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NASA NTRS1985

Impact wear of iron rich superalloys

The impact-sliding wear resistance of chill cast and aligned eutectic Fe-base superalloys against M42 and 17-4 PH steel counterface materials is examined. The aligned material tests were run with carbide fibers perpendicular to the counterface contact surface and the characterization focused on fracture processes as observed in the subsurface microstructure of the worn materials. Metallographic analyses were performed on specimens exposed to various numbers of repetitive impact load cycles, and for comparison, two other aligned composites were tested with the same fiber orientation under identical test conditions against hardened M42 tool steel. A strong improvement was found in wear resistance of an aligned eutectic structure as compared to the corresponding randomized chill cast structure. Experiments with the softer 17-4 counterface were characterized by transfer onto the Fe-base superalloy, and the cracks formed were oriented transverse to the relative sliding direction and occurred in a periodic fashion. Observations in subsurface regions of the worn materials indicated crack intergranularity with an occasional transgranular fracture of the M7C3 carbide phase.

Wayne, S. F.↗

NASA NTRS1988

Use of pure nickel and LiOH for thermal energy storage

The solid to liquid phase transformation of LiOH has been proposed as an ideal candidate thermal energy storage media for a Rankine Cycle powered electrical generation unit envisioned in Space Station based solar dynamic systems. Due to the corrosive nature of molten hydroxides, long term containment of LiOH is of concern. Pure nickel is thought to be a suitably resistant material, and a program has been instituted to measure the effects of prolonged exposure of liquid and gaseous LiOH on the mechanical properties of pure nickel alloys. Results to date indicate that negligible weight and thickness changes occurred in Ni alloys exposed to LiOH for as long as 2500 hr at 775 K, and essentially no difference in 77-900 K tensile properties could be detected between LiOH exposed and vacuum annealed Ni specimens. Although there was little sign of outward damage, microstructural examination revealed that all hydroxide contaminated tensile test specimens had surface connected intergranular cracks along the gage lengths. Two other potential problems, which have strong implications with respect to a LiOH/Ni energy storage system, were also noted during the corrosion experiments. In particular stress corrosion cracking of weld joints in pressurized vessel and permeation of hydrogen through nickel were observed.

Whittenberger, J. D.↗

NASA NTRS1996

Microalloying Improves the Low-Cycle Fatigue Behavior of Powder-Extruded NiAl

There is considerable interest in developing new structural materials in which high use temperatures and strength, coupled with low density, are the minimum requirements. The goal for these new materials is to provide operation well beyond the working range of conventional superalloys. Of the many intermetallics under consideration, NiAl is one of the few systems that has emerged as a promising candidate for further development. This is because of a number of property advantages--including low density, high melting temperature, high thermal conductivity, and excellent environmental resistance. However, binary NiAl lacks strength and creep resistance at elevated temperatures. Also, its poor high-temperature strength results in worse-than-predicted low-cycle fatigue (LCF) lives at low strain ranges at 727 C (1341 F) because of accelerated creep damage mechanisms that result in significant intergranular cracking. One approach for improving these properties involves microalloying NiAl with either Zr or N. As an integral part of this alloy-development program at the NASA Lewis Research Center, the low-cycle fatigue behavior of Zr- and N-doped nickel aluminides produced by extrusion of prealloyed powders was investigated and compared with similarly processed binary NiAl.

Source record↗

NASA NTRS2007

Validation Test Results for Orthogonal Probe Eddy Current Thruster Inspection System

Recent nondestructive evaluation efforts within NASA have focused on an inspection system for the detection of intergranular cracking originating in the relief radius of Primary Reaction Control System (PCRS) Thrusters. Of particular concern is deep cracking in this area which could lead to combustion leakage in the event of through wall cracking from the relief radius into an acoustic cavity of the combustion chamber. In order to reliably detect such defects while ensuring minimal false positives during inspection, the Orthogonal Probe Eddy Current (OPEC) system has been developed and an extensive validation study performed. This report describes the validation procedure, sample set, and inspection results as well as comparing validation flaws with the response from naturally occuring damage.

