Ductile-brittle Transition Behavior Research Articles

高配合率Cr-Cu複合材の引張特性における温度依存性

In order to gain the fundamental knowledge of plastic deformability in a Cr-Cu composite material containing a high fraction of Cr particles, tensile tests of 50mass%Cr-Cu composites were performed at 299K to 473K. Although single-phase Cr generally shows brittle fracture at temperatures lower than approximately 350K, the Cr-Cu composite shows ductile fracture at all tested temperatures. The tensile strength and fracture elongation decreased with increasing testing temperature. These tensile properties of the high-Cr-fraction Cr-Cu composite originate from the properties of a matrix material comprising a Cu alloy with a small amount of dissolved and precipitated Cr. However, the reduction in area was increased with increasing testing temperature. The deformation characteristics of the Cr-Cu composites were considered in terms of the re-arrangement, elongation, and rotation of Cr particles in a ductile Cu matrix. In addition, the ductile-brittle transition behavior of Cr particles would affect the tensile property of Cr-Cu composite at 353K to 373K, which is the range of the ductile-brittle transition temperature (DBTT).

Ductile-brittle transition behaviour of PLA/o-MMT films during the physical aging process

The ductile-brittle transition behaviour of organo modified montmorillonite-based Poly(lactic acid) films (PLA/o-MMT) was analysed using the Essential Work of Fracture (EWF) methodology, Small Punch Tests (SPT) and Enthalpy relaxation analysis. While the EWF methodology could only be applied successfully to de-aged samples, small punch test (SPT) was revealed as more effective for a mechanical characterization during the transient behaviour from duc- tile to brittle. According to differential scanning calorimetry (DSC) results, physical aging at 30°C of PLA/o-MMT samples exhibited slower enthalpy relaxation kinetics as compared to the pristine polymer. Although all samples exhibited an equiv- alent thermodynamic state after being stored one week at 30°C, significant differences were observed in the mechanical performances. These changes could be attributed to the toughening mechanisms promoted by o-MMT.

Effect of Heat Treatment on Ductile-Brittle Transition Behaviour of 9Cr-1Mo Steel

The 9Cr-1Mo steel used in fast breeder reactor is exposed to irradiation during service which severely affects the dynamic fracture resistance by increasing the ductile to brittle transition temperature (DBTT). Thus, even at room temperature, the steel can become brittle and prone to cracking. The microstructural features especially prior austenite grain size (PAGS), martensitic lath and packet size originating from prior processing routes has an effect on the toughness properties of 9Cr-1Mo steel, which is yet to be understood fully. In the present investigation, three different austenitizing temperatures (950°C, 1025°C and 1100°C for 1h) have been selected to vary the PAGS. Impact energy, lateral expansion, dynamic yield stress as the function of temperature has been estimated from Charpy impact tests carried out on normalized and tempered specimens. The results show lower DBTT for the samples reheated at 1025°C followed by tempering.

Brittle–ductile transition behavior of poly(ethylene terephthalate)/poly(ethylene-octene) blend: the roles of compatibility and test temperature

In this study, we attempted to elucidate the Brittle–ductile transition (BDT) behavior of poly(ethylene terephthalate) (PET)/poly(ethylene-octene) (POE) blends under different situations of interfacial compatibility and test temperature. To modulate the compatibility between PET and PEO, maleic anhydride grafted POE (mPOE) was selected as compatibilizer. Three kinds of elastomeric additives, 100 % POE, mPOE/POE (15/85 w/w), and 100 % mPOE, were blended with PET, resulting in three compatibility situations, namely, poor, moderate, and strong interfacial adhesion, respectively. The impact toughness as a function of elastomer content was measured under different interfacial adhesions and test temperature, and microscopic morphology was revealed by scanning electron microscopy and transmission electron microscopy. The results indicated that the interfacial adhesion determines the fashion of microvoiding and even the matrix shear yielding deformation, which will significantly affect the BDT behavior and its response to test temperature. Our study provides not only an effective route to prepare supertoughened PET blends (improved for 20 folds as comparing to the neat PET), but also a fresh insight into the importance of interfacial adhesion on the toughening of thermoplastic/elastomer system.

