Fatigue and Creep of Nickel Alloys

Creep strength and fracture resistance are two properties that are critical in the selection and optimization of high temperature materials. These same properties may be progressively impaired by service exposure and are, therefore, key to the assessment of remaining life of high temperature components. In recent years, a methodology based on accelerated measurements of these properties has been developed. The two critical properties are decoupled in this approach, which allows a clearer separation of the effects of microstructural evolution, damage development and environmental attack.

Materials for applications in e.g. gas turbines are required to withstand harsh operating conditions. They must exhibit good strength to permit their use in lightweight designs and high creep resistance so that operating temperatures can be raised to increase thermal efficiency, while maintaining a long service life of the equipment. In addition, they need a high resistance to fatigue loading including both high cycle fatigue from vibration or rotating bending of the rotor and low cycle fatigue arising predominantly from the steep thermal gradients occurring during rapid start-up and shut-down of the machine.

Since the temperature of the material can vary between sub-zero for the start-up of a turbine located in the open-air to the maximum gas temperature during operation at full load, it is clear that a knowledge of the temperature dependence of these fatigue and creep properties is of paramount importance.

Also, centrifugal forces and vibrations are typical operating conditions of rotating machines for gas turbines. They lead to creep deformation and high-cycle fatigue (HCF)-loading of the blades. Knowledge of creep and HCF properties of blade materials is therefore very important. For the cast nickel base-superalloy 738 LC, a typical gas turbine blade material.

Modern approaches to repair, rejuvenation or replacement of critical components require rapid turnaround and seek accurate assessment of material performance capability. In addition to the need to generate suitable material data in a short time, there are more fundamental objections to the creep rupture test. The test involves an arbitrary combination of deformation processes and fracture processes, both creep strength and fracture resistance are evaluated from the same basic test, and the properties that are being measured are changing during the course of the test.

All these complexities would be acceptable if the test suitably reflected operating conditions. In reality, however, most components experience non-steady stresses and temperatures, multiaxial stresses, cyclic stresses, active and often aggressive gaseous environments, and synergistic interactions among these factors. These concerns are especially applicable to the combustion turbine. Moreover, long-time data have little relevance to cyclic operations where the zero time origin for creep analysis should be reset for each cycle.

With these deficiencies in mind, an alternative approach to testing and evaluation has been developed. The principal features of this Design for Performance methodology may be summarized as follows:
• Creep strength and fracture resistance are de-coupled and measured in separate tests or as distinctly different properties.
• Both properties are measured as current values so that time-dependent changes during the test are minimal.
• Creep strength is evaluated in terms of creep rate rather than time.
• Fracture resistance is evaluated in terms of crack extension or ductility rather than time to failure.
• State of damage is assessed in terms of current values of the critical properties.

In recent years, a new Ni-base superalloy, Allvac 718Plus has been developed to meet the objectives of increasing the temperature capability 55°C higher than that of alloy 718 and with comparable processing characteristics as alloy 718.

Alloy 718Plus™ is a newly developed Ni-base superalloy, strengthened predominantly by γ’ phase. The chemistry and mechanical properties of this alloy have been introduced in a number of publications. Previous studies demonstrated that the processability of this alloy approaches that of alloy 718 and mechanical properties in the solution treated and aged (SA) condition, are comparable to Waspaloy up to 704°C, but with lower cost. It was of interest to determine if direct aging could be effectively applied to alloy 718Plus, potentially increasing its attractiveness for applications such as turbine engine disks.

Previous results showed that fatigue crack propagation (FCP) resistance without holding time has no significant difference between three alloys with 718Plus being the best and 718 the lowest. During the holdtime FCP tests, 718Plus shows comparable results to those of Waspaloy and better than Alloy 718.

The nominal chemical composition of the alloy is listed in Table 1, as compared with that of alloy 718 and other Ni alloys.

Table 1: Nominal chemical composition (wt.%) of various nickel alloys

Alloy	C	Ni	Cr	Mo	Mn	W	Co	Fe	Nb	Ti	Al	Nb+Ta
Nimonic 901	0.08	Bal.	12.7	6.2			0.9	37.4		2.8
718Plus	0.02	Bal.	18.0	3.0		1.0	9.0	10.0	5.4	0.7	1.4
718	0.025	Bal.	18.0	3.0				18.0	5.4	1.0	0.5
Waspaloy	0.035	Bal.	19.5	4.2			13.0			3.0	1.3
600	0.15	min. 72.0	14.0-17.0					6.0-10.0
690	0.05	min. 58.0	27.0-31.0					7.0-11.0
82	0.10	min. 67.0	18.0-22.0		2.5-3.5			3.0				2.0-3.0
182	0.10	min. 59.0	14.0-17.0		5.0-9.5			6.0-10.0				1.0-2.5
52	0.03	73.2	28.6		0.5			9.0
152	0.03	54.2	28.6		3.9			10.2				1.93

Direct aging can result in significant improvement in mechanical properties of many wrought Ni-base superalloys. Its effectiveness is closely related to hot working and aging conditions and is also alloy-dependent. This process is most often used with γ” rather than γ’ strengthening alloys.

