Brittle State Research Articles

The mechanism study on annealing embrittlement of Al88Ce2Ni9Fe1 metallic glass

The mechanism of annealing embrittlement of Al88Ce2Ni9Fe1 metallic glass is studied by using SAXS. For this alloy the activation energy for embrittlement,E b=(160±10) kJ·mol−1 is obtained. It approaches the value of the activation energy for crystallization,E c=(170±10) kJ·mol−1. The significant change in SAXS intensity is observed as the sample transformed from the ductile to the brittle state. About 0.1 volume fraction of Al crystalline particle and 60 m2/cm3 of specific inner surface between crystalline and amorphous phase appear as the sample transformed from the ductile to the brittle state. The annealing embrittlement is caused by crystallization of this alloy.

Temperature dependences of the branching stress intensity factor and the branching crack length in chromium 0.5La 2O 3

The fractographic features of polycrystalline chromium alloys fracture have been studied in a brittle state. The fractographic analysis shows that a brittle fracture of chromium occurs by cleavage, and the cleavage crack branching which is typical of chromium, caused the sample to fracture into three parts. It has been established that the crack shape at the moment of branching is almost square and does not depend upon the cross-section shape of specimens. The temperature dependences of a branching crack length and branching stress intensity factor are obtained for polished and notched specimens. It has been established that the temperature dependences of the notched specimen are controlled by fracture toughness and that of the polished specimens is controlled by the crack initiation stage. The least square method is used for approximation of the temperature dependences. Fracture toughness is estimated with a branching stress intensity factor.

Stereological alterations of non‐metallic inclusions, fracture morphology and alloy impact strength after blowing active powders into liquid steel

Reported here is an analysis of effectiveness of the process of steel refining by the method of injecting active powders. Apart from a brief assessment of the technological aspect, attention is focused principally on changes in morphology and stereological characteristics of the nonmetallic inclusions (NMI). As a result, there are the only appreciable changes due to this refining process. Since, as a consequence, the impact strength of the material is considerably enhanced, the curve of alloy transition to the brittle state is defined by means of parameters describing the set of inclusions. The model obtained in this way describes change in plastic properties against numerical characteristics of the nonmetallic phase for any arbitrary temperature. This enables a quantitative assessment to be made of such refining effects as reduction in number and size of inclusions and differentiation in size of these particles. The consequences of these changes are, respectively: linear improvement in impact strength with temperature increase; exponential rise in impact strength, particularly during work at low temperatures; slowed rate of impact strength decrease at positive temperatures, and increase at low temperatures. The analytical description presented can be an effective tool both for quantitative evaluation of steel refining results and for analysis of the role of NMI in the decohesion process.

High velocity impact test of a C-Mn steel and inertia effect

The fracture toughness of a C-Mn steel and relevant inertia effect in a high velocity impact test were investigated. The inertia force causes oscillations in the force-time curves in impact testing which can be evened up by smoothing the peaks and valleys on the curves. When the impact velocity is in excess of 5 m/s, an unusual stress pulse is displayed which is positive on the back side of the specimen in the first period of vibration. The inertia force contributes to the fracture process, especially in the brittle state of materials. So, the peak force on the force-time curves can be used to calculate the fracture toughness in this state without any corrections. The steel tested exhibits a ductile-to-brittle fracture transition as impact velocity increases.

