this variation in ԑn because of specimen orientation is not very large. In case of hot rolled IF steel the ԑn has been reported [28] as 100 pct and 50.37 pct for as received and 12 pct tensile pre-strained specimens respectively. However, in that study the void nucleation the void nucleation strain was obtained by considering the number density of voids as a function of radial strain.

19

Stage-I

Stage-I Stage-II

Stage-II

(a)

(b)

Stage-I

Stage-I

Stage-II

Stage-II

(c)

(d)

Fig. 12(a-d). Curves showing the variation of areal void density with equivalent plastic strain.

It is well known that voids form by decohesion and/or cracking of particles/inclusions from the matrix. The stress/strain incompatibility at the matrix/particle (or inclusions) interfaces is responsible for decohesion and/or cracking of particles/inclusions and subsequent formation of voids [24-26]. It is also known that when the particle size is very small they remain well bonded with the matrix and hence require higher strain or stress for void nucleation through decohesion or cracking of those small precipitate particles [27]. In the present investigation no evidence for particle (carbides/carbonitrides) cracking could be recorded. But, formation of voids due to removal of particles from the matrix is clearly observed in Fig.13. The size of the 20

inclusion being much larger than the precipitate particle it is logical to consider that the voids nucleate at a lower strain due to decohesion of inclusion/ matrix interfaces (Stage-I). We, however, did not notice voids nucleating due to cracking of inclusions. Moreover, inclusions at large voids do not always present on the polished surface possibly due to removal of inclusions at the polishing stages.

(a)

(b)

Fig.13. (a) Voids forming due to decohesion of a large particle; (b) EDAX spectrum of the particle shown in (a). 3.6. Fractography Examination of the fracture surface of the miniature sized tensile specimens reveals that failure of the investigated IFHS steel occurs in a ductile manner through void nucleation and growth irrespective of tensile pre-straining. Dimple fracture surfaces of the RD(0) and RD(10) specimens are shown in Fig. 14(a-b). Fracture surfaces of TD(0) and TD(10) also revealed similar features as that of RD specimens. Qualitatively, it is observed that due to pre-staining some large voids are formed along with the signature of quasi-cleavage facets.

21

(a)

(b)

Fig. 14 (a-b).Fractographs showing dimple fracture surfaces on miniature sized tensile specimen deformed insitu in scanning electron microscope: (a) RD; (b) RD(10)

4.0 Conclusions: From the present investigation the following conclusions are drawn 1. Plastic deformation of cold rolled IFHS steel sheet under tensile loading occurs heterogeneously by slip; and it does not occur simultaneously in all the grains. 2. During plastic deformation of the IFHS steel sheet considerable undulation of the specimen surface occurs due to out of plane movement of the grains. 3. Intergarnualr cracking, formation of both intergranular and intragranular voids precede the final through thickness shear fracture of IFHS steel. 4. The surface of the miniature sized specimen being in a completely plane stress state, the measured strain at the surface becomes higher, both under as-received and pre-strained conditions. 5. The critical strain for void nucleation depends upon the specimen orientation and whether or not the specimens are prestrained. Pre-straining causes to lower the void nucleation strain of IFHS steel sheet.

22

Acknowledgement The work has been performed using the facility created out of the research grant received from Ministry of Steel, Government of India, Pproject No. 11 (7)/SDF/2007-TW. Partial financial support received from TATA Steel Ltd., India, in carrying out the investigation, is acknowledged. References 1. H. Takechi: Metallurgical Aspects on Interstitial Free Sheet Steel from industrial Viewpoints; ISIJ Int., Vol. 34 (1), 1994, pp.1-8. 2. R. K. Ray, P. Ghosh and D. Bhattacharjee: Effects of composition and processing parameters on precipitation and texture formation in microalloyed interstitial free high strength (IFHS) steels; Mater. Sci. Tech., Vo. 25(9), 2009, pp. 1154-67. 3. P. Ghosh, R. K. Ray, B. Bhattacharya and S. Bhargava: Precipitation and texture formation in two cold rolled and batch annealed interstitial-free high strength steels; Scr. Mater., Vol. 55, 2006, pp. 271-274. 4. P. Ghosh, B. Bhattacharya and R. K. Ray: Comparative study of precipitation behaviour and texture formation in cold rolled-batch annealed and cold rolled-continuous annealed interstitial free high strength steels; Scr. Mater., Vol. 56, 2007, pp. 657-660. 5. S. Chatterjee, S. Mukherjee and S. Kundu: Evolution of microstructure and crystallographic texture during recrystallization of high-strength sheet steel; Mater. Sci. Tech., Vol. 32(18), 2016, pp. 1886-91. 6. Q.N.Han, W.H.Qiu, Y.B.Shang and H.J.Shi: In-situ SEM observation and crystal plasticity finite element simulation of fretting fatigue crack formation in Ni-base single crystal superalloys; Tribol. Int., Vol. 101(9), 2016, pp. 33-42. 7. X.F. Ma and H.J Shi: Insitu SEM studies of the low cycle fatigue behavior of DZ4 superalloy at elevated temperature; Effect of partial recrystallization; Int. J. Fatigue, Vol.61, 2014, pp.255-63. 8. J.C. Liang: Insitu scanning electron microscopy-based high-temperature deformation measurement of nickel-based single crystal superalloy up to 800°C; Optics Laser Engg.,Vol.108, 2018, pp.1-14. 9. Y.Y. Zhang, H.J. Shi HJ, J.L. Gu JL, C.P. Li, K. Kadau and O. Luesebrink: Crystallographic analysis for fatigue small crack growth behaviors of a nickel-based single crystal by in situ SEM observation; Theo. Appl. Fract. Mech., Vol. 69, 2014, pp.80-89. 10. J.Z. Zhang: Direct high resolution in situ SEM observations of small fatigue crack opening profiles in the ultra-fine grain aluminium alloy; Mater. Sci. Engg., Vol. 485 A (1-2), 2008, pp.115-118.

