Creep and fatigue properties and fracture toughness

The maximum stress that can be safely applied to high-strength copper alloys during extended operation at elevated temperatures is less than the yield strength measured during short term exposures (Figs. 7-9), due to the phenomena of creep and fatigue. Figure 10 compares the mechanical stresses which produce creep failure within 100 hours in copper, CuCrZr and CU-AI2O3 during exposure at elevated temperatures [1,11,23,39,43]. Both alloys have significantly higher rupture strengths than pure copper. The 100 hour rupture strength of both MZC CuCrZr [23,43] and GlidCop AL-15 Cu-AI2O3 [11] is approximately 300 MPa at 300°C. However, the rupture strength of CuCrZr decreases rapidly with increasing temperature above 300°C. Oxide dispersion-strengthened copper has superior 100 h rupture strengths at temperatures above 400°C. The higher-strength grades of dispersion-strengthened copper such as GlidCop AL-60 have particularly good stress-rupture strengths. For example, the rupture strengths shown in Fig. 10 for GlidCop AL-60 and AL-15 at 400°C are about 360 and 240 MPa, respectively [11,39]. The impressive creep-rupture strengths of oxide dispersion-strengthened copper alloys such as GlidCop at elevated temperatures is due to the uniform dispersion of thermodynamically stable particles of ~7 nm diameter, which is close to the optimum size for high creep strength [44].

1 ' • • ' 1 1 • • ' 1 ' 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

—•—Harling et al (1981)

—*—Nadkarni (1984)

—o—- Taubenblatt et al (1984) - » — Synk & Vedula (1987)

"-*--• Stephens & Schmale (1987) -Q— Metals Handbook (1990)

FIG.. 10. Elevated temperature stress-rupture strength of copper [1,11], MZC CuCrZr [23,43], Cu-3.5 vol.% A 1203 [43], and GlidCop AL-15 [11,39] and AL-60 [11].

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 Applied Stress (MPa)

FIG. 11. Thermal creep rate in copper and copper alloys [1,18,23,40,45].

171

The steady-state thermal creep rate in copper, CuCrZr and C11-AI2O3 at temperatures between 150 and 400°C is shown in Fig. 11 [1,18,23,40,45]. Pure copper creeps readily at low applied stress levels. For example, the creep rate of copper at 400°C is nearly 10_4/s for an applied stress of 100 MPa, and about 10"8/s for an applied stress of only 15 MPa [46]. CuCrZr and oxide dispersion-strengthened copper alloys can be subjected to much higher stresses at elevated temperature without suffering excessive creep. The available data indicate that creep rates of ~5xl0"9/s are produced in CuCrZr [23] and oxide dispersion-strengthened copper [33] with applied stresses of about 200 MPa at 300°C. Unfortunately, there are not enough data to quantitatively compare the creep behavior of the various high-strength copper alloys over the ITER-relevant temperature range of 20 to 400°C. High strength copper alloys generally have a threshold stress below which creep deformation is negligible [44]. Further study is needed to determine the value of the threshold stress for the various copper alloys at temperatures between 20 and 400°C. The creep data reported by Nadkarni [11] suggest that the threshold stress at 650°C is about 130 and 190 MPa for cold-worked GlidCop AL-15 and AL-60, respectively. The data by Tang et al. [45] indicate that the threshold stress is about 100 MPa for MZC CuCrZr tested at 400°C.

A further factor which must be considered for the precipitation-hardened alloys is that their softening behavior at elevated temperatures due to overaging and recrystallization depends on both the exposure time and temperature. For example, extrapolation of short-term (0.25 to 100 h) annealing results on MZC CuCrZr indicates that the softening temperature is about 410°C for a 100 h exposure, but decreases to ~330°C for a 10 year exposure [10,29]. Therefore, although the creep rate or rupture strength of precipitation-hardened copper alloys such as CuCrZr may be acceptable at temperatures above 300°C during short-term tests, the alloy could become unacceptably soft during extended exposure ( » 1 0 0 h) at these temperatures due to overaging and recrystallization. The softening at elevated temperatures is accelerated by irradiation or the presence of stress [47].

The cyclic fatigue strength of copper alloys generally increases with increasing tensile strength, i.e., metallurgical changes which increase the tensile strength of copper alloys also increase the fatigue strength. Figure 12 shows the room temperature cyclic fatigue strength of cold-worked pure copper, CuCrZr and CuNiBe [1,16,48]. The high-cycle fatigue strength of copper at room temperature is about 100 MPa [1,16,49], which is about 1/3 of the yield strength of the starting material (250 to 300 MPa). The corresponding high-cycle fatigue strength of MZC CuCrZr, with a yield strength of about 550 MPa, was reported to be 170 MPa [50]. There is a lack of fatigue data on CuCrZr and CuNiBe alloys, particularly at high cycles. The room temperature fatigue strengths of Elbrodur CuCrZr and Brush-Wellman CuNiBe alloys at intermediate (105) cycles are -280 and 400 MPa, respectively [48].

