Ductile Transgranular Fracture Research Articles

High Temperature Creep Behavior of Austenitic Fe-Ni-Cr Alloy

Creep damage investigation was carried out in Fe-Ni-Cr alloy at 800°C, 900°C, and 983°C using rectangular section form of specimen. In all the tests conducted on this material, some creep curves showed primary stage, secondary stage and tertiary stage. The creep fracture shows ductile transgranular fracture where separation occurred at the dendrites carbide interface suggesting that the detrimental effect of creep was compounded by precipitation of carbides at matrix. The presence of cavities may be due to the difference in thermal expansion characteristics of the austenite and carbide during high temperature test. Coarsening of carbides lead to cavities formation within the dendrite and carbide interface and form cavities linkage due to formation of crack and finally cause creep fracture. Increase in creep temperature it will lead to increase in creep rate. The fracture modes of creep samples were investigated to predict the failure mode.

Microstructure of Al-Li Alloy Electron Beam Welding Joint after Post-Weld Heat Treatment

Heat treatment was carried out to the 1420 Al-Li alloy electron beam welding (EBW) joints after welding, and the microstructures of welded joints are analyzed systematically before and after post-weld heat treatment (PWHT). The observation of joint microstructure demonstrates that the grain morphology of weldment changes from equiaxed dendrites in as-welded (AW) condition into equiaxed grains after PWHT, and that the fine strengthening phases precipitate within the grain. The XRD analysis of phase constituent and TEM observation of weldment indicate that the main strengthening phases in 1420 Al-Li alloy weldment are spheroidal δ′(Al3Li), β′(Al3Zr) and rod-like T(Al2MgLi) after PWHT. Furthermore, the δ′ phase precipitate free zone (PFZ) is found along the grain boundary. The scanning observation of joint fracture shows that Al-Li alloy EBW joint presents the characteristic of transgranular ductile fracture in AW condition. After PWHT, the Al-Li alloy welded joint presents the pattern of intergranular fracture. The variation of fracture mode is related to dispersed precipitation of δ′ phase and the formation of PFZ at the grain boundary in weldment after heat treatment.

Notch fracture toughness of high-strength Al alloys

In this study, the notch fracture toughness (NFT) of high-strength Al alloys was examined by a non-standardized procedure. The NFT is defined as the critical notch stress-intensity factor (NSIF) Kρ,c, which is determined by using several methods of analysis and computing. A set of specimens with different notch root radii made from overaged 7xxx alloy forging was selected. The influence of the notch radius on the fracture toughness of the material was considered. It was found that the notch radius strongly affects the fracture behavior of forged 7xxx alloy in overaged condition. The notch fracture toughness was higher than the fracture toughness of a cracked specimen and increased linearly with notch radius. The critical notch radius was related to the spacing of intermetallic (IM) particles which promote an intergranular or transgranular fracture mechanism according to their size. It appeared that ductile transgranular fracture generated by the formation of dimples around dispersoids and matrix precipitates was predominant which indicates that intense strains are limited to a much smaller zone than the coarse IM particles spacing. This double mechanism is also operate for crack propagation of ductile fatigue. The nature and morphology of IM particles exert significant effects on the rate of fatigue crack growth and fracture toughness properties.

Effects of microstructure and composition on spall fracture in aluminum

Abstract Spall strength was measured as a function of composition and microstructure in three Al materials: a high-purity Al (Al HP), a commercial-purity Al (AA1100) and an alloy of Al containing 3 wt.% Mg (Al–3Mg). The Al HP and AA1100 materials were tested as single-crystal sheets, and the Al–3Mg alloy was tested as polycrystalline sheets having a variety of controlled grain sizes. A high-intensity laser produced shock loadings to create tensile strain rates ranging from 2 × 10 6 s −1 to 5 × 10 6 s −1 , which caused spall fracture. Crystallographic orientation, relative to the direction of shock propagation, does not discernibly affect spall strength in the Al-HP material. Intermetallic particles, associated with impurity elements, initiate microstructural damage during tensile shock loading and reduce spall strength of the AA1100 material below that of the Al-HP material. The spall strength of the Al–3Mg is lowest among the three materials, and this is a result of the decreased ductility during spall fracture caused by the Mg solid-solution alloying addition. Grain size affects fracture character of the Al–3Mg material, but does not discernibly affect spall strength; the fraction of ductile transgranular fracture, versus brittle intergranular fracture, increases with grain size.

