Low-cycle fatigue behavior of commercially pure titanium

MATERIALS SCIENCE & ENGINEERING

l

ELSEVIER

Materials Science and Engineering A213 (1996) 81-85

Low-cycle fatigue behavior of commercially pure titanium Kenichi Takao, Kazuhiro Kusukawa Yuge National College of Maritime Technology, Yugecho, E/time 794-25, Japan

Abstract Low-cycle fatigue tests have been carried out on annealed commercially pure titanium under strain-controlled conditions. The relation between a plastic strain range and a number of cycles to failure obeyed Manson-Coffin's rule. The initiation of fatigue cracks was observed successively with the aid of a replication technique and microstructural deformation was measured on the surface of a specimen. Results show that fatigue crack initiation is intergranular at strain ranges larger than 1%. On the contrary, slip band cracks appear in lower strain ranges. At specified grain boundaries, microstructural deformation concentrates at or near grain boundaries and irreversible steps between grains form gradually and this leads to intergranular cracking. The above fatigue behavior of pure titanium is mainly due to fewer slip systems (only three) than other metals with bcc or fcc crystal structures.

Keywords: Fatigue; Low-Cycle fatigue: Pure titanium: Crack initiation; IVlicrostructural deformation

1. Introduction

Pure titanium has an excellent resistance to corrosion and a high strength to weight ratio. It has been used frequently for heat exchangers in power stations or reacting containers in various chemical plants. Under such aggressive environments, material is considered to undergo plastic deformation cyclically. Therefore, to make clear low cycle fatigue characteristics is very important for practical use of pure titanium. The crack initiation characteristics in high-cycle fatigue have been reported as follows [1,2]: (i) micro cracks initiate along multiple slip bands within grains and join together to form a crack the size of one grain; (ii) cracks further join with other cracks initiated in neighboring grains to form the main crack which becomes a starting point of final fracture. The behavior is mainly owing to a crystal structure of the metal, hcp, which has fewer slip systems than other metals with bcc or fcc structures and remarkable constraint against intergranular plastic deformation. The constraint is also considered to be

remarkabie under a low-cycle fatigue loading. In the present study, we discuss the crack initiation behavior of an annealed commercially pure titanium through successive observations on the surface of the metal under strain controlled conditions.

2. Material and experimental procedure The material used for this research was a commercially pure titanium bar (JIS TB35, diameter 15 mm). Chemical composition is given in Table 1. Specimen configuration is an hour-glass type as shown in Fig. 1. The specimen has a partial notch at the minimum section for observations of cracking behavior and microstructural deformation. Specimens were annealed at 1073 K for 1 h in vacuum and chemically polished before testing. The mechanical properties after the heat treatment are shown in Table 2.

2-M14

Table 1 Chemical composition (wt.%) 25

H

O

N

Fe

Ti

0.0009

0.I12

0.003

0.042

bal.

_1 -'

100

D e t a i l of notched section

Fig. 1. Dimensions of a specimen.

K. Takao, K, Kusukawa /Materials Science and Engineeriltg A213 (1996) 81-85

82

Table 2 b4echanical properties

0.2% proof stress (MPa) Tenmlestrength (MPa) Elongation C%) Reduction of area (%) True fracturestress (MPa) Young's modulus (GPa) Averagegrain size (p.m) Monotonic strain hardening exponent

243 372

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X 10-3S

-1

Localized deformation at a notch root was measured with the micro grid method (grid separation distance: 20 ~.tm) which was employed by Hatanaka et al. [3]. Strain components were calculate:l by substituting relative displacements ~ and u, of grid points during one cycle, into Euler's equation (Eqs. (1)-(4)). ++ - az

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3.2. Behat, ior of fatigue cracl< initiation Fig. 5 shows an example of successive observations on the surface of the specimen at ,Jet-- 1.6%. Fig. 6 shows the electron micrograph of framed portion in Fig. 5. These observations indicate that several cracks initiate along a grain boundary, they join together to become a crack, and then the crack propagates into grains contacting the boundary. However, at a relatively low Jet ( = 0.6%), micro-cracks initiate along slip bands near the grain boundary as shown in Fig. 7. The latter cracking behavior is similar to that observed in high cycle fatigue [1], 3.3. Localized deformation and crack initiation mecha-

n ism Fig. 8 shows the change in the surface of a specimen when deformed alternatively by strains of 1.6% in tension and compression. Many slip bands are observed within grains after the first tensile loading.

