Fracture mechanics in design and service: ‘living with defects’ - Metallurgical aspects of the toughness of engineering alloys

1981 ◽  
Vol 299 (1446) ◽  
pp. 45-57 ◽  
Keyword(s):  
Tensile Stress ◽  
Crack Extension ◽  
Failure Modes ◽  
Fatigue Loading ◽  
Second Phase ◽  
Cleavage Crack ◽  
Factors Associated ◽  
Second Phase Particles ◽  
Engineering Alloys ◽  
Growth Of Voids

The paper treats micro-mechanical modes of crack extension, classed as ‘cracking’ and ‘rupture’ processes. In ‘cracking’, a cleavage crack nucleus propagates when a critical local tensile stress is attained, the magnitude of the stress being determined by the microstructure. Models for crack propagation from carbides and from martensite/ bainite ‘packets’ are discussed. The ‘rupture’ processes involve the initiation and growth of voids, centred on second-phase particles. Coalescence may arise from ‘internal necking’ or ‘fast shear’ and the factors associated with these two modes are described. Consideration is also given to the ways in which microstructure may produce scatter in toughness values and in growth-rates under fatigue loading, where both cyclic and monotonic failure modes are significant.

scholarly journals Mechanism Of Ductile Rupture In The AL/Sapphire System Elucidated Using X-Ray Tomographic Microscopy

1995 ◽  
Vol 409 ◽  
Author(s):  
Wayne E. King ◽  
Geoffrey H. Campbell ◽  
David L. Haupt ◽  
John H. Kinney ◽  
Robert A. Riddle ◽  
...  
Keyword(s):  
Void Growth ◽  
Fracture Mechanism ◽  
Crack Extension ◽  
Initial Crack ◽  
Fracture Mechanisms ◽  
Second Phase ◽  
Metal Foil ◽  
Ductile Rupture ◽  
X Ray ◽  
Second Phase Particles

AbstractThe fracture of a thin metal foil constrained between alumina or sapphire blocks has been studied by a number of investigators. The systems that have been investigated include Al [1,2], Au [3], Nb [4], and Cu [5]. Except for Al/ Al2O3 interfaces, these systems exhibit a common fracture mechanism: pores form at the metal/ceramic interface several foil thicknesses ahead of the crack which, under increasing load, grow and link with the initial crack. This mechanism leaves metal on one side of the fracture surface and clean ceramic on the other. This has not been the observation in Al/ A12O3 bonds where at appropriate thicknesses of Al, the fracture appears to proceed as a ductile rupture through the metal.The failure of sandwich geometry samples has been considered in several published models, e.g., [6,71. The predictions of these models depend on the micromechanic mechanism of crack extension. For example, Varias et al. proposed four possible fracture mechanisms: (i) near-tip void growth at second phase particles or interfacial pores and coalescence with the main crack, (ii) high-triaxiality cavitation, i.e., nucleation and rapid void growth at highly stressed sites at distances of several layer thicknesses from the crack tip, (iii) interfacial debonding at the site of highest normal interfacial traction, and (iv) cleavage fracture of the ceramic. Competition among the operative mechanisms determines which path will be favored.This paper addresses the question of why the fracture of the A1/A12O3 system appears to be different from other systems by probing the fracture mechanism using X-ray tomographic microscopy (XTM). We have experimentally duplicated the simplified geometry of the micromechanics models and subjected the specimens to a well defined stress state in bending. The bend tests were interrupted and XTM was performed to reveal the mechanism of crack extension.

Brittle cleavage of L12 trialuminides

1990 ◽  
Vol 5 (8) ◽  
pp. 1639-1648 ◽  
Author(s):  
E. P. George ◽  
J. A. Horton ◽  
W. D. Porter ◽  
J. H. Schneibel
Keyword(s):  
Room Temperature ◽  
Crack Nucleation ◽  
Cleavage Plane ◽  
Second Phase ◽  
Cleavage Crack ◽  
Transgranular Cleavage ◽  
Transmission Electron ◽  
Nucleation Sites ◽  
Second Phase Particles ◽  
Cleavage Strength