Ductilization of a Ni<sub>3</sub>(Si,Ti) Intermetallic Alloy by Addition of Interstitial Type Elements

The effect of a concomitant doping of interstitial type elements boron (B) and carbon (C), and boron (B) and nitrogen (N) on tensile properties of a Ni3(Si,Ti) intermetallic alloy was investigated in the temperature range between room temperature and 973 K. It was found that the concomitant doping of (C/B) and (N/B) remarkably improved the intermediate-temperature tensile elongation of the Ni3(Si,Ti) alloy compared with the simple doping of B or C. It was also shown that the fracture surface of the alloy doped with (C/B) and (N/B) exhibited the ductile transgranular fracture mode while that of the alloy doped with only B showed a brittle intergranular fracture mode at 773 K. These results clearly indicate that the concomitant doping of the interstitial type elements are useful for improving the intermediate-temperature tensile ductility of the Ni3(Si,Ti) alloy.

Identification of modes of fracture in a 2618-T6 aluminum alloy using stereophotogrammetry

The identification and the development of a quantification technique of the modes of fracture in fatigue fracture surfaces of a 2618-T6 aluminum alloy were developed during this research. Fatigue tests at room and high temperature (230 Degree-Sign C) were carried out to be able to compare the microscopic fractographic features developed by this material under these testing conditions. The overall observations by scanning electron microscopy (SEM) of the fracture surfaces showed a mixture of transgranular and ductile intergranular fracture. The ductile intergranular fracture contribution appears to be more significant at room temperature than at 230 Degree-Sign C. A quantitative methodology was developed to identify and to measure the contribution of these microscopic fractographic features. The technique consisted of a combination of stereophotogrammetry and image analysis. Stereo-pairs were randomly taken along the crack paths and were then analyzed using the profile module of MeX software. The analysis involved the 3-D surface reconstruction, the trace of primary profile lines in both vertical and horizontal directions within the stereo-pair area, the measurements of the contribution of the modes of fracture in each profile, and finally, the calculation of the average contribution in each stereo-pair. The technique results confirmed a higher contribution of ductilemore » intergranular fracture at room temperature than at 230 Degree-Sign C. Moreover, there was no indication of a direct relationship between this contribution and the strain amplitudes range applied during the fatigue testing. - Highlights: Black-Right-Pointing-Pointer Stereophotogrammetry and image analysis as a measuring tool of modes of fracture in fatigue fracture surfaces. Black-Right-Pointing-Pointer A mixture of ductile intergranular and transgranular fracture was identified at room temperature and 230 Degree-Sign C testing. Black-Right-Pointing-Pointer Development of a quantitative methodology to obtain the percentage of modes of fracture within the fracture surface.« less

Mechanical characterization of cp-Ti produced by investment casting

The influence of hot swaging (SW) and annealing treatment on microstructure and mechanical properties of commercially pure titanium produced by investment casting was evaluated. The as-cast samples showed a typical microstructure consisting of a variety of α-morphologies, while the hot swaged samples exhibited a kinked lamellar microstructure. Annealing at 500 °C did not significantly change this microstructure while annealing at 700 and 870 °C led to recrystallization and formation of equiaxed microstructures. The cast bars exhibited a typical hard α-layer in near-surface regions with a maximum depth and maximum hardness of 720 μm and HV 0.5 660, respectively. Due to SW, the tensile strength of the as-cast material drastically increased from 605 MPa to 895 MPa. Annealing at 500 °C decreased the tensile strength slightly from 895 to 865 MPa while annealing at 700 °C led to a further pronounced drop in tensile strength from 865 to 710 MPa. No additional decrease in tensile strength was noticed with increasing the annealing temperature from 700 to 870 °C. The true fracture strain of the as-cast and hot swaged samples was in the range of 0.05 to 0.12, while the annealed samples showed values in the range of 0.25 to 0.53. In addition, the as-cast and hot swaged samples revealed a brittle cleavage fracture surfaces. However, the annealed samples showed a transgranular ductile fracture with formation of dimples.

