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Microstructure evolution from center to edge of the as-forged and heat treated Inconel-718 disks was investigated. Specifically, the evolution of primary carbides, grain size, γ ″, γ ′, δ , and secondary carbide particles was the focus of the current study. In fact, characterization of these microstructure features is essential for models predicting the creep and fatigue lives of the alloy. Accurate and reliable revealing of the grain boundaries in as-forged and heat treated Inconel-718 was made possible in this study by development of a new method. From microstructure investigations, nonuniformities in grain size, volume fraction, size and inter particle spacing of precipitates from center to edge were observed in both as-forged and heat treated disks. The microstructure nonuniformities resulted in significant variation in hardness from center to edge of the disks.

―Conventionally, the fatigue threshold of a long-crack is obtained by load shedding using a constant normalized K-gradient, as recommended by ASTM E-647. However, this load shedding procedure often causes the crack opening displacement to decrease with increasing crack length, which may trigger crack closure. In this study, load shedding tests were conducted in load control following a power-law load shedding schedule such that the crack opening displacement was kept at a relatively constant level. Using this new testing procedure, it is shown that crack closure is not always as high as that associated with the ASTM recommended procedure at a given AK. Comparisons of fatigue crack growth rates under identical testing parameters, but with different closure levels produced by the two load shedding procedures, have been made for several structural alloys. The extrinsic shielding of the crack tip zone via crack closure has also been examined using an energy approach for these alloys. On the basis of these analyses, the true effective stress intensity range is evaluated for fatigue crack propagation and the role of crack closure in creating a fatigue threshold is re-assessed.

An elastic-plastic crack model is developed for anisotropic crystalline materials exhibiting elastic-perfect plastic behavior, using the continuously distributed dislocation theory (CDDT). The crack and its associated plastic zone are represented by an inverted pile-up of dislocations. The plastic zone size, the crack-tip opening displacement (CTOD) and the energy release rate of the crack are obtained in closed forms, in a fashion similar to the Bilby–Cottrell–Swinden–Dugdale model for isotropic materials, but in terms of material constants that are orientation dependent. Application of the model to the case of small-scale yielding, e.g., long fatigue cracks in Ni-base single crystal superalloys, is discussed.

Laser gas nitriding of Ti-6A1-4V has been investigated with both CO2 and Nd:YAG lasers. Results indicate that Nd:YAG laser in pulse mode provides a better surface finish and a lower cracking severity than CO2 laser. A crack-free nitrided layer has been obtained by optimizing the processing parameters. Titanium nitride (TiN) significantly increases the hardness of the nitrided surfaces. The amount of titanium nitride produced depends on the processing parameters, such as laser pulse energy and nitrogen concentration. With optimized parameters, the nitrided surface is somewhat rougher than the polished base metal but much smoother than the shot peened surface. The shrinkage effect in the laser melt zone produces surface residual tensile stresses regardless of the processing environment. Preheating or stress relieving after laser nitriding can significantly reduce the residual tensile stress level.

The results from a comparative study of erosion resistance of various coatings and some material modification treatments for protecting compressor airfoils made of 17-4 PH stainless steel against sand erosion, are presented. Coatings based on either aluminum, nickel or titanium nitride and material modifications by ion implantation and cryogenic treatment were evaluated by dry particle erosion testing in accordance with the ASTM G76-83 Standard Practice, at different angles of impingement (30°, 60° and 90°). The TiN coatings applied by PVD techniques, were found to be the most erosion resistant. All the PVD deposition techniques appeared capable of producing durable coatings, but in degrees that varied with the deposition method and vendor. All the TiN coatings investigated proved more resistant to erosion at a 30° impingement angle. The implications for compressor airfoil applications are discussed.