Nature of the slow-fracture phenomenon in quenched steels

1. The phenomenon of slow cracking in quenched steels is a process of subcritical development of microscopic cracks (20–50 μm), which are generated during the loading of the samples in the macroelastic region. Upon increasing the loading rate, an effect of the deformation-rate retardation of crack nucleation (DRR effect) is observed which leads to the appearance of an incubation period of the crack nucleation at the tip of the stress concentrator. This point needs to be taken into account while performing the tests. 2. The slow cracking of quenched steels is a particular case of hydrogen embrittlement of materials and is caused by the internal, diffusionally mobile hydrogen, which is formed during the quenching of the steel. Desorption of hydrogen during the course of the postquenching rest eliminates the tendency of steels toward slow fracture. In the example of a model steel, it is shown that the brittle state of the material including grain boundaries is an insufficient condition for the onset of slow cracking and that hydrogen needs to be present. 3. Rest of quenched steels, which causes changes in the mechanical properties, exerts the greatest influence on the value of the crack-nucleation stress. The maximum effect of rest is found to be in the case of the 30KhGSA steel containing ∼0.3% C with a hardness of 50 HRC at a stress-concentrator radius of 0.1–0.2 mm. The kinetics of increase in the mechanical properties σnAE, σF of the quenched samples during the course of the rest is in agreement with the kinetics of desorption of the diffusionally mobile hydrogen to the atmosphere. 4. The cause for the cases of slow cracking of objects and semifinished products observed in practice is the formation and growth of defects (cracks) of a quenching or casting origin on top of a high hydrogen content in the structure of the steel.

Thermal oxidation of anhydride cured epoxies—III. Effect on mechanical properties

Three samples based on a flexibilized anhydride cured epoxy network and respectively unfilled (M), silica filled (S) and hydrated alumina filled (A), were submitted to thermal ageing in air, at temperatures ranging between 125 and 200°C, and their flexural properties were periodically measured. It appeared that for all the samples, the stress-deformation behaviour remains unchanged except for the rupture stage. The ultimate elongation decreases continuously as a result of the formation of a brittle oxidized superficial layer, whereas the stress variation depends upon the shape of the initial stress-elongation curve. The ductile-brittle transition induced by ageing, and the stress decrease versus time in the brittle state, can be interpreted in terms of fracture mechanics. Some consequences of these results on the methodology of lifetime prediction are discussed.

The scope for pressing and sintering diamond-bearing components in shock waves

The most highly stressed materials of that type are diamond-bearing composites having matrices composed of tungsten-cobalt hard alloys (slavutich and tvesal) [2, 3]. Those composites are made by hot pressing and have extremely high wear resistance, but they disintegrate rapidly because of their low strength under conditions of dynamic or cyclic loading. Their physicomechanical properties do not meet the rapidly tightening industrial requirements. The main factor responsible for weakening is the damage to the diamond grains, which is due to the cracking in them when the structure is formed [4, 5]. The physical significance can be evaluated from the proportion of grains containing cracks in unit volume. Grain-size analyses on recovered diamond grains have shown that the damage is functionally related to the mode of sintering, the diamond grade, the grain size, and the diamond concentration [4, 6]. Diamonds are damaged during hot pressing not only because of the solvent metal inclusions they contain, the technological defects, and the state of strain in the mold, but also because the method is used at P and T such that the diamond does not pass from the brittle state to the plastic one. High-pressure equipment has substantially reduced the damage to the diamond grains [5] and thus confirms that P and T influence the damage. A second factor that reduces the strength is the stress concentration near the diamond grains. However, the strength of tvesal as a function of diamond grain size below which stress concentration does not occur and the composite can be considered as a macroscopically homogeneous medium damaged by the joint action of the uniformly distributed residual temperature-dependent and mechanical stresses [6, 7].

Tectonic connection of the Hellenic and Cyprus arcs and related geotectonic elements