Effect of Annealing Temperature on Microstructure and Mechanical Properties of Hot Swaged cp-Ti Produced by Investment Casting

The purpose of this study is to evaluate the influence of hot swaging (SW) and annealing treatment on microstructure and mechanical properties of commercially pure titanium (grade 4) produced by investment casting. During SW at 700 °C, the diameter of the cast titanium bars was reduced from 25 to 8.5 mm in 14 steps. After SW, material was annealed for 1 h at 500, 700, or 870 °C. The as-cast samples showed a typical microstructure consisting of a variety of α-morphologies, while the hot swaged samples exhibited a kinked lamellar microstructure. Annealing at 500 °C did not significantly change this microstructure, while annealing at both 700 and 870 °C led to recrystallization and formation of equiaxed microstructures. The cast bars exhibited a typical hard α-layer in near-surface regions with maximum depth and maximum hardness of 720 μm and 660 HV0.5, respectively. Due to SW, the tensile strength of the as-cast material drastically increased from 605 to 895 MPa. Annealing at 500 °C decreased this tensile strength slightly from 895 to 865 MPa while annealing at 700 °C led to a further pronounced drop in tensile strength from 865 to 710 MPa. No additional decrease in tensile strength was observed by increasing the annealing temperature from 700 to 870 °C. The tensile ductility of the as-cast and hot swaged samples was approximately the same in the range of 0.05 to 0.11, while the annealed samples showed values in the range of 0.25 to 0.53. In addition, the as-cast and hot swaged samples revealed a brittle cleavage fracture surfaces. However, the annealed samples showed a transgranular ductile fracture with formation of dimples.

Stress corrosion cracking susceptibility of austenitic stainless steels in supercritical water conditions

The presented paper summarizes the results of general corrosion and stress corrosion cracking (SCC) susceptibility tests in supercritical water (SCW), studied for austenitic stainless steel 316L, with the aim to identify maximum SCW temperature usability and specific failure mechanisms prevailing during slow strain-rate tensile (SSRT) tests in ultra-pure demineralized SCW solution with controlled oxygen content. The general corrosion tests clearly revealed the applicability of austenitic stainless steel in SCW to be limited to 550 °C as maximum temperature as oxidation rates of austenitic stainless steels 316L increase dramatically above 550 °C. The SSRT tests were performed using a step-motor controlled loading device in an autoclave at 550 °C SCW. Besides the strain rate (resp. crosshead speed), the oxygen content was varied in the series of tests. The obtained results showed that even at the lowest strain rate, a serious increase of SCC susceptibility, as typically characterized by IGSCC crack growth, was not observed. The fractography confirmed that failure was due to a combination of transgranular SCC and transgranular ductile fracture. Based on fractographic findings a phenomenological map describing the SCC regime of SSRT test parameters could be proposed for AISI 316L.

Effects of second phases on fracture behavior of Mg-10Gd-3Y-0.6Zr alloy

The effects of second phases on the fracture behavior of Mg-10Gd-3Y-0.6Zr alloy were investigated. The results show that the fracture mode can be generally described as ductile transgranular fracture in as-extruded condition and intergranular fracture in peak-aged condition. In as-extruded condition, the ductile transgranular fracture occurs by the formation and transgranular propagation of the microcrack from the broken primary phases. However, as the collaboration effects of precipitates inside grains and on the grain boundaries have the tendency to reduce the cohesive strength of the grain boundary, and make the grain boundaries the favorable path for crack propagation, the intergranular fracture occurs in peak-aged condition.

Understanding the mechanical properties of 2198 Al-Li-Cu alloy in relation with the intra-granular and inter-granular precipitate microstructure

3rd generation Al-Li-Cu alloys such as AA2198 are known to present extremely high strength levels because of the precipitation state of the material. The final precipitation state is reached via a complex sequence that can involve notably solute clusters, GP zones, θ', T1 and δ' phases. In this work, we have studied the evolution of mechanical properties during heat treatment. Microhardness and tensile tests have shown a direct link with the evolution of the size of intra-granular T1 precipitates. The evolution of the fracture mode in relation with the precipitation state has been followed by the so-called short bar test that allows to load relatively thin plates in the short transverse direction, which is particularly sensitive to the inter-granular fracture mode. Along the heat treatment, from the naturally aged T351 state and along ageing at 155°C, we show a clear transition from a ductile transgranular fracture mode to a ductile inter-granular fracture mode, controlled by the precipitation of the T1 phase at the grain boundaries.