Effect of Interstitial Elements on Ductile-Brittle Transition Behavior of Austenitic Fe-18Cr-10Mn-2Ni Alloys

The effect of interstitial elements on the ductile-brittle transition behavior of austenitic Fe-18Cr-10Mn-2Ni alloys with different nitrogen and carbon contents was investigated in this study. All the alloys exhibited ductile-brittle transition behavior because of unusual low-temperature brittle fracture, even though they have a faced-centered cubic structure. With the same interstitial content, the combined addition of nitrogen and carbon, compared to the sole addition of nitrogen, improved the low-temperature toughness and thus decreased the ductile-brittle transition temperature (DBTT) because this combined addition effectively enhances the metallic component of the interatomic bonds and is accompanied by good plasticity and toughness due to the increased free electron concentration. The increase in carbon content or of the carbon-to-nitrogen ratio, however, could increase the DBTT since either of these causes the occurrence of intergranular fracture that lead to the deterioration of the toughness at low temperatures. The secondary ion mass spectroscopy analysis results for the observation of carbon and nitrogen distributions confirms that the carbon and nitrogen atoms were significantly segregated to the austenite grain boundaries and then caused grain boundary embrittlement. In order to successfully develop austenitic Fe-Cr-Mn alloys for low-temperature application, therefore, more systematic study is required to determine the optimum content and ratio of carbon and nitrogen in terms of free electron concentration and grain boundary embrittlement.

Influence of Cu and Ni on Ductile-Brittle Transition Behavior of Metastable Austenitic Fe-18Cr-10Mn-N Alloys

The influence of Cu and Ni on the ductile-brittle transition behavior of metastable austenitic Fe-18Cr-10Mn-N alloys with N contents below 0.5 wt.% was investigated in terms of austenite stability and microstructure. All the metastable austenitic Fe-18Cr-10Mn-N alloys exhibited a ductile-brittle transition behavior by unusual low-temperature brittle fracture, irrespective of Cu and/or Ni addition, and deformation-induced martensitic transformation occasionally occurred during Charpy impact testing at lower temperatures due to reduced austenite stability resulting from insufficient N content. The formation of deformation-induced martensite substantially increased the ductile-brittle transition temperature(DBTT) by deteriorating low-temperature toughness because the martensite was more brittle than the parent austenite phase beyond the energy absorbed during transformation, and its volume fraction was too small. On the other hand, the Cu addition to the metastable austenitic Fe-18Cr-10Mn-N alloy increased DBTT because the presence of <TEX>${\delta}$</TEX>-ferrite had a negative effect on low-temperature toughness. However, the combined addition of Cu and Ni to the metastable austenitic Fe-18Cr-10Mn-N alloy decreased DBTT, compared to the sole addtion of Ni or Cu. This could be explained by the fact that the combined addition of Cu and Ni largely enhanced austenite stability, and suppressed the formation of deformation-induced martensite and <TEX>${\delta}$</TEX>-ferrite in conjunction with the beneficial effect of Cu which may increase stacking fault energy, so that it allows cross-slip to occur and thus reduces the planarity of the deformation mechanism.

Effect of quenching process on mechanical properties and ductile-brittle transition behavior of 28CrMnMoV steel

Effect of quenching process on the microstructure and mechanical properties of a kind of seamless tubes of steel 28CrMnMoV was investigated. Then, an investigation on the influence of two different quenching processes on the ductile-brittle transition behavior of this steel was undertaken. The ductile-brittle transition temperatures of the steel by two different quenching processes were also determined. The results show that a good combination of mechanical properties can be obtained through austenitizing experimental steel at 800 °C or 890 °C followed by tempering at 630 °C. Ductile-to-brittle transition temperature of 28CrMnMoV steel austenitized at 800 °C followed by tempering at 640 °C is about −73 °C, which is much lower than the value −37 °C when the steel was austenitized at 890°C and then tempered at 650 °C. This indicates that subcritical quenching process could decrease largely the ductile-to-brittle transition temperature of 28CrMnMoV steel.

Effect of Thermal Aging on Ductile-Brittle Transition Temperature of Modified 9Cr-1Mo Steel Evaluated with Reference Temperature Approach Under Dynamic Loading Condition

The effect of thermal aging on the ductile-brittle transition behavior has been assessed for a modified 9Cr-1Mo steel (P91) using the reference temperature approach under dynamic loading condition (T 0 dy ). The steel in normalized and tempered (NT) condition and in different levels of subsequent cold work (CW) was subjected to thermal aging at temperatures of 873 K and 923 K (600 °C and 650 °C) for 5000 and 10,000 hours. For the NT and all the cold work conditions of the starting material, a drastic increase in T 0 dy has been noticed after aging at 923 K (650 °C) for 10,000 h. A moderate increase was observed for the NT steel aged at 873 K (600 °C) for 5000 hours and for the 10 pct CW steel aged at 873 K (600 °C) for 10,000 h. A detailed transmission electron microscope (TEM) study of the embrittled materials aged at 923 K (650 °C)/10,000 hours and 873 K (600 °C)/10,000 hours has indicated presence of hexagonal Laves phase of Fe2(Mo,Nb) type with different size and spatial distributions. The increase in the T 0 dy is attributed to the embrittling effect of a network of Laves phase precipitates along the grain boundaries.

