Showing papers on "Fractography published in 2003"

Effect of standard heat treatment on the microstructure and mechanical properties of hot isostatically pressed superalloy inconel 718

Abstract: Ni–Fe base superalloy, Inconel 718, was processed through powder metallurgy (P/M) hot isostatic pressing (HIP) route. In order to balance the strength and ductility, the HIPed material was given the standard heat treatment, viz. solution treatment at 980 °C for 1 h/water quenched (WQ) to room temperature and a two-step ageing treatment consisting of 720 °C for 8 h/furnace cooling (FC) at 55 °C h−1 to 620 °C and holding at 620 °C for 8 h before air cooling (AC) to room temperature. Optical microscopy and scanning electron microscopy (SEM) studies on the heat treated alloy have shown a homogeneous microstructure with fine grain size (25 μm) along with the presence of prior particle boundary (PPB) networks. Transmission electron microscopy (TEM) on the heat treated material has revealed the presence of oxides, MC carbides and δ-precipitates at the grain boundaries and a uniform precipitation of fine γ″ and γ′ strengthening phases in the matrix. Tensile and stress rupture tests were performed on the heat treated material. While the yield strength (YS) and ultimate tensile strength (UTS) of the HIPed and heat treated alloy at room temperature and 650 °C were comparable to those of conventionally processed wrought IN 718, its ductility was lower. The stress rupture life of the HIPed alloy improved marginally due to heat treatment and met the minimum specification requirement of life hours but the rupture ductility was found to be inferior to that of the wrought material. The fractography of the failed samples has revealed the transgranular ductile mode of fracture in the as-solution treated alloy, while intergranular mode of failure with the decohesion of PPBs occurred more predominantly in the aged condition. This change of fracture mode with ageing treatment shows the ductility dependence on the relative strength of the matrix and PPBs. The TEM studies on the deformed alloy have revealed that the brittle oxides and carbides at the prior particle boundaries coupled with the fine γ″ and γ′-precipitates in the matrix are responsible for low ductility at 650 °C. The investigations of the present study have led to better understanding of the structure–property relationships in HIP+heat treated alloy 718 and suggest that the standard heat treatment recommended for wrought IN 718 is not suitable for HIPed alloy and has to be modified to realise optimum properties.

Influence of interfacial intermetallic compound on fracture behavior of solder joints

Abstract: This paper studies the solder joints of an Sn–Ag lead-free solder system with pure copper wires. The study focuses upon the interrelationships, which exist between the adhesive strength of the joint, its shear strength, the formation of interfacial intermetallic compounds (IMC) and the fractographic morphology. Additionally, the paper determines how these characteristics, and the relationships between them, are influenced by the storage duration and the storage temperature. Experimental results show that both the adhesive strength and the shear strength of the solder joints decrease significantly following short-term thermal storage. As the storage time is increased, it is noted that both the thickness and the roughness of the interfacial IMC layers increase. Regarding the fracture of the solder joints, fractographic observation reveals that fracture morphology under adhesive loading are similar to those observed under shear loading conditions. In the as-soldered condition, the fracture surface appears to be flat, and some broken Cu6Sn5 and residual solder pieces are evident. When the total thickness of the IMC layer lies within the range 1–10 μm, it is observed that the fracture morphology gradually becomes a dimple-like structure. This phenomenon may be attributed to the residual stresses caused by phase transformation, and by the increasing roughness of the IMC layers which causes an increase in the stress concentration within the Cu6Sn5 layer, and which ultimately results in fracturing of this layer. When the total thickness of the interfacial IMC layers exceeds 10 μm, the roughness of the IMC layers and the residual stress between them and the solder both continue to increase. Eventually this results in a fracture being initiated and propagated within the Cu6Sn5 layer. Fractographic observation shows the fracture to have a cleavage-like morphology.