The fatigue strengths of normal and overaged CuNiBe were found to be comparable (-400 MPa) at 105 cycles, whereas the initial yield strengths were 770 and 570 MPa, respectively [48]. Additional low-cycle (<3xl04) fatigue data on CuNiBe have been reported by Rosenwasser and coworkers [37,42] using a minimum to maximum stress ratio of R=0. Their fatigue strengths were 200 to 250 MPa higher than the data reported by Bushnell and Ellis (Fig. 12) for a stress ratio of R=-l [48].

The room temperature cyclic fatigue strength of GlidCop C11-AI2O3 is shown in Fig. 13 [16,32, 48,51,52]. The fatigue data from refs. [48,52] were obtained under stress-controlled conditions, whereas the remaining data were obtained under strain-controlled conditions and converted to equivalent stress. All of the data were obtained on GlidCop AL-15 except for Singhal et al. [51], who examined GlidCop AL-25. The fatigue strengths of AL-15 and AL-25 appear to be comparable at intermediate (104 to 106) cycles. The high-cycle fatigue strength of CU-AI2O3 with a yield strength of 330 MPa is about 175 MPa [16]. Nadkarni [11] reported somewhat higher fatigue strengths for GlidCop AL-15 and AL-60 compared to the data shown in Fig. 13 over the range of 3X104 to 2xl07

cycles. His rotating-bending cantilever beam fatigue measurements indicated fatigue strengths at 2xl07 cycles of about 205 and 230 MPa for AL-15 and AL-60, respectively. As discussed elsewhere [16], this discrepancy may be due to dynamic hysteresis effects associated with the rotating-bending beam measurements, or to the higher frequency (167 Hz vs. 1 to 10 Hz) of these measurements compared to the intermediate-to-high cycle data shown in Fig. 13. Alternatively, the higher fatigue strength may be due to a higher initial yield strength of Nadkarni's material (470 MPa for AL-15).

800

—a— Bushnell & Ellis (1990)

—•—Metals Handbook (1990)

FIG. 12. Room temperature cyclic fatigue strength of copper [1,16,48], Elbrodur CuCrZr [48] and CuNiBe [48]. Fully reversed stress amplitude (R=-l). All data except for refs.

[1,16] were stress-controlled. The data points with arrows denote specimens which did not fail during the test lifetime.

10° 10¹ 1 0² 10³ 1 0⁴ 1 0^s 1 0⁶ 10⁷ Cycles to Failure

FIG. 13. Room temperature cyclic fatigue strength of Cu-Al203 [16,32,48,51,52]. All of the data was obtained under fully reversed

(R=-1) stress conditions except for ref. [16], which used a minimum to maximum stress ratio of R=0.

The limited amount of data obtained on CuCrZr (Fig. 12) indicates that the room temperature fatigue behavior of this alloy is comparable to that of Cu-Al203 (Fig. 13).

There have only been a few studies of the fatigue behavior of CuCrZr [18,45,53,54], CuNiBe [37,42], and oxide dispersion-strengthened copper [16,18] at elevated temperatures. The high-cycle (>106) fatigue strength of GlidCop AL-15 was measured to be about 75 MPa at 600°C [16]. Figure 14 summarizes the fatigue results obtained on high-strength copper alloys under strain-controlled conditions at temperatures between 20 and 300°C [18,32,37,42,51]. Additional low-cycle strain-controlled data have been obtained on MZC CuCrZr at 400°C, with the results reported only in terms of the plastic strain range [45,53]. The low-cycle strain-controlled fatigue behavior of CuCrZr and oxide dispersion-strengthened copper appears to be very similar over the temperature range of 20 to 300°C.

The results by Rosenwasser and coworkers [37,42] suggest that CuNiBe has superior low-cycle fatigue properties compared the other copper alloys. There does not appear to be a strong dependence on temperature between 20 and 300°C when the data are evaluated according to strain amplitude. All of the alloys have comparable or better fatigue properties than pure copper. For example, the fatigue lifetime of pure copper tested in the temperature range of 20 to 300°C decreases from about 105 cycles to 103 cycles as the total strain amplitude increases from about 0.15% to 0.6% [55,56].

_i ' • • ' — I I - I I m l i I I-J

ioz 103 104 105

Cycles to Failure

106

FIG. 14. Strain-controlled cyclic fatigue behavior of copper alloys [18,32,37,42,51].

173

We are not aware of any published measurements of the fracture toughness or fatigue crack growth behavior of CuCrZr, CuNiBe, or commercial oxide dispersion-strengthened copper. The room temperature fracture toughness of a medium-strength (Gy=250 MPa) Cu-Zr alloy that should be comparable to CuCrZr was measured to be an impressive 420 MPaVm, and the tearing modulus was 440 [57]. The fatigue crack growth rate was also found to be much better than pure copper. On the other hand, unpublished measurements on a cold-worked GlidCop AL-15 alloy (Gy=330 MPa) indicate that the room temperature fracture toughness was only 20 to 50 MPaVm [52,58], which is similar to values for high strength aluminum alloys [59].