Showing papers on "Fractography published in 2004"
TL;DR: In this paper, the authors investigated the nanoscopic nature of surface/interphases in terms of topography, fractography, adhesion and stiffness, and showed that variations in both adhesive and attractive forces on oxidized high modulus and intermediate modulus (IM) carbon fiber surfaces appear to result from the coating layer.
152 citations
15 Aug 2004-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the effect of isothermal treatment on the embrittlement of a 2205 duplex stainless steel (with 45 ferrite-55 austenite, vol.%) has been investigated.
Abstract: The effect of isothermal treatment (at temperatures ranging between 400 and 500 °C) on the embrittlement of a 2205 duplex stainless steel (with 45 ferrite–55 austenite, vol.%) has been investigated. The impact toughness and hardness of the aged specimens were measured, while the corresponding fractography was studied. The results show that the steel is susceptible to severe embrittlement when exposed at 475 °C; this aging embrittlement is analogous with that of the ferritic stainless steels, which is ascribed to the degenerated ferrite phase. High-resolution transmission electron microscopy reveals that an isotropic spinodal decomposition occurred during aging at 475 °C in the steel studied; the original δ-ferrite decomposed into a nanometer-scaled modulated structure with a complex interconnected network, which contained an iron-rich BCC phase (α) and a chromium-enriched BCC phase (α′). It is suggested that the locking of dislocations in the modulated structure leads to the severe embrittlement.
147 citations
TL;DR: In this article, the fatigue properties of two types of cold-work tool steels tempered at various temperatures were evaluated and the microstructure and fracture surface morphology were correlated to the fatigue behavior.
Abstract: The fatigue properties of two types of cold-work tool steels tempered at various temperatures were evaluated. The microstructure and fracture surface morphology were correlated to the fatigue behavior. Cold-work tool steels using this study were a conventional tool steel (JIS SKD11; 1.4C-11Cr-0.8Mo-0.2V) and its modified steel (M-SKD11; 0.8C-8Cr-2Mo-0.5V). The fatigue strength of the M-SKD11 steel increased 20 pct over that of the SKD11 steel for any number of cycles. This is attributed to the refinement of primary M7C3 carbides. These M7C3 carbides fractured during fatigue and were found at the sites of fatigue crack initiation. Change in crack initiation behavior was confirmed by acoustic emission testing. The S-N curves of the steels are similar to those of most structural steels. However, the subsurface fatigue crack initiation was dominant at lower alternating stresses. This study points to a general approach of carbide refinement that can be used for the enhancement of fatigue properties.
122 citations
25 Aug 2004-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the damage and rupture mechanisms of thin sheets of 2024 aluminum alloy (Al containing Cu, Mn, and Mg elements) are investigated, and two grades are studied: a standard alloy and a high damage tolerance alloy.
Abstract: The damage and rupture mechanisms of thin sheets of 2024 aluminum alloy (Al containing Cu, Mn, and Mg elements) are investigated. Two grades are studied: a standard alloy and a high damage tolerance alloy. The microstructure of each material is characterized to obtain the second phase volume content, the dimensions of particles and the initial void volume fraction. The largest particles consist of intermetallics. Mechanical tests are carried out on flat specimens including U-notched (with various notch radii), sharply V-notched and smooth tensile samples. Stable crack growth was studied using “Kahn samples” and pre-cracked large center-cracked tension panels M(T). The macroscopic fracture surface of the different specimens is observed using scanning electron microscopy. Smooth and moderately notched samples exhibit a slant fracture surface, which has an angle of about 45° with respect to the loading direction. With increasing notch severity, the fracture mode changes significantly. Failure initiates at the notch root in a small triangular region perpendicular to the loading direction. Outside this zone, slant fracture is observed. Microscopic observations show two failure micromechanisms. Primary voids are first initiated at intermetallic particles in both cases. In flat regions, i.e. near the notch root of severely notched samples, void growth is promoted and final rupture is caused by “internal necking” between the large cavities. In slanted regions these voids tend to coalesce rapidly according to a “void sheet mechanism” which leads to the formation of smaller secondary voids in the ligaments between the primary voids. These observations can be interpreted using finite element simulations. In particular, it is shown that crack growth occurs under plane strain conditions along the propagation direction.
