Low cycle fatigue behavior of Ti-6AI-2Sn-4Zr-6Mo: Part II. Cyclic deformation behavior and low cycle fatigue

A series of strain-controlled low cycle fatigue tests have been carried out on Ti-6-2-4-6 at room temperature primarily for solution-treated and aged material. Tension-compression testing at R = 1 produced softening, irrespective of the morphology of the primary α. The amount of softening increased with increasing strain. Tension-tension testing for R = 0 produced strengthening. Unaged specimens showed behavior similar to aged material for both R = ?1 and R = 0 testing. Annealing at 621 °C produced hardening for R = ?1 and softening for R = 0 testing. Aging at 210 °C of R = ?1 specimens produced strengthening. Fatigue life data revealed longer fatigue life for equiaxed (E) structures than for Widmanstätten plus grain boundary (W +GB)α structures. ForEα, increasing α particle size tends to reduce fatigue life. For W +GBα alloys the situation is more complex, and both a decrease and increase in fatigue life may be seen for increasing particle sizes. There is also a prior β grain size dependency. As a result of softening, the slope of the log Δε _p/2 vs logN _f curve continuously decreases with increasingN. Explanations of the softening and hardening behavior in terms of dislocation rearrangement are offered. An explanation of the role of microstructure on fatigue life has been offered in the companion paper.     

Low Cycle Fatigue Behavior of Ti-6AI-2Sn-4Zr-6Mo: Part I. The Role of Microstructure in Low Cycle Crack Nucleation and Early Crack Growth

Widmanstätten + grain boundary (W + GB) α and equiaxed (E) structures of different α particle sizes were produced in smooth bar specimens of a Ti-6Al-2Sn-4Zr-6Mo alloy, heat treated to produce a 0.2 pct yield stress of about 1100 MPa. Specimens were cycled at room temperature under total strain control. For both W + GB and Eα structures at low strains crack initiation occurred at α-β interfaces and in the aged β matrix. Profuse extrusions were found in the W + GBα structure. At higher strains for both structures, crack initiation took place preferentially within slip bands in α. Surface crack nucleation and crack link-up occurred more readily in W + GBα structures, which had a shorter life²⁸ than Eα structures, a phenonmenon thought to be due to the much longer surface lengths of the cracks in W + GBα structures. Larger prior β grain size permitted surface cracks to be developed more readily, but the role of β grain size is an indirect one. At large plastic strains colonies of Wα at large angles to the direction of crack propagation can cause the crack to change directions. An explanation for this behavior is offered.     

The structure and properties of thermomechanically treated β-III titanium

Beta-III titanium (Ti-11.5Mo-5.5Zr-4.5Sn) was solutionized above the β-transus, water-quenched and deformed by rolling at room temperature. The deformation accelerated the aging kinetics at all temperatures up to the β transus. The thermomechanically treated (TMT) alloy always had higher strength than the conventionally heat treated (CHT) alloy; the effect being most marked when the aging product was normally α in a β matrix. In addition, the ductility and notched impact resistance of TMT β-III was greater than that of the CHT alloy in the over-aged condition. The TMT did not alter the morphology of the ellipsoidal α phase formed at low aging temperatures and short aging times, respectively. Here the strengthening increase is attributed to strain hardening of the initial β + ω microstructure. The deformation did substantially change the morphology of the Widmanstätten α phase that formed at higher aging temperatures. In particular, the Widmanstätten α plates were much finer and the β grain boundaries were no longer a preferred precipitation site following TMT. At the highest aging temperatures, the TMT material developed a cell structure of about 1 μ in diameter.     

Effect of the initial hydrogen concentration and vacuum-annealing modes on the structure, phase composition, and mechanical properties of sheet billets of the VT6 alloy: Part 2. Effect of the phase composition and grain size on mechanical properties of sheet billets of the VT6 alloy

The effect of the size of the α grain and the phase composition on the mechanical properties of sheet billets of the VT6 alloy after hot rolling with the use of reversible alloying by hydrogen is investigated. To evaluate the contribution of various hardening mechanisms to the total hardening of the alloy, the Hall-Petch relationship σ_y = σ _i + k _h d ^?1/2 is used. The possibility for its use to evaluate the intragrained and grain-boundary hardening of the VT6 α + β alloy in comparison with the Ti-6Al α alloy and commercial titanium is shown.     

