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Studies of sprayed tungsten carbide hardfacing deposits on three types of industrial component have revealed a number of different wear mechanisms. The wear mechanism operating at any one location on a component may be explained in terms of the interaction of the hardfacing with the abrasive environment, factors such as angle and speed of impingement, particle size and shape and hardness being important. The distinctive wear mechanisms may be separated according to whether wear of the carbide or of the matrix is rate controlling. Best wear resistance is obtained in the former case, but the latter is more common. Some factors which control the interfacial and transgranular fracture behaviour of the carbide are discussed.

The probability of fracture initiation, propagation, and arrest is one of the important problems facing designers, analysts, and operators of modern structures including nuclear reactors. The question of fracture requires special considerations which include the random nature of the loading and the statistical nature of the material's response to fracture under the imposed service loads. In most structural applications it is essential to know not only if and when fracture will occur under normal and off-normal operating conditions, but also to have some knowledge of the fracture propagation configurations and their resulting influence on the integrity of the structure. In this paper we address the problem of multiple fracture propagation configurations in structures under service conditions. This is accomplished by introducing a generalized energy criterion for multiple brittle fracture in nonhomogeneous and anisotropic materials. The fracture criterion is expressed in terms of the so-called Hartz function H which measures the difference between the total instantaneous strain energy release rate G T and the rate at which energy is required for the formation of new fracture surfaces R T . Strain energy release rates are computed for a variety of symmetric as well as asymmetric fracture propagation configurations from finite element solutions of incrementally related boundary value problems. These solutions yield a deterministic influence parameter α which is used to relate the applied loading to the probabilistic expression for the strain energy release rate. A similar treatment is given to the function R T for which the influence parameter β must be determined experimentally. The parameters, α and β, depend upon the relative locations, sizes and orientations of the primary and secondary cracks as well as flaws, and secondary imperfections present in the material. In addition the parameter β depends upon the relative values of the specific surface energy associated with the possible primary and secondary fracture paths. This is important for anisotropic materials such as ceramics mixed-oxide fuels and concrete, and for materials experiencing stress-corrosion fracture, where the energies associated with intergranular and transgranular fracture, for example, may differ significantly. From the probabilistic expressions for G T and R T , the Hartz function is determined as a random variable h which describes the multiple fracture process. The formulations developed in this presentation are applied to a single edge notched panel experiencing multiple fracture under assumed random loadings. Some interesting symmetric as well as asymmetric fracture configurations are studied and the results are related to reported experimental observations. The approach presented here may have applications in the areas of fatigue crack propagation, stress corrosion cracking, and fail-safe design optimization. Current studies are aimed at simulating the influence of grain structure anisotropy, intergranular corrosive attack, and creep deformation on multiple fracture in polycrystalline materials.

1. We have established that under the effect of elastic vibrations in molybdenum a mosaic structure arises with large angles of misorientation (3–5 °). The mean subgrain size is about 2 μ. 2. We determined that the formation of misoriented mosaic structure in molybdenum, chromium, and tungsten takes place upon cyclic loading with amplitudes and exposure times at which cracks due to fatigue appear. 3. It was shown that the application of intermediate anneals up to 800 °C and electrolytic polishing in order to remove the surface layer can substantially decrease the number of microcracks observed in the surface layer. 4. It was shown that the hardening achieved in molybdenum by low-frequency cyclic loading is comparable to the hardening that can be achieved by 90% plastic deformation in rolling. The yield stress of cyclically loaded samples amounts to 110–120 kg/mm2 as compared to 50–60 kg/mm2 in the initial state. The transition temperature to cold brittleness is lowered by 20–30 °C. 5. We have established that cyclic loading of molybdenum samples leads to a change of the character of brittle fracture. While in the initial, recrystallized state the fracture is primarily intergranular, after cyclic loading we observed transgranular fracture.