3. Experimental results and discussion

3.1. Low cycle fatigue properties The cyclic stress-strain curve shown in Fig. 2 is obtained by using a multiple step method. The cyclic strain hardening exponent, n' was determined by using the least squares method for log-log plots in the figure. Fig. 3 shows the relationship between strain ranges (total strain range; Act, plastic ,;train range; ASp and elastic strain range: As+) and fatigue life Nr. The ASp vs. Nr relation of the metal obeys ]vlanson-Coffin's rule (Eq. (5)) as observed in other metals. j

ep(Nr)044 = 0.19

(5)

The variation of stress ranges during strain cycling with a constant strain range is shown in Fig. 4. This shows that fatigue softening and hardening are not remarkable in pure titanium. The amount of variation in the stress range is within 7% of initial stress ranges.

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Fig. 9 shows observations on the surface of the specimen scribed in a grid when deformed alternatively by 1.6%. Fig. 10 shows localized shear strain, ?w, obtained from measurements of relative displacements between grid points. Fig. 10(a) shows Y,y after the first half cycle (from 0 through + 1.6% to 1.6%) and (b) the one after the full cycle (from 0 through + 1.6% and - 1 . 6 % to 4-1.6%). The measurement leads to the following. During the first half cycle, large strains (shown by the height of the bars) develop within grains. Most of them disappear after the next half cycle, however, strain near boundaries remains almost intact. The irreversible strain accumulated by strain cycling near the boundary is responsi-

Fig. 4. Variation of stress ranges.

The remarkably deformed regions concentrate nearby grain boundaries. At the following compressive load, twins are observed, however, concentrated deformations show no remarkable change. From stereoscopic measurements on SEM micrographs, the height of step between grains framed in Fig. 8 is about 0.6 p.m under the first tensile load. In the following compressive load, it still remains at 0.4 pro. Therefore, localized deformation nearby the grain boundary is considered to be irreversible and it grows at subsequent cycles.

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K. Takao, K, Kusukawa / Materials Scie~ce a~d Engineeri~tg A213 (I996) 81-85

84

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ble for the large step at the grain boundary leading to cracking as shown in Fig. 5. As described above, it has been shown quantitatively that the irreversible deformation concentrates near grain boundaries. Pure titanium having a hexagonal close-packed lattice, has only three slip systems (prismatic plane, conical plane and basal plane). Since the number of slip systems of pure titanium is less than other metals with fcc or bcc lattice, it is considerably

more difficult to maintain the continuity of the plastic deformation between grains. In other words, slip band growth beyond the boundary is obstructed by surrounding grains. Therefore, excessive deformation tends to concentrate near grain boundaries. As described previously, these deformations are irreversible and their severities increase with loading cycle. Consequently, crack initiation is most likely to occur owing to slip concentration and accompanying step formation.

K. Takao, K. Kusukawa / Materials Science and Engineering A213 (i996) 81-85

85

4. Conclusions

References

(1) Pure titaniuna exhibits neither fatigue softening nor hardening. A ep vs. Nt- relation obeys MansonCoffin's rule as observed in other metals. (2) Fatigue crack initiations occur along grain boundaries where irreversible deformation accumulates with loading cycles. (3) The irreversible deformation is owing to the slip constraint between grains.

[I]K. Takao and H. Nisitani, Fatigue crack initiation and notch sensitivity of commercial purity titanium, Trans. Jptz. Soc. Mech. E~zg., 50 (1984) 1049-1053 (in Japanese). [2] K. Takao and K. Kusukawa, Fatigue notch characteristics of commercially pure titanium, Proc. 6th Int. Co@ on Mechanical Behaviot~r of ~I'Iaterials, Vol. 2. I991, pp. 445-450. [3] K. Hatanaka, Y. Yoshida and T. Ishikawa, Measurements of localized strain around crack tip during strain cycling and some considerations on fatigue crack extension in copper, Trans. jpn. Soc. Mech. E~zg., 59 (I993) 674-681 (in Japanese).