Kagawa, Japan. Abstract

Transcription

1 Fracgraphic study on fatigue crack initiation and early crack growth processes for pure titanium implanted with nitrogen ions with variable notch radii K. Futagami/R. Murakami,* Y. Koichi,*A. Kawahi,^ T. Kayahara,' W. G. Ferguson/ M. Yoneda/M. Katumura^ "Department of Mechanical Engineering, University oftokushima, 2-1 Minami-josanjima-cho, Tokushima, 770, Japan ^Miura Co. Ltd., 7-Horie-cho, Matsuyama, Ehime, , Japan ^Miura Insitute of Research and Development Co. Ltd., 7-Horie-cho, Matsuyama, Ehime, , Japan ^Department of Chemistry and Mechanics, The University of Auckland, Private Bag* 92019, Auckland, New Zealand Kagawa, Japan Abstract The aim of this work was study fracgraphically the fatigue crack initiation and early crack growth mechanism for pure titanium metal implanted with nitrogen ions while varying the notch radii. A maximum surface roughness, Rmax and the factual dimension, D, were calculated from the fatigue fracture surface and were related crack initiation and early crack growth patterns. The main results obtained are summarized as follows; (1) When crack length is <lmm, Rmax decreased but/) increased with decreasing notch radius. (2) When crack length is > 1mm, the tendency of R^ax and D were keep close constant values. These constant values were about O.lmm and 1.2, respectively. (3) The second type of early crack growth easily occurred when the value of D was high. 1. Introduction Ion implantation is an effective process for changing the surface sensitive properties of materials without adversely affecting its desirable bulk properties [ 1, 2]. This surface modification has been reported greatly increase resistance wear and corrosion of materials [3, 4, 5]. Since the fatigue crack initiation of metals was delayed by this type of modification, fatigue life increased and fatigue strength increased also [6,7,8]. Yet it has been reported that the fatigue life was decreased by the surface modification [9,10]. The authors investigated the effect of ion implantation and notch radius on crack initiation life, early crack growth behavior and fatigue life for pure titanium implanted with nitrogen ions [11,12]. It was reported that the fatigue life of pure titanium was decreased by the N+ ion implantation, and early crack growth was very sensitive the microstructure of the metal. Early crack growth patterns

2 300 Localized Damage were divided in two types which were controlled by the mechanical condition The aim of this work was study fracgraphically the fatigue crack initiation and early crack growth mechanism for pure titanium metal implanted with nitrogen ions while varying the notch radii. The effects of notch radius and ion implantation on the fatigue crack initiation were investigated using an optical microscope and a scanning electron microscope (SEM). A maximum surface roughness, R^ax and fractal dimension, D, were calculated from the fatigue fracture surface and measured using a surface roughness measurement instrument. These parameters were related crack initiation and early crack growth patterns. 2. Experimental Procedure The material used in this work was pure titanium (JIS TP35) supplied as a 4 mm thick plate. The mechanical properties of the Ti plate consisted of a yield stress of 268 MPa, a tensile strength of 329 MPa and an elongation of 34 %. The fatigue specimens used smooth and notched specimens. Firstly, specimens were machined a width of 20mm and a length of 80mm. Machined notches were located in the central zone of the specimens with the notch depth constant at 2mm and the notch root radius, p, either O.lmm, 0.25mm, 1mm or 2mm. The stress concentration facr, Kt, which was calculated using the finite element method was 10, 6.8, 4.2 and 2.9 respectively. All specimens were polished with #1000 grit emery paper and after being kept for 1 h at 773 K remove residual stress they were cooled in the furnace. After the heat treatment, the specimens were chemically polished. The conditions for N+ ion implantation were based on Yano et al.'s results [13], The implantation treatment was performed by using a dynamic mixing machine (HITACHI IX ) using a dose of 2.5 x 10 ion/cm^ at 25keV in a vacuum of 9 x 10 Torr. Non-implanted specimens were also prepared assess the effects of N+ ion implantation on the other specimens. In this case, these were smooth and notched specimens having p=0.25mm. Three point bending fatigue tests were carried out using an electro-hydraulic fatigue testing machine with a frequency of 2()Hz, R=0 and at room temperature obtain S-N curves. The plastic replicas were taken from the vicinity of the notch root at intervals of about 2000 cycles and were then evaporately gold coated in a vacuum. The plastic replicas were then observed using an optical microscope verify fatigue crack initiation and early crack propagation. Results and Discussion: Analysis of the surface layer after N* i on implantation ; The average grain size of implanted specimens was about 34jim in circumference which is almost equal that of non-implanted specimens. The implanted layer and substrate of the pure titanium was analyzed using XPS (KRATOS XSAM 800). The both surface layer included oxygen, these concentrations of oxygen were almost equal. These analysis was guessed that oxygen diffuses in the interior of the titanium substrates. The binding energies of these elements suggest that the N* implanted layer consists of predominantly

