Modeling thermomechanical fatigue life of high-temperature titanium alloy IMI 834

A microcrack propagation model was developed to predict thermomechanical fatigue (TMF) life of high-temperature titanium alloy IMI 834 from isothermal data. Pure fatigue damage, which is assumed to evolve independent of time, is correlated using the cyclic J integral. For test temperatures exceeding about 600 °C, oxygen-induced embrittlement of the material ahead of the advancing crack tip is the dominating environmental effect. To model the contribution of this damage mechanism to fatigue crack growth, extensive use of metallographic measurements was made. Comparisons between stress-free annealed samples and fatigued specimens revealed that oxygen uptake is strongly enhanced by cyclic plastic straining. In fatigue tests with a temperature below about 500 °C, the contribution of oxidation was found to be negligible, and the detrimental environmental effect was attributed to the reaction of water vapor with freshly exposed material at the crack tip. Both environmental degradation mechanisms contributed to damage evolution only in out-of-phase TMF tests, and thus, this loading mode is most detrimental. Electron microscopy revealed that cyclic stress-strain response and crack initiation mechanisms are affected by the change from planar dislocation slip to a more wavy type as test temperature is increased. The predictive capabilities of the model are shown to result from the close correlation with the microstructural observations.     

A study of the mechanisms of fatigue life improvement in an ion implanted nickel-chromium alloy

Wires of the alloy Ni20Cr with and without carbon ion implantation have been tested in tension-tension fatigue. A 17% increase in endurance limit was found with implantation. The fatigued surface was examined by SEM, and the wire then nickel plated so that transverse sections could be made for TEM study. It was found that bulk slip was unaffected by implantation but slip in surface grains was unable to penetrate the implanted layer to significant degree. Slip band cracking which was found in unimplanted specimens was replaced by grain boundary cracking in the implanted specimens.     

Stray Grain Formation in Welds of Single-Crystal Ni-Base Superalloy CMSX-4

Autogenous welds on the single-crystal (SX) alloy CMSX-4 were prepared over a wide range of welding parameters and processes to investigate the formation and behavior of stray grains (SGs). The quantity and location of SGs in the welds were analyzed by orientation imaging microscopy (OIM). Heat- and fluid-flow modeling was conducted to understand the influence of welding parameters on the local solidification conditions and resultant SG formation tendency. The results indicate that constitutional supercooling and SG formation are generally reduced in low-power, high-travel-speed welds. Because of the complex effect of travel speed on temperature gradient and solidification velocity, the worst conditions for SG formation in alloy CMSX-4 for the conditions examined here occur at intermediate travel speeds of ~6 mm/s. These findings were corroborated with heat-transfer/fluid-flow modeling simulations that were coupled with SG predictions. These calculations also indicate that SG formation will be greatest where different regions of dendrite growth intersect, due to the so-called off-axis heat flow. For a given set of welding conditions, the amount of SGs will also vary with substrate orientation. This effect is attributed to differences in the number and location of dendrite growth intersection regions within the melt pool that occur with changes in substrate orientation.     

Quantifying Equiaxed vs Epitaxial Solidification in Laser Melting of CMSX-4 Single Crystal Superalloy

Metallurgical and Materials Transactions A - The competition between epitaxial vs. equiaxed solidification has been investigated in CMSX-4 single crystal superalloy during laser melting as...     

The Geometrical Effect on Freckle Formation in the Directionally Solidified Superalloy CMSX-4

In order to better understand the geometrical effect on freckle formation in superalloys, samples with uniform increases and decreases in their cross sections were directionally solidified in a Bridgman furnace. In comparison to our conventional knowledge, some new features of freckle appearance have been observed. Freckles could occur at sloped surfaces where the freckle pattern is no longer roughly parallel to the direction of gravity but has the same slope as the surface. At significantly angled surfaces, the freckles appear as discrete flakes, having the shape of tree roots, instead of the long and narrow chains which are usually observed. The component portions having inward sloping surfaces are very freckle prone while those with outward sloping surface are mostly freckle free, although the completely opposite freckling tendency is indicated by the simulated solidification condition. Therefore, as an independent factor the geometrical feature of the components can more effectively affect the freckle formation than the local thermal conditions.     

Control of residual stresses affecting fatigue life of pulsed current gas-metal-arc weld of high-strength aluminum alloy

The influence of pulse parameters on residual stresses of the gas-metal-arc (GMA) weld of a 10-mm-thick extruded section of high-strength Al-Zn-Mg alloy has been analyzed. The role of pulse parameters affecting the residual stresses of the weld joint has been studied by considering a summarized influence of pulse parameters defined by a dimensionless factor ϕ=[(I _b /I _p ) ft _b ]. The reason for the variation in residual stresses of the weld joint with a change in ϕ under different mean currents (I _m ) has been studied by correlating the extent of weld metal deposition and weld size with the ϕ. It is observed that the increase of ϕ reduces the longitudinal and transverse stresses of the weld joint. The nature of variation in residual stresses of the weld joint with ϕ shows an agreement to the trend of variation in its size with ϕ. In conformation of an earlier work, it is proposed that the use of a pulsed current gas-metal-arc welding (GMAW) at proper pulse parameters giving desired ϕ may produce a weld joint having comparatively lower residual stresses with improved fatigue life than that of the weld joint produced by conventional GMAW process through its influence on weld size.     

