Microstructure and mechanical properties relationships in the Ti-11 alloy at room and elevated temperatures

To establish correlations between microstructure and mechanical properties for the Till alloy, twelve different combinations of hot die forging and heat treatment, in the α+β and β phase regions, were investigated. The resulting heat treated forgings were classified into four distinct categories based on their microstructural appearance. The room temperature tensile, post-creep tensile, fracture toughness and fatigue crack propagation properties were measured along with creep and low cycle fatigue at 566°C. The creep, tensile, fatigue crack propagation and fracture toughness properties, grouped in a manner similar to the microstructural categories. The fracture appearance and behavior of the cracks during propagation in fatigue and in fracture toughness tests were examined, and correlations with the microstructure discussed. In the case of the fully transformed acicular microstructure, it was found that the size and the orientation of colonies of similarly aligned α needles are dominant factors in the crack behavior.     

损伤容限型钛合金TA15 ELI 43 mm厚板组织与性能

研究了TA15 ELI合金43mm厚板双态组织和片层组织的室温拉伸性能、断裂韧性KIC以及疲劳裂纹扩展速率da／dN等损伤容限性能。讨论了显微组织对该合金损伤容限性能的影响。结果表明，相对双态组织而言，该合金片层组织在较少损失拉伸强度的前提下，提高了合金的断裂韧性，降低了合金的疲劳裂纹扩展速率，具有更好的损伤容限性能。     

The strength,fracture toughness,and low cycle fatigue behavior of 17-4 PH stainless steel

The influence of microstructure on the strength, fracture toughness and low cycle fatigue behavior of 17-4 PH stainless steel has been examined. Aging hardening involves initial formation of coherent copper-rich clusters which transform to incoherent fee ∈-copper precipitates upon further aging. The changes in strength level and strain hardening rates observed during aging are consistent with previously suggested models for precipitation hardening based on differing elastic moduli. The fracture toughness and fatigue crack growth rates were shown to be a function of microstructure and environment. At equivalent strength levels overaging resulted in a higher fracture toughness than did underaging. The fatigue crack growth rates increased with increasing strength level and humidity but were not a function of toughness level. Attempts to correlate the fatigue crack growth rates with monotonie tensile properties were unsuccessful. However when final failure obeyed a critical strain criteria, the fracture toughness behavior could be reasonably described and related to preferential void nucleation and growth at δ-ferrite-matrix interfaces.     

Fatigue and fracture behavior of a fine-grained lamellar TiAl alloy

The fatigue and fracture resistance of a TiAl alloy, Ti-47Al-2Nb-2Cr, with 0.2 at. pct boron addition was studied by performing tensile, fracture toughness, and fatigue crack growth tests. The material was heat treated to exhibit a fine-grained, fully lamellar microstructure with approximately 150-μm grain size and 1-μm lamellae spacing. Conventional tensile tests were conducted as a function of temperature to define the brittle-to-ductile transition temperature (BDTT), while fracture and fatigue tests were performed at 25 °C and 815 °C. Fracture toughness tests were performed inside a scanning electron microscope (SEM) equipped with a high-temperature loading stage, as well as using ASTM standard techniques. Fatigue crack growth of large and small cracks was studied in air using conventional methods and by testing inside the SEM. Fatigue and fracture mechanisms in the fine-grained, fully lamellar microstructure were identified and correlated with the corresponding properties. The results showed that the lamellar TiAl alloy exhibited moderate fracture toughness and fatigue crack growth resistance, despite low tensile ductility. The sources of ductility, fracture toughness, and fatigue resistance were identified and related to pertinent microstructural variables.     

Effects of high temperature fatigue on microstructure and mechanical properties of a nickel-base precipitation hardened alloy

Effects of high temperature strain controlled push-pull fatigue on the microstructure and mechanical properties of a nickel-base precipitation hardened alloy were studied. The fatigue deformation alone at 700 °C did not impair the mechanical properties of this alloy; however, a hold period ranging from one minute to one hour at tension-peak decreased the tensile ductility and the fracture toughness significantly. This was mainly attributed to grain boundary cavitation. Continuous fatigue resulted in dislocation bands, whereas hold-time fatigue caused a coherency loss iny’ precipitates. Implications of these microstructural changes for the residual mechanical properties are discussed. Formerly with the Brookhaven National Laboratory, Upton, NY 11973. Formerly with the Brookhaven National Laboratory.     