Multiscale modeling of ductile failure in metallic alloys

Abstract Micromechanical models for ductile failure have been developed in the 1970s and 1980s essentially to address cracking in structural applications and complement the fracture mechanics approach. Later, this approach has become attractive for physical metallurgists interested by the prediction of failure during forming operations and as a guide for the design of more ductile and/or high-toughness microstructures. Nowadays, a realistic treatment of damage evolution in complex metallic microstructures is becoming feasible when sufficiently sophisticated constitutive laws are used within the context of a multilevel modelling strategy. The current understanding and the state of the art models for the nucleation, growth and coalescence of voids are reviewed with a focus on the underlying physics. Considerations are made about the introduction of the different length scales associated with the microstructure and damage process. Two applications of the methodology are then described to illustrate the potential of the current models. The first application concerns the competition between intergranular and transgranular ductile fracture in aluminum alloys involving soft precipitate free zones along the grain boundaries. The second application concerns the modeling of ductile failure in friction stir welded joints, a problem which also involves soft and hard zones, albeit at a larger scale.

The effect of film thickness on the failure strain of polymer-supported metal films

We perform uniaxial tensile tests on polyimide-supported copper films with a strong (1 1 1) fiber texture and with thicknesses varying from 50 nm to 1 μm. Films with thicknesses below 200 nm fail by intergranular fracture at elongations of only a few percent. Thicker films rupture by ductile transgranular fracture and local debonding from the substrate. The failure strain for transgranular fracture exhibits a maximum for film thicknesses around 500 nm. The transgranular failure mechanism is elucidated by performing finite element simulations that incorporate a cohesive zone along the film/substrate interface. As the film thickness increases from 200 to 500 nm, a decrease in the yield stress of the film makes it more difficult for the film to debond from the substrate, thus increasing the failure strain. As the thickness increases beyond 500 nm, however, the fraction of (1 0 0) grains in the (1 1 1)-textured films increases. On deformation, necking and debonding initiate at the (1 0 0) grains, leading to a reduction in the failure strain of the films.

Microstructure and properties of an Al-Zn-Mg-Cu alloy pre-stretched plate under various ageing conditions

The microstructures after various ageing treatments and their relation to the strength, fracture toughness, and corrosion behavior of an Al-Zn-Mg-Cu alloy pre-stretched plate were investigated. The results show that retrogression and reaging (RRA) treatment led to a combination of high strength and stress corrosion cracking (SCC) resistance of the alloy. The TEM microstructure of the RRA-treated alloy is a distribution of very fine precipitates in the aluminum matrix grains, similar to that obtained under T6 condition, and the distribution of coarse η MgZn 2 precipitates on the grain boundaries similar to that obtained by T7 temper. SEM observations revealed that most of the intergranular fracture characteristics were present on the fracture surface of both the T6 and RRA-treated specimens. On the contrary, the fractographs of the T7 treated specimens mainly consisted of dimple-type ductile transgranular fracture with minor intergranular cracking.

Fe-Ni-Co 코바 합금의 고온변형거동에 미치는 합금원소(Mn, Mo, B) 첨가의 영향

The effect of alloying elements(Mn, S, Mo, B) on the high temperature deformation behavior of Fe-29%Ni-17%Co (Kovar) alloy were investigated. And the effect of high temperature oxidation on the hot ductility was also studied. The hot ductility of Kovar alloy was drastically increased with the addition of Mn and lowering of S content. It has been found that the brittle intergranular fracture at high temperature cracking is closely associated with the FeS sulfide along the grain boundary. When Mn was added, the type of sulfide was changed to MnS from FeS and ductile intergranular fracture and transgranular fracture were promoted. The formation of oxide layer was found to have minimized the hot ductility of the Kovar alloy significantly. Grain boundary micro-cracks in the internal oxide region were noted following deformation due to high temperature, one of which acting as a notch that caused the poor hot workability of the oxidized specimen. The addition of Mo to the Kovar alloy could also retard the decrease in the hot ductility of the oxidized specimen through the prevention of notching due to internal oxidation. Hot ductility was remarkably improved by the addition of Boron. The improvement of hot ductility results from the grain boundary migration mainly due to the dynamic recrystallization at lower temperature range (<TEX>$900{\sim}1000^{\circ}C$</TEX>).