3 TiN with some TiO2. Localized Damage 301 S-N curves; The S-N curves for both smooth and notched N* implanted specimens are shown in Fig. 1 and compared with the results for the non-implanted specimens, both smooth and notched (p=0.25mm). Fig. 1 shows that the fatigue life increases with increasing notch radius except for Aa<2()()MPa. N* ion implantation did not contribute increases in the fatigue life, regardless of whether the specimen is smooth or notched. Crack initiation; In order investigate the effect of the notch root radius, p, and stress amplitude, Aa, on crack initiation, replicas taken of the notch root surface were observed using an optical microscope and the fracture appearances observed using a scanning electron microscope (SEM). On the p surface, microcracks were initiated at grain boundaries or the inside of the grain and served as the starting points. Microcracks were tending be observed at a grain boundary at increasing Aa (in other words; with decreasing NJ. The SEM observation results of the fracture near the p surface are shown in Fig. 2. As can be seen in Fig. 2, many transgranular cleavage like facets are observed whether N* implanted or not. Though the crack initiated at a grain boundary, intergranular cleavage like facets were not observed. Judging from these observation results, it is clear that the form of crack initiation is transgranular fracturing. From the experimental results of the three point bending fatigue tests, it was clear that the fatigue life was decreased by the ion implantation. It can be considered that the ion implantation formed a very thin TiN layer on the surface of pure titanium. Such a TiN layer is thought play a role in obstructing dislocation movement at the interface between the thin layer and the bulk material. In fact, the slip bands were not observed on the surface of implanted titanium, as shown in Fig. 2. Thus as a result of accumulating dislocations at the interface, a high stress concentration may be introduced. The observation results of the notch root surface showed that the decrease of fatigue life results from a cracked thin surface layer. This thin layer consists of mixed microstructures of TiN and TiCh. There is a significant incoherence between TiN and TiCb. This incoherent thin layer easily failed after a few cycles of stress resulting in cracking leading flatand smooth fractures in the very p surface. Early crack growth; After the fatigue crack initiation, the early crack growth, which was controlled by a o/aa value of about 1.5, was very sensitive microstructure and its progress showed a zig zag pattern. In the region of early crack growth, there were microstructure sensitive fracture appearances such as like cleavage cracking, as shown in Fig. 2. In order relate unevenness of the fracture surface crack initiation, the unevenness of the fracture surface was measured using a surface roughness measurement instrument at a constant distance from the p surface, X'. The measured results were printed with the output expanded 20 times in the v direction (transversely along the crack) and 200 times in the z direction (90* normal they direction) showing the fracture relief pattern. An example of this