The IAR Nanomechanical Probe (NMP) was used in the depth sensing indentation mode to evaluate the Vickers hardness (HV) and elastic modulus E of five commercial TiN coatings applied to a 17-4 PH stainless steel substrate. The HV values of the TiN layers at depth to thickness (DTT) ratios of less than 0.1 varied between about 28 and 39 GPa, depending on the deposition process. The elastic modulus was within the range of 300 to 400 GPa, corresponding to published values, at DTT ratios of less than or equal to 0.05. At higher DTT values, the elastic modulus decreased with increasing DTT ratio due to larger and larger interference from the less rigid stainless steel substrate. Depth sensing scratch tests were also performed on the samples to determine the critical load crvalues needed to cause spallation of the coatings. For each coating, the interfacial fracture toughness Kic was calculated from crand used to describe the adhesive strength. Distinct differences between the Kic values of the different samples were observed, reflecting differences in adhesive strength. The significance of these results is discussed in terms of the application of titanium nitride coatings to gas turbine engines.

A multi-variable test method, known as the Orthogonal Array, was used to optimize the parameters for the laser gas nitriding process (LGNP) to avoid surface cracking. Based on the fundamental requirement of a crack-free condition, the processing parameters were further optimized to improve the surface finish and to obtain a reasonable hardened depth. The effects of processing parameters have also been investigated with respect to the characteristics of the laser nitrided layer. Two types of lasers, i.e., CO2 and Nd:YAG lasers were used. The CO2 laser was operated in both the continuous as well as the pulse mode, while the Nd:YAG laser was used only in the pulse mode. A Nd:YAG laser in the pulse mode provided a better surface finish and lower cracking severity.

The characteristics of laser nitrided layers formed on Ti-6Al-4V are presented in this investigation. It has been determined that titanium nitride (TiN) is formed, which significantly increases the hardness of the nitrided surfaces. The amount of titanium nitride produced depends on the processing parameters such as laser pulse energy and nitrogen concentration. Nitrided layers are much smoother along the laser pass direction than perpendicular to this direction. The shrinkage effect in the laser melt zone produces surface residual tensile stresses in Ti-6Al-4V samples regardless of whether the processing environment is Ar, N2, or a mixture of these gases. Pre-heating or stress relieving after laser nitriding significantly reduces the residual tensile stress level.

A constitutive model is developed for grain boundary sliding (GBS) at serrated grain boundaries. Based on a previously developed GBS model, using the dynamics of grain boundary dislocation pile-up, the present model takes the average of the sliding rate over the characteristic dimensions of grain boundary serrations. Thus, a geometric factor φ is introduced to account for the effects of serration wave length and amplitude on the GBS rate, as compared to the GBS rate at planar boundaries. By considering the role of grain boundary shear stress in stress balancing, the proposed model removes the singularity at planar boundaries which exists in the diffusion-controlled GBS model at serrated grain boundaries. The modified model describes very well the transient creep of complex Ni-base superalloys with and without grain boundary serrations and should be suitable for other engineering alloys (with the exception of columnar grained and single crystal alloys).

A creep Deformation Mechanism Map (DMM) of an engineering alloy can be an effective tool for developing physics based prognostics systems. Many classical diffusion based rate equations have been developed for time dependent plastic flow where dislocation glide, dislocation glide-plus-climb and vacancy diffusion driven grain boundary migration (diffusion creep) are rate controlling. Long term creep testing and analysis of complex engineering alloys has shown that power law breakdown phenomenon is related to the dominance of Grain Boundary Sliding (GBS) as opposed to diffusion creep. Rate equations are now available for GBS in complex alloys and, in this paper, a DMM is constructed for Waspaloy (a Nickel-Based Superalloy) and validated by comparison with a collection of experimental data obtained from the literature. The GBS accommodated by wedge type cracking is considered dominant at low homologous temperatures (0.3 to 0.5Tm - temperature in Kelvin) whereas GBS accommodated by power-law or cavitations creep dominates above 0.55Tm.Copyright © 2013 by ASME

