X-RAY FRACTOGRAPHIC STUDY OF SINTERED Fe-Cr STEEL/ TiN SYSTEM COMPOSITE MATERIALS

Transcription

1 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol X-RAY FRACTOGRAPHIC STUDY OF SINTERED Fe-Cr STEEL/ TiN SYSTEM COMPOSITE MATERIALS Shigeki Takago, Toshihiko Sasaki, Masaharu Miyano2 and Yukio Hirose Department of Materials Science and Engineering Kanazawa University Kakuma-machi, Kanazawa , Japan RIKEN Corporation Suehiro, Kumagaya, Saitama , Japan INTRODUCTION High chromium steel containing titanium nitride (TiN) when prepared by powder metallurgy has high heat- and wear-resistance. Valve seats in automobile diesel engines are made of this steel. When external stress is applied to a composite material, microscopic stresses 2 occur due to the misfit of physical and mechanical properties between the constituents in the material. Residual stress on a fracture surface is an important subject in an X-ray fractographic study3-5. X-ray fractographic studies are used to analyze fracture mechanisms in fracture toughness and fatigue tests and especially in high strength steels4 5. This technique is useful in X-ray stress measurement methods6 of fracture analysis. X-ray fractography was used in this study to measure fracture of a Fe-Cr/TiN sintered composite material. Fracture toughness tests were conduced on notched specimens. The residual phase, macro- and microstresses distributed beneath the fracture surface were measured by X-ray diffraction using phase x-ray elastic constants (PXEC). The relation between fracture toughness, macro- and microstress is discussed. EXPERIMENT Materials and Specimens Fe-12% Cr steel (Matrix) containing a second phase (TiN) prepared by powder metallurgy were studied. Sintering conditions used for the materials are shown in Fig. 1. Specimens were held at 923K for loomin, then at 1505K for 120 min. followed by rapid cooling in N2 gas. They were then tempered at 983K for 120 min. Table 1 shows the chemical components for both the powders, for the matrix, and the second phase. Young s modulus of the specimen is 219 GPa. Bending strength is 765 MPa. Fig. 2 shows the microstructure obtained from a section of material. TiN particles (under 45um in size) were seen to disperse uniformly. No voids were observed.

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol KX 12Omin. N,GAS Cooling ~-- Time --- Fig. 1 Sintering conditions adopted for specimens used in this study. Table 1. Chemical components of powder. Phase IFe Cr MO Si V N 0 Ti Matrix Bal Inclusion Bal. Fracture Toughness Test The geometry of specimens used for fracture toughness test7 is shown in Fig. 3. The notch radius p was between 0.1 and 0.75 mm. Specimens were notched with diamond blades. Crack opening displacement was measured using a clip gauge. All specimens were fractured by three-point bending as shown in Fig. 4. The cross head speed used was 5 mm/min. The onset of crack extension was detected by measuring a change in AC electrical potentia18. The fracture toughness is expressed in terms of the stress intensity factor which was calculated using the initial notch length as the crack length for notched snecimens. E&ailofA I _ EPM circuit Fig. 3. Geometry of specimen. Fig. 4. Schematic illustration of fracture toughness test. X-ray Stress Measurement Conditions An X-ray stress measurement instrument (RIGAKU MSF-2M) with parallel beam optics was used. The conditions used for the experiment are stated in Table 2. Co-Ka radiation was used. Peak positions were measured using a full width half maximum point method. Raw powders of each constituent were used to measure the peak position of a stress free

4 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol material. The residual stress during crack growth was measured by the sin2v technique. The distribution of residual stress beneath the fracture surface was obtained by irradiating the new surface revealed by successive electrolytic polishing. Fig. 5 shows the schematic illustration of the X-ray irradiated area on the fracture surface. The electrolytic polishing liquid used was Perchloric acid + Acetic anhydride (composition ratio 1:4)6. A micrometer was used to measure the area polished. Table 2 X-ray stress I X-ray optics Radiation Wavelength, nm Filter Tube voltage, kv Tube current, ma Irradiated area, mm2 Incident angles v, deg Diffraction plane, hkl Diffraction angle, deg Peak position measurement conditions. Parallel beam Co-Ka Fe foil x4 0, 18, 27, 33, 39, 45, 51 Matrix (Fe-Cr steel):3 10 TiN: Full width half maximum point method The residual stress of each phase in composite material is obtained by o;, -o:, +Jphcote;[ a;$;). (1) The composition phase was expressed by i, and it is assumed that the matrix is i=m, and the second phase is i=i. The 1,3 axes are assumed to be the principal stress directions. Symbol (El[l+v 1) iph means phase x-ray elastic constant (PXEC). Measurement of the x-ray elastic constant was done by the four-point bending test device. The applied stress was measured by means of a strain gauge bonded to the specimen on the face next to the x-ray irradiated one and expressed as a mechanical Young s modulus. The X-ray elastic constant was calculated from the applied strain E ir* and slope of 28 - sin2 t,u diagram. AIncident X-ray act i 01 Fig. 5 Coordinate system of X-ray stress measurement and symbols used in this study.