Wincheski, Russell A.↗

NASA NTRS2007

Dynamics of Nanoscale Grain-Boundary Decohesion in Aluminum by Molecular-Dynamics Simulation

The dynamics and energetics of intergranular crack growth along a flat grain boundary in aluminum is studied by a molecular-dynamics simulation model for crack propagation under steady-state conditions. Using the ability of the molecular-dynamics simulation to identify atoms involved in different atomistic mechanisms, it was possible to identify the energy contribution of different processes taking place during crack growth. The energy contributions were divided as: elastic energy, defined as the potential energy of the atoms in fcc crystallographic state; and plastically stored energy, the energy of stacking faults and twin boundaries; grain-boundary and surface energy. In addition, monitoring the amount of heat exchange with the molecular-dynamics thermostat gives the energy dissipated as heat in the system. The energetic analysis indicates that the majority of energy in a fast growing crack is dissipated as heat. This dissipation increases linearly at low speed, and faster than linear at speeds approaching 1/3 the Rayleigh wave speed when the crack tip becomes dynamically unstable producing periodic dislocation bursts until the crack is blunted.

Yamakov, V.↗

NASA NTRS1968

Study of crack initiation phenomena associated with stress corrosion of aluminum alloys

Study of stress corrosion cracks in aluminum alloys reveals that crack initiation is greatly influenced by boundary orientation and directionality of the structure. In all crack susceptible materials, intergranular corrosion and stress corrosion cracking started and progressed in boundaries oriented perpendicularly to the stressing direction.

Hunter, M. S.↗

NASA NTRS2012

Oxidation and the Effects of High Temperature Exposures on Notched Fatigue Life of an Advanced Powder Metallurgy Disk Superalloy

Oxidation and the effects of high temperature exposures on notched fatigue life were considered for a powder metallurgy processed supersolvus heat-treated ME3 disk superalloy. The isothermal static oxidation response at 704 C, 760 C, and 815 C was consistent with other chromia forming nickel-based superalloys: a TiO2-Cr2O3 external oxide formed with a branched Al2O3 internal subscale that extended into a recrystallized - dissolution layer. These surface changes can potentially impact disk durability, making layer growth rates important. Growth of the external scales and dissolution layers followed a cubic rate law, while Al2O3 subscales followed a parabolic rate law. Cr- rich M23C6 carbides at the grain boundaries dissolved to help sustain Cr2O3 growth to depths about 12 times thicker than the scale. The effect of prior exposures was examined through notched low cycle fatigue tests performed to failure in air at 704 C. Prior exposures led to pronounced debits of up to 99 % in fatigue life, where fatigue life decreased inversely with exposure time. Exposures that produced roughly equivalent 1 m thick external scales at the various isotherms showed statistically equivalent fatigue lives, establishing that surface damage drives fatigue debit, not exposure temperature. Fractographic evaluation indicated the failure mode for the pre-exposed specimens involved surface crack initiations that shifted with exposure from predominately single intergranular initiations with transgranular propagation to multi-initiations from the cracked external oxide with intergranular propagation. Weakened grain boundaries at the surface resulting from the M23C6 carbide dissolution are partially responsible for the intergranular cracking. Removing the scale and subscale while leaving a layer where M23C6 carbides were dissolved did not lead to a significant fatigue life improvement, however, also removing the M23C6 carbide dissolution layer led to nearly full recovery of life, with a transgranular initiation typical to that observed in unexposed specimens.