Fabrication, characterization, and dynamic behavior of polyester/TiO2 nanocomposites

Abstract: Unsaturated polyester resin specimens embedded with small loadings of 36 nm average diameter TiO 2 particles were fabricated using a direct ultrasonification method to study the effects of nanosized particles on nanocomposite bulk mechanical properties. The ultrasonification method employed produced nanocomposites with excellent particle dispersion as verified by transmission electron microscopy (TEM). Quasi-static fracture toughness, tension, and compression testing was carried out. The presence of the particles had the greatest effect on fracture toughness; negligible influence was observed in the remaining quasi-static properties. Scanning electron microscopy (SEM) of fracture surfaces was carried out to identify toughening mechanisms. The inadequacy of the bond between the filler and the matrix and the presence of minor particle agglomerations in specimens containing higher volume fractions of particles were believed to be responsible for a consistent decrease in property values beyond a volume fraction of 1 vol.%. Dynamic fracture toughness testing was carried out, and an increase in dynamic fracture toughness relative to quasi-static fracture toughness was observed. High strain rate testing conducted using a split Hopkinson pressure bar (SHPB) apparatus revealed a moderate stiffening effect with increasing particle volume fraction, although no marked effect was observed on the ultimate strength.

Crack initiation and propagation in 50.9 at. pct Ni-Ti pseudoelastic shape-memory wires in bending-rotation fatigue

Abstract: The structural fatigue of pseudoelastic Ni-Ti wires (50.9 at. pct Ni) was investigated using bending-rotation fatigue (BRF) tests, where a bent and otherwise unconstrained wire was forced to rotate at different rotational speeds. The number of cycles to failure (N f ) was measured for different bending radii and wire thicknesses (1.0, 1.2, and 1.4 mm). The wires consisted of an alloy with a 50-nm grain size, no precipitates, and some TiC inclusions. In BRF tests, the surface of the wire is subjected to tension-compression cycles, and fatigue lives can be related to the maximum tension and compression strain amplitudes (ɛ a ) in the wire surface. The resulting ɛ a -N f curves can be subdivided into three regimes. At ɛ a > 1 pct rupture occurs early (low N f ) and the fatigue-rupture characteristics were strongly dependent on ɛ a and the rotational speed (regime 1). For 0.75 pct < ɛ a < 1 pct, fatigue lives strongly increase and are characterized by a significant statistical scatter (regime 2). For ɛ a < 0.75 pct, no fatigue rupture occurs up to cycle numbers of 106 (regime 3). Using scanning electron microscopy (SEM), it was shown that surface cracks formed in regions with local stress raisers (such as inclusions and/or scratches). The growth of surface cracks during fatigue loading produced striations on the rupture surface; during final rupture, ductile voids form. The microstructural details of fatigue-damage accumulation during BRF testing are described and discussed.

Formation of carbides and their effects on stress rupture of a Ni-base single crystal superalloy

Abstract: Creep tests of a nickel–base single crystal superalloy with minor C addition and non-carbon were carried out at different temperatures and stresses. Correlations between microstructural change and testing temperature and stress were enabled through scanning electron microscopy (SEM) and transmission electron microscopy (TEM), detailing the rafting microstucture and carbides precipitation. The results showed that minor carbon addition prolonged the second stage of creep strain curves and improved creep properties. Some carbide was precipitated during creep tests in modified alloy. M23C6 carbide precipitated at lower temperature (871–982 °C), while (M6C)2 carbide precipitated at higher temperature (>1000 °C), all of which was considered to be beneficial to creep properties. A small amount of MC carbide formed during solidification and its decomposition product (M6C)1 were detrimental to mechanical properties, which together with micropores provided the site of initiation of cracks and led to the final fracture.

Short crack initiation and growth at 600 °C in notched specimens of Inconel718

Abstract: The natural initiation and growth of short cracks in Inconel®718 U-notch specimens has been studied at 600 °C in air. U notches were introduced through broaching, and hardness traces and optical microscopy on cross-sections through the U notch broaching showed that the broaching process had introduced a deformed, work hardened layer. Fatigue tests were conducted under load control using a 1-1-1-1 trapezoidal waveform, on specimens with as-broached and polished U-notches. Multi-site crack initiation occurred in the notch root. Many of the cracks initiated at bulge-like features formed by volume expansion of oxidising (Nb,Ti)C particles. In unstressed samples, oxidation of (Nb,Ti)C particles occurred readily, producing characteristic surface eruptions. Scanning electron microscopy on metallographic sections revealed some sub-surface (Nb,Ti)C oxidation and localised matrix deformation around oxidised particles. A mechanism for crack initiation by carbide expansion during oxidation is discussed. Surface short crack growth rates in the notch root of polished specimens were measured using an acetate replica technique. Observed short-crack growth rates were approximately constant across a wide range of crack lengths. However, there was a transition to rapid, accelerating crack growth once cracks reached several hundred micrometers in length. This rapid propagation in the latter stages of the fatigue life was assisted by crack coalescence. Polishing the U-notch to remove broaching marks resulted in a pronounced increase in fatigue life.