Grain size and grain growth in an equiaxed alpha-beta titanium alloy

Methods of revealing grain size in a two-phase α-β titanium alloy have been examined and observations on beta grain growth in the presence of alpha have been carried out. The technique proposed by Greenfield and Margolin¹ for revealing β matrix grain sizes has been shown not to produce grain growth. However, for grain sizes of about 10 μm the G.M. technique does not reveal all the grains because of the similarity in orientation in neighboring grains. These clusters of similarly oriented grains are shown to persist as grain growth takes place but the misorientation between grains within a cluster decreases. Both the beta grain growth and alpha particle coarsening follow the same time dependency from which it is shown that a linear relationship exists between α particle size and β grain size. It is proposed that α particles must dissolve from theβ grain edges for β grain growth to occur. The linear dependency between beta grain size,D _β, and alpha particle size,d _α, can be rationalized either on the basis of geometrical or surface tension considerations. Formerly with New York University. Formerly Graduate Student with New York University.     

The effect of matrix strength on void nucleation and growth in a widmanstätten alpha-beta titanium alloy,CORONA-5

This paper presents the results of a study of the effect of matrix yield strength, at a constant Widmanstätten α microstructure, on the nucleation and growth of voids in an α-β titanium alloy, CORONA-5, Ti-5Al-4.5Mo-l.5Cr. Four microstructures with a retained β matrix and involving coarse or fine Widmanstätten α particles in coarse or fine β grains were used in different heat treatment conditions resulting in yield strengths from 765 to 1018 MPa. Void nucleation occurred atα/α boundaries, grain boundary α/matrix interfaces, α twin/matrix interfaces, and α twin/untwin boundaries. The void nucleation strain varied from 0 to 0.26 and was a function of both microstructure and yield strength. Void growth rates increased with yield strength for all microstructures except for those containing coarse α which decreased with yield strength. Intense shear was observed in colonies of fine α structures and was considered to be the cause of the observed rapid void growth rates in the fine β+ fine α microstructures. Surface cracking occurred in several aged conditions during straining. This cracking was attributed to strain concentration in α.     

Low-Cycle Fatigue of Ultra-Fine-Grained Cryomilled 5083 Aluminum Alloy

The cyclic deformation behavior of cryomilled (CM) AA5083 alloys was compared to that of conventional AA5083-H131. The materials studied were a 100 pct CM alloy with a Gaussian grain size average of 315 nm and an alloy created by mixing 85 pct CM powder with 15 pct unmilled powder before consolidation to fabricate a plate with a bimodal grain size distribution with peak averages at 240 nm and 1.8 μm. Although the ultra-fine-grain (UFG) alloys exhibited considerably higher tensile strengths than those of the conventional material, the results from plastic-strain-controlled low-cycle fatigue tests demonstrate that all three materials exhibit identical fatigue lives across a range of plastic strain amplitudes. The CM materials exhibited softening during the first cycle, similar to other alloys produced by conventional powder metallurgy, followed by continual hardening to saturation before failure. The results reported in this study show that fatigue deformation in the CM material is accompanied by slight grain growth, pinning of dislocations at the grain boundaries, and grain rotation to produce macroscopic slip bands that localize strain, creating a single dominant fatigue crack. In contrast, the conventional alloy exhibits a cell structure and more diffuse fatigue damage accumulation.     

The influence of grain size on the erosion rate of metals

The objective of this work was to understand the influence of grain size on solid impingement erosion behavior characterized by deformation at high strain rates and large strains. Experiments were carried out at a velocity of 40 m/s, impact angle of 90 deg with 300 to 450 μm steel shot as erodent on iron, copper, and titanium with varying grain sizes. The results indicate that the erosion rate is independent of grain size in iron and copper while it is apparently grain size dependent in titanium. The results are rationalized in terms of the negligible contribution of the Hall-Petch component to the flow stress at large strains in the case of copper and iron. The decreasing erosion rate in titanium with increasing grain size was due to the increased interstitial content picked up during thermal treatment and consequent increase in strain hardening and strain rate hardening and not due to increased grain sizeper se. Adiabatic shear bands were observed in coarse-grained iron under actual erosion conditions.     