A creep Deformation Mechanism Map (DMM) of an engineering alloy can be an effective tool for developing physics-based prognostics systems. Many classical diffusion based rate equations have been developed for time dependent plastic flow where dislocation glide, dislocation glide-plus-climb and vacancy diffusion driven grain boundary migration (diffusion creep) are rate controlling. These creep rate equations have been proven experimentally for simple metals and alloys and form the basis of constructing an Ashby’s DMM. Long term creep testing and analysis of complex engineering alloys has shown that power law breakdown phenomenon is related to the dominance of Grain Boundary Sliding (GBS) as opposed to diffusion creep. Rate equations are now available for GBS in complex alloys and, in this paper, a DMM is constructed for a fine grained Alloy 718 and this is validated by comparison with a collection of experimental data obtained from the literature. The GBS accommodated by wedge type cracking is considered dominant at low homologous temperatures (0.3 to 0.5Tm i. e. melting temperature in Kelvin) whereas GBS accommodated by power-law or cavitation creep dominates above 0.55Tm.Copyright © 2012 by ASME

Stress intensity factors for thru-thickness and thumb-nail cracks in the double edge notch specimens, containing two different notch radius (R) to specimen width (W) ratios ( R/W = 1 8 and 1 16 ), are calculated through finite element analysis. The finite element results are compared with predictions based on existing empirical models for SIF calculations. The effects of a change in R/W ratio on SIF of thru-thickness and thumb-nail cracks are also discussed.

SUMMARYFour chromosomes in the karyotype of a clone of Iris variegata L. were heteromorphic. On the basis of somatic chromosome morphology and meiotic associations, it is established that the heteromorphicity is due to translocation involving shifting of a large segment of the long arm of the longest chromosome with nearly median centromere on the short arm of the smallest chromosome with submedian centromere, giving rise to two medium sized chromosome with nearly median and submedian centromeres. Bridge-fragment configurations at metaphase I also suggest the involvement of paracentric inversion heterozygosity. While the basic karyotype of I. variegata appears to be similar to that of the other species of bearded irises with 24 chromosomes, structural alterations, similar to the one presently recorded, have probably been responsible for the reported variations in its karyotype.

This article describes the investigation of failures in a series of diesel locomotive axles. It was found that embrittlement of the axle material occurred by liquid babbitt metal embrittlement following intermittent lubrication breakdown. The embrittlement led to the formation of surface cracks that eventually propagated by fatigue until final failure occurred. The microstructure and fractographic analyses are discussed in some detail, the failure mechanisms observed differing in some aspects from those quoted in the literature in other detailed studies.

The basic assumptions of the θ projection concept for creep life prediction of nickel-based superalloy components are critically analysed by using databases available for IN-100 and IN-738 turbinr blade materials. The θ projection concept in its present form does not account for the shape changes of a creep curve that can take place as a result of service-induced microstructural degeneration in nickel-based superalloy components. Long-term rupture lives can be underpredicted by factors of 3–8 through this approach.

The Vickers hardness HV, elastic modulus E and interfacial fracture toughness K{sub ic} of four commercial TiN coatings applied to a 17-4 PH stainless steel substrate were evaluated using a Nanomechanical Probe (NMP). The HV values of the TiN layers deposited by different methods varied between about 28 and 41 GPa, depending on the deposition process. The elastic modulus, on the other hand, appeared to remain constant at 320-330 GPa for all coatings examined. Finally, distinct differences between the K{sub ic} values representing the bond strength of the coating/substrate interface were observed.

