Time-Dependent Fatigue Crack Propagation Behavior of Two Solid-Solution-Strengthened Ni-Based Superalloys—INCONEL 617 and HAYNES 230

The fatigue crack propagation (FCP) as well as the sustained loading crack growth (SLCG) behavior of two solid-solution-strengthened Ni-based superalloys, INCONEL 617 (Special Metals Corporation Family of Companies) and HAYNES 230 (Haynes International, Inc., Kokomo, IN), were studied at increased temperatures in laboratory air under a constant stress-intensity-factor (K) condition. The crack propagation tests were conducted using a baseline cyclic triangular waveform with a frequency of \frac13 \frac{1}{3} Hz. Various hold times were imposed at the maximum load of a fatigue cycle to study the hold time effect. The results show that a linear elastic fracture mechanics (LEFM) parameter, stress intensity factor (K), is sufficient to describe the FCP and SLCG behavior at the testing temperatures ranging from 873 K to 1073 K (600 °C to 800 °C). As observed in the precipitation-strengthened superalloys, both INCONEL 617 and HAYNES 230 exhibited the time-dependent FCP, steady SLCG behavior, and existence of a damage zone ahead of crack tip. A thermodynamic equation was adapted to correlate the SLCG rates to determine thermal activation energy. The fracture modes associated with crack propagation behavior were discussed, and the mechanism of time-dependent FCP as well as SLCG was identified. Compared with INCONEL 617, the lower crack propagation rates of HAYNES 230 under the time-dependent condition were ascribed to the different fracture mode and the presence of numerous W-rich M₆C-type and Cr-rich M₂₃C₆-type carbides. Toward the end, a phenomenological model was employed to correlate the FCP rates at cycle/time-dependent FCP domain. All the results suggest that an environmental factor, the stress assisted grain boundary oxygen embrittlement (SAGBOE) mechanism, is mainly responsible for the accelerated time-dependent FCP rates of INCONEL 617 and HAYNES 230.     

Material Interactions in a Novel Pinless Tool Approach to Friction Stir Spot Welding Thin Aluminum Sheet

The requirement for a probe, or pin, in friction stir spot welding (FSSW) leads to an undesirable keyhole and “hooking,” which can influence the fracture path and weld strength. Furthermore, the full weld cycle for FSSW is typically longer than ideal for the automotive industry, being 2 to 5 seconds. Here, it is shown that using a novel pinless tool design it is possible to achieve high lap shear strength (~3.4 kN) in thin aluminum sheet (~1 mm thick), with short weld cycle times (<1 second). Several techniques have been exploited to study the material flow and mechanisms of weld formation in pinless FSSW, including high-resolution X-ray tomography, to understand the role of the tool design and weld parameters. Despite the “simple” nature of a pinless tool, material flow in the weld zone was found to be surprisingly complex and strongly influenced by surface features on the tool, which greatly increased the penetration of the plastic zone into the bottom sheet. Because of the rapid thermal cycle and high level of grain refinement, the weld zone was found to develop a higher strength than the parent material with little evidence of a heat affected zone (HAZ) after postweld natural aging.     

Microstructural modification in full penetration and partial penetration electron beam welds in INCONEL-718 (IN-718) and its effect on fatigue crack initiation

Detailed microstructural analysis, as well as fatigue crack initiation evaluation, was carried out for electron beam (EB) welded IN-718. Fatigue test specimens were EB welded (full penetration) along their length, and a second weld pass, incorporating a slope-out from full to zero penetration along the gage length, was also applied. The specimens were fatigue tested at 523 °C and maximum stress (R=0) in the range 579 to 820 MPa. Early fatigue failure (<100,000 cycles at 0.25 Hz) was directly associated with the initiation at solidification porosity formed during “spiking” in the partial penetration weld metal at the start of the slope-out. The base metal, full penetration weld metal, and slope-out region were characterized using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM), which indicated that the microstructures of the base metal and full penetration weld metal should give good fatigue resistance. The rapid solidification of the full penetration weld metal gave an interdendritic terminal solidification product consisting of γ+NbC+Laves phase instead of the usually reported eutectic γ+Laves phase. Microstructural and chemical heterogeneities in the full penetration weld metal, combined with the sharp perturbations in penetration and solidification conditions (spiking) in the partial penetration weld metal, resulted in locally embrittled regions and interdendritic regions containing large numbers of fine pores as well as a higher volume fraction of mixed, hard interdendritic phases. These features would be consistent with a lower resistance to fatigue crack propagation in the partial penetration weld metal.     