The Cyprus Arc System forms part of a well-defined, presently active, plate boundary and is a critical area for understanding the evolution of the Eastern Mediterranean Sea. The available seismic reflection profiles are utilised in order to establish the nature and origin of the main tectonic features of the arc, which include: 1. (a) The Cyprus Trench, displaying evidence of underthrusting and morphostructural features typical of subduction zones. 2. (b) The Pytheus Trench, associated with right-lateral strike-slip movement and connecting the western end of the Cyprus Trench to the Strabo Trench sector of the Hellenic Trench System. 3. (c) A predominantly transtensional fault zone connecting the easternmost part of the Cyprus Trench via the Baer Bassit overthrust zone in Syria to the Dead Sea Transform Fault System. 4. (d) A zone of complex deformation, involving thrust and transform elements, running from the Misis Mountains of southeast Turkey through the Kyrenia Range in Cyprus to the Gulf of Antalya and transected by roughly N-S major shears (notably the Cape Gelidonya-Anaximander Mountains strike-slip zone), which appears to form the western boundary of the present active system of the Cyprus Arc. The nature of the shallow structures associated with the Cyprus Arc is consistent with a geotectonic model involving some post-Miocene rotation of the South Anatolian-Cyprus Block, probably induced by “escape dynamics” resulting from continent-continent collision in eastern Turkey and the development of the Maras triple junction in southeastern Turkey. The alternation of plate boundary sectors respectively characterised by thrusting and strike-slip displacement suggests that the shallow crust in this part of the eastern Mediterranean is in a compartmented and relatively brittle state, allowing accumulated boundary stresses to be accommodated largely by displacements and deformation along secondary features.

Laser speckle decorrelation as a noncontact method for brittle-ductile fracture differentiation

Quasi-static fracture in four-point-bend specimens of both brittle and ductile materials was examined using a method which required only photographic access to the specimen surface. Decorrelation of laser speckle patterns was used as a means to map out two-dimensional regions of high surface strain associated with crack propagation. ASTM A515 grade 70 steel was tested, at temperatures above and below its brittle-ductile transition temperature, by double-exposure speckle photography of the area ahead of the crack tip. The regions where the two speckle patterns were uncorrelated, determined by pointwise spatial filtering of the speckle interferograms, have been observed and are related to plastic deformation of the specimen surface near the crack tip. A subsequent comparison of the decorrelation zones resulting from brittle versus ductile states showed differences as expected in both zone size and shape.

Processing, structure and performance of RS “classical” silver-base brazing alloys

Abstract Rapidly solidified (RS) ribbons of silver-base BAg-series alloys were produced in brittle and ductile states over a wide range of compositions. The brittle fracture mode observed in these normally very ductile materials is due to dispersion hardening resulting from the appearance of RS metastable phases and crystalline morphologies in thin (30–50 μm) strip. Both states may have an important practical application as powder and foil brazing filler metals.

Application of ultrasonic inspection data in strength calculations for nuclear power plant equipment

Several kinds of test specimens were produced with three types of defects of defined sizes and positions in the particular localities of weld joints. Such specimens have been used for the assessment of defect parameter characterization by ultrasonic testing. The principles for schematization of such defects and the formulae for the stress intensity factor calculations for elliptical and semielliptical cracks have been worked out. The methods for defining the sizes of defect which are acceptable have been designed for use on the nuclear power plant equipment in the operational stage and take account of the mutual effects of the force, thermal and residual stresses. The method can be used in the brittle, transitional and tough material state.

Precipitation–segregation mechanism for high temperature temper embrittlement of steels revealed by Auger electron spectroscopy and internal friction measurements

A precipitation–segregation mechanism of high temperature (reversible) temper embrittlement has been suggested on the basis of Auger electron spectroscopy, internal friction measurements, and scanning and transmission electron microscopy (SEM, TEM) of 0·3C–Si–Mn–Cr steel after various heat treatments. It was found that P segregation takes place during heating for quenching and tempering. The intensity of P segregation does not change when the steel transforms from a ductile (600°C tempered for 2 h with water cooling) to a brittle (600°C tempered with furnace cooling) state. Isothermal aging at 500°C for 200 h leads to further segregation of P, but the intensity remains unchanged after a reductilising treatment (600°C reheating for 2 h with water cooling). (The term reductilising is used to describe the treatment employed to attempt to return the material to its original ductile state.) The internal friction measurements and TEM examinations indicated that the temper embrittlement of the steel is always accompanied by precipitation of very fine particles of Fe3C(N) causing dead pinning of dislocations within the grains while the reductilised state corresponds to the re-solution of these fine particles back into the a solid solution which explains the reversibility of the phenomenon. Therefore, the P segregation is a necessary factor for grain boundary fracture of the steel in the temper embrittled state, but is still not sufficient for the actual appearance of this mode of fracture. The leading factor is the precipitation of Fe3C(N) particles on dislocations and a precipitation–segregation mechanism is suggested which can be used to explain all the specific features of high temperature temper embrittlement.MST/707