Ambient and elevated temperature mechanical properties of hot-pressed fused silica matrix composite

Abstract: The ambient and elevated temperature mechanical properties of two kinds of hot-pressed fused silica matrix composites, SiO 2 +5 vol% Si 3 N 4 and SiO 2 +5 vol% Si 3 N 4 + 10 vol% C f , were investigated Si 3 N 4 additions greatly enhanced the ambient strength and fracture toughness, while, further incorporation of chopped carbon fibers only but sharply increased the fracture toughness value from 122 to 24 MPa m 1/2 The strength of the two composites synchronously exhibited anomalous gains at certain elevated temperature range especially from 1000 to 1200 °C, and reached their maximum values at 1000 °C, 1689 and 1306 MPa, which were 770 and 774% higher than their ambient strength, respectively The two composites exhibited catastrophic fracture even at 1000 °C, but manifested prominent plastic deformation at 1200 °C and usually no fracture occurred during the strength test Vickers’ indentation crack propagation behavior, combined with fractographs studies, suggested that toughening from carbon fiber was attributed primarily to the fiber bridging, pull-out and crack deflection

Threshold and growth mechanism of fatigue cracks under mode II and III loadings

Abstract: The fatigue crack growth behaviour of 0.47% carbon steel was studied under mode II and III loadings. Mode II fatigue crack growth tests were carried out using specially designed double cantilever (DC) type specimens in order to measure the mode II threshold stress intensity factor range, ΔK IIth . The relationship ΔK IIth >ΔK Ith caused crack branching from mode II to I after a crack reached the mode II threshold. Torsion fatigue tests on circumferentially cracked specimens were carried out to study the mechanisms of both mode III crack growth and of the formation of the factory-roof crack surface morphology. A change in microstructure occurred at a crack tip during crack growth in both node II and mode III shear cracks. It is presumed that the crack growth mechanisms in mode II and in mode III are essentially the same. Detailed fractographic investigation showed that factory-roofs were formed by crack branching into mode I. Crack branching started from small semi-elliptical cracks nucleated by shear at the tip of the original circumferential crack.

Investigations on the fracture toughness of austempered ductile irons austenitized at different temperatures

Abstract: Ductile cast iron was austenitized at four different temperatures and subsequently austempered at six different temperatures. Plane strain fracture toughness was evaluated under all the heat treatment conditions and correlated with the microstructural features such as the austenite content and the carbon content of the austenite. Fracture mechanism was studied by scanning electron microscopy. It was found that the optimum austempering temperature for maximum fracture toughness decreased with increasing austenitizing temperature. This could be interpreted in terms of the microstructural features. A study of the fracture mechanism revealed that good fracture toughness is unlikely to be obtained when austempering temperature is less than half of the austenitizing temperature on the absolute scale.

The tensile behavior of two magnesium alloys reinforced with silicon carbide particulates

Abstract: In this paper is reported the results of a study aimed at establishing an understanding the role of particulate reinforcement on tensile deformation and fracture behavior of magnesium alloys discontinuously-reinforced with silicon carbide (SiC) particulates. An increase in particulate reinforcement content was observed to decrease ultimate tensile strength and ductility of the composite when compared to the unreinforced counterpart. Cracking of the individual and clusters of reinforcing particulates present in the microstructure dominated tensile fracture of the composite, on a microscopic scale. Final fracture occurred as a result of crack propagation through the matrix between particulate clusters. The fracture behavior of the composite is discussed in light of the concurrent and mutually interactive influences of intrinsic microstructural effects, deformation characteristics of the metal matrix and the particulate reinforcement, nature of loading and local stress state.