This paper reports the results of a demonstration computer program carried out to determine the influence of the sensitivity and reliability of non-destructive inspection (NDI) techniques on the damage-tolerance-based life assessment of aeroengine compressor discs. The program was carried out on AM-355 martensitic stainless-steel compressor discs from an aeroengine. The safe inspection intervals (SII) for the compressor discs are calculated using probabilistic fracture mechanics (PFM) principles. These calculations involve the use of the NDI data, finite-element analysis and the experimental fatigue crack growth rate (FCGR) data base generated on compact tension specimens machined from safe-life-expired discs. The PFM analysis, which randomly uses the distribution of undetected crack sizes for a given NDI technique to choose an ai value for fracture mechanics calculations, demonstrates that the worst-case combinations of ai and FCGR do not occur during 7000 disc simulations. The PFM results also indicate that it may be possible to obtain cost effective SII if the sensitive eddy current and the automated ultrasonic leaky surface wave techniques are used to inspect the discs. However, the behaviour of short cracks would have to be characterized prior to obtaining usable PFM-based SII.

A modified heat treatment has been developed for the nickel-base superalloy, PWA 1113, to create a damage tolerant microstructure @TM) using mechanistic microstructural design concepts. The DTM was designed with the aim of imparting improved fatigue crack growth resistance without forfeiture of other vital properties such as tensile strength, stress rupture life, and low cycle fatigue lifetimes. This was achieved by optimizing the material’s grain size, grain boundary morphology, and the intragranular precipitate size and distribution. Mechanical testing demonstrated that when compared to the conventional microstructure (CM), the short crack growth rate for the DTM was slower by a factor of 3 at room temperature, and 2.2 times slower at 482 “C. Creep test results showed that at 690 MPa (100 ksi) and 705 “C, the creep-rupture life was extended by a factor of almost 4 for the new DTM. Tensile test results indicated minimal strength losses for the DTM with respective YS and UTS values of 80% and 90% of the CM baseline values at both test temperatures. Introduction Emerging safety standards for rotating engine components are now starting to demand properties in engineering materials, that may not meet the required specifications’. 2. In particular, the damage tolerance specifications imposed upon turbine disc materials have prompted researchers to investigate ways to improve crack growth properties of presently used alloys without compromising the traditional safe life limits. As a result, a microstructural design philosophy has been successfully developed that is aimed at improving the damage tolerance of conventional disc materials.3 Damage tolerance, by definition, is the ability of the disc material to exhibit greater resistance to the growth of inherent or service induced flaws under creep and/or fatigue loading conditions, while still maintaming adequate low cycle fatigue pro erties, tensile strength, as well as stress rupture strength. ! Consequently, improving the damage tolerance of conventional Ni-base disc alloys has become an area of substantial interest amongst disc material manufacturers and engine designers alike. Superalloys 2000 Edited by T.M. Pollock, R.D. Kissinger, R.R. Bowman, K.A. Green, M. McL.eaa, S. Olson, aad J.J. Schina TMS (The Minerals, Metals &Materials Society), 21X0

The influence of heat treatments on the microstructure and mechanical properties of a powder metallurgy (P/M) turbine disc alloy MERL76, was studied. A damage tolerant microstructure (DTM) was designed for the alloy using a modified heat treatment (MHT) sequence. Comparisons of mechanical properties between the DTM and the conventional microstructure (CM) were made on the basis of tensile strength, low cycle fatigue (LCF) life, creep rupture life, and fatigue crack growth rates (FCGRs). The DTM significantly improved the creep rupture life (310 times) at 760°C but resulted in a marginal loss. in tensile strength. The FCGR tests showed that FCGRs of the DTM at 700°C were much lower than the CM material by a factor of 10 at a frequency of 0.15 Hz whereas the FCGRs at a frequency of 1 Hz were comparable over a wide range of AK values. The LCF lives of the two microstructures at 700°C were also comparable.

The influence of a modified heat treatment (MHT) and the standard heat treatment (SHT) on the microstructure and mechanical properties of Alloy 7 18 turbine disc material has been studied over a range of temperatures varying between room temperature and 650°C. The influence of these heat treatments has been studied on creep, low cycle fatigue (LCF), and fatigue crack growth rate (FCGR) properties. Test results have shown that the MHT improves the creep rupture life at 650°C but results in a loss of LCF crack initiation life at room temperature and 525°C. The FCGRs of MHT and SHT materials are very similar at room temperature but the MHT improves the FCGR’s of Alloy 718 at 525°C and 650°C over a wide range of U values. The MHT material is cyclically more stable than the SHT material at all test temperatures. The notch sensitivity of Alloy 718 is also discussed.