The role of mechanical properties in low-stress fatigue crack propagation

An experimental investigation was undertaken to study the relationship between mechanical properties and low stress fatigue crack propagation. Attention was focused on the “fatigue” or “reversed plastic zone” at the crack tip, since it was felt that material properties in this region were of prime importance in the crack propagation process. An effort was made to simulate this region through fully reversed strain-cycling tests on tensile specimens. Mechanical properties obtained from a number of materials before and after strain cycling were correlated with crack propagation data from the same materials. Evidence indicated that while monotonic tensile properties are inadequate for correlation purposes, the cyclic strain-hardening coefficient, the cyclic yield strength, and the elastic modulus appear to be important parameters. This was felt to be an indication of the importance of strain cycling in the reversed plastic zone in influencing the rate-governing mechanisms in fatigue crack growth. Formerly Research Assistant, Department of Metallurgy and Materials Science, Lehigh University, Bethlehem, Pa.     

Global and Local Mechanical Properties and Microstructure of Friction Stir Welds with Dissimilar Materials and/or Thicknesses

This article studies the properties of a wide range of friction-stir-welded joints with dissimilar aluminum alloys or thicknesses. Two aluminum alloys, namely, 2024-T3 and 7075-T6, are selected for the study and are welded in ten different combinations of alloys and thicknesses. The welding parameters are optimized for each configuration, and a systematic study of the effects of material and thickness combinations on the microstructural features, global and local mechanical properties, and fracture mechanisms of the welds is carried out. It is shown that dissimilar alloys are extruded into each other, the texture is heterogeneous in the weld zone, and that there is no significant diffusion of alloying elements between the alloys. For most configurations, the local and global mechanical properties decrease as the thickness ratio increases. The local yield strength and plasticity parameters substantially vary next to the weld centerline, hence requiring their implementation in finite element method (FEM) models. Machining to obtain a constant thickness significantly influences the mechanical properties of the welds. The fracture mechanism is found to be a mixture of ductile and brittle fractures and to qualify as “quasi-cleavage.”     

Topologically close-packed phase precipitation and thermal stability in alloy 22

The thermal stability of Alloy 22 affected by the precipitation of topologically close-packed (TCP) phases was investigated by means of experimental measurements and thermodynamic calculations. Both mill-annealed and welded specimens were exposed to various thermal aging and solution-annealing treatments. Microstructural analyses showed progressive precipitation of TCP phases in both the mill-annealed and welded specimens upon aging at 870 °C. Solution annealing of the welded material results in homogenization of the fusion zone; however, a high annealing temperature of 1300 °C leads to undesirable grain growth. All aging and solutionizing treatments of the welded material enhance precipitation of the secondary phases. Thermodynamic calculations predicted a solvus temperature of 1271 °C for the TCP phase in the interdendritic region as a result of Mo segregation during the solidification of the welded metal. This model prediction is consistent with experimental results showing that precipitates are observed in the welded material after various solution-annealing treatments. Results obtained from the present study suggest that solution-annealing treatments for Alloy 22 disposal containers should be carefully evaluated to assure that a homogeneous single-phase structure will be obtained. This article is based on a presentation made in the symposium “Effect of Processing on Materials Properties for Nuclear Waste Disposition,” November 10–11, 2003, at the TMS Fall meeting in Chicago, Illinois, under the joint auspices of the TMS Corrosion and Environmental Effects and Nuclear Materials Committees.     

Understanding the potential and pH dependency of high-strength β-titanium alloy environmental crack initiation

An explanation for the strong dependency of crack initiation of precracked high-strength β-titanium alloys in room-temperature 0.6 M NaCl on applied potential and bulk-solution pH is presented. It is proposed that environment-assisted cracking (EAC) susceptibility in neutral aqueous NaCl results from (1) film rupture due to plastic deformation at actively deformed crack tips, (2) accelerated dissolution of titanium, (3) crack tip acidification by hydrolysis of titanium ions, (4) crack tip potential excursions toward bare metal open-circuit potentials (OCPs) during film rupture due to large ohmic voltages in the crack solution, (5) accelerated crack tip proton or water reduction concurrent with titanium dissolution, (6) bare surface-dominated hydrogen ingress into a fracture process zone, and (7) crack initiation by hydrogen embrittlement. Evidence for each of the above stages of the crack initiation scenario is presented, with emphasis on crack tip electrode kinetics and ohmic voltage calculations which govern process zone-controlled hydrogen uptake. The seven stages are consistent with the strong dependencies of crack initiation and growth in precracked high-strength β-titanium alloys on (1) solution pH, (2) applied potential, and (3) strain rate, and they explain the “apparent” EAC resistance of smooth- and blunt-notch specimens. The latter lack both occluded crack tip geometries to promote acidification and ohmic voltage drops below reversible hydrogen, as well as localization of dynamic plastic strain. Hydrogen uptake is, subsequently, limited.     