An investigation was made of the metallurgical factors responsible for the improved impact resistance of 5150 steel processed by modified ausforming. It was found that if the austenite is unrecrystallized prior to quenching to martensite, a change occurs from a primarily intergranular to a transgranular fracture path. While some increase in impact strength seems associated with this change, the major factor responsible for improved impact strength is lamellate fracture,i.e., blunting of a propagating crack by splitting on planes parallel to the rolling plane. Such fracture is not observed unless banded alloy segregation exists, in which case splitting occurs in a brittle manner along sulfide interfaces in bands of high alloy content. While lamellate fracture produces a marked improvement in longitudinal impact strength, there is a corresponding deterioration of transverse impact properties.

Since an establishment of the preparation method of the CrO2 powder, various efforts have been made to obtain a compact body of the oxide to measure its electrical and magnetic properties. Since the oxide decomposes to nonmagnetic Cr2O3 at temperatures above 400°-450°C in the atmospheric pressure, a sintered body could not be obtained by the usual technique.The present paper describes the preparation method of the highly dense sintered bodies of CrO2 by hot-pressing at mechanical pressure of 30000kg/cm2 and at 450°C. A high pressure apparatus used in this experiment is of a modified internal heating type. A starting powder of CrO2 was obtained by the thermal decomposition of CrO3 under an oxygen pressure of about 350kg/cm2 at 400°C. The obtained powder was coldpressed into a disk having the diameter of 6mm and the thickness of 3mm. The pressed body was heated at 450°C for 2hours at various pressures ranging from 10000 to 30000kg/cm2 in the pressing apparatus.An electronmicroscopic observation of fractured surfaces of the samples hot-pressed at pressures less than 15000kg/cm2 showed an intergranular fracture and the original shape of each grain remained unchanged. In the samples pressed at above 25000kg/cm2 a transgranular fracture was observed and therefore the samples conclusively exhibited a strong bonding between grains. The applied pressures higher than 30000kg/cm2 remarkably promoted the sintering of CrO2 even at low temperature as 450°C.The bulk density of the samples pressed at 30000kg/cm2 was higher than 99% of the X-ray density. The electrical properties were quite different from the result obtained by CrO2 powder. The electrical resistivity showed a metal-like behavior, i.e. it increased monotonously with increasing temperature in the range -196°C to 300°C and there existed a small change in a slope of temperature dependence of resestivity at the Curie temperature of CrO2. The result is consistent with the metallic behavior of CrO2 as predicted by a Goodenough model.

The fractographic changes occurring as a result of the interaction of titanium and methanol/HCl have been investigated in a long series of tests in which specimens were removed from the solution and broken in air after a range of treatments incorporating combinations of washing procedures and aging before fracture at different temperatures in different environments. It is shown that the extensive transgranular striations observed, associated with regions of planar slip, arise from the influence of absorbed hydrogen during stress corrosion or oxygen during aging. Aging can cause the dispersion of the absorbed hydrogen and a complete absence of striations. Charging with hydrogen can cause their appearance. These regions occur also in Ti-5Al-2.5Sn alloy with cleavage. It is concluded that all the transgranular fracture in stress corrosion arises from absorbed hydrogen and that the occurrence of striations arises from a low energy tear process which can result in rapid stress corrosion fracture by a completel...

Alumina is normally regarded as truly brittle below ∼ 1000°C. Contrary to this view, it is argued that plastic deformation at the crack tip plays a necessary part in the fracture. This is used to explain the minimum observed at 250°C in the fracture stress of polycrystalline plates containing drilled cracks and in the stress intensity required for crack-branching. Other details of the fracture of single crystal and polycrystalline alumina, including changes in the proportion of intergranular and transgranular fracture with temperature, crack length and crack-branching, are also explained by the involvement of plastic deformation.