Fatigue crack initiation life of Alloy 718 specimens, subjected to conventional and modified heat treatments, was determined using ASTM E606 small cylindrical specimens and an AGARD designed double edge notch (DEN) specimen in load control at 650°C. Crack initiation lives were monitored for the DEN specimens using a direct current potential drop technique while the ASTM E606 specimens were cycled to rupture. The DEN specimens were tested in two surface conditions. In one set of specimens the notch was lathe cut and in the other set of specimens the notch was electron discharge machined. It is shown that the notch surface condition can dominate the crack initiation life in Alloy 718 at 650°C. It is also shown that the modified heat treatment forms a notch sensitive microstructure which results in a loss in high cycle fatigue life of two orders of magnitude. However, under high strain low cycle fatigue conditions, normally encountered in aircraft gas turbine engine discs, the loss in fatigue life is less severe. Superalloys 718,625 and Various Derivatives Edited by Edward A. Lmia The Minerals, Metals & Materials Society, 1993

Powder metallurgy processing of aerospace alloys is an important activity at the National Research Council of Canada. Inert gas atomized, nickel base superalloys using hot isostatic pressing and forging has matured to production status, while basic studies are also underway with titanium aluminides to prepare for the likely introduction of intermetallics. The PM research team reviews its findings in both these areas.

Cytological studies on Phyllactinia show both inter- and intra-specific polyploidy. Changes of chromosome number may or may not result in morphological changes which lead to the formation of new species. These intra-specific variants have been termed chromosome races; P. guttata has five while P. dalbergiae has two. The importance of these variants in relation to pathogenicity, host range, species delimitation and the basic chromosome number in the genus Phyllactinia are discussed.

Grain boundary serration is studied as a function of cooling rate in a number of Ni-based superalloys, e.g. IN 738, Nimonic 115 and Nimonic 105. The mechanism is related to γ′ precipitation in these alloys and a higher γ′ solvus temperature than the M 23 C 6 solvus temperature appears to be a prerequisite for the development of serrations. A model based on the grain boundary primary γ′ particle movement causing the displacement of the local grain boundary segment and initiating serrated grain boundary formation has been proposed.

The effects of second phase particles, e.g. M23C6, MC and M(C, N) carbides on the grain growth phenomenon of FeNiCr alloys have been determined. Various theoretical models on grain coarsening have been compared. The grain size at all stages of grain coarsening was dependent on the undissolved carbide particle size (r), the volume fraction (f), and the nature of the carbides. The nature of M23C6 carbides varied, since Fe, Ni and Mo entered the M23C6 unit cell; and complex carbides such as (Cr15Fe4Ni2Mo2)C6 were formed. Gladman's equation was verified for predicting the observed grain size values to a significant degree, and other models were less successful.

Creep life prediction techniques such as the Larson-Miller parameter, life trend diagrams, the Monkman-Grant relationship and the Dobes-Milicka relationship are critically reviewed. Problems associated with the scatter in creep life data and the inability of these relationships to accommodate for service-induced material degeneration are discussed. These effects are rationalized in terms of the erratic tertiary creep behaviour of complex engineering alloys. A new creep design relationship based on the sum of the primary creep life tp plus the secondary creep life ts, normalized by the sum of the primary creep strain ϵp plus the secondary creep strain ϵs, varying as a function of secondary creep rate ϵ has been proposed. The relationship is described by t p +t s ϵ p +ϵ s ϵ M =K where M and K are constants. The equation is accurate over a wide range of stresses (350–700 MPa) and temperatures (760–890°C) in IN-738LC. The method systematically reveals the creep degeneration effects with increasing service life. The relationship appears to be independent of any changes in the predominant deformation mechanism.