Near threshold fatigue crack growth behavior of a titanium alloy: Ti-6A1-4V

Near threshold fatigue crack growth behavior of Ti-6A1-4V alloy was investigated as function of Widmanstatten microstructures. In particular, the effect of colony size on ΔK_th, ΔK_eff,th and K_cl,th has been studied. It has been found that crack growth rates are strongly affected by the size of microstructural features such as colonies and α laths. However, the microstructural units controlling crack growth are colonies in fast cooled microstructures consisting of fine Widmanstatten colonies while they are α laths in relatively slow cooled ones with coarse colonies. As a result, the projection in literature of increased fatigue crack growth resistance at large colony sizes in titanium alloys can not be generalized. This distinction appears to be brought about by the thick continuous interplatelet β phase present in slow cooled structures. In fast cooled structures, thin discontinuous β phase is seen to be ineffective in arresting slip or crack. However, in slow cooled ones thick β phase appears to effectively retard slip/crack in fatigue. The thickness, composition, intrinsic properties such as modulus, ductility of β phase and an added environmental effect have been suggested to be important in this respect. The crack growth rates and the magnitudes of ΔK_th and ΔK_eff,th can be uniquely ordered when compared in terms of the controlling microstructural units in respective microstructures. However, crack closure levels at threshold appear to be dependent on colony size. In addition, an increase in the intrinsic crack growth resistance ΔK_eff,th appears to exist when the cyclic plastic zone size approaches the thickness of α laths in the microstructure.     

High-temperature low-cycle fatigue behavior of K40S cobalt-base superalloy

An investigation was made on the strain-controlled low-cycle fatigue (LCF) of K40S cobalt-base superalloy at 900 °C in ambient atmosphere. The results show that K40S alloy possesses high LCF resistance in comparison with X-40 alloy. Under the testing conditions in this study, K40S alloy exhibits a cyclic stress response of initial hardening followed by softening. The cyclic stress response behavior has been attributed to dislocation-dislocation interactions and dislocation-precipitate interactions. The high response stress can lead to a large stress concentration at locations where inelastic strains of high amplitude accumulate, which account for the decreasing fatigue life with increasing strain rate. The well-distributed carbide particles are the “secondary” crack initiation sites. The secondary crack initiation relaxes the stress concentration at the crack tip, reducing the driving force of crack propagation. High-temperature LCF failure of K40S alloy results from the interaction of the mechanical fatigue and environmental oxidation.     

The Effect of ITMT’s and P/M Processing on the Microstructure and Mechanical Properties of the X7091 Alloy

The influence of microstructure and texture on the monotonic and cyclic properties of X7091-T651 was investigated. The various structures were developed from conventional ingot metallurgy (I/M), powder metallurgy (P/M) and intermediate thermal mechanical treatments (ITMT). Powder metallurgy produced a finer grain structure and particle distribution than I/M. Intermediate thermomechanical treatment produced a recrystallized, coarse grain structure with a weak texture, compared to the unrecrystallized grain structure and sharp texture obtained with conventional processing (CP). All materials had comparable monotonic properties. The resistance to fatigue crack initiation (FCI) increased with both a reduction in grain size and a finer particle distribution. Smaller grain sizes and finer particle distributions reduced the degree of cyclic strain localization. The CP-P/M alloy had the poorest ductility and FCI resistance of all the materials, although the slip was fairly homogeneous. This may be due to the presence of oxides at the grain boundaries and a sharp texture. The threshold stress intensity, ΔK_th, and the fatigue crack growth rate (FCGR) roughly follow a grain size dependence with the resistance of fatigue crack propagation (FCP) increasing with increasing grain size. It appears that large grains allow more reversible slip and reduce the amount of accumulated plastic strain within the reverse plastic zone. It is also believed that a greater degree of fatigue crack closure, which may be associated with large grains and a rough FCP surface, results in a lower FCGR in the lowΔK region. The intermediate thermomechanical treatment of P/M X7091 produced the optimum microstructure giving the best combination of mechanical properties. The important features include a small recrystallized grain structure, a fine particle distribution, a weak texture, and a low concentration of oxides at grain boundaries. Formerly Director, Fracture and Fatigue Research Laboratory, Georgia Institute of Technology, Atlanta, GA.     