Influence of heat treatment on the fatigue crack growth rates of a secondary hardening steel

The relationships between microstructure and fatigue crack propagation behavior were studied in a 5Mo-0.3C steel. Microstructural differences were achieved by varying the tempering treatment. The amounts, distribution, and types of carbides present were influenced by the tempering temperature. Optical metallography and transmission electron microscopy were used to characterize the microstructures. Fatigue fracture surfaces were studied by scanning electron microscopy. For each heat treatment the fatigue crack growth properties were measured under plane strain conditions using a compact tension fracture toughness specimen. The properties were reported using the empirical relation of Paris [da/dN = C_oΔK^m]. It was found that secondary hardening did influence the fatigue crack growth rates. In particular, intergranular modes of fracture during fatigue led to exaggerated fatigue crack growth rates for the tempering treatment producing peak hardness. Limited testing in a dry argon atmosphere showed that the sensitivity of fatigue crack growth rates to environment changed with heat treatment.     

Microstructure and mechanical behavior of Cr-Cr2Hfin situ intermetallic composites

A detailed investigation of the effects of microstructural changes on the mechanical behavior of twoin situ intermetallic composites with Cr and Cr₂Hf phases in the Cr-Hf system was performed. The nominal compositions (at. pct) of the alloys were Cr-5.6Hf (hypoeutectic) and Cr-13Hf (eutectic). The study included evaluations of strength, ductility, and fracture toughness as a function of temperature and creep behavior. Two microstructures in each alloy were obtained by heat treatments at 1250 ‡C (fine microstructure) and 1500 ‡C (coarse microstructure). A decrease in elastic strength (stress at the onset of inelastic response in the load-deflection curve) with the coarsening of the microstructures was noted for both alloys below 1000 ‡C. The Cr-13Hf alloy retained strength to a higher test temperature, relative to Cr-5.6Hf alloy, under both microstructural conditions. The alloys showed no evidence of ductility at room temperature. However, in the coarse microstructure of the Cr-5.6Hf alloy, the primary Cr exhibited ductility at and above 200 ‡C; ductility in primary Cr could be seen only at and above 1000 ‡C for the fine microstructure. In other words, the temperature at which ductility was first observed decreased from about 1000 ‡C to about 200 ‡C due to high-temperature heat treatment in this alloy. Both microstructures of Cr-5.6Hf alloy showed a significant increase in fracture toughness with increasing test temperature. However, the increases in fracture toughness with temperature for the Cr-13Hf alloy microstructures were relatively small. Both alloys showed about four orders of magnitude reduction in steady-state creep rates relative to pure Cr at 1200 ‡C. The results are analyzed in the light of deformation characteristics and fracture micromechanisms. The effects of microstructural factors, such as the size and continuity of phases, solubility levels of Hf as well as interstitial elements in Cr, on the observed mechanical behavior are discussed. Formerly Research Scientist, Materials and Processes, UES, Inc.     

Fatigue cracking characteristics of β-annealed large colony Ti-11 alloy

In order to better understand the large scatter in the fatigue results associated with β-annealed microstructures of α+ β titanium alloys, the fatigue crack initiation and propagation behavior of thin center notched Ti-11 specimens with a large colony α platelet microstructure was investigated. Colonies with a mean intercept diam of greater than 1 mm were grown in 2 mm thick specimens by means of a vacuum β-annealing process. This enabled crack path morphologies and crack propagation rates to be determined within single colonies by means of optical microscopy on the polished and etched surfaces. The results showed that the fracture is related to a shear mechanism across the colonies from the initiation stage through the overload fracture. Intense shear bands were observed ahead of and in the same direction as the propagating cracks. The density of the shear bands increased with increasing stress intensity. Since the colonies are randomly oriented, the fatigue cracks propagated at various angles with respect to the tensile axis. The crack propagation rate across a single colony is no faster than the propagation rate in the equiaxed α+ β microstructure of the same material. However, cracks were halted at the colony boundaries and forced to reinitiate through a cycle consuming process into the next colony. It is mainly this reinitiation process and microstructurally dependent growth which are responsible for the slower crack growth rates and large scatter band obtained for the β-annealed microstructures when compared to the α+ β microstructures. It is suggested that by reducing the colony size the crack growth rates will be reduced and the fatigue scatter band will be narrowed. Formerly with the Metals and Ceramics Division, Air Force Materials Laboratory, Wright-Patterson Air Force Base.     

