MEASUREMENT OF RESIDUAL PHASE STRESS OF THE METAL, MATRIX COMPOSITE MATERIAL USING SYNCHROTRON RADIATION

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1 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol MEASUREMENT OF RESIDUAL PHASE STRESS OF THE METAL, MATRIX COMPOSITE MATERIAL USING SYNCHROTRON RADIATION Shigeki Takago, Toshihiko Sasaki, 2Koichi Akita, 3Yasuo Yoshioka and Yukio Hirose Department of Materials Science and Engineering, Kanazawa University, Japan 2Department of Mechanics Engineering, Tokyo Metropolitan University, Japan 3Department of Energy Science and Engineering, Musashi Institute of Technology, Japan The purpose of this study, is evaluating the residual stress in each phase of a composite material using synchrotron radiation. Diffraction experiment with synchrotron radiation was performed at the Photon Factory at the high-energy accelerator research organization, Japan. Residual stress measurement using X-ray diffraction with synchrotron source is a relatively new and rapidly developing technique. We prepared Fe-Cr alloy containing titanium nitride composite materials for the samples. Difference of mechanical and physical properties generates microscopic stress. Influence of the TiN on phase stress was discussed. From adjusting wavelength with silicon single crystal monochromator, some diffraction profiles of specimen were obtained. Results of experiment were compared with micromechanics model. Furthermore, X-ray fractography technique was used to investigate the fracture mechanism of specimen. It was found that residual phase stresses in the matrix were dependent on stress intensity factor. INTRODUCTION Sintered composite materials excel in function and performances, because of they are reinforced by hard particle. On the other hands, existence of microscopic residual stress due to the misfit of the mechanical properties between each phase was related to the material strength [1,2]. It is necessary to evaluate residual stress in each phase (as called phase stress) accurately. For such a microscopic stress state, a theoretical estimation was reported by the equivalent inclusion method by Eshelby and Mori-Tanaka [3,4]. Moreover, the theory is confirmed using the X-ray diffraction technique [5, 61. However, there is no paper on the influence of the free surface on the residual stress. When the mean inter-particle spacing is smaller than the X-ray penetration depth, material has the triaxial stress state. Since stress was relaxed by a free surface, residual stresses were different from surface to inside of materials. In this study, the X-ray diffraction technique with synchrotron radiation was used to solve

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3 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol this problem. Synchrotron radiation has the feature is that high resolution beam. The double silicon (111) monochromator was made, the wavelength was adjusted to fix the diffraction angle 2 8. The influence of the free surface on the phase stress during bend loading. was discussed. EXPERIMENT PROCEDURE Materials and specimens Specimens were made from the SKDll and the TiN powders. Table I shows the chemical elements of powder. There are 3 kinds of Fe-Cr steel/en sintered composite materials. The maximum grain size of powder is 45,U m. The molding pressure is 588MPa. They were held at 1505 K for 120 min. TiN volume and the bulk density were listed in Table II respectively. Volume fraction is calculated from the bulk density and the weight ratio. Figure 1 shows the microstructure obtained from the section of each specimen. TiN particle were seen disperse uniformly. Table I. Chemical components of powder. Phase Fe Cr MO Si V N 0 Ti Matrix Bal Inclusion Bal. Table II. Mechanical properties of specimen. Parameters TiNO TiNlO Weight fraction (wt.%) 0 10 Volume fraction (vol.%) Mechanical Young s modulus (GPa) Bulk density (Mg/m3) TiN I TiNO TiNlO t Figure 1. Photographs of microscopic structure.