102 citations
25 Mar 2004-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the ductility and bendability of AA6111 aluminum alloy have been investigated as a function of iron content ranging from 0.06 to 0.68 wt.
Abstract: The ductility and bendability of the AA6111 aluminum alloy have been investigated as a function of iron content ranging from 0.06 to 0.68 wt.%. A significant decrease in ductility (as measured by the reduction of area at fracture) is observed as the iron level increases. Both the low and high iron-containing alloys fail in a shear mode due to void sheeting. The low iron alloy can be bent fully, although it develops significant grooves on the tensile surface linked to shear bands. The high iron alloy, however, develops cracks, running parallel to the bend axis. Both shear bands and cracks propagate along lines of maximum shear, i.e. close to 45° to the maximum tensile direction. These processes are closely linked. Observations suggest that this material is damage-sensitive such that once voids nucleate in shear bands they rapidly develop shear cracks that propagate through a void sheeting process. This results in a significant loss of both tensile ductility (as defined by the reduction of area) and bendability with increasing Fe content.
88 citations
TL;DR: In this article, tensile tests of polymethylmethacrylate (PMMA) at quasi-static loading conditions and at intermediate strain rates were performed on MTS810 materials test system and on a self-developed intermediate strain rate materials testing apparatus, respectively.
86 citations
25 Mar 2004-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the results of an experimental study of the fatigue mechanisms of Duocel® open cell aluminum foams and the effects of heat treatment on foam fatigue behavior were presented.
Abstract: This paper presents the results of an experimental study of the fatigue mechanisms of Duocel® open cell aluminum foams and the effects of heat treatment on foam fatigue behaviour. The macro-/micro-mechanisms of fatigue were studied for the foams in the as-fabricated (F), annealed (O) and T6-strengthened (T6) conditions. The effects of annealing and T6-strengthening on the stress–strain behavior and plastic collapse strengths of foams were introduced before presenting the results of compression–compression fatigue experiments. The formation of localized deformation bands were investigated using an in-situ digital camera. Scanning electron microscopy (SEM) revealed clear evidence of the surface crack nucleation in the individual struts, prior to the abrupt strain jumps. Fractographic analysis of the failed struts also revealed fatigue striations and surface crack nucleation mechanisms in the struts. Finally, a simple compression–compression fatigue mechanism is proposed to link the observed macro- and micro-scale fatigue mechanisms in open cell aluminum foams.
85 citations
TL;DR: In this article, two Ti-activated brazes have been investigated for joining of a Si 3 N 4 /TiN ceramic composite to steel, and an average bending strength of 466 MPa was achieved when joining ceramic to ceramic and 398 MPa when joining ceramics to metal.
67 citations
25 Feb 2004-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the authors measured the Charpy impact energy of Al-Si-Cu AA319-type alloys in terms of the total absorbed energy, and showed that strontium-modification enhances the impact properties, so that the Sr-modified AA319 alloy exhibits the highest impact properties compared to the base, and other alloys at any given dendrite arm spacing.
Abstract: The Charpy impact energy of Al–Si–Cu AA319-type alloys was measured in terms of the total absorbed energy. The Charpy specimens were machined from end-chilled castings to incorporate the effect of cooling rate on the impact properties. Unnotched specimens were used to increase the accuracy of the measurements, and to emphasize the effect of microstructure. The influence of the microconstituents on the impact strength was investigated by adding various alloying elements (i.e. Sr, Fe, and P) to the AA319 base alloy, and applying two different heat treatments (T5, and T6). The results show that strontium-modification enhances the impact properties, so that the Sr-modified AA319 alloy exhibits the highest impact properties compared to the base, and other alloys at any given dendrite arm spacing (DAS). The impact energy increases with increase in cooling rate, while iron, and phosphorus additions have a detrimental influence due, respectively, to the formation of β-Al5FeSi, and phosphorus oxide particles during solidification. T6 treatment assists in the even distribution, and dissolution of the microconstituents (including the block-like CuAl2 particles) into the aluminum matrix. With more Cu available for strengthening during aging, the impact toughness is greatly enhanced. In the unmodified AA319 base alloy, crack initiation, and propagation occur mainly through Si-particle fracture, and the mechanism of void coalescence. In the Sr-modified, 1.2% Fe-containing 319 alloys, however, crack initiation takes place through fragmentation of β-Al5FeSi, Si, and CuAl2 or Cu2FeAl7 particles. Crack propagation occurs through cleavage of the β-Fe platelets, and fracture of the Cu-intermetallics, and brittle Si particles. Such samples exhibit very low impact energies.