Effects of HIPping on high-cycle fatigue properties of investment cast A356 aluminum alloys

Abstract: This study is concerned with the effects of HIPping on high-cycle fatigue properties of investment cast A356 Al alloys. Tensile and high-cycle fatigue tests were conducted on cast alloys, two of which were HIPped, and then the test data were analyzed in relation with microstructures, tensile and fracture properties, and fatigue fracture mode. Eutectic Si particles were homogeneously dispersed in the matrix of the casting A356 Al alloys, but there were many large pores formed as casting defects. The high-cycle fatigue test results indicated that fatigue strength of the HIPped alloys was higher than that of the non-HIPped alloys because of the significant reduction in volume fraction of pores by HIPping. In the non-HIPped specimens, fatigue cracks initiated at large pores adjacent to the specimen surface and then propagated down to several hundreds micrometers depth while coalescing with other large pores. On the other hand, the HIPped specimens, where pores did not affect the fatigue much, fatigue cracks initiated at eutectic Si particles and propagated along them, thereby leading to improved fatigue strength by 40 to ∼50% over the non-HIPped specimens.

Effect of microstructure on properties of ADI and low alloyed ductile iron

Abstract: Microstructure, tensile, impact, hardness, fractography and wear characteristics were investigated for: (1) Austempered ductile iron (ADI); and (2) low alloyed ductile iron. Comparison has been made between the properties of these two types and that of conventional ductile iron. Detailed analysis, of the fracture mode for the 3 types of ductile iron, which failed under tensile and impact testing, were presented using the SEM. The wear properties were determined using pin-on-ring machine, under dry sliding conditions. The variation of mass loss and coefficient of friction with sliding distance, at different loads and speeds were presented and discussed. The wear mechanisms were investigated by means of subsurface observations. Microhardness test was used to study the change in the matrix strength with distance from the worn surface due to plastic deformation.

Microstructure and tensile properties of in situ synthesized (TiB+Y2O3)/Ti composites at elevated temperature

Abstract: A novel titanium matrix composites reinforced with TiB and rare earth oxides (Y2O3) were prepared by a non-consumable arc-melting technology. Microstructures of the composites were observed by means of optical microscope (OM) and transmission electron microscope (TEM). X-ray diffraction (XRD) was used to identify the phases in the composites. There are three phases: TiB, Y2O3 and titanium matrix alloy. TiB grows in needle shape, whereas Y2O3 grows from near-equiaxed shape to dendritic shape with increase of yttrium content in the composite. The interfaces between reinforcements and titanium matrix are very clear. There is no interfacial reaction. Tensile properties of the composites were tested at 773, 823 and 873 K. Both the fracture surfaces and longitudinal sections of the fractured tensile specimens were comprehensively examined by scanning electron microscope (SEM). The fracture mode and fracture process at different temperatures were analyzed and explained. The results show that the tensile strength of the composites has a significant improvement at elevated temperatures. The predominant fracture mode of composites is cleavaged at 773 and 823 K. Fracture occurs by ductile failure at 873 K.

Toughness and strength characteristics of Nb-W-Si ternary alloys prepared by Arc melting

Abstract: This article describes the room-temperature and high-temperature mechanical properties and failure modes of series Nb-W-Si alloys—Nb-10W, Nb-10Si, Nb-10Si-5W, Nb-10W-5Si, and Nb-10W-10Si—prepared by arc melting. For the Nb-10W alloy, the microstructure was a monolithic Nb solid solution (Nbss) with a grain size up to a few hundred microns, while the other four alloys consisted of primary Nbss and a eutectic of Nbss/Nb5Si3 (5-3 silicide) as a result of replacing Nb with Si. Among all alloys, the Nb-10W showed the highest fracture toughness of about 15.3 MPa√m1/2 and the lowest 0.2 pct yield compressive strength of 90 MPa at 1670 K. Conversely, the Nb-10Si-10W had the highest 0.2 pct yield strength of about 330 MPa at 1670 K and the lowest fracture toughness of 8.2 MPa√m1/2. It is suggested that toughness is supplied by the metallic Nbss phase, while high-temperature strength is mainly provided by the brittle silicide phase. For the Nb-10W alloy with the monolithic Nbss, intergranular cleavagelike crack propagation is the fracture mode at room temperature, and dislocation movement within the grains and grain-boundary sliding are the dominant modes of high-temperature failure. With two-phase Nbss/Nb5Si3 microstructures, the compressive damage of all four alloys at high temperature was dominated by debonding of the interfaces between the Nbss and the silicide; however, the fracture mode at room temperature is transgranular, controlled by the primary Nbss cleavage.