The effects of grain size in the range of 9–200 μm and of precipitation parameters on the creep rupture properties of Inconel alloy X-750 at 700°C have been investigated. Superior creep properties of 113 μm grain size material are associated with very low minimum creep rates. This in turn is a consequence of an optimum combination of microstructural parameters—viz., a wide precipitate free zone (PFZ), discontinuous precipitates along grain boundaries, together with a fine distribution of intragranular γ' (Ni3 Al, Ti), precipitates. At low temperature, the material was relatively ductile and exhibited cup and cone type transgranular fracture, while at intermediate temperatures the fracture mode was intergranular. At intermediate temperatures, fracture occurred by cavitation, failure resulting late in the tertiary stage of creep when a crack of critical size to propagate spontaneously formed. At high temperatures, large areas of fracture surface were created by plastic tearing. Many cracks were present in the small (9 μm) grain size material, owing to the large number of triple junctions which acted as barriers to crack propagation while in coarse grained material no cavitation damage was observed in failed specimens. Water quenching during heat treatment in general produced continuous grain boundary precipitates which are detrimental to the creep rupture properties. In cases where continuity of grain boundary precipitates and the precipitate free zone (PFZ) were such as to have opposing effects on creep rupture properties, the former played the dominant role in limiting rupture life.

Zirconium diboride and hafnium diboride were fabricated by hot-pressing at 1800°C and 120,000 psi. Bend strengths were measured on the fully dense materials from 25° to 1400° C in an argon atmosphere. These diboride compounds do not exhibit any gross plastic flow in the temperature range studied. The bend strengths go through a maximum between 700° and 1000°C and vary from 39,000 to 68,000 psi for HfB2 and 30,000 to 56,000 psi for ZrB2. The maxima in strength correspond to maxima in the fraction of transgranular fracture. The bend strength and room-temperature elastic modulus measurements were combined with available thermal conductivity and expansion data to calculate thermal stress resistance parameters. Under steady-state heat flow conditions, the calculated thermal stress resistance parameters of the borides are higher than those calculated for other refractory compounds.

Electron microscopy, fractography, and notched tear tests have been used to investigate the effects of heat treatment upon the fracture behavior of aged 7075 aluminum alloy sheet. Toughness, as measured by crack propagation energy, decreases as the yield stress increases; the toughness of an overaged structure is inferior to that of an underaged structure at the same yield stress. The decrease of toughness with increased aging time is accompanied by a change in fracture mode from predominantly transgranular to intergranular. Transgranular fracture proceeds by dimple rupture and is facilitated by chromium-rich particles which are dispersed throughout the microstructure. Intergranular fracture proceeds by the fracture of grain boundary precipitate particles. The variation of fracture mode with aging time is attributed to a steady decrease of the intergranular fracture stress relative to the transgranular fracture stress, due to increasing grain boundary particle size. A possible explanation of this effect is discussed using the stress concentration due to colinear crack arrays as an analogy. The effects of quenching variations and two-step aging are discussed. It is shown that, in aged 7075, microstructural variables such as the width of precipitate-free zones and the nature of the matrix precipitate do not have a controlling effect on toughness.

Fracture surface of several austenitic stainless steels in stress corrosion cracking (SCC) in MgCl2 and CaCl2 solution was investigated fractographically by scanning electron microscopy. Effect of Mo, applied stress, and testing temperature on the fracture mode was studied. Type 304 steel and 310 steel fractured transgranularly in MgCl2 solution boiling at 143 C (289 F). Transgranular fracture was characterized by a fine parallel pleat pattern. This pattern seemed to be traces of dissolution of metals. Intergranular SCC occurred in Type 316 steel and 16Cr-15Ni-2~4Mo steels, and it was also observed in Type 304 steel fractured at temperatures lower than 125 C (257 F). Intergranular fracture tended to occur with increasing Mo content, applied stress, and lowering testing temperature.

Polycrystalline AgCl, normally ductile in air when deformed in tension, is observed to suffer as much as an 80% loss of ductility with a corresponding decrease in fracture stress when deformed in specific aqueous environments. The fracture path is observed to be intergranular as opposed to a transgranular fracture which is observed when fracture occurs in air or non-embrittling solutions. The selectivity of the embrittling solutions is related to the adsorption of ions which are compatible with Ag+ or Cl− in the host lattice. A mechanism for the embrittlement based on the lowering of the strain energy by replacement of strained bonds at a crack nucleus by strain-free adsorbed-ion bonds is proposed. The adsorbed-ion double layer also serves to stabilize the crack faces.