Near-threshold fatigue crack growth in 8090 Al-Li alloy

Near-threshold fatigue crack growth was studied in 8090-T8771 Al-Li alloy tested in moist laboratory air. The testing was conducted using (1) the ASTM E-647 load-shedding procedure, (2) a power-law load-shedding procedure, and (3) a constant-amplitude (CA) loading procedure. Crack closure in the three procedures was analyzed. In reconciling fatigue crack growth rates (FCGRs) with different crack closure levels under identical testing parameters, the conventional ΔK _eff (=K _max —K _op) fails to correlate the test data and the modified ΔK eff (=K _max - _χK_op, where χ is the shielding factor, defined by an energy approach) is proven to be the true crack driving force. A parallel slip-rupture model is proposed to describe the mechanism of near-threshold fatigue crack growth in this alloy. The model explains the mode transition from crystallographic slip band cracking (SBC) to subgrain boundary cracking (SGC)/brittle fracture (BF) in terms of a microstructure-environment synergy. The transition is related to the material’s short-transverse grain size.     

Damage evolution during thermal fatigue in fiber-reinforced light-metal-matrix composites

Thermal cycling of a composite material creates thermal stresses in the composite because of thermal-expansion mismatch between the fiber and the matrix. These stresses can be quite large in the case of ceramic-fiber-reinforced light-metal-matrix composites, leading to plastic deformation of the metal matrix. Other modes of damage include an increased reaction zone at the interface, cavitation at the interface, and, finally, fiber fracture. Mechanical cyclic loading of the composite concomitant with thermal cycling can aggravate the situation even more. Large reductions in the strength of the composite were observed under thermal-fatigue conditions. This degradation was due to the microstructural damage of the fiber/matrix interface and fiber fracture. Under conditions of cavitation at the interface, it is possible to use quantitative damage parameters based upon loss in modulus or change in density to evaluate the damage.     

Mechanisms of high-temperature fatigue failure in alloy 800H

The damage mechanisms influencing the axial strain-controlled low-cycle fatigue (LCF) behavior of alloy 800H at 850 °C have been evaluated under conditions of equal tension/compression ramp rates (fast-fast (F-F): 4 × 10⁻³ s⁻¹ and slow-slow (S-S): 4 × 10⁻⁵ s⁻¹) and asymmetrical ramp rates (fast-slow (F-S): 4 × 10⁻³ s⁻¹ / 4 × 10⁻⁵ s⁻¹ and slow-fast (S-F): 4 × 10⁻⁵ / 4 × 10⁻³ s⁻¹) in tension and compression. The fatigue life, cyclic stress response, and fracture modes were significantly influenced by the waveform shape. The fatigue lives displayed by different loading conditions were in the following order: F-F > S-S > F-S > S-F. The fracture mode was dictated by the ramp rate adopted in the tensile direction. The fast ramp rate in the tensile direction led to the occurrence of transgranular crack initiation and propagation, whereas the slow ramp rate caused intergranular initiation and propagation. The time-dependent processes and their synergistic interactions, which were at the basis of observed changes in cyclic stress response and fatigue life, were identified. Oxidation, creep damage, dynamic strain aging, massive carbide precipitation, time-dependent creep deformation, and deformation ratcheting were among the several factors influencing cyclic life. Irrespective of the loading condition, the largest effect on life was exerted by oxidation processes. Deformation ratcheting had its greatest influence on life under asymmetrical loading conditions. Creep damage accumulated the greatest amount during the slow tensile ramp under S-F conditions. Formerly USA National Research Council Associate, NASA-Lewis Research Center This article is based on a presentation made at the “High Temperature Fracture Mechanisms in Advanced Materials” symposium as a part of the 1994 Fall meeting of T.S., October 2-6, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow and Fracture Committee.     

Mode III fatigue crack propagation in low alloy steel

To provide a basis for estimating fatigue life in large rotating generator shafts subjected to transient oscillations, a study is made of fatigue crack propagation in Mode III (anti-plane shear) in torsionally-loaded spheroidized AISI4340 steel, and results compared to analogous behavior in Mode I. Torsional S/N curves, determined on smooth bars containing surface defects, showed results surprisingly close to expected unnotched Mode I data, with lifetime increasing from 10⁴ cycles at nominal yield to 10⁶ cycles at half yield. Fatigue crack growth rates in Mode III, measured on circumferentially-notched samples, were found to be slower than in Mode I, although still power-law related to the alternating stress intensity(△K _III) for small-scale yielding. Mode III growth rates were only a small fraction (0.002 to 0.0005) of cyclic crack tip displacements(△CTD _III) per cycle, in contrast to Mode I where the fraction was much larger (0.1 to 0.01). A micromechanical model for Mode III growth is proposed, where crack advance is considered to take place by a Mode II coalescence of cracks, initiated at inclusions ahead of the main crack front. This mechanism is consistent with the crack increment being a small fraction of △CTD_III per cycle. Formerly with Massachusetts Institute of Technology, Cambridge, MA Formerly with M.I. T.     