Anisotropic fracture behaviour of cold drawn steel: a materials science approach

Abstract: In this paper the fracture performance of axisymmetric notched samples taken from pearlitic steels with different levels of cold drawing is studied. To this end, a real manufacture chain was stopped in the course of the process, and samples of all intermediate stages were extracted. Thus the drawing intensity or straining level (represented by the yield strength) is treated as the fundamental variable to elucidate the consequences of the manufacturing route on the posterior fracture performance of the material. A materials science approach to the phenomenon is proposed, so that the strongly anisotropic fracture behaviour of the steels with high level of strain hardening (which exhibit a 90° step in the fracture surface) is rationalized on the basis of the markedly oriented pearlitic microstructure of the drawn steels which influences the operative micromechanism of fracture in this case.

Constitutive response of welded HSLA 100 steel

Abstract: The dynamic mechanical behavior of a welded joint of high-strength, low alloy (HSLA) 100 steel was investigated by both quasistatic (103 s � 1 ) and high strain rate (103 s � 1 ) tension loadings at ambient and low temperatures. The constitutive responses for the microstructurally different weld and base steels, along with the interface, which included the heat-affected zone (HAZ), were analyzed and compared. This response is successfully modeled by the mechanical threshold stress (MTS) constitutive equation for different regions of the welded joint, which shows qualitatively different behavior. The necking and failure occurred uniformly within the weld metal but not in the HAZ. The main mechanism for the failure of the welded joints is void growth. Microstructural characterization revealed that the nucleation of voids occurred mainly at the interface between the base and the weld metal, and initiated at inclusions. Measurements of damage distributions across HAZ were made to evaluate the contribution of porosity variation to the constitutive response. In both the quasi-static and dynamic tests, the deformation localization in the form of necking first appeared in the weld metal. Fractographic observation demonstrates that void evolution is a dominant factor in the macroscopic mechanical response. The Gurson � /Tvergaard model was included in the modeling effort to incorporate the effect of void opening on the mechanical response as well as tensile instability. The MTS constitutive model was successfully implemented to the tensile regime of loading. # 2003 Elsevier Science B.V. All rights reserved.

Plane-Strain Fracture Toughness on Thin AZ31 Wrought Magnesium Alloy Sheets

Abstract: There are few reports about plane-strain fracture toughness on wrought magnesium alloys. Also, there are a little data, for example plane-strain fracture toughness, that evaluates such as reliability and safety in magnesium alloys. Therefore, in this study, plane-strain fracture toughness, K IC , on thin AZ31 wrought magnesium alloy sheets was analyzed. As a result, appropriate plane-strain fracture toughness was not obtained by plane-strain fracture toughness test. It was because specimens used in this study were too thin to satisfy small scale yielding condition. But, as a result of stretched zone analysis, appropriate plane-strain fracture toughness, K IC , was obtained and the values of K IC were 16.5-18.4 MPam 1/2 . According to the result of this study, it is concluded that stretched zone analysis Was one of effective ways to evaluate fracture toughness of AZ31 wrought magnesium alloy appropriately. And the values of fracture toughness on AZ31 wrought magnesium alloys were equal or higher than that of cast magnesium alloy.