Sudbrack, Chantal K.↗

NASA NTRS1975

Fractography of the high temperature hydrogen attack of a medium carbon steel

Microscopic fracture processes were studied which are associated with hydrogen attack of a medium carbon steel in a well-controlled, high-temperature, high-purity hydrogen environment. Exposure to a hydrogen pressure and temperature of 3.5 MN/m2 and 575 C was found to degrade room temperature tensile properties with increasing exposure time. After 408 hr, yield and ultimate strengths were reduced by more than 40 percent and elongation was reduced to less than 2 percent. Initial fissure formation was found to be associated with manganese rich particles, most probably manganese oxide, aligned in the microstructure during the rolling operation. Fissure growth was found to be associated with a reduction in carbide content of the microstructure and was inhibited by the depletion of carbon. The interior surfaces of sectioned fissures or bubbles exhibit both primary and secondary cracking by intergranular separation. The grain surfaces were rough and rounded, suggesting a diffusion-associated separation process. Specimens that failed at room temperature after exposure to hydrogen were found to exhibit mixed mode fracture having varying amounts of intergranular separation, dimple formation, and cleavage, depending on exposure time.

Melson, H. G.↗

NASA NTRS1982

On relationship between microfissuring and microstructure in the HAZ of Inconel 718

Inconel 718, as well as many other metals, is susceptible to intergranular hot cracking in the weld heat-affected-zone (HAZ). These cracks form near the solidus temperature of the metal during the welding process. This problem is of particular concern to NASA/MSFC because the SSME is primarily constructed of welded Inconel 718. The present program studied microfissuring in the weld HAZ of Inconel 718 by simulating HAZ thermal cycles with the Gleeble machine. Previous researchers have studied the instantaneous mechanical properties of the HAZ using the Gleeble. The present study examines the instantaneous microstructure of the HAZ. This approach showed that second phase structures, high in niobium, melt and wet the grain boundaries in the HAZ during welding. It is postulated that the resultant HAZ grain boundaries, enriched in niobium, act as preferred sites for microfissure nucleation as the weld zone cools.

Thompson, R. G.↗

NASA NTRS1998

Creep of Uncoated and Cu-Cr Coated NARloy-Z

Stress rupture creep tests were performed on uncoated and Cu-30vol%Cr coated NARloy-Z copper alloy specimens exposed to air at 482 C to 704 C. The results showed that creep failure in air of unprotected NARloy-Z was precipitated by brittle intergranular surface cracking produced by strain assisted grain boundary oxidation (SAGBO) which in turn caused early onset of tertiary creep. For the protected specimens, the Cu-Cr coating remained adherent throughout the tests and was effective in slowing down the rate of oxygen absorption, particularly at the higher temperatures, by formation of a continuous chromium oxide scale. As the result of reducing oxygen ingress, the coating prevented SAGBO initiated early creep failure, extended creep deformation and increased the creep rupture life of NARloy-Z over the entire 482 C to 704 C test temperature range.

Walter, R. J.↗

NASA NTRS1984

Bending strength studies on hot-pressed silicon carbide

The 4-point bending strength of 4 grades of hot-pressed SiC was determined at different temperatures. With a transgranular mode of fracture the values for bending strength are retained up to high temperatures. For intergranular fracture the decrease of strength is governed by subcritical crack growth. The intergranular fracture is caused by a high content of silicate glassy phase at the grain boundaries of hot-pressed SiC.

Kriegesmann, J.↗

NASA NTRS2009

The Development of Directional Decohesion Finite Elements for Multiscale Failure Analysis of Metallic Polycrystals

Atomistic simulations of intergranular fracture have indicated that grain-scale crack growth in polycrystalline metals can be direction dependent. At these material length scales, the atomic environment greatly influences the nature of intergranular crack propagation, through either brittle or ductile mechanisms, that are a function of adjacent grain orientation and direction of crack propagation. Methods have been developed to obtain cohesive zone models (CZM) directly from molecular dynamics simulations. These CZMs may be incorporated into decohesion finite element formulations to simulate fracture at larger length scales. A new directional decohesion element is presented that calculates the direction of Mode I opening and incorporates a material criterion for dislocation emission based on the local crystallographic environment to automatically select the CZM that best represents crack growth. The simulation of fracture in 2-D and 3-D aluminum polycrystals is used to illustrate the effect of parameterized CZMs and the effectiveness of directional decohesion finite elements.