Preparation and properties of fine grain β- CuAlNi strain- memory alloys

A method has been developed to produce grain sizes as small as 5 μm in alloys of β-CuAlNi. The alloys were of eutectoid composition and a procedure was developed for determining the composition of a eutectoid alloy having any required value for transition temperature (M _s ). The thermo-mechanical treatment involved two sequential stages of warm rolling followed by recrystallization. The alloys produced were single phase β-type with no second phase being present. Characteristic two-stage stress-strain curves were obtained for most of the specimens. It was generally found that the tensile strength and strain to failure increased with decreasing grain size according to a Hall-Petch type relationship down to a grain size of 5 μm. A fracture strength of 1200 MPa and a fracture strain of 10 pct were obtained in the best alloy. It was found that the major recovery mode, whether pseudoelastic or strain-memory, did not have any significant effect on the total recovery obtained. Recovery properties were not affected significantly by decreasing grain size, and 86 pct recovery could still be obtained at a grain size of around 10 μm. Grain refinement improved the fatigue life considerably, possibly due to the high ultimate fracture stress and ductile fracture mode. A fatigue life of 275,000 cycles could be obtained for an applied stress of 330 MPa and a steady state strain of 0.7 pct. At fine-grain sizes most of the fractures were due to transgranular-type brittle fracture and micro void-type ductile fracture, depending on the alloy composition. It was suggested that the difference between the alloys was due to differences in oxygen segregation at the grain boundaries.     

The effect of matrix strength on the fracture resistance of an alpha-Beta titanium alloy,corona-5

This paper presents the results of a study of the effect of matrix yield strength, at constant Widmanstätten α microstructure, on the fracture resistance of an α-β Ti alloy, CORONA-5. Fracture initiation resistance,J _q, and the stable crack growth resistance,T, were evaluated by the single specimen, unloading compliance method for four different microstructures and three yield strengths. The microstructures involved coarse or fine Widmanstätten α particles in a heat treated β-matrix; the yield strength ranged from 765 to 1018 MPa. It was found thatJ _q/σ_Y, where σ_Y is the effective yield strength, decreased with increasingσ _Y.T/σ _Y also decreased with increasing σ_Y for fine structures. For the coarse α structures, however, T/σ_Y revealed intermediate maxima. Coarser structures, in general, revealed higher values ofJ _q/σ_Y andT/σ _Y. The cause was found primarily to be due to the effect of increased α particle thickness. The effect of grain size was secondary. J_q/σ_Y increased with increasing tensile strain hardening rate, obtained at the onset of void nucleation. T/σ_Y was found to decrease with increasing tensile void growth rate. In general, J_q/σ_Y and T/σ_Y revealed different relationships with microstructure. Fatigue precrack front- and the stable crack length-tortuosities did not yield any general relationship to fracture resistance at different yield strengths.     

Effect of SiC volume fraction and particle size on the fatigue resistance of a 2080 Al/SiC p composite

The effect of SiC volume fraction and particle size on the fatigue behavior of 2080 Al was investigated. Matrix microstructure in the composite and the unreinforced alloy was held relatively constant by the introduction of a deformation stage prior to aging. It was found that increasing volume fraction and decreasing particle size resulted in an increase in fatigue resistance. Mechanisms responsible for this behavior are described in terms of load transfer from the matrix to the high stiffness reinforcement, increasing obstacles for dislocation motion in the form of S’ precipitates, and the decrease in strain localization with decreasing reinforcement interparticle spacing as a result of reduced particle size. Microplasticity was also observed in the composite, in the form of stress-strain hysteresis loops, and is related to stress concentrations at the poles of the reinforcement. Finally, intermetallic inclusions in the matrix acted as fatigue crack initiation sites. The effect of inclusion size and location on fatigue life of the composites is discussed.     

热变形及热处理过程中TC17钛合金组织与取向的关联性

为了进一步研究热压缩及热处理过程对组织及取向变化的关联性, 通过对TC17进行热压缩变形及后续热处理, 利用光学显微镜和背散射电子衍射等分析方法, 结合晶粒尺寸、织构分布图、极图以及反极图, 研究变形后及热处理后的TC17的组织结构、晶粒尺寸的变化和取向的演变规律以及两者之间的关联性.结果表明: 随着变形温度升高, 初生α相含量大幅减小, 尺寸减小, 大部分α相晶粒分散分布, 且位于高温β相晶粒的三叉晶界上; 热处理后, α相和β相组织特征清晰, 界限明显, 初生α相依旧存在, 且趋于等轴化, 亚稳定β相发生转变, 形成片层状β转变组织; 热变形使α相织构极密度值减小, 且随之温度增加, α相织构极密度值也变小; 热变形后的α相已不存在明显的强织构, 热变形对α相晶粒的取向影响较大, 很明显的改善了其取向的均匀性; 热变形同样使β相织构极密度值减小, 但效果不明显.β相仍存在取向集中现象, 取向均匀性相对较差.      