Laser-induced spallation of aluminum and Al alloys at strain rates above 2×106s−1

Material microstructure is a significant determinant of the tensile stress at which materials fail. Using a high-energy laser to drive shocks in thin slabs, we have explored the role material microstructure plays on the spall strength of high-purity and alloyed aluminum at strain rates of (2–7.5)×106s−1. Slabs of pure recrystallized Al and recrystallized or cold worked Al+3wt% Mg were shock driven using the Z-Beamlet Laser at Sandia National Laboratories. Velocity interferometer measurements determined the spall strength of the materials, and postshot target analysis explored the microscopic fracture morphology. We observed the greatest spall strength for large-grained, recrystallized high-purity aluminum, with the dominant failure mode being ductile and transgranular. We observe for the first time at these strain rates fracture features for a fine-grained Al+3wt% Mg that were a combination of brittle intergranular and ductile transgranular fracture types. Postshot analysis of target cross sections and hydrocode simulations indicate that this mixed-mode failure results from spall dynamics occurring on spatial scales on the order of the grain size. Differences in spall strength between these Al samples were experimentally significant and correlate with the damage morphologies observed.

Deformation behavior and microstructure evolution of 7050 aluminum alloy during high temperature deformation

Deformation behavior and microstructure evolution of 7050 aluminum alloy were investigated by tensile tests conducted at different temperatures (340, 380, 420, and 460 °C) with different strain rates of 1.0 × 10 −4, 1.0 × 10 −3, 1.0 × 10 −2 and 0.1 s −1. The results show that the stress level of the alloy can be presented by a Zener–Holloman parameter in a hyperbolic sine-type equation with the hot activation energy of 256.6 kJ/mol. Ductile transgranular fracture transforms progressively into ductile intergranular (or inter-subgranular) fracture with the decrease of Z value. At the same time, the soften mechanism of the alloy during high temperature deformation transforms from dynamic recovery to continuous dynamic recrystallization with decreasing Z value. The main deformation mechanism is usual slipping when the alloy was deformed at high Z value. Grain boundaries sliding takes part in deformation with low Z value. Grain boundaries migration plays the role with medium Z value.

Effect of precipitation aging on the fracture behavior of Mg–11Gd–2Nd–0.4Zr cast alloy

The effect of precipitation aging on fracture behavior of Mg–11Gd–2Nd–0.4Zr cast alloy at room temperature has been studied in this paper. The transition of fracture mode as a function of temper state can be generally described as follows: ductile transgranular fracture in as-quenched condition, transgranular cleavage fracture in peak-aged condition and intergranular fracture in overaged condition. The transgranular cleavage fracture is attributed to the planar slip arising from the dislocation shearing of strengthening precipitates in matrix. The intergranular fracture is explained to occur as a result of the growth of grain boundary precipitates (GBPs) and precipitate free zones (PFZs) via two different types of mechanism, i.e. decohesive rupture accompanied by intergranular facets with ledges in slightly overaged specimen with fine GBPs and narrow PFZs, and dimple rupture in substantially overaged specimen with coarse GBPs and wide PFZs.

Relationship between Microstructure and Tensile Strength in the Directionally Solidified (23-27) at. % Al-Ni Alloys

Directional or single crystal technique was applied to enhance the ductility, and two phases of γ (Ni) phase or β (NiAl) phase in γ‘(Ni3Al) matrix were also considered to increase the strength and ductility. In this study, directionally solidified rods were prepared at the solidification rate of 50µm/s in 23-27 at.% Al-Ni alloys, and tensile strengths of these rods were analyzed at room temperature. Directionally solidified samples showed the γ dendrite fibers formed in the Ni3Al matrix in the hypo eutectic composition of 23 at.% Al, the γ‘ single phase in the eutectic composition of 24.5 at. % Al, and the β dendrite fibers in the γ‘ matrix in the hyper eutectic compositions of 25, 26, 27 at.% Al. The hypoeutectic alloy including γ dendrites with γ‘ matrix exhibited a large elongation of over 70% with ductile transgranular fracture at room temperature. With increasing Al contents, the γ dendritic microstructure changed to the β dendrite in the γ‘ matrix, which resulted in decreasing the elongation by increasing the volume fraction of the brittle β dendrites in the ductile γ’ matrix.

Small crack behavior and fracture of nickel-based superalloy under ultrasonic fatigue

Fracture and small crack behavior in the very high cycle domain of 10 9 cycles were investigated with a nickel-based superalloy under ultrasonic fatigue in ambient air at room temperature. The influence of ultrasonic frequency is examined by comparing the results with those in conventional low frequency fatigue. It is found that fatigue strength increases as frequency is raised up to 19.5 kHz and the most of fatigue life is consumed in nucleating and propagating small cracks up to 100 μm. Transition of fracture mode from transgranular ductile fracture to cleavage-dominated fracture occurs beyond a critical stress intensity factor range of approximately 21 MPa m , leading to the catastrophic failure under ultrasonic fatigue.