Strain Rate Dependency of Ductile-Brittle Transition Behavior in Ductile Cast Iron

Strain Rate Dependency of Ductile-Brittle Transition Behavior in Ductile Cast Iron

Anisotropy in tensile and ductile–brittle transition behavior of ODS ferritic steels

Anisotropic fracture behavior of SOC-1 oxide dispersion strengthened (ODS) ferritic steel has been investigated for a hot-extruded bar by tensile tests and Charpy impact tests. These mechanical properties are better in the longitudinal direction than in the transverse directions against extrusion direction (ED). Fracture surface observations by scanning electron microscopy and auger electron spectroscopy indicated bundle-like morphology with existence of segregation/precipitation/inclusions along ED. Pole figures of the hot-extruded bar characterized using electron back scattering diffraction (EBSD) technique and X-ray diffraction exhibited 〈110〉 fiber texture formation along ED. The EBSD orientation map showed a complex bundle-like grain morphology which consists of elongated grains having a specific orientation 〈110〉// ED and relatively isotropic and small grains having other orientation. The results conclude that the combined effects of observed elongated grain morphology and these small grains with segregation/precipitation/inclusions along ED can explain the anisotropic fracture behavior of the hot-extruded ODS ferritic steel.

Progress in the development of reduced activation ferritic-martensitic steels and fabrication technologies in India

The Indian reduced activation ferritic-martensitic (RAFM) steel, meeting the stringent chemical composition requirements with respect to radiologically undesirable elements as well as trace and tramp elements, has been produced by vacuum induction melting followed by vacuum are refining. Also, the optimum combination of normalising and tempering heat treatments was determined. Detailed investigations on the microstructure, including thermodynamic and kinetic features of phase transformations, and tensile, creep and ductile–brittle transition behaviour of this steel indicate that the Indian RAFM steel has the required microstructure, phase stability, creep rupture strength, elevated temperature tensile strength as well as low ductile-to-brittle transition temperature. Systematic investigations have been conducted to optimise the welding parameters to produce crack-free welds using laser and electron beam welding techniques. Salient features associated with materials development, fabrication technologies and characterization of Indian RAFM steels are highlighted.

The Effect of Severe Plastic Deformation on the Brittle-Ductile Transition in Low Carbon Steel

Brittle-ductile transition (BDT) behaviour was investigated in low carbon steel deformed by an accumulative roll-bonding (ARB) process. The temperature dependence of its fracture toughness was measured by conducting four-point bending tests at various temperatures and strain rates. The fracture toughness increased while the BDT temperature decreased in the specimens deformed by the ARB process. Arrhenius plots between the BDT temperatures and the strain rates indicated that the activation energy for the controlling process of the BDT was not changed by the deformation with the ARB process. It was deduced that the decrease in the BDT temperature by grain refining was not due to the increase in the dislocation mobility controlled by short-range barriers. Quasi-three-dimensional simulations of dislocation dynamics, taking into account of crack tip shielding due to dislocations, were performed to investigate the effect of a dislocation source spacing along a crack front on the BDT. The simulation indicated that the BDT temperature is decreased with decreasing in the dislocation source spacing. Molecular dynamics simulations revealed that moving dislocations were impinged against grain boundaries and were reemitted from there with increasing strain. It indicates that grain boundaries can be new sources in ultra-fine grained materials, which increases toughness at low temperatures.

Mechanical Properties of Ni-Free High-Nitrogen Austenitic Stainless Steel Made by Metal Injection Molding with Nitrogen Absorption Method

Microstructure and mechanical properties of high nitrogen steels containing 17Cr-12Mn-3Mo produced by means of metal injection moulding and nitrogen absorption methods were examined. The compacts were sintered at 1300°C, then furnace-cooled to 1200°C and were held at that temperature for 2 to 20 hrs. Subsequently, they were furnace-cooled. Thereafter, solution treatment was done at 1200°C. The solution treated compacts held at 1200°C for above 5 hrs had homogeneous microstructure. Increasing the holding time at 1200°C, ferrite phase disappeared due to the increment of nitrogen content and grain size became larger. Lattice constant of austenite phase also increased. Yield and tensile strength increased with increasing nitrogen content. Yield and tensile strength of the compact held at 1200°C for 10 hrs were 758 MPa, 1172 MPa respectively and elongation was 41 %. However, ductile-brittle transition (DBT) behaviour was seen, and the DBT temperature of every compact was about -20°C. From the fracture surface analysis, brittle fracture surface was identified as (111).