抗拉强度1200 MPa贝氏体钢轨的开发及其在铁路上的应用

 对鞍钢生产的贝氏体钢轨组织及性能的稳定性进行了全面的分析研究,针对钢轨矫直残余应力比较高的特点,重点进行了断裂韧性、疲劳裂纹扩展速率、残余应力等方面的研究,结合钢轨试铺考察了残余应力对钢轨的影响。研究结果表明：贝氏体钢轨组织形态及性能稳定,并具有较高的强韧性。对试铺的观测结果表明：其耐磨及表面耐剥离性能优于热处理珠光体钢轨。     

The influence of SiC particulates on fatigue crack propagation in a rapidly solidified Al-Fe-V-Si alloy

The fatigue crack propagation properties of a rapidly solidified aluminum alloy are compared with those of a metal matrix composite (MMC) made of the same base alloy with the addition of 11.5 vol pct SiC particulate. The high-temperature base material, alloy 8009 produced by Allied-Signal, Inc. (Morristown, NJ), is solidified and processed using powder metallurgy techniques; these techniques yield a fine-grained, nonequilibrium microstructure. A direct comparison between the fatigue crack propagation properties of the reinforced and unreinforced materials is possible, because alloy 8009 requires no postprocessing heat treatment. As a consequence, this comparison reflects the influence of the SiC particulate and not differences in microstructure that could arise during processing and aging. The experimental data demonstrate that the SiC-reinforced material exhibits modestly superior fatigue crack propagation properties: slower crack growth rates for a given ΔK, at near-threshold crack growth rates. Even when the data are corrected for crack closure using an effective stress intensity factor, ΔK_eff, the composite exhibits lower crack propagation rates than the unreinforced matrix alloy. Microscopic evidence shows a rougher fracture surface and a more tortuous crack path in the composite than in the base alloy. It is argued that the lower crack growth rates and higher intrinsic threshold stress intensity factor observed in the composite are associated with crack deflection around SiC particles. Formerly Graduate Research Assistant, University of California-Davis     

Effect of Duplex Aging on the Initiation and Propagation of Fatigue Cracks in the Solute-rich Metastable <Emphasis Type="Italic">β</Emphasis> Titanium Alloy Ti 38-644

Aging of highly β-stabilized titanium alloys commonly leads to the formation of precipitate-free zones being susceptible to fatigue crack initiation. Duplex aging improves the fatigue properties of metastable β titanium alloys by enhancing a homogeneous α phase formation. In this study a duplex-aging cycle was designed for Ti 38-644 (β-C). Depending on the prior processing history heat treatment parameters were adapted on the basis of microstructure studies, hardness measurements and comparative tensile tests. The fatigue limit and fatigue crack growth threshold were determined for duplex-aged β-C. The results indicate that duplex aging promotes a homogeneously precipitated α phase providing excellent values of the fatigue limit. Surface-related fatigue crack initiation was observed. Comparing the fracture surfaces of direct- and duplex-aged β-C a transition of the tensile fracture mode from intergranular to predominantly transgranular was observed accompanied by a gain in ductility at comparable yield strengths. This was assumed to be the reason for the slightly improved fatigue crack growth behavior of duplex-aged as compared to direct-aged β-C. Along the entire heat treatment cycle the microstructure response was evaluated with regard to the particular effects on the fatigue properties. The results indicate clearly that key to success is a completely recrystallized β microstructure and the reasonably controlled aging response.     

Fatigue and fracture of porous steels and Cu-infiltrated porous steels

The effects of Cu infiltration on the monotonic fracture resistance and fatigue crack growth behavior of a powder metallurgy (P/M) processed, porous plain carbon steel were examined after systematically changing the matrix strength via heat treatment. After austenitization and quenching, three tempering temperatures were chosen (177 °C, 428 °C, and 704 °C) to vary the strength level and steel microstructure. The reductions in strength which occurred after tempering at the highest temperature were accompanied by the coarsening of carbides in the tempered martensitic steel matrix, as confirmed by optical microscopy and by microhardness measurements of the steel. Each steel-Cu composite, containing approximately 10 vol pct infiltrated Cu, had superior fracture toughness and fatigue properties compared to the porous matrix material given the same heat treatment. Although the heat treatments given did not significantly change the fatigue behavior of the porous steel specimens, the fatigue curves (da/dN vs ΔK) and fracture properties were distinctly different for the steel-Cu composites given the same three heat treatments. The fracture toughness (K _IC and J _IC ), tearing modulus, and ΔK _TH values for the composites were highest after tempering at 704 °C and lowest after tempering at 177 °C. In addition, the fracture morphology of both the fracture and fatigue specimens was affected by changes in strength level, toughness, and ΔK. These fractographic features in fatigue and overload are rationalized by comparing the size of the plastic zone to the microstructural scale in the composite. This article is based on a presentation made in the symposium “Fatigue and Creep of Composite Materials” presented at the TMS Fall Meeting in Indianapolis, Indiana, September 14–18, 1997, under the auspices of the TMS/ASM Composite Materials Committee.     