Slow crystallographic fatigue crack growth in a nickel-base alloy

Fatigue crack propagation rates at very low cyclic stress intensity levels (1 to 3 MNm^-372) have been measured in cube-oriented, planar slip nickel-base superalloy monocrystals using a high frequency (20 kHz) resonant fatigue testing technique. It is found that crack propagation is entirely along the crystallographic slip planes and the crack growth rate does not drop off into a threshold behavior but follows a power law with a power law exponent close to 4, which is similar to the functional dependency observed at higher cyclic stress intensity levels in similar superalloys. The observed behaviors are discussed with respect to a new theory on threshold and the effects of strong crystallographic constraints on crack propagation behavior.     

The influence of the coating technique on the high cycle fatigue life of alumina coated Al 6061 alloy

The primary objective of this study is to evaluate the influence of coating technique on the high cycle fatigue of an Al6061 alloy. Towards this purpose, Al6061 alloy fatigue samples have been coated with Al₂O₃ utilising the detonation spray, air plasma spray, micro arc oxidation and hard anodizing techniques. The high cycle fatigue life of these coated samples has been evaluated over a range of alternating stress values and compared with the fatigue life of the uncoated Al6061 alloy. It is observed that the detonation spray coated sample exhibits a higher fatigue life than the uncoated sample. In contrast, the samples coated using the other techniques exhibit poorer fatigue life compared to the uncoated sample especially at lower alternating stress values. These results have been explained on the basis of the nature of the coating-substrate interface which is strongly determined by the coating technique used to deposit the Al₂O₃ coatings.     

The importance of surface layer on fatigue behavior of a Ti- 6AI- 4V alloy

The effects of precycling and surface removal on the fatigue life and fatigue limit of a Ti-6A1-4V alloy were investigated. It was shown that both the fatigue life and fatigue limit were strongly dependent on the severity of precycling. The fatigue limit lost its significance if the alloy was subjected to a precycling treatment with a high stress amplitude. Cycling with stress amplitude below the fatigue limit after precycling showed a dependence of the logarithmic number of cycles to failure on the fraction of prefatigue damage. The interdependence of fatigue life and fatigue limit to precycling history was attributed to microcrack formation, principally restricted to a surface layer of less than 100 μm. Depending on the severity of precycling and on the magnitude of the applied cyclic stress, the fatigue damage could be either partially or totally eliminated by surface removal. The α/β interphase region of the surface layer appeared to offer preferred sites for dislocation pile-ups and crack initiation.     

应力比对TC4钛合金超高周疲劳失效机理的影响

采用频率为100 Hz的电磁谐振疲劳试验机进行疲劳拉伸试验, 研究了两种应力比(R=0. 1和-1) 对TC4钛合金的超高周疲劳失效机理的影响.结果表明, 两种应力比下的S-N曲线都呈现"双线"型, 但各自表示的意义及失效机理不同.当R=0. 1时, TC4钛合金的疲劳失效形式有两种, 即由加工缺陷诱发的表面失效和内部鱼眼失效, 这两种失效形式都伴随着颗粒平面(Facet) 出现; 而当R=-1时, 仅存在表面失效, 且无Facet的出现.基于断裂力学的讨论可知, 在正应力比及真空环境下, 对应小裂纹扩展的门槛值更低, 更有利于裂纹扩展及Facet的形成. TC4钛合金的整个内部疲劳失效过程及机理可解释为: (1) 滑移线或滑移带在部分α晶粒上的出现; (2) 微裂纹的萌生和接合; (3) 颗粒亮区(GBF) 的形成; (4) 鱼眼的形成; (5) 鱼眼外的失稳裂纹扩展; (6) 最终的瞬时断裂.      

Observations on microstructurally sensitive fatigue crack growth in a widmanstätten Ti-6Al-4V alloy

Fatigue crack growth rates in commercial purity Ti-6A1-4V can be substantially reduced with a beta anneal from levels associated with the mill anneal, owing primarily to crystallographic crack bifurcation in the Widmanstätten packets. This microstructurally sensitive fatigue crack growth occurs when the reversed plastic zone is less than the packet size and results in a fracture surface with a faceted morphology. It appears from replica and scanning electron microscopic examination that the facets are comprised of three superposed features,viz cleavage-like river-line patterns, very fine striations and traces of slip lines (and slip-band cracks). Limited X-ray evidence suggests a facet orientation some 8 to 10 degrees off the basal plane.