5 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Macro- and Microstresses A microscopic stress (so called microstress) occurred due to the misfit of physical and mechanical properties (e. g. Young s modulus, coefficient of thermal expansion etc.) between the constituents in the material, when external stress is applied to composite material. 0 m, cr o as microstresses in the matrix and in the second phase, CT A is macrostress (i.e. overall stress) within the range of the measurement. CT M, CJ I as phase stresses in the matrix and in the second phase. Using the equilibrium condition, we have: GA =(1-f)C.r +fcr 0 =f(o -0 ) OR = (f-l&? 4) i (l-fjzr +fon =o Where f denotes the volume fraction of the second phase. If the phase stresses in all constituents are determined, we obtain microstress in both constituents. RESULT Fracture Toughness Test Fig. 6 shows the relation of load P and electrical potential difference (AV vs. displacement u as p=o.35 mm). The relationship between load and clip-gauge displacement is linear up to the fracture point. Point P(shown by the symbol arrow) was assumed to be a fracture load corresponding to the crack initiation point when AV changed steeply. A fracture toughness value of the notch material was obtained. The fracture toughness of notched specimens, K p, is plotted against the square root of the notch radius, fi, in Fig. 7. As seen in Fig. 7. The relationship can be roughly approximated by a straight line. KP is proportion to fi. (2) Displacement u,mm Fig. 6 Relation between load value and crack-opening displacement at p =0.35 mm.

6 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol I I I I Square root of notch radius (Smm) Fig. 7. Fracture toughness value K p as a function of square root radius of notch tip. Determination of Phase X-ray Elastic Constant (PXEC) The PXEC in the matrix material (TiNO%, Fe-Cr single-phase specimen) which consist of only matrix was used. The relationship between diffraction angle 28 and sin2v is shown in Fig. 8. As seen in Fig. 8, changes in the slope of 20 -sin21y diagram decreased as applied strain E 11 * increased. Fig. 9 shows the slope of 28 -sin2v diagram vs. the applied strain ranged from 0 to 800~10~~ in intervals of 250~10~~. The vertical bars in Figure shows the confidence limit of 68.3%. PXEC was obtained from eq( 1). In the case of the TiN phase, it was calculated using the elastic constant of single crystalline by using Krijner model. Obtained PXEC ([ l+v ]/E)ph were 7.54TPa (matrix) and 2.27TPa- (TIN) respectively. tp a t-4 2 bll B 161.m g & r, ': m L" a Sin * WI Applied strain E,I*,( X 10e6) Fig sin2v diagram of matrix phase. Fig. 9. Changes of slope in 28 -sin2v diagram. Residual Stress near Fracture Surface Residual macro- and microstresses near a fracture toughness surface were measured using eqs (l)(2). The result of p =0.75 is shown in Fig. 10. The residual phase stress and microstress measured on the fracture surface were tensile in the matrix. As the depth from fracture surface increased, the residual stress gradually decreased and changed from tensile to compressive. At a certain depth, it became a local maximum compressive stress