23

11. R. Jiang, F. Pierron, S. Octaviani and P.A.S. Reed: Characterisation of strain localisation processes during fatigue crack initiation and early crack propagation by SEMDIC in an advanced disc alloy; Mater. Sci. Engg., Vol. 699A, 2017, pp.128-44. 12. W.P. Li: Fracture mechanisms of a Mo alloyed CoCrFeNi high entropy alloy; Insitu SEM investigation, Mater. Sci. Engg., Vol. 723 A, 2018, pp.79-88. 13. C.S. Tan, Q.Y. Sun, L. Xiao, Y.Q. Zhao and J. Sun: Characterization of deformation in primary α phase and crack initiation and propagation of TC21 alloy using insitu SEM experiments; Mater. Sci. Engg., Vol. 725A, 2018, pp.33-42. 14. S. Lavenstein, B.Crawford, G.D. Sim, P.A. Shade, C.Woodward and J.A. El-Awady: High frequency insitu fatigue response of Ni-base superalloy Rene-N5 microcrystals; Acta Mater., Vol. 144, 2018, pp.154-163. 15. G.C.Chai, R.L. Peng and S.Johansson: Fatigue behaviors in duplex stainless steel studied using in-situ SEM/EBSD method; Procedia Mater. Sci., Vol. 3, 2014, pp.174853. 16. L.Yang and JK.Fan: Insitu SEM study of fatigue crack initiation and propagation behavior in 2524 aluminum alloy; Mater. Des., Vol. 110, 2016, pp.592-601. 17. W. Zhang and Y.M. Liu: Investigation of incremental fatigue crack growth mechanisms using in situ SEM testing; Int. J. Fatigue, Vol.42, 2011, pp.14-23. 18. M.L. Zhu, F.Z. Xuan and S.T. Tu: Observation and modeling of physically short fatigue crack closure in terms of insitu SEM fatigue test; Mater. Sci. Engg., Vol. 618A, 2014, pp.86-95. 19. M. Ghosh, R. Santosh, S.K. Das, G. Das, B. Mahato, J. Korody, S. Kumar and P.K. Singh: Effect of structural heterogeneity on in situ deformation of dissimilar weld between ferritic and austenitic steel; Metal. Materl. Trans., Vol. 46A (8), 2015, pp. 3555-68. 20. R R. Nivas, G. Das, S.K. Das, B. Mahato, S. Kumar, K. Sivaprasad, P.K. Singh and M. Ghosh: Effect of stress relief annealing on microstructure and mechanical properties of welded joints between low alloy carbon steel and stainless steel; Metal. Materl. Trans., Vol. 48A (1), 2017, pp. 240-45. 21. D. Chandrasekaran and M. Nygards: A study of the surface deformation behaviour at grain boundaries in an ultra-low-carbon steel; Acta Materl., Vol.51, 2003, 5375-84. 22. H.-P. GaÈnser, E. A. Werner and F. D. Fischer: Plasticity and ductile fracture of IF Steels; experiments and micromechanical modelling; Int. J. Plast., Vol. 14, No. 8, pp. 789-803, 1998. 23. S.H. Goods and L.M. Brown: Overview No. 1: The nucleation of cavities by plastic deformation; Acta Metall. 27(1), (1979) pp. 1-15. 24. B. Dodd and Y. Bai: Ductile Fracture and Ductility; Academic Press, London, 1987. 24