Influence of the second phase on the room-temperature tensile and creep deformation mechanisms of α-β titanium alloys: Part I. Tensile deformation

The effects of α and β phase interactions on the room-temperature tensile and creep deformation behavior of α + β titanium alloys with Widmanst?tten microstructures were studied using Ti-6.0 wt pct Mn and Ti-8.1 wt pct V as the model two-phase alloy systems. This article, Part I, deals with tensile deformation. It was found that when the α phase is present as thin (<10-μm) plates in the α + β alloys, significant twinning occurs. No significant twinning was observed in single-phase alloys with the same chemistry and similar grain size. Additionally, the β phase of Ti-8.1 V deforms by stress-induced hexagonal martensite (α′), while only twinning occurs in the single-phase β alloy with the same chemistry. Twinning in the α phase in association with stress-induced martensite (SIM) in the β phase was observed for the first time in a two-phase titanium alloy. This behavior is explained in terms of a number of factors including elastic interaction stresses between the α and β phases, coherency between the α phase and hexagonal martensite, and β phase stability.     

The influence of grain size on the erosion rate of metals

The objective of this work was to understand the influence of grain size on solid impingement erosion behavior characterized by deformation at high strain rates and large strains. Experiments were carried out at a velocity of 40 m/s, impact angle of 90 deg with 300 to 450 μm steel shot as erodent on iron, copper, and titanium with varying grain sizes. The results indicate that the erosion rate is independent of grain size in iron and copper while it is apparently grain size dependent in titanium. The results are rationalized in terms of the negligible contribution of the Hall-Petch component to the flow stress at large strains in the case of copper and iron. The decreasing erosion rate in titanium with increasing grain size was due to the increased interstitial content picked up during thermal treatment and consequent increase in strain hardening and strain rate hardening and not due to increased grain sizeper se. Adiabatic shear bands were observed in coarse-grained iron under actual erosion conditions.     

The effect of initial carbide morphology on abnormal grain growth in decarburized low carbon steel

The effect of fine or coarse carbide particles on abnormal grain growth in a low carbon steel subjected to small deformation was studied. Grain growth was induced in a decarburizing atmosphere at 1450 ‡F (788 ‡C) in order to study the effect of the initial carbide particle morphology on the subsequent motion of the grain boundaries at temperature. In this manner, the primary influence of second phase particles during straining could be evaluated. Transmission electron microscopy revealed that the large carbide particles exhibit a larger dislocation density area around them than the small carbide particles after straining up to ten percent. At higher strains, cell formation begins and the dislocation distribution is more uniform. Both coarse and fine carbide particle samples demonstrated three stage abnormal grain growth at strains below 10 pct. An incubation stage, which decreases with increasing strain, is followed by an abnormal grain growth stage, in which certain selected grains grow at the expense of other grains. Finally an equilibrium stage is reached in which the largest grain size is obtained for the lowest strain. At the lowest strains coarse carbide material has a longer incubation time and coarser Stage III grain size than the fine carbide alloy. Formerly a Summer Student with Bethlehem Steel Corporation, is now an Engineer with AT&T Technologies, Allentown, PA.     

The effect of grain size, strain rate, and temperature on the mechanical behavior of commercial purity aluminum

Commercial purity aluminum AA1050 was subjected to equal channel angular extrusion (ECAE) that resulted in an ultrafine-grained (UFG) microstructure with an as-received grain size of 0.35 μm. This UFG material was then annealed to obtain microstructures with grain sizes ranging from 0.47 to 20 μm. Specimens were compressed at quasi-static, intermediate, and dynamic strain rates at temperatures of 77 and 298 K. The mechanical properties were found to vary significantly with grain size, strain rate, and temperature. Yield stress was found to increase with decreasing grain size, decreasing temperature, and increasing strain rate. The work hardening rate was seen to increase with increasing grain size, decreasing temperature, and increasing strain rate. The influence of strain rate and temperature is most significant in the smallest grain size specimens. The rate of work hardening is also influenced by strain rate, temperature, and grain size with negative rates of work hardening observed at 298 K and quasi-static strain rates in the smallest grain sizes and increasing rates of work hardening with increasing loading rate and grain size. Work hardening behavior is correlated with the substructural evolution of these specimens.