Three trialuminide alloys, binary Al–25Sc, ternary Al–25Zr–6Fe, and quaternary Al–23Ti–6Fe–5V, all having the cubic L12 structure, were investigated. All three alloys fracture in a brittle manner (fracture toughness, 2–3 MPa m½), predominantly by transgranular cleavage. Of nineteen cleavage facets examined in binary Al3Sc, seventeen were of the {110} type and only two were of the {100} type, consistent with our earlier work which showed that the cleavage plane occurring most frequently in quaternary Al–23Ti–6Fe–5V is also {110}. The room-temperature hardnesses and yield strengths (100–200 DPH and 100–270 MPa, respectively) of all three alloys are quite low (comparable to ductile L12 alloys like Ni3Al), indicating that there is significant dislocation activity in these materials. Consistent with this, transmission electron microscopy identified several APB-coupled dislocations with b - a/2〈110〉 gliding on the {111} planes in both binary Al–25Sc and quaternary Al–23Ti–6Fe–5V. The separations between the superpartials in Al–25Sc and Al–23Ti–6Fe–5V were measured to be 3.7 and 4 nm, respectively, giving APB energies of 313 and 274 mJ/m2, respectively. Auger analyses failed to detect any impurities on the cleavage facets themselves, or on second phase particles (or other potential cleavage crack nucleation sites). It is therefore concluded that brittle fracture in these alloys is not impurity-induced. Based on all the results obtained to date we conclude that the unusual brittleness of L12 trialuminides is related to their intrinsically low cleavage strength. Possible reasons for their low cleavage strength are discussed.

Effects of a second phase on the impact strength of structural steels

1978 ◽  
Vol 36 (1) ◽  
pp. 604-605
Author(s):  
T. Gréday ◽  
H. Mathy
Keyword(s):  
Fracture Mechanisms ◽  
Second Phase ◽  
Cleavage Crack ◽  
X Ray ◽  
Steel Microstructure ◽  
Thin Foils ◽  
Hsla Steels ◽  
Second Phase Particles ◽  
Second Phases ◽  
The Impact

Both steel producers and users are faced with the problem of evaluating steel toughness. This property depends on the steel microstructure and, more particularly, on the presence of second phase particles. Consequently is important the knowledge of how fracture mechanisms are related to those particles. Also, quantitative relationships between second phase particles and properties are requested. Results have been obtained using a SEM equipped with peripherals. X-ray spectrometers and image analysis (1,2) as well as a TEM working on thin foils. Studied second phases range from microprecipitates in HSLA steels to coarser particles such as : inclusions, cementite, pearlite and bainite or martensite islands. In each case, characterization has been attempted of their size, shape, nature and distribution. Toughness is defined by transition temperatures or by shelf energy. Pearlite islands can initiate, stop or deflect a local crack. Deflection brings into relief 2 facets of the pearlite island when it deflects a cleavage crack originated by 2 adjacent grains (Fig.l).

Polishing process effect on the image of dispersed precipitates

1984 ◽  
Vol 42 ◽  
pp. 534-535
Author(s):  
C.T. Hu ◽  
C.W. Allen
Keyword(s):  
Matrix Material ◽  
Specimen Preparation ◽  
Second Phase ◽  
Precipitate Particle ◽  
Polishing Process ◽  
Second Phase Particles ◽  
The Matrix ◽  
Surface Profiles ◽  
Thinning Process

One important problem in determination of precipitate particle size is the effect of preferential thinning during TEM specimen preparation. Figure 1a schematically represents the original polydispersed Ni3Al precipitates in the Ni rich matrix. The three possible type surface profiles of TEM specimens, which result after electrolytic thinning process are illustrated in Figure 1b. c. & d. These various surface profiles could be produced by using different polishing electrolytes and conditions (i.e. temperature and electric current). The matrix-preferential-etching process causes the matrix material to be attacked much more rapidly than the second phase particles. Figure 1b indicated the result. The nonpreferential and precipitate-preferential-etching results are shown in Figures 1c and 1d respectively.