4 302 Localized Damage measurement for p=0.25mm and Aa=200MPa is shown in Fig.3. The maximum surface roughness, Rmax, is defined as the difference in the highest and lowest points of the mesured plofiles at the distance from the notch root, X. Fig. 4 shows the relationship between Rmax and X for Ao=200MPa with different notch root radii. Rmax depends on the fracture' s appearance. At X <lmm, Rmax decreases with decreasing p and is related a small region of crack initiation. At X >lmm, it was the tendency of Rmax stay close a constant value of about O.lmm. In order calculate the fractal dimension, the measured profiles on the fracture surface were divided by lattices of squares (6x6) which are changed from loiim 50iim. N(6) was defined as the number of lattices which could cover all profile of fracture surface, and L(6) was defined as a product of 6 by N(6). The relation between L(6) and 6 was closed by a formula, L(6) = c X 6^~. The D value can be calculated from the slope of form both logarithms graph. As an example, the relation between L(6) and 6 are shown in Fig. 6 for t=2mm, p=0.25, Ao=200MPa. The D were calculated from Fig. 5. The relationship between D andx for Acr=20()MPa is shown in Fig. 6. AtZ <lmm, it shows that D increased with decreasing p. It means that the unevenness of the fracture surface which has large D, was microscopically bigger than that which has small D. In this region, a striation was not observed, but some intergranular cleavage like facets were observed using a SEM. So, it was fracgraphically considered that the crack growth was very sensitive microstructure. When p was small, the second type of early crack growth easily occurred. As a result, D value of fracture surface of the second type of early crack growth was high. So, it is considered that the early crack growth patterns can be evaluated by the fractal dimension. When the D value was high, the second type of early crack growth easily occurred. When^T is about 1mm or greater, it stayed close a constant value of about 1.2. In this region, some striations were not observed, and crack growth was not affected by the specimen' s microstructure. Conclusions; (l)the maximum surface roughness R^, depended on fracture appearance and increased with increasing notch radius. It is related the region of crack initiation (2)At X<lmm, the fractal dimension, D, increased with decreasing p. It was tendency of D stay close a constant value of about 1.2 over a crack length of about 1mm. (3)The value for D of fracture surface of the second type of early crack growth was high. References 1. M. Iwaki, (1992), Surface Modification and Ion Beam Technology, Journal of the Surface finishing Soc. of Japan, 43-12, K. Okabe (1988), Ion Implanters and Related Machines for Surface-Layer Modification, Journal of The Society of Metal Surface Techniques, 39-10, 555

5 Localized Damage M. Iwaki (1988), Surface Layer Modification by Ion Implantation, Journal of The Society of Metal Surface Techniques, 39-10, M. Sai (1988), Ion Implantation in Metals, Journal of The Society of Metal Surface Techniques, 39-10, M. Hirano (1988), Mechanical Properties of Ion - Implanted Materials, Journal of The Society of Metal Surf ace Techniques, 39-10, R. Murakami, M. Yoneda, and M. Katumura (1992), Effect of Laminated Layer Fatigue Properties of Steel Modified by Dynamic Mixing Process, The 41 JSMS Spring Annual Meeting, R. Murakami, Y. Morikawa, Y. Morimo, M. Yoneda, and M. Katumura, (1994), Fatigue Properties and Crack Behavior of Steel with TiNx Films Laminated by Dynamic Mixing Method, J.Soc.Mat.Sci., , H. Bakhru, W. Gibson, C. Burr, (1981), Nuclear Instruments and Methods, R. Ehara (1981), The 183 JSME Meeting of Fatigue 10.M. Hashimo, S. Nagashima, M. Sirari, (1989), An x-ray Study on Residual stress and Fatigue Characteristics of TiC Coated Steels, J.Soc.Mat.Sci., , 150 U.K. Futagami, R. Murakami, Y. Morikawa, A. Kawahi, W. G. Ferguson, M. Yoneda, and M. Katumura (1995), Effects of notch radii on fatigue strength and crack initiation of nitrogen ions implanted pure titanium, 7th Int Conference on Mechanical Behavior of Materials, K. Futagami, R. Murakami, Y. Morikawa, A. Kawahi, W. G. Ferguson, M. Yoneda, and M. Katumura (1995), Effects of notch radius on fatigue life and early crack growth behavior of pure titanium implanted with nitrogen ions, The JSME, A, T. Yano (1989), Formation of TiN Films by Dynamic Mixing Method, Journal of High Temperature Society, 15-6, 279

6 304 Localized Damage < D t=2mm + P=0.1mm, N "^ p =0.25mm, Non O p=().25mm, N "^ O Smooth, N + Smooth Non, N p=2mm, N o o# 100,0" 10" 10" Nf (cycle) Fig. 1 S-N curves of implanted and non-implanted specimens. Fig. 2 Observation of transgranular cleavage facet using a SEM (p=0.25mm, N+ ion, Ao=200MPa)

7 Localized Damage 305 X rt 7\ t=2mm p=0.1mm N + Ao=200MPa =0.2mm o TZ 3 00 o 73 (^ Fig. 3 The measured results of unevenness of the fracture appearance. =0.1mm =0.25mm =lmm 10' 10" 10' 10' Disance from notch root X (mm) Fig. 4 The relationship between Jf and R^x -

8 CO o era Fractal dimension D 3 crs' p" Ln p- G 8 sr cr o eg o OQ CX5 Q. v^ CL OQ