55 citations
Book•
30 Aug 2004
TL;DR: In this paper, the authors present a failure analysis approach to failure analysis using a failure analyst and a failure model, with the aim of finding the causes of failure in a specific component.
Abstract: INTRODUCTION Engineering Products and Their Performance Engineering Properties of Materials Classes of Engineering Alloys Structure of Engineering Alloys Failure of Engineering Products Imperfect vs. Defective Products Definition and Objective of Failure Analysis Investigations Approach to Failure Analysis Investigations Background Requirements of the Failure Analyst: Scope of the Book ENGINEERING DESIGN-FABRICATION-PERFORMANCE Introduction Stages of Engineering Design Material Selection Fabrication of Engineering Alloys Solidification of Ingots Cold Working Recrystallization Thermomechanical Processing Primary Fabrication Techniques Secondary Fabrication Techniques Joining Techniques Service Performance Common Causes of Failure PRINCIPLES OF MECHANICS Introduction Concepts of Mechanics Concepts of Mechanical Force Concepts of Work and Energy Force and Motion Conservation of Energy Concept of Machines State of Mechanical Equilibrium Concept of Strain Concept of Stress Hook's Law PROPERTY EVALUATION Introduction Nondestructive Tests Destructive Tests: Measurement of Mechanical Properties STRESS ANALYSIS Introduction Uniaxial State of Stress Generalized State of Stress Multiaxial Stress-Strain Relationship Loading Conditions and Stress Thermal Stress Type of Stress Required to Produce Plastic Deformation Maximum Stresses Design Stresses Criterion for the Onset of Plastic Deformation (Yielding) Stress Concentration Criteria for Mechanical Failure Applications: Analysis of Stresses in Specific Components Solved Problems MACROSCOPIC ASPECTS OF FRACTURE AND FRACTURE MECHANICS Definition of Fracture Objective of Fracture Mechanics Use of the Terms Brittle and Ductile in Fracture Crack Loading Modes and Macroscopic Morphology of Fracture Surfaces Crack Propagation Under a Plane Strain Condition Crack Propagation Under a Plane Stress Condition Crack Propagation Under a Mixed State of Plane Strain and Stress Sequence of Events Leading to Fracture Classification of Crack Propagation Modes According to Loading Conditions Variables Affecting Fracture Behavior Basic Principles of Fracture Mechanics Linear Elastic Fracture Mechanics (LEFM) Use of Fracture Mechanics in Design Concept of Allowable Crack Size Use of Fracture Mechanics in Failure Analysis Selection of Materials Resistant to Fracture STRUCTURE OF ENGINEERING ALLOYS Introduction Principles of Thermodynamics Elements of Internal Structure Structure of the Atom Significance of the Electronic Structure of Atom Electronic Structure and Chemical Properties: Classes of Elements Origin of Interatomic Binding Forces Types of Interatomic Binding Forces Bond Strength and Properties of Materials Arrangement of Atoms in Perfect Crystals Understanding the Microscopic Plasticity of Perfect Crystals Crystal Imperfections Understanding the Microscopic Plasticity of Real Crystals Alloy Phases and Phase Change Equilibrium Phase Diagrams Methods of Strengthening Engineering Alloys MATERIALS CHARACTERIZATION Introduction Techniques for Microstructural Characterization Techniques for Chemical Analysis Microstructural Engineering Alloys CORROSION Introduction Low-Temperature Aqueous Corrosion High-Temperature Corrosion METALLURGICAL ASPECTS OF FRACTURE AND FRACTOGRAPHY Introduction Microscopic Aspects of Crack Nucleation Microscopic Mechanisms of Crack Propagation Fracture Modes and Fractography FAILURE ANALYSIS PROCEDURE Introduction Definition of the Problem Technical Background Experimental Program and Analysis Mode of Failure vs. Cause of Failure Data Interpretation and Terminology Recommendations Failure Analysis Reports CASE STUDIES Introduction Failure of Engineering Alloys Due to Improper Processing Practice Failure of Engineering Products During Manufacturing Effect of Variations in Design on Service Performance Failure of Engineering Products During Service Because of Unanticipated Service Conditions Failure of Engineering Products During Service Because of Improper Material Selection Failure of Engineering Products During Service Because of Improper Service Conditions APPENDIX A: CHEMICAL COMPOSITION AND CLASSIFICATION OF SELECTED STEELS APPENDIX B: UNITS OF MEASUREMENTS IN MECHANICS APPENDIX C: MOMENT OF INERTIA OF SELECTED CROSS SECTIONS INDEX
53 citations
05 Feb 2004-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: Widmanstatten and equiaxed microstructures containing very fine 2 particles were obtained by aging a Ti-6Al-4V alloy, and their quasi-static and dynamic deformation behavior was investigated in comparison to that of unaged micro-structures as discussed by the authors.