Precipitation–segregation mechanism for high temperature temper embrittlement of steels revealed by Auger electron spectroscopy and internal friction measurements

A precipitation–segregation mechanism of high temperature (reversible) temper embrittlement has been suggested on the basis of Auger electron spectroscopy, internal friction measurements, and scanning and transmission electron microscopy (SEM, TEM) of 0·3C–Si–Mn–Cr steel after various heat treatments. It was found that P segregation takes place during heating for quenching and tempering. The intensity of P segregation does not change when the steel transforms from a ductile (600°C tempered for 2 h with water cooling) to a brittle (600°C tempered with furnace cooling) state. Isothermal aging at 500°C for 200 h leads to further segregation of P, but the intensity remains unchanged after a reductilising treatment (600°C reheating for 2 h with water cooling). (The term reductilising is used to describe the treatment employed to attempt to return the material to its original ductile state.) The internal friction measurements and TEM examinations indicated that the temper embrittlement of the steel is always accompanied by precipitation of very fine particles of Fe3C(N) causing dead pinning of dislocations within the grains while the reductilised state corresponds to the re-solution of these fine particles back into the a solid solution which explains the reversibility of the phenomenon. Therefore, the P segregation is a necessary factor for grain boundary fracture of the steel in the temper embrittled state, but is still not sufficient for the actual appearance of this mode of fracture. The leading factor is the precipitation of Fe3C(N) particles on dislocations and a precipitation–segregation mechanism is suggested which can be used to explain all the specific features of high temperature temper embrittlement.MST/707

Processes of failure of polymers in various types of loading in the brittle and quasibrittle state

Processes of failure of polymers in various types of loading in the brittle and quasibrittle state

Strength calculations of full-size components in the presence of cracks for the brittle and quasibrittle state of the material. Report 1

Strength calculations of full-size components in the presence of cracks for the brittle and quasibrittle state of the material. Report 1

Effect of rhenium on the brittle fracture threshold of molybdenum

1. Addition of rhenium in quantities from 0.5 to 36% into molybdenum does not result in monotonic reduction of the metal cold brittleness threshold in its cast condition (Tbu), as this is usually suggested in technical literature [5, 6, 9]. 2. In introducing rhenium into molybdenum an increase is observed in the temperature range of transition of cast metal from plastic to brittle state. This increases Tbu and reduces Tbl with the exception of alloys containing 4–6% Re whose Tbu corresponds to Tbu of pure molybdenum. 3. An increase of Tbu in adding rhenium to molybdenum is connected with increased values of σ0·2 in the higher temperature region of the cold brittleness range, while a reduction of Tbl is caused by a marked weakening of the temperature sensitivity σ0·2. 4. The retention of plasticity down to lower temperatures with a high rhenium content (36%) is associated with the additional mechanism of strain-twinning. In alloys with a lower rhenium content (0.5–20%) twinning is not observed. 5. At all temperatures below Tbu the character of fracture of alloys, regardless of their rhenium content, is brittle, transcrystalline and differs little when the test temperature changes from Tbu to Tbl. An increase in plasticity during transition from Tbl to Tbu leads to the appearance in the fracture of traces of a preliminary plastic strain with the general brittle nature of fracture retained. At temperatures above Tbu the specimens bend with α=120°C without fracture, consequently, the Mo−Re alloys like pure molybdenum develop no tough fracture.