Effect of weld heat input and creep strain on the elevated temperature and crack growth properties of austenitic steels

A potential material class for use at 600°C and more, e.g. for steam turbines with improved thermal efficiency, are austenitic steels. Using these steels with welded joints, it is to be considered that, by superposition of weld residual stresses and service stresses, extensive creep strains – and in the worst case crack formation – can occur locally. To assess the influence of these effects on service behaviour, different material states of CrNi-steels and Incoloy 800 were investigated with respect to strength, ductility and, especially, to crack and creep crack growth in the temperature range around 600°C. It is shown that creep embrittlement, not microstructural changes as effected by weld heat input, causes heat affected zone (HAZ)-reheat cracking. Creep embrittlement can be avoided by special design and fabrication rules.     

Plastic flow and fracture of metallic glass

The tensile flow and fracture behavior of three Pdo.₈Sio2-based alloys in the glassy, “microcrystalline,” and fully crystalline condition has been studied. The glassy alloys flow plastically to a total strain of approximately 0.5 pct e, and exhibit proportional limit stresses of approximatelyE x 10~² whereE is Young’s modulus. This plastic flow is accompanied by the formation of shear deformation bands on the specimen surfaces. Fully crystalline alloys are extremely brittle and fracture via intergranular cracking. Fracture surfaces of the amorphous and “microcrystalline” alloys are inclined at 45 deg to the tensile axis and exhibit two morphologically distinct zones. One zone is relatively featureless while the other contains a “river” pattern of local necking protrusions. Detailed comparison of opposing surfaces indicates that fracture is preceded by large local plastic shear which produces the smooth zone while the local necking pattern is produced during rupture. These observations form the basis for the hypothesis that plastic flow in the glassy material occurs via localized strain concentrations and that fracture is initiated by catastrophic, “adiabatic” shear. Formerly Postdoctoral Associate, Yeshiva University, New York, N. Y.     

Fracture in equiaxed two phase alloys: part ii. fracture in alloys with isolated plastic particles

A model, describing fracture of two phase equiaxed alloys containing isolated plastic particles within an elastically deforming matrix, has been developed. The model relates fracture toughness parameters to the microstructure and mechanical characteristics of the individual phases. The model utilizes the concepts of a process zone and crack closing forces in the process zone along with recent developments in the fracture mechanics of toughened ceramics. One adjustable parameter, either the extent of the process zone or the effective “gauge length” of plastic particles within the process zone, is used in the analysis. The values of these parameters, as deduced from experimental fracture mechanics studies in Co-CoAl alloys, are reasonable in their magnitude and depend on alloy microstructure in the manner predicted from the analysis. M. A. PRZYSTUPA, formerly Graduate Research Assistant, Department of Metallurgical Engineering, Michigan Technological University     

Fracture behaviours of fracture toughness testing specimens with metallurgical heterogeneity along crack front

Safe use of welded structures is dependent on fracture mechanics properties of welded joints. In present research, high strength low alloyed HSLA steel in a quenched and tempered condition, corresponding to the grade HT 80, was used. The fluxo cored arc welding process (FCAW), with CO₂ as shielding gas, was used and two different tubular wires were selected. The aim of this paper is to analyse fracture behaviour of undermatched welded joints, and also to determine relevant parameters which contribute to higher critical values of fracture toughness. Towards this end three differently undermatched welded joints were analysed using results of testing the composite notched specimens with through thickness crack front positioned partly in the weld metal, partly in heat affected zone (HAZ) and partly in base material (BM).The presence of different microstructures along the pre‐crack fatigue front has an important effect on the critical crack tip opening displacement (CTOD). This value is the relevant parameter for safe service of welded structure. In the case of specimens with through thickness notch partly in the weld metal, partly in the heat affected zone and partly in the base material, i.e. using the composite notched specimen, fracture behaviour strongly depends on a partition of ductile base material, size and distribution of mismatching factor along vicinity of crack front. If local brittle zones occur in the process zone, ductile base metal can not prevent pop‐in instability, but it can reduce it to an insignificant level while the fracture toughness parameter is higher and the weakest link concept can not be applied.     