Abstract: Widmanstatten and equiaxed microstructures containing very fine 2 particles were obtained by aging a Ti–6Al–4V alloy, and their quasi-static and dynamic deformation behavior was investigated in comparison to that of unaged microstructures Quasi-static and dynamic torsional tests were conducted on them using a torsional Kolsky bar, and torsionally deformed areas beneath fracture surfaces were observed to investigate various microstructural factors determining the deformation behavior and effects of 2 precipitation The dynamic torsional test results indicated that maximum shear stress and fracture shear strain of the aged Widmanstatten and equiaxed microstructures were higher than those of the unaged microstructures The number of voids initiated in the aged Widmanstatten and equiaxed microstructures was five times greater than those in unaged microstructures because of the 2 precipitation This indicated that the aging treatment had a homogenizing effect, ie, less likelihood of developing a region of concentrated strain that preceded the adiabatic shear band formation, thereby reducing the possibility of the adiabatic shear band formation Fine 2 precipitation by aging was effective in the improvement of quasi-static and dynamic torsional properties and in the reduction of the adiabatic shear banding, which provided a new idea to improve ballistic performance of Ti alloy armor plates © 2003 Elsevier BV All rights reserved
TL;DR: In this paper, a model is developed that quantitatively predicts fracture surface roughness and roughness-induced crack closure stress intensity values from measurements of slip line spacing in a compression specimen.
25 Nov 2004-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the tensile properties of AA5182 aluminum alloys at temperatures close to the solidus temperature have been measured using optical and scanning electron microscopy, and the fracture surfaces and microstructures of the tested specimens have been examined using optical-and scanning-emitting microscopy in an attempt to correlate tensile property with fracture behaviour and changes in microstructure.
Abstract: In response to the demand for accurate mechanical property data in the partially solidified state, an experimental apparatus has been developed to perform tensile measurements of aluminum alloys at temperatures close to the solidus temperature. Measurements of the tensile properties of an industrially direct chill cast AA5182 aluminum alloy have been carried out at temperatures between 500 and 580 °C, at a range of strain rates between ∼10 −2 and ∼10 −4 s −1 . The fracture surfaces and microstructures of the tested specimens have been examined using optical and scanning electron microscopy in an attempt to correlate tensile properties with fracture behaviour and changes in microstructure. These properties have also been linked to the liquid fraction present in the specimen based on data found in the literature.
15 Aug 2004-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, failure mechanisms in air plasma sprayed thermal barrier coatings for land-based gas turbines have been studied by finite element simulations and fractographic investigations of low cycle fatigue (LCF) tested material, here chosen as an 350μm thick partially stabilised zirconia top coat (TC) together with a 150 μm thick NiCo-Co-Cr-Al-Y bond coat (BC) on a nickel base substrate (Haynes 230).
Abstract: In the present paper failure mechanisms in air plasma sprayed thermal barrier coatings for land-based gas turbines have been studied. This has been done by finite element simulations and fractographic investigations of low cycle fatigue (LCF) tested material, here chosen as an 350 μm thick partially stabilised zirconia top coat (TC) together with a 150 μm thick NiCoCrAlY bond coat (BC) on a nickel base substrate (Haynes 230). Both LCF testing, modelling results and fractographic investigations point in the same direction. An increased thickness of the thermally grown oxide (TGO) does decrease the LCF life of a coated structural alloy. Several points of crack initiation were found, in the TGO at the TC/BC interface, at the oxide network within the BC and at oxide inclusions between BC and substrate. During LCF tests the initiated cracks will grow radially into the substrate material. The behaviour is explained by increased TC/BC delamination stresses and changed oxidation behaviour with increased oxidation times.
TL;DR: In this article, high-cycle fatigue properties at 4 K, 77 K and 293 K were investigated in forged-INCONEL 718 nickel-based superalloy with a mean gamma (� ) grain size of 25 mm.
Abstract: High-cycle fatigue properties at 4 K, 77 K and 293 K were investigated in forged-INCONEL 718 nickel-based superalloy with a mean gamma (� ) grain size of 25 mm. In the present material, plate-like delta phase precipitated atgrain boundaries and niobium (Nb)-enriched MC type carbides precipitated coarsely throughout the specimens. The 0.2% proof stress and the ultimate tensile strength of this alloy increased with decreasing temperature, without decreasing elongation or reduction of area. High-cycle fatigue strengths also increased with decreasing temperature although the fatigue limit at each temperature didn't appear even around 10 7 cycles. Fatigue cracks initiated near the specimen surface and formed faceted structures around crack initiation sites. Fatigue cracks predominantly initiated from coarse Nb-enriched carbides and faceted structures mainly corresponded to these carbides. In lower stress amplitude tests, however, facets were formed through transgranular crack initiation and growth. These kinds of distinctive crack initiation behavior seem to lower the high-cycle fatigue strength below room temperature in the present material.
TL;DR: In this article, the repair effect of hot work tool steel (SKD6) by laser-melting process has been evaluated by rotating bending fatigue test, and the fracture surface of heat-treated specimen is similar to that of base metal, while that of laser processed one shows different appearance.
TL;DR: In this paper, the room-temperature tensile and high-cycle fatigue behavior of orthorhombic Ti-22Al-27Nb alloy with varying lamellar morphology was investigated.
Abstract: The room-temperature tensile and high-cycle fatigue (HCF) behavior of orthorhombic Ti-22Al-27Nb alloy with varying lamellar morphology was investigated. Varying lamellar morphology was produced by changing the cooling rate after annealing in the single B2 phase region. A slower cooling rate of 0.003 K/s, for example, resulted in several large packets or colonies of similarly aligned O-phase lamellae and a nearly continuous massive α
2 phase at the prior B2 grain boundaries, while a faster cooling rate of 0.1 K/s led to the refinement of colony sizes and the O-phase lamellae. The interface of O-phase lamellae and B2 phases was semicoherent. Water quenching produced a very fine tweed-like microstructure with a thin continuous O phase at the prior B2 grain boundaries. The 0.2 pct yield stress, tensile strength, and HCF strength increased with increasing cooling rate. For example, the tensile strength and HCF strength at 107 cycles of 0.003 and 0.1 K/s-cooled were 774 and 450 MPa, and 945 and 620 MPa, respectively. Since the fatigue ratio, which is the ratio of HCF strength at 107 cycles to tensile strength, did not show a constant value, but instead increased with increasing cooling rate, part of the fatigue improvement was the result of improved resistance to fatigue associated with the microstructural refinement of the lamellar morphology. Fatigue failure occurred by the subsurface initiation, and every initiation site was found to contain a flat facet. Concurrent observation of the fatigue initiation facet and the underlying microstructure revealed that the fatigue crack initiated in a shear mode across the colony, irrespective of colony size, indicating that the size of the initiation facet corresponded to that of the colony. Therefore, the colony size is likely a major controlling factor in determining the degree of fatigue improvement due to the microstructural refinement of lamellar morphology. For the water-quenched specimens, fatigue crack initiation appeared to be associated with shear cracking along the boundary between the continuous grain boundary O phase and the adjacent prior B2 grain.
15 Aug 2004-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the fracture toughness of bainitic RPV steels with similar chemical compositions but different manufacturing processes was examined in view of cleavage fracture stress at crack-tip.