4 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol Measurement of residual phase stress Phase stress was measured using X-ray diffraction with SR beam. Experiment was performed in the Photon Factory (the beam line 3A) at the High-energy accelerator research organization, Japan. Table III shows the wavelength used in this study. The 211, 220 and 310 diffractions for the matrix (Fe-Cr) and the 420 diffraction for TiN phase were adopted. Wavelength is controlled using the monochromator, which was Si(ll1) single crystal, and the diffraction angle 2 6 is set 154 deg. Figure 2 shows the optical system in the experiment. the incident beam size is defined by slits as 2 X 8 mm2. X-ray elastic constant of the single phase is needed to obtain the phase stress. Matrix (Fe-Cr) was determined from the TiN 0% specimen. In the case of TiN, it was calculated using Kroner model[7] with elastic constant of single crystal. Phase stress was measured from the slope of the 2 8 -sin2 ti diagram: where E is Young s modulus, and v indicate Poisson s ratio, angle $ indicate the direction cosine between incident beam and the normal of the diffraction plane. Each phase is expressed by symbol i, and matrix is i=m, and the inclusion is?=i. (2/s&h is X-ray and elastic constant. To obtain elastic strain, E, from which one must make measurement strain free lattice plane spacing da, we have Ad E =-=-cot8;ao (2) d where 8 0 is a diffraction angle of unstressed material. (1) 15.2m 20.2m 23.2m 27.5m Figure 2. A schematic layout of beam line (Measurement system of the BG3A).

5 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol Phase hkl Fe-Cr 211 Fe-Cr 220 Fe-Cr 310 TiN 420 Table III. Wavelength used in this experiment. Lattice spacing (nm) Wavelength (mn) Tensile loading test The phase stresses were measured using x-ray stress measurement methodology under applied stress by means of a four-point-bending device. The applied stress was monitored with a strain gauge bonded to the specimen on the face opposite to the are indicated by x-ray. These gauges have a range from 0 to 800 X 10T6 with an interval of 200 X 10m6 step. The distribution of residual stress on the fracture surface Residual stress measurement on the fracture surface after the fracture toughness test had been examined. The area irradiated by X-rays was 2 mm widths and 2 mm length at the middle of the thickness of fracture surface. The direction of residual stress measured is along the crack extension as shown in Figure 3. Normal of specimen RESULTS AND DISCUSSION Change of phase stress during tensile loading Figure 3 Coordinate system of X-ray stress Figure 4 shows phase stress obtained from the measurement near fracture surface. Fe211 diffraction of the TiN20 specimen. Phase stress increases proportionally with the applied stress. Phase stress in the matrix is smaller than applied stress. An infhrence of TiN on the ratio of the phase stress to the applied stress was examined. In the case of X-ray, the relation between them agree the micromechanics model [8]. At this time, the loading stress is multiplied by the Young s modulus (Table I). There is a methodology by Sasaki [5,6] based on the work of Eshelby [3] and Mori et al. [4] about a theoretical estimation of deformation behavior in the multi phase material. The dashed line in Figure 5 is calculated using Eshelbyl MO&Tanaka model [9]. In this study, the shape of inclusion was assumed to be spherical. Tensile stress in the matrix was found to decrease with volume fraction of TiN for all specimens. Results of experiment agree well with theoretical prediction. Tensile stress in the matrix phase was relaxed by existence of TiN phase. The influence of the free surface on the residual stress measurement value of the Fe-Cr/TiN sintered composite materials by the synchrotron quantitatively was clarified.