An investigation of the fatigue and fracture behavior of a Nb-12Al-44Ti-1.5Mo intermetallic alloy

This article presents the results of a study of the fatigue and fracture behavior of a damage-tolerant Nb-12Al-44Ti-1.5Mo alloy. This partially ordered B2 + orthorhombic intermetallic alloy is shown to have attractive combinations of room-temperature ductility (11 to 14 pct), fracture toughness (60 to 92 MPa√m), and comparable fatigue crack growth resistance to IN718, Ti-6Al-4V, and pure Nb at room temperature. The studies show that tensile deformation in the Nb-12Al-44Ti-1.5Mo alloy involves localized plastic deformation (microplasticity via slip-band formation) which initiates at stress levels that are significantly below the uniaxial yield stress (∼9.6 pct of the 0.2 pct offset yield strength (YS)). The onset of bulk yielding is shown to correspond to the spread of microplasticity completely across the gage sections of the tensile specimen. Fatigue crack initiation is also postulated to occur by the accumulation of microplasticity (coarsening of slip bands). Subsequent fatigue crack growth then occurs by the “unzipping” of cracks along slip bands that form ahead of the dominant crack tip. The proposed mechanism of fatigue crack growth is analogous to the unzipping crack growth mechanism that was suggested originally by Neumann for crack growth in single-crystal copper. Slower near-threshold fatigue crack growth rates at 750 °C are attributed to the shielding effects of oxide-induced crack closure. The fatigue and fracture behavior are also compared to those of pure Nb and emerging high-temperature niobium-based intermetallics.     

mechanisms of fatigue crack retardation following single

In ingot metallurgy (IM) alloys, the number of delay cycles following a single tensile overload typically increases from a minimum at an intermediate baseline stress intensity range, ΔK_B, with decreasing ΔK_B approaching threshold and increasing ΔK_B approaching unstable fracture to produce a characteristic “U”-shaped curve. Two models have been proposed to explain this behavior. One model is based on the interaction between roughness and plasticity-induced closure, while the other model only utilizes plasticity-induced closure. This article examines these models using experimental results from constant amplitude and single overload fatigue tests performed on two powder metallurgy (PM) aluminum alloys, AL-905XL and A A 8009. The results indicate that the “U”-shaped curve is primarily due to plasticity-induced closure, and that the plasticity-induced retardation effect is through-thickness in nature, occurring in both the surface and interior regions. However, the retardation effect is greater at the surface, because the increase in plastic strain at the crack tip and overload plastic zone size are larger in the plane-stress surface regions than in the plane-strain interior regions. These results are not entirely consistent with either of the proposed models.     

The relationship between microstructure and fracture behavior of fully austenitic type 316L weldments at 4.2 K

Multipass weld deposits produced with a Mn-modified Type 316L filler material exhibited fracture toughness nearly 100 MPa√m less than that of a conventional 316L filler material when tested at 4.2 K. Although fracture in both materials occurred by ductile rupture, the crack path in the Mn-modified weld metal was microstructure-specific. The resultant fracture surface exhibited a “corduroy” morphology which reflected the underlying solidification pattern. Corresponding fracture surfaces in compact tension and tensile specimens from the standard 316L weld deposits showed little tendency for microstructure-specific fracture. A model is proposed which relates the fracture morphology and fracture toughness to the microstructural stability of the austenite during testing at 4.2 K. Partitioning of manganese and molybdenum to cellular dendritic boundaries during weld solidification tends to stabilize the austenite and suppress martensite formation in these regions. As a result, fracture occurs preferentially along these boundaries in the Mn-modified weld deposits, giving rise to the “corduroy” fracture morphology and providing less resistance to fracture than in weld deposits where martensite formation is more homogeneous. Formerly with Sandia National Laboratories, Livermore, CA 94550. Formerly of Lawrence Livermore National Laboratory.     

Changes in Precipitate Distributions and the Microstructural Evolution of P24/P91 Dissimilar Metal Welds During PWHT

The effect of post-weld heat treatments (PWHTs) on the evolution of precipitate phases in dissimilar metal welds made between 9 pct Cr P91 alloy and 2.25 pct Cr T/P24-type weld metal has been investigated. Sections of multi-pass fusion welds were analyzed in their as welded condition and after PWHTs of 2 and 8 hour duration at 1003 K (730 °C). Thin foil specimens and carbon extraction replicas have been examined in transmission electron microscopes in order to identify precipitate phases and substantiate their distributions in close proximity to the fusion line. The findings of these studies confirm that a carbon-depleted region develops in the lower alloyed weld material, adjacent to the weld interface, during thermal processing. A corresponding carbon enriched region is formed, simultaneously, in the coarse grain heat affected zone of the P91 parent alloy. It has been demonstrated that carbon depletion from the weld alloy results in the dissolution of M₇C₃ and M₂₃C₆ chromium carbides. However, micro-alloying additions of vanadium and niobium which are made to both the P24 and P91 alloys facilitate the precipitation of stable, nano-scale, MX carbonitride particles. This work demonstrates that these particles, which are of key importance to the strength of ferritic creep resistant alloys, are retained in carbon-depleted regions. The microstructural stability which is conferred by their retention means that the pernicious effects of recrystallization are largely avoided.     