25. G. Straffelini: Ductility of materials with ferritic matrix; Materials Science and Engineering A342 (2003) 251-/257. 26. L.M. Brown and W.M. Stobbs: The work-hardening of copper-silica v. equilibrium plastic relaxation by secondary dislocations; Philos. Mag., 1976, vol. 34, pp. 351-72. 27. G.E. Dieter: Mechanical Metallurgy; SI metric edition, Mc-GrawHill Book Co.,1989. 28. S. Majumdar and K.K. Ray: Effect of prestrain on the ductile fracture behavior of an Interstitial-Free Steel; Metal. Materl. Trans., Vol. 39A (7), pp.3541-53. 29. O. L. García, R. Petrov and L. A.I. Kestens: Void initiation at TiN precipitates in IF steels during tensile deformation; Mater. Sci. Engg.,Vol.527A, 2010, pp. 4202-09. 30. M.N. James: Intergranular crack paths during fatigue in interstitial-free steels; Engg. Fract. Mech., Vol. 77, 2010, pp. 1998-2007. 31. T. Makota, N. Soichiro and O. Kunio: Evaluation of fatigue property for automobile steel sheet after plastic working; Proc. Annual Cong. Japan Soc. Automotive Engineers, Vol.10.3, 2003, pp. 1-3. 32. P. S. De, A. Kundu and P.C. Chakraborti: Effect of prestrain on tensile properties and ratcheting behaviour of Ti-stabilised interstitial free steel; Mater. Des., Vol.57,2014, pp. 87-97. 33. P. Atoine, S. Vandeputte and G.B.Vogt: Effect of microstructure on strain-hardening behaviour of Ti-IF steel grade; ISIJ Int., Vol 45(3),2005, pp.399-404. 34. Q.Z. Chen and B.J. Duggan: On cells and microbands formed in an interstitial-free steel during Cold Rolling at low to medium reductions; Metal. Materl. Trans.,Vol. 35A (11), 2004, pp.3423-30. 35. D. Raabe, M. Sachtleber, Z. Zhao, Z, F. Roters and S. Zaefferer: Micromechanical and macromechanical effects in grain scale polycrystal plasticity experimentation and simulation; Acta Mater., Vol. 49(17),2001, pp. 3433-41. 36. N. Allain-Bonassoa, F. Wagner, S. Berbenni and D. P. Field: A study of the heterogeneity of plastic deformation in IF steel by EBSD; Mater. Sci. Engg., Vol. 548A, 2012, pp. 56-63. 37. S. Banerjee, T. Dasgupta, S. Mukherjee, M. Shome, P. C. Chakraborti and S. K. Saha: Digital image correlation for grain scale strain measurement in interstitial free high strength steel; Materl. Sci. Tech., Vol. 32(4), 2016, pp. 328-37. 38. K. Balusua, R. Kelton, E.I. Meletis and H. Huang: Investigating the relationship between grain orientation and surface height changes in nickel polycrystals under tensile plastic deformation; Mech. of Materl., Vol.134, 2019. pp.165-75. 39. S.W. Banovic and T. Foecke: Evolution of strain-snduced microstructure and texture in commercial aluminum sheet under balanced biaxial stretching; Metal. Materl. Trans., Vol. 34A (3), 2003, pp. 657-71. 25

40. D.Raabe, Michael Sachtleber, Hasso Weiland, Georg Scheele and Z. Zhao: Grain-scale micromechanics of polycrystal surfaces during plastic straining; Acta Mater., Vol.51, 2003, pp.1539-1560. 41. O.Wouters, W.Vellinga, R. Tijum and j. Hosson: On the evolution of surface roughness during deformation of polycrystalline aluminium alloys; Acta Mater., 53, 2005, pp.4043-50. 42. H. Mughrabi, K. Herz and X. Stark: Cyclic deformation and fatigue behaviour of a-iron mono-and polycrystals; Int. J. Fract, 17(2),1981, pp.193-220. 43. C. Sommer, H. Mughrabi and D. Lochner: Influence of temperature and carbon content on the cyclic deformation and fatigue behaviour of α-iron. Part I: cyclic deformation and stress-behaviour; Acta Metall., 46(5), 1998, pp.1527-36. 44. S.Majumdar, D. Bhattacharjee and K.K. Ray: Mechanism of fatigue failure in interstitial-free and interstitial-free high-strength steel sheets; Scr. Mater., Vol. 64, 2011, pp. 288-91. 45. T.Matsuno, D. Maeda, H. Shutoh, A. Uenishi and M. Suehiro: Effect of Martensite Volume Fraction on Void Formation Leading to Ductile Fracture in Dual Phase Steels; ISIJ Int. Vol. 54 (2014), No. 4, pp. 938–944.

26

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: