High temperature,high strain rate embrittlement of Al-Mg-Mn alloy: evidence of cleavage of an fcc alloy

Abstract

The fracture behaviour in tension of an Al-Mg -Mn alloy has been investigated. At high temperatures and strain rates, intergranular brittle fracture is observed along with cleavage fracture. Intergranular fracture is related to local melting at the grain boundaries. Cleavage occurs in equal proportions on the {100} and {110} crystallographic planes. The area fraction of cleavage facets on the fracture surface has been quantified. Their initiation is shown to be related both to the liquid metal embrittlement of the grain boundaries and to the presence of brittle Mn containing particles at the grain boundaries. Cleavage fracture in an aluminium alloy also requires an inhibition of plastic flow which prevents plastic blunting at the crack tip. It is proposed that this modification of the plastic behaviour is provided by the decrease in stacking fault energy at high temperatures in Al-Mg alloys.     

Grain boundary crystallography in two Fe-C-P alloys

Abstract

Misorientation, grain growth and brittle fracture were investigated in two iron - carbon alloys containing 0.06 wt-% phosphorus (0.06P) and 0.12 wt-% phosphorus (0.12P) after selected heat treatment schedules. A 'fracture surface serial sectioning' technique was devised and combined with misorientation measurements to reconstruct specimens after fracture. Anomalous grain growth occurred in the 0.06P specimen only, after 1000°C annealing. This was attributed to the inhomogeneous distribution of phosphorus at the interfaces. No evidence was found for the direct influence of misorientation angle distributions or coincidence site lattice distributions on anomalous grain growth. The proportion of Σ3s increased greatly after annealing at 1000°C, attributed to the twinning that developed in the austenite range. There was strong evidence that Σ3s were in general more resistant to brittle fracture than were random boundaries. It is suggested that alloys of this type could be 'grain boundary engineered' to improve fracture resistance.     

Effect of Microstructure on the Mirror-Like Surface Quality of FCC and BCC Metals

The microstructure of machined metals changes near the tool-affected zone. This paper presents some new results concerning mirror-like surface cutting of aluminum, copper and tungsten. The microstructure of aluminum and copper represents polycrystalline mild metals with face centered cubic (FCC) crystal structure. Examination of a mirror-like surface by optical microscopy, scanning electron microscopy (SEM), electron backscattered diffraction and atomic force microscopy revealed that grain boundaries and twin boundaries were present, which separates two domains for different crystal orientation. The Young's modulus that depends on orientation can change considerably on these boundaries and, consequently, the value of elastic deformation of the layer under the machined surface. This effect modified the roughness. Ultraprecision machining of tungsten, which is a body centered cubic (BCC) metal, proved useless using diamond and/or boron-nitride tools. Since tungsten is a very brittle metal at room temperature, its ductile to brittle transition temperature is much higher. Therefore, in contrast to normal cutting, the material that is incapable of plastic deformation will cause brittle fracture of the chip and bad surface quality.     

Influence of swaging on hydrogen charged tensile fracture of a 12 wt-%Cr steel

Abstract

The 12 wt-%Cr secondary hardening steel considered in this work is being evaluated for use in the first wall of fusion reactors. As the service temperature can approach 500°C, the microstructure of greatest interest has been a quenched and tempered structure obtained by tempering at 750°C after air cooling from the austenitizing temperature of 1050°C. This structure is susceptible to grain boundary failure whether internal hydrogen has been introduced by cathodic charging or not. In the uncharged condition failure is ductile, but follows prior austenite grain boundaries. Hydrogen charging results in a severe loss of ductility, and tensile fractures which are 30% brittle intergranular. This susceptibility to grain boundary fracture has been attributed both to phosphorus segregation to these grain boundaries and to a nearly continuous array of grain boundary carbides. This tendency for grain boundary fracture can be eliminated and the embrittlement associated with the introduction of internal hydrogen greatly reduced by swaging and subsequently retempering the quenched, and tempered microstructure. The improved properties of the swaged and retempered conditions are attributed to the effects of swaging on the prior austenite grain boundary structure and the orientation of the grain boundaries with respect to the tensile axis.

MST/376     

Deformation and fracture in Al–Li-base alloys

Abstract

Aluminium–lithium-base alloys are of considerable interest because of their low density and high modulus. However, they have been shown to have low ductility and poor fracture toughness. This has been attributed to a variety of factors, including intense shear band formation, segregation to grain boundaries, and weakened grain boundaries due to precipitation and precipitate-free zones. The authors have investigated the deformation structures observed in binary and more complex commercial alloys. As would be expected, considering the microstructure of the alloys, extensive strain localization and shear band formation occurs in these alloys. However, it is shown that the commercial alloys are less sensitive to strain localization than the model binary alloy systems investigated. The stresss–train behaviour has been investigated. The alloys exhibit jerky flow, which is indicative of negative strain rate sensitivity, and strain rate change tests showed this to be the case. This is consistent with the deformation structures observed. The effect of weakened grain boundaries due to precipitation and precipitate-free zones has been studied by comparing the fracture characteristics of aged and unaged material. It is shown that the mode of failure is identical under appropriate conditions. It is concluded that segregation to grain boundaries is the major cause of the lower ductility and toughness of Al–Li alloys. This possibility has been investigated using in situ fracture surface analysis techniques. Results are presented on grain boundary segregation, and methods of reducing its influence on fracture behaviour are indicated.

MST/570     

Ductility and fracture of copper bicrystals with boundary SiO2 particles

Abstract

High temperature deformation andfracture ofCu-SiO₂ bicrystals with [001] twist boundaries of various misorientation angles were investigated under the condition of non-activation of grain boundary sliding. As the misorientation angle increases, the bicrystals became more susceptible to intergranular brittle fracture. Clear intermediate temperature embrittlement was observed in bicrystals with a random high angle boundary. The boundary segregation of O atoms was found to enhance intergranular fracture. Although the boundary SiO₂ particles provide stress concentration sites which cause early formation of boundary cavities, the boundary dependent deformation and fracture behaviour is essentially determined by inherent boundary strength, which is afunction of the misorientation angle.

MST/1969     

Isothermal embrittlement of Fe–8Mn alloys at 450°C

Abstract

Two Fe–8Mn alloys, one of which is alloy 193, stabilised with 0·17%Ti and 0·18%Al, were austenitised at 900°C, ice brine quenched and their DBTTs determined. In this condition, brittle fractures were predominantly cleavage, and thermodynamic calculations on alloy 193 showed that there were 0·0025 wt-%C and <0·03 ppm N in solid solution. Alloys were tempered for 6 min, 1 h and 10 h at 450°C and their DBTTs again determined; in this case, brittle fractures were mainly intergranular. In alloy 193, DBTT rose from 27 to 125°C in 6 min. Hardness values at 450°C were also monitored and the variation of hardness with time is discussed. It is thought that brittle fracture in alloy 193 is due to segregation of Mn per se to prior austenite grain boundaries, unlike an earlier investigation of a pure Fe–8Mn alloy (K1525), where embrittlement was due to a Mn–N and to a lesser extent a Mn–P interaction at prior austenite grain boundaries. The driving force for Mn segregation to prior austenite grain boundaries is thought to be the initial formation of reverted austenite at such sites.     

Cleavage of tungsten rod

Abstract

This paper presents a study of the causes of cleavage in swaged and drawn tungsten rod. The results show that cleavage fracture is nucleated by separation of grain boundaries that are transverse to the direction of swaging and drawing. It is further shown that heat treatments that cause either an increase in the density of transverse grain boundaries or a significant widening of the grains lead to a reduction of fracture stress because they increase the probability of nucleating a crack of critical size at a low stress. These results can all be interpreted in terms of the Griffith model for cleavage fracture.

MST/1383     

On mechanism of fatigue fracture in interstitial free steel sheet

Abstract

The mechanism of fatigue fracture in an interstitial free steel sheet has been studied. The process can be divided into four regimes:

(i) the fatigue crack initiates on the specimen surface, from the mesocracks along the grain boundaries in stage I

(ii) propagates mostly in an opening mode through grain boundaries in stage II

(iii) propagates through microscopic striations and transverse intergranular cracking briefly in stage III

(iv) the crack path changes from flat to slant along with through thickness necking and it propagates to failure through discrete crack jumps in stage IV. The crack jumps are associated with crack progression marks (CPMs), the spacing of which increases exponentially from few micrometres to few hundred micrometres with crack length.     

The effect of nickel content on the fracture behaviour of Cu-Al-Niβ-phase alloys

The fracture behaviour of Cu-14 wt% Al alloys has been studied as a function of nickel content varying from 0 to 10 wt%. It was found that the presence of a brittle phase ₂ at the grain boundaries is responsible for intergranular fracture in low nickel alloys. Severe intergranular embrittlement exhibited by high nickel alloys in not associated with any precipitate at the grain boundaries. In fact when high nickel alloys are cooled slowly, a ductile phase () forms along the grain boundaries that resists the propagation of crack through grain boundaries and the fracture is transgranular.     

Influence of directionality on strength and impact behaviour of high strength steels

Abstract

The directionality of the strength and impact behaviour of control rolled 110 grade high strength steel has been examined. Tensile specimens tested along the rolling direction were found to give strengths of ～60 MPa lower than when tested transversely in the rolling direction. Charpy V notch impact samples, in contrast, gave lower impact transition temperatures when taken in the rolling direction (35°C lower). The difference in strength between the two directions is believed to be mainly as a result of the strong texture produced on control rolling. The difference in impact behaviour is due to the more severe fissuring on the fracture surface of the longitudinally tested specimens. Fissuring is more marked on the longitudinally tested sample because of the greater grain elongation in that direction, the initial fracture path being along the grain boundaries.     

Effects of aging at 700°C on ductile fracture of AISI 316 stainless steel

Abstract

The micromechanisms of ductile fracture have been studied in a commercial AISI 316 austenitic stainless steel. Tensile, Charpy impact, and ductile fracture toughness testing have been performed on unaged material and samples aged at 700°C for times up to 4380 h. Examination of the specimens after testing has shown that the microstructural changes occurring at grain boundaries are responsible for the observed losses of ductility and crack growth resistance. The relative magnitude of the observed changes in mechanical properties has been explained using a simple model to describe the ductile fracture process.

MST/1001     

Effect of aging on high temperature brittle intergranular fracture in austenitic stainless steels

Abstract

The present paper describes the effect of aging on crack growth at 550–750°C in a series of 316 and 347 based stainless steels. Crack initiation parameters and crack growth rates have been measured, and detailed fractography and microstructural characterisation carried out. The study shows that the high temperature brittle intergranular fracture mechanism operates in these alloys, as expected from incidences of cracking in austenitic stainless steels used in power plant. High temperature brittle intergranular fracture leads to lower crack tip opening displacements at initiation, and slightly higher crack growth rates than ductile intergranular failure. Susceptibility to high temperature brittle intergranular fracture is enhanced by aging. This increased susceptibility is explained in terms of the increased hardness, the reduction in dissolved C, and grain boundary precipitation. The effects of temperature, composition, and loading mode on the behaviour of the aged alloys are determined.

MST/3100     

Influence of grain boundary carbide thickness and grain size on cleavage fracture strength and Charpy impact behaviour of steels

Abstract

The cleavage fracture strengths and Charpy impact transition temperatures of plain C–Mn steels (0·12%C and 1–1·4%Mn) having ferrite–pearlite microstructures have been determined for ranges of grain size and grain boundary carbide thickness. Using appropriate heat treatments, ferrite grain size and carbide thickness were varied independently. Refining the ferrite grain size or grain boundary carbide thickness increased the fracture strength and decreased the impact transition temperature. Of the current theories for brittle fracture, an equation recently derived by Petch was found to give the most satisfactory agreement with the experimental data for cleavage strength.

MST/1424     

Metallurgical Analysis of Premature Failure of a Phosphor Bronze Worm Wheel

Worm drives are commonly used where large reduction in speed and greater transfer of torque are required within a small space. Phosphor bronze is normally used material for this application owing to its lower coefficient of friction and good wear and fatigue properties. This work presents a failure analysis of a worm wheel. Analysis revealed that around 40% of the teeth of worm wheel were broken. Failure mode was identified to be intergranular brittle fracture using scanning electron microscopy. A number of casting voids could be observed. In addition, network of intermetallic phases were present along the grain boundaries. These phases were identified to be Cu₃Sn and Ni₃P using elemental mapping through wavelength dispersive spectroscopy technique. These phases were found to have significantly higher hardness compared to the matrix and their precipitation along grain boundaries made the alloy susceptible to intergranular fracture even under small increase in service stress than nominal level.     

Method for determining the elastic modulus at grain boundaries for polycrystalline materials

Abstract

On the basis of the model of non-equilibrium grain boundary segregation induced by tensile stress, a set of kinetic equations is derived to formulate this process. These kinetic equations allow excellent simulation of the grain boundary segregation of phosphorus and sulphur observed in steels subjected to low tensile stresses. In the present paper, based on such a widely approved model, a new approach is proposed to quantify the elastic modulus at grain boundaries for polycrystalline materials. Using the observation of Misra, the grain boundary elastic modulus E _gb = 2.03 × 10⁹ Pa at 883 K for Cr-Mo-V-2.6Ni steel is obtained for the first time. This result shows excellent agreement with the local elastic constants simulated theoretically by Kluge et al., and indicates that the grain boundary elastic modulus for a polycrystalline material is much lower than the commonly assumed value.     

Intergranular fracture in cleavage fracture temperature range

Abstract

The fracture of α iron and C–Mn submerged arc weld metal at temperatures below the brittle–ductile transition temperature is examined. The probability of intergranular fracture occurring in a regime where cleavage fracture is observed is considered with respect to simple theoretical arguments that take into account the geometry of grains and the relative energy of the two fracture processes. The experimental observations are examined in relation to the predictions of these arguments. Simple geometrical arguments demonstrate that a small proportion of intergranular fracture is expected to accompany cleavage fracture in most specimens of polycrystalline α iron and ferritic steels fractured at temperatures below the brittle–ductile transition temperature.

MST/1943     

Intergranular fracture of Al–Li–Cu–Mg alloy resulting from non-equilibrium eutectic melting during solution treatment

Abstract

This investigation has examined intergranular fracture during heat treatment and deformation of an Al–Li–Cu–Mg alloy and of an Al–Li–Cu alloy. When solution treatment of the Al–Li–Cu–Mg alloy was initiated by rapid heating to temperatures ≥ 545°C, non-equilibrium eutectic melting of a grain boundary precipitate phase occurred and the liquid spread along grain boundaries as a thin film. On quenching, intergranular cracks were observed at grain boundaries into which a liquid film had penetrated during solution treatment. For less rapid heating rates, non-equilibrium eutectic melting did not occur and no intergranular cracks were observed after quenching. No evidence of non-equilibrium eutectic melting was observed in the Al–Li–Cu alloy irrespective of the rate of heating to 550°C. During tensile testing of as quenched and quenched and aged specimens of the two alloys, intergranular fracture occurred in most specimens, whether or not non-equilibrium eutectic melting had taken place during solution treatment, indicating that at least one additional mechanism of intergranular fracture was initiated by deformation.

MST/947     

Failure mechanisms in rephosphorised sheet steel spotwelds

Abstract

The influence of phosphorus and sulphur on the microstructures and cross-tension strength of spotwelds in sheet steels has been studied. In steels containing 0·06%C and 0·33%Mn (wt-%), the addition of phosphorus had two effects on the microstructure. An addition of 0·10%P increased the hardenability, resulting in a martensitic weld and heat affected zone. A further increase to 0.15%P resulted in the stabilisation of ferrite in the weld and fusion zones. The ferrite along grain boundaries in the fusion zone aided the initiation of cleavage and decreased the cross-tension strength. The addition of sulphur to a steel containing 0·15%P decreased the cross-tension strength. This effect was associated with a change from ductile to brittle behaviour at the interface between the sheets in the heat affected zone, caused by increased wetting of the interface and grain boundaries by oxysulphides as the sulphur content was increased.

MST/920     

Shape effect of ultrafine-grained structure on static fracture toughness in low-alloy steel

Abstract

A 0.4C-2Si-1Cr-1Mo steel with an ultrafine elongated grain (UFEG) structure and an ultrafine equiaxed grain (UFG) structure was fabricated by multipass caliber rolling at 773 K and subsequent annealing at 973 K. A static three-point bending test was conducted at ambient temperature and at 77 K. The strength–toughness balance of the developed steels was markedly better than that of conventionally quenched and tempered steel with a martensitic structure. In particular, the static fracture toughness of the UFEG steel, having a yield strength of 1.86 GPa at ambient temperature, was improved by more than 40 times compared with conventional steel having a yield strength of 1.51 GPa. Furthermore, even at 77 K, the fracture toughness of the UFEG steel was about eight times higher than that of the conventional and UFG steels, despite the high strength of the UFEG steel (2.26 GPa). The UFG steel exhibited brittle fracture behavior at 77 K, as did the conventional steel, and no dimple structure was observed on the fracture surface. Therefore, it is difficult to improve the low-temperature toughness of the UFG steel by grain refinement only. The shape of crystal grains plays an important role in delamination toughening, as do their refinement and orientation.