Burst tests were conducted at 900°F on specimens prepared from 75% UO2 −25% PuO2 fuel pins clad with 0.250-in. o.d. × 0.016-in. wall Type-304 stainless steel (PNL 1-3 and 1-6) that were irradiated in EBR-II to ∼8500 MWd/MTM at 8 to 9 kW/ft. Argon gas was used internally to pressurize the fuel pin sections. The fuel was left intact in order that the effects of the fuel and fission products on the properties of the cladding could be determined.Specimens taken from above the midplane but within the fuel column exhibited brittle, intergranular fracture at drastically lower stresses (∼30 000 psi) than would be expected from Type-304 stainless steel irradiated under similar conditions. The measurable strain in these specimens was

The fracture mechanics of polycrystalline alumina have been investigated using the double cantilever beam cleavage method. The fracture resistance of alumina was measured at temperatures up to 1500°C. The fracture resistance of alumina increased with increasing grain size for all temperatures studied. Electron fractography studies of the fractured surface indicate that the large grain alumina contains more transgranular fracture than the small grain material. This preponderance of transgranular fracture is shown to accompany a higher fracture resistance in the material. It is thus concluded that one mechanism of variation of strength in these materials is through variation in the relative amounts of transgranular and intergranular fracture. The lower softening point of SiO2 causes alumina with this material as a binder to lose its strength at lower temperatures than does MgO bonded material. The mechanism of fracture in these polycrystalline ceramics leads to a suggested method for improved mechanical strength.

Sensitive extension measurements on direct-loaded wires of a Mg-7Alz.sbnd;1Zn alloy have shown discontinuous extension on a scale of 100–200A. The fracture path has been found to depend solely on the microstructural condition of the material. In both intergranular and transgranular fracture surfaces there are indications of alternating slow and fast stages of crack propagation over distances of 0·1 and 0·7 μ respectively. A model for transgranular cracking is proposed.

Transgranular cleavage fracture experiments were carried out on MgO bicrystals with various misorientations. Although the general bicrystal must be characterized by five parameters it was.found that only one was needed to describe transgranular cleavage. This parameter is defined as the angle δ between the lines of intersection of each of the two respective cleavage planes and the grain boundary. During transgranular cleavage, steps formed at the boundary increased in height with increasing values of δ. The large steps caused the originally straight and continuous crack front to break into tongue-shaped segments which overlapped one another. For δ>12° reinitiation of the crack front occurred at the grain boundary. Two energy absorbing mechanisms were observed. Both were related to the segmentation of the crack front. They are (a) the increase in area formed by overlapping cracks and (b) plastic deformation that occurred adjacent to the steps. Estimates of the energy absorbed during fracture were ...

Fractographic studies by electron microscopy have been carried out on WC-Co alloys subjected to bending, indentation and sliding friction of a micro-indenter. Examples have been found of intracrystalline fracture originating from slip line intersections, of transgranular fracture emanating from intergranular fracture after deformation by slip, and of slip transfer from one region to another.

The fracture energy of brittle materials can be defined as the energy absorbed during the extension of a crack over a unit area of surface formed during the fracture process1. Measurements of fracture energy for polycrystalline ceramic materials have resulted in a wide range of reported values for the same material. This is due to some extent to the method used for measurement1–3, however, more important is the strong dependence of fracture energy on such microstructural variations as grain size4–9 and porosity2,6.

Fracture topography is a permanent record of the fracture process, providing valuable information on crack propagation and failure mechanisms. This paper describes mostly features observed in flexural failure at room temperature. Previous papers1–3 are complemented by emphasizing features and techniques for finding and characterizing fracture origins to aid in use of such important, too often neglected, steps in strength studies.