A HIP rejuvenation study for an aluminide coated, internally cooled IN 738, turbine blade from an aero engine is described. The study,which involved assessing changes in blade shape, microstructure and creep properties of the airfoil portions of the blades as a function of service time, has shown that airfoils have a tendency to lengthen along their longitudinal axis during service and also that the rate of lengthening increases with increasing service life. The paper discusses on that basis, when to apply HIP rejuvenation for cost-effective blade life extension. It is also shown that the microstructure of the blades is substantially modified by service. The 7' particles coarsen through agglomeration, continuous networks of M23c6 carbides form along the grain boundaries and the blade surface along the internal cooling passages oxidizes. The design of a HIP rejuvenation/heat treatment cycle to recover microstructures and creep properties is discussed. Finally, a rejuvenation cycle that incorporates a diffusion treatment for recoating the blade is proposed that is shown to restore the loss of creep ductility induced by service, while improving time to rupture by a factor of 3 relative to new blades. Superalloys 1988 Edited by S. Reichman, D.N. Duhl, G. Maurer, S. Antolovich and C. Lund The Metallurgical Society, 1988

This paper examines the influence of serrated grain boundary morphology and dislocation substructures on the fatigue crack growth resistance of Alloy 718 at 650"~ in laboratory air. The serrated grain boundary work was conducted on experimental plate stock whereas the influence of dislocation substructures was examined on specimens machined from service exposed discs that were retrieved from aero engines. Serrated grain boundaries retard the fatigue crack growth rates (FCGR'S) by suppressing grain boundary sliding and providing a tortuous path for crack growth. The influence of dislocation substructures on FCGR's can vary markedly with the type of substructure present. High dislocation densities in the form of planar arrays dramatically increase the FCGR's relative to cellular dislocation arrangements. The origin of dislocation substructures in relation to remnant forging strains or service induced LCF damage is considered in detail. The influence of stress cycling on substructure development in service exposed discs is also discussed.

Low cycle fatigue lives of as-hipped P/M Rene 95 in four microstructural conditions are compared at 650°C and 0.3 Hz in laboratory air. Results indicate that lives are influenced primarily by microporosity.

SUMMARY Constant load (90 MPa) creep properties of specimens machined from new and service exposed IN-738LC turbine blades are reported for testing temperatures in the range of 899 to g96OC. The rupture lives in these tests varied between 250 to 10,000 hours. There appears to be a transition temperature (- 960°C) above and below which intragranular and grain boundary sliding deformation mechanisms predominate in IN-738LC at 90 MPa. Under intragranular deformation conditions, service exposed blades exhibit an increase in em and Er relative to new blades because the coarse 7' precipitates in service exposed blades facilitate flow. Under grain boundary sliding deformatipn conditions, however, the service exposed blades exhibit a decrease in Em and E, relative to new blades because service induced break down of MC carbides produces continuous networks of grain boundary M23C6 carbides which suppress the sliding more effectively during creep testing. The rupture life in both new and service exposed blades appears to be governed by stress assisted environmental cracking rather than any deformation mechanisms 'per se.' Fracture in both materials occurs through the link-up of environmentally induced surface cracks with the creep induced internal cracks. Final fracture occurs by transgranular shear. The service exposed blades contain slightly lower Cr content in the grain boundary regions because

The interrelationship between flow strength and microstructure at isothermal forging temperatures and strain rates in high γ'volume fraction nickel-base superalloys is discussed. A deformation mechanism map for these materials is introduced that has been developed as a basis for the formulation of constitutive relation for flow in this class of alloys.

A new modified heat treatment has been developed for Inconel 718. This heat treatment leads to substantial improvements in elevated temperature crack propagation resistance with apparently limited loss in resistance to LCF crack initiation as compared to the conventional heat treatment for this alloy. This is a result of tailoring the microstructure to obtain the optimum combination of grain size, grain boundary structure and matrix precipitate morphology.