Saether, Erik↗

NASA NTRS1983

Fatigue crack initiation and propagation in several nickel-base superalloys at 650 C

The modes of crack initiation and propagation of several nickel-base superalloys have been examined after fatigue and creep-fatigue testing at 650 C. In fatigue, crack initiation was transgranular and frequently associated with porosity or inclusions in the higher strength alloys. These defects were usually located at the surface, except for tests at low strain ranges where larger, internal defects often initiated failure. Although fatigue crack initiation was transgranular, in those alloys with grain sizes of less than 15 microns, fatigue crack growth quickly became intergranular. This transition was environmentally assisted and did not occur for subsurface cracks until the crack broke through to the atmosphere. In the creep-fatigue cycle, which included a 900 s tensile dwell, crack initiation and propagation were both intergranular in all alloys.

Gayda, J.↗

NASA NTRS1986

Grain boundary oxidation and oxidation accelerated fatigue crack nucleation and propagation

Fatigue life at elevated temperatures is often shortened by oxidation. Grain boundary oxidation penetrates deeper than the surface oxidation. Therefore, grain boundary oxide penetration could be the primary cause of accelerated fatigue crack nucleation and propagation, and the shortened fatigue life at elevated temperatures. Grain boundary oxidation kinetics was studied and its statistical scatter was analyzed by the Weibull's distribution function. The effects of grain boundary oxidation on shortened fatigue life was analyzed and discussed. A model of intermittent microruptures of the grain boundary oxide was proposed for the fatigue crack growth in the low frequency region. The proposed model is consistent with the observations that fatigue crack growth rate in the low frequency region with hold time at K sub max is inversely proportional to cyclic frequency and that crack growth is intergranular.

Liu, H. W.↗

NASA NTRS1986

Axial and torsional fatigue behavior of Waspaloy

The cyclic flow response and crack growth behavior of Waspaloy at room temperature and 650 C under tensile loading and torsional loading was studied, for two conditions of Waspaloy: fine grain, large gamma prime size; coarse grain, small gamma prime size. The fine grain material showed 5 to 10 percent hardening after about 10 percent of life, with sequent softening to failure at both themperature levels. The coarse grain material showed either stable response or monotonic softening to failure. Early crack initiation was observed on planes of maximum shear, with eventual branching to principle planes under torsional loading; cracks were always normal to load axis under tensile loading. Also, crack paths were intergranular at 650 C, mostly transgranular at room temperature.

Zamrik, S.↗

NASA NTRS1986

Grain boundary oxidation and fatigue crack growth at elevated temperatures

Fatigue crack growth rate at elevated temperatures can be accelerated by grain boundary oxidation. Grain boundary oxidation kinetics and the statistical distribution of grain boundary oxide penetration depth were studied. At a constant delta K-level and at a constant test temperature, fatigue crack growth rate, da/dN, is a function of cyclic frequency, nu. A fatigue crack growth model of intermittent micro-ruptures of grain boundary oxide is constructed. The model is consistent with the experimental observations that, in the low frequency region, da/dN is inversely proportional to nu, and fatigue crack growth is intergranular.

Liu, H. W.↗

NASA NTRS1986

Grain boundary oxidation and fatigue crack growth at elevated temperatures

Liu, H. W.↗

NASA NTRS2017

Creep, Fatigue and Environmental Interactions and Their Effect on Crack Growth in Superalloys

Complex interactions of creep/fatigue/environment control dwell fatigue crack growth (DFCG) in superalloys. Crack tip stress relaxation during dwells significantly changes the crack driving force and influence DFCG. Linear Elastic Fracture Mechanics, Kmax, parameter unsuitable for correlating DFCG behavior due to extensive visco-plastic deformation. Magnitude of remaining crack tip axial stresses controls DFCG resistance due to the brittle-intergranular nature of the crack growth process. Proposed a new empirical parameter, Ksrf, which incorporates visco-plastic evolution of the magnitude of remaining crack tip stresses. Previous work performed at 704C, extend the work to 760C.

environmental interactions↗