Abstract: The fracture behaviors in the ductile–brittle transition region of reactor pressure vessel (RPV) steels with similar chemical compositions but different manufacturing processes were examined in view of cleavage fracture stress at crack-tip. The steels typically had a variation in grain size and carbide size distribution through the different manufacturing processes. Fracture toughness was evaluated by using a statistical method in accordance to the ASTM standard E1921. From the fractography of the tested specimens, it was found that fracture toughness of the steels increased with increasing distance from the crack-tip to the cleavage initiating location, namely cleavage initiation distance (CID, X f ) and its statistical mean value ( K JC(med) ) was proportional to the cleavage fracture stress ( σ f ) determined from finite-element (FE) calculation at cleavage initiating location. On the other hand, σ f could also be calculated by applying the size of microstructural parameters, such as carbide, grain and bainite packet, into the Griffith’s theory for brittle fracture. Among the parameters, the σ f obtained from the mean diameter of the carbides above 1% of the total population was in good agreement with the σ f value from the FE calculation for the five different steels. The results suggest that the fracture toughness of bainitic RPV steels in the transition region is mostly influenced by only some 1% of total carbides and the critical step for cleavage fracture of the RPV steels should be the propagation of this carbide size crack to the adjacent ferrite matrix.
15 Aug 2004-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the authors evaluate the cracking resistance at different locations of the ingot during direct chill casting of aluminum ingots and find that cracking resistance increases in the region with higher solidification rate.
Abstract: In direct chill (DC) casting of aluminum ingots uneven cooling rates at different regions of the ingot generate thermal stresses, which cause solidification cracks that might propagate to failure. As the microstructure changes in different parts of the ingot due to the different cooling rates, it is important to evaluate the cracking resistance at different locations of the ingot during DC ingot casting. Quench cracking tests, which simulate the cold crack propagation of the cast ingots under thermal stresses, were conducted for Al 2024 and Al 3004 alloys to determine the cracking resistance. Coupon specimens which represent the different locations of the cast ingot having different solidification rates were tested under specific thermomechanical loading conditions. The cracking resistance was observed to increase in the region with higher solidification rate and the cracking resistance of Al 3004 was observed to be higher than Al 2024 for a given solidification rate. The variations were investigated and found to correlate well with the microstructural and fractography analyses. The results provide further understanding of the often-observed center crack failure of casting ingot at the production runs.
TL;DR: In this article, the authors evaluated the stress corrosion cracking behavior of cast and heat-treated Al-6%Zn-1%Mg (Al-6-1) and Al- 6% Zn−1%mg −1%Ag (Al 6 1 1 ) composites.
TL;DR: In this paper, the fatigue properties of Pb-free solder alloys, such as Sn-3.5Ag, Sn-0.7Cu, and Bi-containing alloys were studied at room temperature using specimens with printed circuit board (PCB)/solder/PCB structure under total displacement of ±10 µm, 12 µm.
Abstract: Low-cycle, lap-shear fatigue behavior of Sn-based, Pb-free solder alloys, Sn-3.5Ag, Sn-3.5Ag-Cu, Sn-3.5Ag-Bi, and Sn-0.7Cu, were studied at room temperature using specimens with printed circuit board (PCB)/solder/PCB structure under total displacement of ±10 µm, 12 µm, 15 µm, and 20 µm. The fatigue lives of various solder joint materials, defined as 50% load drop, were correlated with the fracture paths and analyzed using the Coffin-Manson relation, Morrow’s plastic-energy dissipation model, and Solomon’s load-drop parameter. The Sn-3.5Ag, Sn-0.7Cu eutectics, and Sn-3.5Ag-Cu ternary alloys showed the same level of fatigue resistance, while Bi-containing alloys showed substantially worse fatigue properties. Cross-sectional fractography revealed cracks initiated at the solder wedge near the solder mask and subsequently propagated into the solder matrix in the former group of alloys, in contrast with the crack propagation along the solder/under bump metallurgy (UBM) interfaces in the Sn-3.5Ag-Bi alloys. Inferior fatigue resistance of Bi-containing alloys was ascribed to high matrix hardness, high stiffness, possible Bi segregation to the interface, and high residual stress in the interfacial area.
25 Oct 2004-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the growth behavior of an AISI 316 austenitic stainless steel (SS) annealed using a CO2 laser was evaluated under various environments (lab air, gaseous hydrogen and saturated hydrogen sulfide solution).