Fractographic studies of B4C and AlB12 ceramics subjected to ballistic impact are presented. Results suggest that macroscopic textures of fracture-exposed surfaces are indicative of stress states occurring during the fracture event, whereas microscopic topography is strongly influenced as well by microstructural features. Variation observed as a function of distance from the impact axis allows some differentiation between the strain rate sensitivity of the fracture processes of the two materials.

a unified, structural mechanism is proposed to explain the phenomena of intergranular and transgranular stress corrosion cracking. the dependence of failure on composition and stress in copper alloys andin stainless steels is analyzed in terms of two general conditions: (1). a chemically reactive path and (2). a mechanism of concentrating stress across the reactive path. intergranular fracture in a chemical environment is considered in terms of the normal stress distribution at the head of a group of dislocations piled up against a grainboundary. the available data are consistent with the proposed model with respect to (1). a grain size dependence (2). stacking fault energy, which defines the number of coherent twin boundaries per grain and thereby the stress concentration at the boundary and (3) the existence of an endurance stress, approximately equal to the macroscopic yield stress, below which the stress concentration at the boundary is insufficient to cause fracture. the nucleating sites for transgranular fracture in single crystals, and in polycrystalline aggre gates that fail transgranularly, are shown to be cottrell-lomer barriers. a detailed crystallographic mechanism of fracture is proposed which dependson (1) orientation with respect to applied stress and (2). the strength of the barrier, which is related to the stacking fault energy.

At high temperatures, close to above 0.5Tm, the absolute melting temperature, metal behavior becomes sensitive to strain rate. Strength and ductility values, deformation modes, and fracture characteristics all respond in important degrees to strain rate. This makes it necessary to understand the nature of grain boundaries (and subgrain boundaries) and how their behavior influences mechanical properties. Short-time mechanical tests, such as tension, impact, and hardness tests, represent values only for the specific conditions of that test. Accordingly, it is necessary to perform tests which take into account the rate of straining at high temperatures. This has resulted in the development of stress-rupture, creep-rupture, and creep tests as the most simple types of tests to evaluate the performance of materials for high-temperature service. In contrast to low-temperature deformation, which is referred to as homogeneous deformation, high-temperature deformation is heterogeneous. In addition to slip (and twinning), one finds at high temperatures several new deformation modes, the most important of which is grain-boundary sliding (shear). Coupled with the broader spectrum of deformation modes, one finds that recovery (in several forms) and even recrystallization can occur, leading to softening of work-hardened regions. The most spectacular of these recovery modes is grain-boundary migration. Deformation is therefore cyclical and is composed of periods of work hardening and periods of recovery. The net effect, of course, is that heterogeneous deformation results in heterogeneous fracture. Fractures become intergranular, a process which is as normal as transgranular fracture is at low temperatures. The degree to which cracking and final fracture are intergranular is strongly dependent on structure as well as on temperature and strain rate. Of particular interest, but largely unsolved, is the nucleation of intergranular cracks, and numerous theories have been proposed based on as yet inadequate data. Growth of cracks is slightly better understood, but because of the heterogeneous nature of both crack nucleation and growth, the roles of all the important variables on the behavior require considerable additional study. In substance, high-temperature creep-fracture studies have been underway a relatively short time and will require much more effort before the topic is as well understood as is low-temperature ductile fracture in simple tension-type loading.

Studies of the structural changes associated with stress corrosion and delayed failure in AISI 4340 steel have shown that significant changes take place in both the internal structure and the fracture-sufrace morphology as a result of specific treatments, particularly the cathodic portion of the corrosion reaction. The incipient stage of stress cracking may be the result of stress corrosion, hydrogen embrittlement, or both, inasmuch as either corrosion or cathodic charging regenerates visible stacking faults in martensite. The regeneration of these stacking faults is believed to be the cause of transgranular fracture in the initial stages of stress-corrosion cracking. Examination of the fracture-surface morphology of stress-corrosion cracked and hydrogen-cracked alloys indicates that, after the initial reaction, hydrogen diffuses to and along prior-austenite grain boundaries and causes intergranular failure. The most likely mechanism for intergranular failure appears to be reduction of prior-austenite grain boundary energy as a result of adsorbed hydrogen. The analysis of fracture morphology of stress-corrosion failures is compared with that of a hydrogen-cracked fracture surface modified by a subsequent anodic treatment in the corrosion medium. (Author)