6 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol ;;i g2oc- 35cb Fe211 TiN20,,,J',,, y ; 'OC- p" i? I' z 5 - /,' 2 ) 2 I I I I Applied stress u IIA (MPa) Figure 4. Change of phase stress in the TiN 20 specimen. b L lo z z B o- TIN volume fraction f -----, I I ls CA Figure 5. Influence of TiN on deformation behavior Residual stress near fracture surface Relation between stress intensity factor and residual phase stress was examined. The Kit value was stress intensity factor when crack propagation occurred. If the X-ray parameter (e.g. residual stress) was dependent on stress intensity factor, we can evaluate the toughness of material. Figure 6 shows phase stress near fracture and the Kro vs. the TiN volume fraction. The Krc value and phase stresses in the matrix decrease with TiN content respectively. Phase stresses in the matrix, of the TiNlO and TiN20 specimens are about same value. It is thought initial residual stress is different. The tensile residual stress remained due to the misfit of the coefficient of thermal expansion. We concluded that the value of Krc increase with phase stress near fracture surface. Residual stress obtained from the 310 diffraction of the TiNO specimen was twice as the 211 and 220. Wavelength used when 310 is measured is the smallest, i.e. the penetration depth is the largest. And, depth to the TiNO specimen was greater than the other TiNlO and 20 specimens. Therefore, this result suggests that it be necessary to consider the penetration depth. Furthermore, a microscopic stress generated in the material, which contains TiN. When the stress in the mean inter-particle spacing is assumed to be CJ 33 (normal direction of specimen surface) to 0 11, o ii and CJ 33 are normal stresses respectively as for the mean stress in the direction of the crack growth in the X-ray penetration depth. If the depth of X-ray is shallower than the mean size of inter-particle spacing [lo], ~~33 is almost zero. In the case of that the volume fraction of TiN is 13.8% and 27.0%, the spacing are about 57 pm and 29 /J m respectively. The depth of the synchrotron X-ray beam is range from 5 to 10 pm. Moreover, The results of the TiNlO and the TiN20 were equaled each other. In the range of the experiment,

7 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol it thought that phase stress was independent on the stress component o33. Because mean spacing is larger than the penetration depth. The relaxation of o 33 due to free surface occurred. Furthermore, we concluded that it is necessary to consider penetration depth to evaluate the residual phase stress of composite materials. I I TiN volume fraction TiN volume fraction (a) Residual stress (b) Stress intensity factor Figure 6. Influence of TiN volume traction on phase residual stress near the fracture surface. 3 CONCLUSIONS Residual stress of sintered Fe-Cr/TiN composite materials were measured. Matrix (Fe-Cr) 211,220,310 diffractions and TiN: 420 reflections were obtained using synchrotron diffraction. The deformation behavior of the phase stress in the matrix showed a corresponding to the Eshelby/Mori-Tanaka model. Residual phase stresses near fracture surface were measured. There tensile stress in the matrix, and the compressive stress in the TiN phase. Residual stress decreased with TiN content. One with a large penetration depth indicated large value of residual stress in the TiNO specimen. It is necessary to consider the influence of the penetration depth on residual stress for the single phase material with a large tensile residual stress. The mean interpartcle spacing is larger than the X-ray penetration depth. Moreover, phase stress corresponding to the results of TiNlO and TiN20, and it is thought that the influence of the free surface caused for o 33 can be disregarded. A good correlation was found between the phase stress in the matrix and stress intensity factor &c. REFERENCES [l] Noyan, I. C., A4etallurgical Transactions A. vol. 1983,14A, [2] Predecki, P. K., Abuhasan, A., Barrett, C. S.,Adv. X-ray Anal. 1988,31, [3] Eshelby, J. D., Proc. Roy. Sot. London, 1957, A241, [4] Mori, T. and Tanaka, K.Acta Metallurgica, 1973, vol. 21,

8 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol [5] Sasaki, T., Rin, Z. and Hirose, Y, Transactions of the Japan Sot. of Mech. Eng. (A), 1996, vol. 62,604, [6] Sasaki, T, Rin, Z, and Hirose, Y, Transactions of the Japan Sot. of Mech. Eng. (A), 1996, vol. 62 No 604, [7] KrGner, E, 2. Physik, Bd151,1958, [8] Takago, S., Sasaki, T. and Hirose, Y, Int. Sot. Ojjfshore &Polar Erg. 99 Best vol. 4,1999, [9] Sasaki, T., Yasukawa, S., Takago, S. and Hirose, Y., Adv. X-ray Anal., 1997,41, [lo] Hanabusa, T, Nishioka, K. and Fujiwara, H., 2. Metallkzule. Bd. 74,1983,