Abstract: Fatigue crack growth behavior of an AISI 316 austenitic stainless steel (SS) annealed using a CO2 laser was evaluated under various environments—lab air, gaseous hydrogen and saturated hydrogen sulfide solution. The laser-annealed specimen revealed no change in microstructures in various regions of the specimen. The results of fatigue crack growth tests indicated the laser-annealed specimen had a higher resistance to fatigue crack growth in the region preceding the laser-annealed zone (LAZ) independent of the test environments. Meanwhile, crack growth results also suggested that AISI 316 SS showed a low level of sensitivity to hydrogen-accelerated crack growth. X-ray diffraction pattern of the fatigue-cracked surface revealed that partial austenite to martensite transformation occurred within a narrow depth. The presence of residual austenite in the highly strained region trapped a large amount of hydrogen, which helped reduce hydrogen embrittlement susceptibility and hydrogen-accelerated crack growth in the alloy. Fatigue fractography of the specimens tested in air showed predominantly transgranular fatigue fracture with some flat facets (FFs). In case of specimens tested in the H2S solution or gaseous hydrogen at low loading frequency, quasi-cleavage (QC) fracture was correlated with hydrogen-enhanced crack growth. Moreover, the presence of obvious striations on the fracture surface of embrittled specimens could be attributed to the hydrogen-activated slip processes ahead of the crack front.
TL;DR: In this article, a procedure was developed to fabricate oxide-fiber-reinforced minicomposites with a dense matrix and evaluate two oxidation-resistant interface coatings, porous oxide (zirconia-silica mixture) and monazite.
Abstract: A procedure was developed to fabricate oxide-fiber-reinforced minicomposites with a dense matrix and evaluate two oxidation-resistant interface coatings, porous oxide (zirconia-silica mixture) and monazite. The coatings were evaluated using NextelTM 720-fiber-reinforced BlackglasTM-matrix minicomposites. Boron nitride (BN) coated and uncoated fibers were used as controls for comparison. The evaluation was based on ultimate failure strengths, fractography, and fiber pushin tests. All the composites that used fiber coatings had ultimate strengths significantly better than the control that used uncoated fibers. In addition, porous-oxide-coated fibers were found to be similar to BN-coated fibers in strength, fractography, and fiber pushin behavior. Monazite-coated fibers resulted in similar ultimate strengths but showed no appreciable fiber pullout. Fiber pushin tests showed that monazite debonds readily but frictional resistance is higher than for BN or porous oxide fiber coatings.
TL;DR: In this paper, the microstructural modification including the formation of hard precipitates and hardened matrix in the surface composite layer improved hardness, and fracture toughness was determined mainly by the volume fraction of precipitates working as fracture initiation sites and partly by the matrix property interrupting the crack propagation.
Abstract: Correlation of microstructure with hardness and fracture toughness of (TiC,SiC)/Ti–6Al–4V surface composites fabricated by high-energy electron-beam irradiation was investigated in this study. The mixtures of TiC, SiC, or TiC+SiC powders and CaF 2 flux were placed on a Ti–6Al–4V substrate, and then the electron beam was irradiated on these mixtures using an electron-beam accelerator. The surface composite layers of 1.2∼2.1 mm in thickness were formed, and contained a large amount (up to 66 vol.%) of precipitates such as TiC and Ti 5 Si 3 in the martensitic matrix. This microstructural modification including the formation of hard precipitates and hardened matrix in the surface composite layer improved hardness. In situ observation of the microfracture process revealed that microcracks primarily initiated at precipitates, and that shear bands formed in the matrix between these microcracks. Thus, fracture toughness was determined mainly by the volume fraction of precipitates working as fracture initiation sites and partly by the matrix property interrupting the crack propagation.
TL;DR: In this article, in situ observations of fracture processes combined with one-to-one observation of fracture surfaces and finite element method (FEM) calculations are carried out on notched tensile specimens of two-phase polycrystalline TiAl alloys, and the results reveal that most cracks are initiated and propagated along the interfaces between lamellae before plastic deformation.