Allvac 718Plus™ alloy is predominantly a γ’ strengthening alloy which shows excellent mechanical properties and high thermal stability up to 704°C in the solution treated and aged condition. It was uncertain if direct aging could be effective in this alloy. A test program was initiated to explore if direct aging could improve the alloy’s performance and to determine the optimum process conditions. The variables evaluated included forging temperature, forging reduction, forging heating time and post-forging cooling rate. Mechanical tests and microstructural study after different direct aging treatments demonstrated that direct aging is feasible at a production scale for alloy 718Plus and a significant improvement in performance can be achieved under well-controlled conditions.

Significant effect of direct aging was observed in some cases such as small size bar rolled at low temperature. Compared to the SA condition, both strength and rupture resistance were remarkably improved, and this improvement was not significantly diminished even after long-term thermal exposure. For the billet products, a strength increase occurred in the smaller size and a loss in the larger. These results clearly demonstrate that direct aging can significantly improve the mechanical properties of alloy 718Plus, but the processing conditions will be critical. The increase in rupture life for all of these samples also was especially interesting and worthy of further study.

Among the process parameters studied, the forging temperature appears to be the most critical. The optimum temperature range for DA (direct aging) processing for alloy 718Plus appears to be from about 954°C to 1038°C. Forging temperature dependencies of strength and of rupture life are different. As expected, yield strength increased with decreasing forging temperature. The smaller increase at the lowest forging temperature was probably due to adiabatic heating. There was almost no change in rupture life at the lower forging temperatures.

Reactor vessel internal components made of Ni–base alloys are susceptible environmentally assisted cracking (EAC). A better understanding of the cause and mechanisms of this cracking may permit less conservative estimates of damage accumulation and requirements on inspection intervals. The resistance of high–Ni alloys, e.g., Alloys 600 and 690, as well as their welds, i.e., Alloys 82, 182, 52, and 152, to EAC in simulated light water reactor (LWR) environments has been evaluated. Existing crack growth rate (CGR) data for these materials under cyclic and constant loads have been analyzed to establish the effects of alloy chemistry, cold work, and thermal treatment, temperature, water chemistry, load ratio, and applied stress intensity on CGRs.

The fatigue crack growth data in air have been analyzed to develop correlations for estimating the fatigue CGRs of Alloys 600 and 690 as a function of stress intensity factor range ∆K, load ratio R, and temperature. The results indicate that in air, the CGRs of these materials are relatively insensitive to changes in frequency. For cyclic loads, the experimental CGRs in high–temperature, high–purity water are compared with CGRs that would be expected in air under the same mechanical loading conditions to obtain a qualitative understanding of the degree and range of conditions for significant environmental enhancement in growth rates.

The fatigue CGRs of Alloy 600 are enhanced in high–dissolved–oxygen (DO) water; the environmental enhancement of growth rates does not appear to depend on the material condition. In contrast, environmental enhancement of CGRs of Alloy 600 in low– dissolved–oxygen water seems to depend on material conditions such as yield strength and grain boundary coverage of carbides. Material with high yield strength and/or low grain boundary coverage of carbides exhibit enhanced CGRs. Correlations have been developed for estimating the enhancement of CGRs in LWR environments relative to the CGRs in air under the same loading conditions.

For Alloy 690, the data suggest some enhancements of CGRs in boiling water reactor (BWR) water. Limited data indicate no environmental effects on CGRs in pressurized water reactor (PWR) water. However, the existing database for Alloy 690 is small and additional tests are needed to verify these results.

The enhancement of fatigue CGRs of high–Ni alloy welds in LWR environments has been determined relative to the CGRs that would be expected under the same loading conditions for Alloy 600 in air. Fatigue CGRs of Alloy 82 and 182 welds are enhanced in PWR and BWR environments with normal water chemistry (NWC). The results in PWR water show significant scatter, growth rates of welds may be up to a factor of 10 higher than the CGRs predicted for Alloy 600 in air. Hydrogen water chemistry has a beneficial effect on growth rates; CGRs are decreased by a factor of 5–10 when DO level is decreased from 200 to 10 ppb. The results in a NWC BWR environment show good agreement with the predicted curve for Alloy 600; the data in a PWR environment are higher.