The microstructure and fracture behavior of the dissimilar alloy 690-SUS 304L joint with various Nb addition

Abstract: This study investigates the microstructure and fracture behavior of dissimilar weldments of alloy 690 and SUS 304L for various additions of niobium (0.1, 1.03, 2.49, and 3.35 wt pct) in the flux. With identical parameters and procedures, weldments were butt welded by the shielding metal arc welding (SMAW) process using three layers, with each layer being deposited in a single pass. The results indicate that the microstructure of the fusion zone was primarily dendritic and that the contents of Ni, Cr, and Fe within this zone remain relatively constant and resemble alloy 690. With Nb addition, it is noted that the microstructure changes from a cellular to columnar dendrite and equiaxed dendrite. Meanwhile, the dendrite arm spacing reduces and the secondary arms grow longer. Moreover, the composition of the interdendritic phase, whose precipitate volume percentage increases from 5 to 25 pct, changes from Al-Ti-O to Nb rich. The spread of the interdendritic phase is less in the root bead than in the cap bead due to the greater influence of base metal dilution in this region. Mechanical tests indicate that Nb addition increases the average hardness of the weldment and reduces its elongation prior to rupture. However, the tensile strength is essentially unchanged by Nb addition. It is found that the average hardness of the root bead is generally lower than the cap bead, and that the tensile specimens all rupture in the fusion zone, with the fracture surfaces exhibiting ductile features. It is noted that the cap bead tends to rupture interdendritically with increasing Nb addition. Finally, fractography shows that the dimples in the root become larger and shallower with Nb addition and are rich with an interdendritic phase.

The influence of reinforcement morphology on the tensile response of 6061/SiC/25p discontinuously-reinforced aluminum

Abstract: In order to study the effect of particle morphology on the tensile response of discontinuously-reinforced aluminum (DRA), two P/M 6061/SiC/25p materials were fabricated using established powder blending, compaction and extrusion techniques. One of the materials contained abrasive-grade SiC (F-600) whilst the second material was fabricated using a less angular SiC particulate with a lower aspect ratio, selected to give an overall higher bulk density (HBD) in the as-blended form. Care was taken to ensure that each material contained the same size and volume fraction of SiC particles, and that each material experienced an identical processing route. Mechanical testing was completed at ambient temperature, to measure the effect of particle morphology (F-600 vs. HBD) on both the elastic and plastic tensile response of the DRA. Specimens were tested in as-extruded (F), peak-aged (T6) and over-aged heat treatment (OA) conditions. The DRA produced with the HBD reinforcement consistently showed improved tensile elongation over the DRA containing the F-600 reinforcement, with the most significant effect being observed in the as-extruded (F) condition. The Considere criterion was used to show that different damage mechanisms may be operating in each material. Extensive microstructural and fractographic analyses were also carried out on the as-processed and as-tested specimens, using optical and electron microscopy. The dominant damage and failure mechanisms in each material are discussed in the light of these results.

The role of air in fatigue load interaction

Abstract: Natural fatigue crack formation and growth were studied in notched Al-Cu alloy coupons through high-resolution SEM fractography. The experiments were conducted under programmed loading conditions designed to induce microscopic marking of the crack formation and growth process under varying stress ratio and closure-free crack tip conditions. Control experiments were performed by switching between an air and vacuum environment. In air, varying the stress ratio from 0.74 down to 0.64 retards crack growth by up to a factor of five. This 'closure-free' stress ratio history effect totally disappears in vacuum, suggesting a significant environmental influence on stress ratio and its history. Crack-tip stress state appears to moderate environmental action, revealing a potential mechanism sensitive to residual stress. Consequently, crack closure, residual stress and crack front and plane orientation are identified as major load interaction mechanisms whose synergistic action controls fatigue under variable amplitude loading. The study also appears to suggest that as a consequence of the crack seeking the path of least resistance, load-sequence sensitive crack plane and front orientation may only induce retardation effects.

Formation of the breaking surface of alumina in laser cutting with a controlled fracture technique

Abstract: Laser cutting using the controlled fracture technique is based on the thermal breaking principle. The laser beam is applied to the surface of a ceramic substrate; the substrate is then controllably separated along the moving path of the laser beam. The fractography and cutting surface formation are studied in this work. It is found that the breaking surface can be divided into four regions. The first region is the laser evaporation region produced by the heat concentration. The second region is the columnar grain region produced by resolidification of the melted material. The third region is the intergranular fracture region produced by anisotropic thermal expansion. The fourth region is the transgranular fracture region characterized by unstable fractures. These experiments are conducted on alumina ceramics using a CO2 laser. The fracture mechanism is analysed using stress analysis and fractographic observation. The tensile stress generated on the surface due to the laser beam separates the mater...