Fatigue crack growth mechanisms at the microstructure scale in Al-Si-Mg cast alloys: Mechanisms in regions II and III

The fatigue crack growth behavior in Regions II and III of crack growth was investigated for hypoeutectic and eutectic Al-Si-Mg cast alloys. To isolate and establish the mechanistic contributions of characteristic microstructural features (dendritic α-Al matrix, eutectic phases, Mg-Si strengthening precipitates), alloys with various Si content/morphology, grain size level, and matrix strength were studied; the effect of secondary dendrite arm spacing (SDAS) was also assessed. In Regions II and III of crack growth, the observed changes in the fracture surface appearance were associated with changes in crack growth mechanisms at the microstructural scale (from a linear advance predominantly through primary α-Al to a tortuous advance exclusively through Al-Si eutectic Regions). The extent of the plastic zone ahead of the crack tip was successfully used to explain the changes in growth mechanisms. The fatigue crack growth tests were conducted on compact tension specimens under constant stress ratio,R=0.1, in ambient conditions.     

Oligodendrocytes from forebrain are highly vulnerable to AMPA/kainate receptor-mediated excitotoxicity

STATEMENT OF PROBLEM: Titanium and its alloys are more commonly used in prosthodontics and welding has become the most common modality for their joining. Studies on the welding of titanium and its alloys have not quantified this value, though its importance has been suggested. PURPOSE: This study compared the strength and properties of the joint achieved at various butt joint gaps by the arc-welding of cast Ti-6Al-4V alloy tensile bars in an argon atmosphere. MATERIAL AND METHODS: Forty of 50 specimens were sectioned and welded at four gaps. All specimens underwent tensile testing to determine ultimate tensile strength and percentage elongation, then oxygen analysis and scanning electron microscopy. RESULTS: As no more than 3 samples in any group of 10 actually fractured in the weld itself, a secondary analysis that involved fracture location was initiated. There were no differences in ultimate tensile strength or percentage elongation between specimens with weld gaps of 0.25, 0.50, 0.75, and 1.00 mm and the as-cast specimens. There were no differences in ultimate tensile strength between specimens fracturing in the weld and those fracturing in the gauge in welded specimens; however, as-cast specimens demonstrated a higher ultimate tensile strength than welded specimens that fractured in the weld. Specimens that fractured in the weld site demonstrated less ductility than those that fractured in the gauge in both welded and as-cast specimens, as confirmed by scanning electron microscopy examination. The weld wire showed an oxygen scavenging effect from the as-cast parent alloy. CONCLUSIONS: The effects of the joint gap were not significant, whereas the characteristics of the joint itself were, which displayed slightly lower strength and significantly lower ductility (and thus decreased toughness). The arc-welding of cast titanium alloy in argon atmosphere appears to be a reliable and efficient prosthodontic laboratory modality producing predictable results, although titanium casting and joining procedures must be closely controlled to minimize heat effects and oxygen contamination.     

Effect of thermomechanical processing on fatigue crack propagation

The effect of thermomechanical processing on fatigue crack propagation (FCP) is examined for 70/30 brass and 305 stainless steel. It is found that grain size and cold work induced changes in yield strength, ductility, and preferred orientation have a minor effect on FCP. Rather, cyclically stabilized properties of material in the crack tip plastic zone are believed to control the FCP process. Although mechanical processing fails to significantly alter the rate of FCP, it is apparently responsible for the unique fracture path observed in specimens oriented at an angle(A) to the rolling direction. Deviation of the crack path out of the plane of maximum net section stress is believed to be associated with mechanical fibering andJor crystallographic texturing effects. The complex fracture mode transition observed in cold worked 70/30 brass also is associated with the deformation texture of the starting material. For the cold-worked 305 stainless steel, striation spacings are correlated with the stress intensity range for specimens tested in the longitudinal, transverse, and “angle” orientations. Comparison of these data with corresponding macroscopic data indicate that an approximately one-to-one correspondence exists between macroscopic and microscopic fatigue crack growth rates over the range investigated.