Abstract: In-situ observations of fracture processes combined with one-to-one observations of fracture surfaces and finite-element method (FEM) calculations are carried out on notched tensile specimens of two-phase polycrystalline TiAl alloys. The results reveal that most cracks are initiated and propagated along the interfaces between lamellae before plastic deformation. The driving force for the fracture process is the tensile stress, which is consistent with a previous study.[1] In specimens with a slit notch, most cracks are initiated directly from the notch root and extended along lamellar interfaces. The main crack can be stopped or deflected into a delamination mode by a barrier grain with a lamellar interface orientation deviated from the direction of crack propagation. In this case, new cracks are nucleated along lamellar interfaces of grains with favorable orientation ahead of the barrier grain. The main crack and a new crack are then linked by the translamellar cleavage fracture of the barrier grain with increasing applied load. In order to extend the main crack, further increases of the applied load are needed to move the high stress region into the ligament until catastrophic fracture. The FEM calculations reveal that the strength along lamellar interfaces (interlamellar fracture) is as low as 50 MPa and appreciably lower than the strength perpendicular to the lamellae (translamellar fracture), which shows a value higher than 120 MPa. This explains the reason why cracks nucleate and preferably extend along the lamellar interfaces.
TL;DR: In this paper, the effect of microstructures of materials reinforced with brittle hard particles on their fracture behavior and toughness was studied using microstructure-based finite element meshes and an element elimination method.
Abstract: This paper presents a systematical computational study of the effect of microstructures of materials reinforced with brittle hard particles on their fracture behavior and toughness. Crack growth in particle-reinforced materials (here, in high speed steels) with various artificially designed arrangements of brittle inclusions is simulated using microstructure-based finite element meshes and an element elimination method. The following types of brittle inclusions arrangements are considered: (simple microstructures) net-like continuous, band-like, random with different inclusion sizes, and (complex microstructures) layered and clustered arrangements, with different inclusion sizes and orientations. Crack paths, force-displacement curves, fracture toughness and fractal dimension of fracture surfaces are determined numerically for each microstructure of the materials. It is demonstrated that extensive crack deviations from the initial cracking directions and an increase in the fracture toughness can most efficiently be achieved by using complex microstructures, such as alternated layers of fine and coarse inclusions.
TL;DR: In this article, the roles of colony boundary and crack orientations in the fracture resistance of two-phase lamellar TiAl alloys (Ti−46.5Al and Ti−47Al−2Nb-1.6Cr−1V, all in at%) were investigated.
TL;DR: In this paper, the authors investigated the crack growth behavior of Inconel 718 by high temperature moire interferometry (HTMI), coupled with SEM/EDAX, and proposed that the mechanism is caused by the segregated Nb, which couples with the oxygen diffusing into the grain boundaries in front of the crack tip and forms an NbO layer on the grain boundary, thereby causing the brittle elastic cracking behavior.
Abstract: The effect of environment on creep crack growth behaviors of many nickel-base superalloys is a well-documented and serious problem. Stress accelerated grain boundary oxidation (SAGBO) is accepted as the prior mechanism of the environment effect. In this paper, the crack growth behavior of Inconel 718 was investigated by high temperature moire interferometry (HTMI), coupled with SEM/EDAX. Based on the results obtained from this research, the mechanism is proposed to be caused by the segregated Nb, which couples with the oxygen diffusing into the grain boundaries in front of the crack tip and forms an NbO layer on the grain boundaries, thereby causing the brittle elastic cracking behavior.
TL;DR: In this paper, the authors conducted constant amplitude axial fatigue tests on SA533B3 steels with four levels of sulfur content at room temperature and 300 °C and found that inclusions near the specimen surface served as the crack initiation site for a majority of the fatigued specimens.
TL;DR: In this article, the influence of temperature on the fracture behavior of epoxy-based composites was studied using an instrumented Charpy impact approach, and it was found that a proper hybridisation of composites is possible to achieve a better impact performance compared to single-filler/epoxy.
Abstract: The influence of temperature on the fracture behaviour of epoxy-based composites was studied using an instrumented Charpy impact approach. A series of epoxy reinforced with short carbon fibres (SCF) and additionally filled with various amounts of PTFE and graphite particles was considered in this study. Unnotched specimens were tested at −196 °C, 20 °C, and 70 °C, respectively. It was found that, for specimens with the same matrix content, a proper hybridisation of composites was possible to achieve a better impact performance compared to single-filler/epoxy. For example, 10 vol.%PTFE+10 vol.%SCF/epoxy exhibited a higher impact resistance than that of 20 vol.%SCF/epoxy at all measured temperatures. Failure mechanisms at different temperatures were discussed with SEM fractography.