Analytical Electron Microscopy Study of Second-Phase Particles in 7075 and 7475 Aluminum Alloys

1985 ◽  
Vol 43 ◽  
pp. 348-349
Author(s):  
M. Raghavan ◽  
J. Y. Koo ◽  
J. W. Steeds ◽  
B. K. Park
Keyword(s):  
Aluminum Alloys ◽  
Silicon Oxide ◽  
Analytical Electron Microscopy ◽  
Microscopy Study ◽  
Electron Microscopy Study ◽  
Partial Substitution ◽  
Second Phase ◽  
X Ray ◽  
Second Phase Particles ◽  
Almost All

X-ray microanalysis and Convergent Beam Electron Diffraction (CBD) studies were conducted to characterize the second phase particles in two commercial aluminum alloys -- 7075 and 7475. The second phase particles studied were large (approximately 2-5μm) constituent phases and relatively fine ( ∼ 0.05-1μn) dispersoid particles, Figures 1A and B. Based on the crystal structure and chemical composition analyses, the constituent phases found in these alloys were identified to be Al7Cu2Fe, (Al,Cu)6(Fe,Cu), α-Al12Fe3Si, Mg2Si, amorphous silicon oxide and the modified 6Fe compounds, in decreasing order of abundance. The results of quantitative X-ray microanalysis of all the constituent phases are listed in Table I. The data show that, in almost all the phases, partial substitution of alloying elements occurred resulting in small deviations from the published stoichiometric compositions of the binary and ternary compounds.

Development of Second-Phase Particles during Solidification and Homogenization of Aluminium Alloy AlMn1

2019 ◽  
Vol 56 (5) ◽  
pp. 317-341 ◽  
Author(s):  
O. Engler ◽  
K. Kuhnke ◽  
K. Westphal
Keyword(s):  
Aluminium Alloy ◽  
Second Phase ◽  
Second Phase Particles

scholarly journals Evidence of hydrogen trapping at second phase particles in zirconium alloys

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Christopher Jones ◽  
Vidur Tuli ◽  
Zaheen Shah ◽  
Mhairi Gass ◽  
Patrick A. Burr ◽  
...  
Keyword(s):  
Density Functional ◽  
Secondary Ion Mass Spectrometry ◽  
Zirconium Alloys ◽  
Nuclear Industry ◽  
Second Phase ◽  
Hydrogen Trapping ◽  
Functional Theory ◽  
Second Phase Particles ◽  
Hydrogen Isotope Ratios ◽  
Dft Modelling

AbstractZirconium alloys are used in safety–critical roles in the nuclear industry and their degradation due to ingress of hydrogen in service is a concern. In this work experimental evidence, supported by density functional theory modelling, shows that the α-Zr matrix surrounding second phase particles acts as a trapping site for hydrogen, which has not been previously reported in zirconium. This is unaccounted for in current models of hydrogen behaviour in Zr alloys and as such could impact development of these models. Zircaloy-2 and Zircaloy-4 samples were corroded at 350 °C in simulated pressurised water reactor coolant before being isotopically spiked with 2H2O in a second autoclave step. The distribution of 2H, Fe and Cr was characterised using nanoscale secondary ion mass spectrometry (NanoSIMS) and high-resolution energy dispersive X-ray spectroscopy. 2H− was found to be concentrated around second phase particles in the α-Zr lattice with peak hydrogen isotope ratios of 2H/1H = 0.018–0.082. DFT modelling confirms that the hydrogen thermodynamically favours sitting in the surrounding zirconium matrix rather than within the second phase particles. Knowledge of this trapping mechanism will inform the development of current understanding of zirconium alloy degradation through-life.

scholarly journals On the variety and formation sequence of second-phase particles in nickel-based superalloys fabricated by laser powder bed fusion

Materialia ◽  
2021 ◽  
Vol 15 ◽  
pp. 101037
Author(s):  
A. Després ◽  
C. Mayer ◽  
M. Veron ◽  
E.F. Rauch ◽  
M. Bugnet ◽  
...  
Keyword(s):  
Second Phase ◽  
Powder Bed Fusion ◽  
Laser Powder Bed Fusion ◽  
Powder Bed ◽  
Second Phase Particles
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