7 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol that gradually decreased and approached a constant value. In the case of the TiN phase, the residual phase stress and microstress on the fracture were compressive. As the depth increased, the compressive residual stress gradually decreased and approached a constant value. The residual stress was caused by local plastic strain near the crack-tip. The influence of notch radius on the plastic zone size was examined. Fig. 11 shows the phase stress distribution in the direction of depth from the fracture. For the matrix, residual stresses near fracture surface saturate after it decreased. It considered that the saturation depth is a plastic zone size, w y. In the case of the TiN phase, data are shallow compared with the matrix. Strain sensitivity becomes small when a low diffraction angle and the Young s modulus are comparatively high. In addition, the TIN content is about 10 wt.% and diffraction intensity is small. The depth at which the phase stress and microstress changed to increase the information volume of the judgment was obtained respectively in this study, and the plastic zone size of the TIN phase was decided from those mean values. n 4 b L b" lo (b) p =0.7.5 mm 5-5 ~~:-~~~~~~=::~~r:::::~~--~~~~~ il@ & ---* --*-W f 4-100(-y i 2~, Leo I ye?j,-i Depth from surf&e, I.4 m (a) Macro and phase stresses Fig. 10 Macro- and microstresses E Depth from surfsce,,u m (b)microstress vs. depth from fracture surface. Depth from surface,,u m Depth from surface,,u m Fig. 11 The distribution of residual stress near fracture surface. (Left-matrix phase, Right-TiN phase) DISCUSSION Fracture Toughness and Notch Radius It is generally said that Kp is proportional to p 4,5. However, the extension of the straight line does not correspond to the starting point though Kp in Fig. 7. We believe the TIN particle becomes an obstruction of a crack growth as nodular cast iron, and thus has some influence on the effect of the notch; i.e., the stress concentration in TiN phase increases as the notch radius increase, so enhancing crack growth in the interface between TiN and the matrix.

8 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Plastic Zone Size The distribution of the direction of depth is shown in Fig. 12. A value, almost equal to the strain free powder, was obtained right below the fracture in the second phase. A large value was indicated about the half value width of the matrix under the fracture compared with the powder. We believe it is the major cause of plastic strain in the matrix, has hardly any effect in the second phase. s -# o 1. ; Depth from surface, /.L m Depth from surface, p m Fig. 12 Full width at half maximum intensity vs. depth of surface. The plastic zone size o, can be defined as the depth where the residual stress or the half value width approaches a constant value. In this study, we obtained the half value width for the TiN phase by using the residual stress in the matrix. In Fig. 13, the size of o,, was plotted against the fracture toughness divided by the bending strength, K p /CT B. my was proportional to the square of K p / 0 B, i. e. wy = a(k, /IT,)~ (3) Where a =O. 13 for matrix phase and cc=o.o64 for TiN phase, it is small when compared with the value that in case of steels varies around 0.12 to * 5. Eq(3) is extremely important for determinng the fracture toughness of these materials from the X-ray parameter near the fracture surface. The correlation of the macrostress and the phase stress is found by using the micromechanics theory16. In future work, we hope to study the relationship between elastic-plastic deformation behavior and the plastic size.

9 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol E Y k 3 s -2 g N.r$ a ia Matrix310 + W,/od2, m ~ Fig. 13. Relation between plastic zone size and stress intensity factor divided by bending stress. CONCLUSIONS 1. Fracture toughness was found to be proportional to the square root of the notch radius in composite material (Fe-Cr/TiN). 2. The distribution of macro- and microstress under a fracture toughness fracture was obtained by using PXEC. 3. In a matrix (Fe-Cr steel) phase, the residual phase and microstresses on the fracture surface measured by X-rays were tension. The tensile residual stress gradually decreased and went into compression as the depth increased. For the TIN phase, the stresses were compressive and gradually decreased as depth below the fracture surface increased. At a certain depth the residual stress becomes almost constant. 4. The plastic zone size, o y was determined from the residual stress (TiN) and the half value breath (Matrix) distribution beneath the fracture surface. w, was found to be related to both fracture toughness K p and the bending stress 0 B by the following wy = a(k, 10, y equation; where a depends on the material. a =O. 13 for matrix, a =0.064 for TIN phase. REFERENCES 1. I. C. Noyan, Met. Trans., A14, 1907 (1983). 2. P. K. Predecki, et. al., Adv.X-ray Anal.,3 1, 23 1 (1988). 3. K. Tanaka, Establish of Analysis of Accident Fracture by X-Ray Fractography, Science study expenses subsidy synthsis report(a), 46 (1987). 4. Y. Hirose and Tanaka K. Jpn. sot. mat. sci., 29, 828(1979). 5. Y. Hirose, K. Tanaka and K. Okabayashi, Jpn. sot. mat. sci., 27, 545(1978). 6. Japan Sociey of Materials Science, X-ray Stress Measurement method, 54( 198 l), Youkendou.

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