氢蚀程度对钢疲劳断裂性能的影响

研究了国产钢经不同温度和时间氢暴露后的力学性能、疲劳性能和断裂韧性,用扫描电镜证实了氢蚀后断裂机制发生了变化.研究表明:随氢蚀程度增加,20G钢抗拉强度和塑性降低明显,CrMo钢抗拉强度略有降低,塑性变化不大.氢蚀使20G钢的门槛值有一个最小值,而断裂韧性随氢蚀程度升高而降低,在氢蚀程度较低时,断裂韧性下降程度大;在氢蚀程度较高时,断裂韧性下降程度变缓.碳钢的疲劳性能变化是由于材料损伤作用和氢蚀造成的裂纹表面引起的闭合效应增加二者共同作用的结果.     

Grindability of Ti alloys

The grindability of Ti-based alloys was analyzed by considering the fracture behavior of individual alloys in response to the stress field of a grinding wheel. First, the stress field under a grinding wheel was computed by treating the grinding wheel as a cylindrical disk with a flat region acting on a flat substrate. The initiation and propagation of microcracks in the substrate was then examined on the basis of the contact stress field and one of two fracture criteria: (1) a critical stress criterion for the onset of cleavage crack initiation, and (2) a critical stress intensity factor criterion for the initiation and propagation of shear cracks. Grindability was computed as a function of grinding speed and microstructure for several Ti-based cast alloys containing α, α + β, or β microstructure with or without the intermetallic precipitates. Model predictions indicated that the grindability of Ti alloys increases with decreasing fracture toughness or tensile ductility. The theoretical results are compared against experimental data in the literature to elucidate the roles of microstructure in grindability. The comparison revealed that alloying addition that leads to the formation of brittle intermetallics enhances grindability by reducing fracture toughness, tensile ductility, and the resistance to crack initiation and propagation.     

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.     

Mechanisms of Slow Fatigue Crack Growth in High Strength Aluminum Alloys: Role of Microstructure and Environment

The role of microstructure and environment in influencing ultra-low fatigue crack propagation rates has been investigated in 7075 aluminum alloy heat-treated to underaged, peak-aged, and overaged conditions and tested over a range of load ratios. Threshold stress intensity range, ΔK₀, values were found to decrease monotonically with increasing load ratio for all three heat treatments fatigue tested in 95 pct relative humidity air, with ΔK ₀ decreasing at all load ratios with increased extent of aging. Comparison of the near-threshold fatigue behavior obtained in humid air with the data forvacuo, however, showed that the presence of moisture leads to a larger reduction in ΔK₀ for the underaged microstructure than the overaged condition, at all load ratios. An examination of the nature of crack morphology and scanning Auger/SIMS analyses of near-threshold fracture surfaces revealed that although the crack path in the underaged structure was highly serrated and nonlinear, crack face oxidation products were much thicker in the overaged condition. The apparent differences in slow fatigue crack growth resistance of the three aging conditions are ascribed to a complex interaction among three mechanisms: the embrittling effect of moisture resulting in conventional corrosion fatigue processes, the role of microstructure and slip mode in inducing crack deflection, and crack closure arising from a combination of environmental and microstructural contributions.     

Effect of strontium modification on near- threshold fatigue crack growth in an Al-Si-Cu die cast alloy

The effects of strontium modification on microstructure and fatigue properties in a die cast com-mercial aluminum-silicon alloy are demonstrated. Strontium additions of 0.010 and 0.018 wt pct drastically change the morphology of the eutectic silicon. The influence of these microstructural changes on fatigue properties is evaluated through fatigue crack growth testing. Examination of the fracture surfaces and the crack path establish distinct fatigue fracture modes for the modified and unmodified eutectic structures. Changes in fracture mode and crack path are correlated to the mi-crostructure changes. A higher energy fracture mode and increased crack path tortuosity explain the observed improvement in fatigue properties for the modified alloys. Strontium modified alloys exhibit a 10 to 20 pct higher fatigue crack growth threshold compared to an unmodified alloy for testing at a load ratio of 0.5. No difference was observed for testing at a load ratio of 0.1. Formerly Research Project Engineer, Briggs & Stratton Corporation, Milwaukee, WI 53222     

Microstructural evolution and mechanical properties of a forged gamma titanium aluminide alloy

A two-phase gamma titanium aluminide alloy, Ti-47Al-1Cr-1V-2.5Nb (in at.%), was studied under forged and various subsequent heat treatment conditions, to investigate the microstructural evolution and the effect of microstructure on room temperature (RT) tensile properties and fracture toughness behavior. Four classes of microstructure and three types of lamellar formation were identified, and their formation mechanisms were analyzed using various analytical techniques including metallography, electron optics, differential thermal analysis (DTA), and crystallography. It was found that both tensile and toughness behavior were profoundly affected by the microstructural variations.     

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.