A correlation between the structural parameters and the mechanical properties of aluminum-beryllium alloy

i. A. A. Uglov, "High-activity heat sources in treatment of inorganic materials," Fiz.-Khim. Obrab. Mater., No. 3, 3-14 (1976). 2. A. G. Lanin, V. V. Borunov, V. S. Egorov, and V. P. Popov, "Failure of cylindrical bodies made of brittle materials in thermal loading," Probl. Prochn., No. 3, 56-60 (1973). 3. V. S. Egorov, A. G. Lanin, and I. I. Fedik, "Failure of thermally loaded disks of materials in the elastic brittle state," Probl. Prochn., No. 3, 48-54 (1981). 4. S. Sato, K. Sato, and Y. Ymamura, "Determination of the thermal shock resistance of graphite by arc discharge heating," Carbon, 13, No. 4, 309-316 (1975). 5. V. P. Popov, A. G. Lanin, and N. A. Bochkov, "Method of testing heat resistance of specimens of brittle electrically conducting materials using electron beam heating," Probl. Prochn., No. 9, 77-81 (1984). 6. V. V. Vlasov, "The method of initial functions in the asymmetric problem of elasticity theory," in: Calculation of Plates and Shells [in Russian], Tr. Mosk. Inzh. Stroit. Inst., No. 34, 31-45 (1963). 7. V. V. Apollonov, A. I. Barchukov, N. V. Karlov, et al., "Thermal effect of powerful laser radiation on the surface of solid," Kvantovaya Elektron., ~, No. 2, 380-390 (1975). 8. R. A. Andrievskii, A. G. Lanin, and G. A. Rymashevskii, Strength of Refractory Compounds [in Russian], Metallurgiya, Moscow (1974).

Optimized design of a high pressure compound vessel by fem

The problem of safety optimization of the high pressure compound vessel is presented. The safety for both assembly and working states is considered. Two different objective functions are taken into account, corresponding to two methods of loading from actual stress state to the limiting brittle fracture state. For the vessel of the given type consisting of n parts (the matrix and n−1 steel rings), the number of design parameters is 2 n−1. Design parameters were preoptimized on the cylindrical compound vessel model. Because of the cost-effectiveness of finite element calculations, only two variables having the greatest influence on the matrix safety were taken into account. These were the radius and the overlap of the first ring. For optimization the variable metric method is used.

Small angle X-ray scattering from an embrittling metallic glass

The metallic glass Fe 79.3B 16.4Si 4.0C 0.3 was examined by small angle X-ray scattering (SAXS) in the as-quenched condition and after low temperature annealing. In all cases, the annealing temperature was well below the crystallization temperature of the material. There was no significant change in SAXS intensity as the sample transformed from the ductile to the brittle state. Partially crystallized specimens prepared by up-quenching were used as calibration samples to estimate an upper limit of 0.9 vol% of a possible second phase with the electron density of amorphous Fe 3B in the most embrittled samples. It was concluded that phase separation is not the cause of annealing embrittlement in this system. The weak SAXS intensity suggests that the phase separation and chemical inhomogeneity in both the as-quenched and the annealed state are limited.

Estimating the remanent creep life of power plant components

This paper examines the current status of remanent creep life assessment methods for power plant components. Consideration is given mainly to predictive techniques based on post-service examination and testing with application to low alloy ferritic components in fossil plant. The requirements for producing methodologies, namely the development of mechanistic and parametric models for creep damage and failure, are discussed together with aspects on the measurement of the relevant creep damage feature or property. Techniques considered include physical and mechanical property measurement, metallographic examination, strain measurement, and accelerated creep and rupture testing. Methods based on accelerated testing are discussed in detail; extrapolative techniques and application of the life fraction rule are considered both from an experimental and mechanistic viewpoint. Finally, attention is given to the choice of representative stress to apply to uniaxial data. The influence of material parameters on the representative stress is emphasized and upper and lower bounds appropriate to creep brittle and creep ductile material states are considered.