Brittle-Ductile Transition in Low Carbon Steel Deformed by the Accumulative Roll Bonding Process

Brittle-ductile transition (BDT) behaviour was investigated in low carbon steel deformed by an accumulative roll-bonding (ARB) process. The temperature dependence of its fracture toughness was measured by conducting four-point bending tests at various temperatures and strain rates. The fracture toughness increased while the BDT temperature decreased in the specimens deformed by the ARB process. Arrhenius plots between the BDT temperatures and the strain rates indicated that the activation energy for the BDT did not change due to the deformation with the ARB process. It was deduced that the decrease in the BDT temperature by grain refining was not due to the increase in the dislocation mobility controlled by short-range obstacles. Molecular dynamics simulations revealed that moving dislocations were impinged against grain boundaries, creating a shielding field, and were reemitted from there with increasing strain. Grain refining led to an increase in the fracture toughness at low temperatures and a decrease in the BDT temperature. In the present paper, the roles of grain boundaries have been discussed in order to explain the enhancement in the fracture toughness of fine-grained materials at low temperatures, and the decrease in the BDT temperature.

Statistical evaluation of anisotropic fracture behavior of ODS ferritic steels by using small punch tests

Small punch (SP) tests were performed to evaluate the anisotropy in the ductile–brittle transition behavior for the oxide dispersion strengthened (ODS) ferritic steel for the advanced nuclear systems. The upper-shelf energy and scattering of SP fracture energy for the specimens where the surfaces are perpendicular to the extruded direction was larger and smaller than that for the other direction, respectively. The anisotropic fracture behavior appears to be mainly caused by the favorable cracking path along the elongated grain. Data scattering of SP fracture energy could be explained by combination of microstructural observations and the statistical evaluation method based on the Weibull distribution of SP fracture energy, indicating that brittle inclusions along elongated grain was responsible to the unexpected initial failure type fracture mode.

Effects of helium on ductile-brittle transition behavior of reduced-activation ferritic steels after high-concentration helium implantation at high temperature

The effects of He on the fracture behavior of reduced-activation ferritic/martensitic steels, including oxide dispersion-strengthened (ODS) steels and F82H, was determined by characterizing the microstructural evolution in and fracture behavior of these steels after He implantation up to 1000 appm at around 550 °C. He implantation was carried out by a cyclotron with a beam of 50 MeV α-particles. In the case of F82H, the ductile-to-brittle transition temperature (DBTT) increase induced by He implantation was about 70 °C and the grain boundary fracture surface was only observed in the He-implanted area of all the ruptured specimens in brittle manner. By contrast, no DBTT shift or fracture mode change was observed in He-implanted 9Cr-ODS and 14Cr-ODS steels. Microstructural characterization suggested that the difference in the bubble formation behavior of F82H and ODS steels might be attributed to the grain boundary rupture of He-implanted F82H.

Effect of nucleating agent on the brittle–ductile transition behavior of polypropylene/ethylene–octene copolymer blends

AbstractIn the present work, α‐form nucleating agent 1,3:2,4‐bis (3,4‐dimethylbenzylidene) sorbitol (DMDBS, Millad 3988) is introduced into the blends of polypropylene/ethylene–octene copolymer (PP/POE) blends to study the effect of the nucleating agent on the toughness of PP/POE blends through affecting the crystallization behavior of PP matrix. Compared with the PP/POE blends, in which the toughness of the blends increases gradually with the increasing content of POE and only a weak transition in toughness is observed, addition of 0.2 wt % DMDBS induces not only the definitely brittle‐ductile transition at low POE content but also the enhancement of toughness and tensile strength of the blends simultaneously. Study on the morphologies of impact‐fractured surfaces suggests that the addition of a few amounts of DMDBS increases the degree of plastic deformation of sample during the fracture process. WAXD results suggest that POE induces the formation of the β‐form crystalline of PP; however, DMDBS prevents the formation of it. SEM results show that the addition of DMDBS does not affect the dispersion and phase morphologies of POE particles in PP matrix. DSC and POM results show that, although POE acts as a nucleating agent for PP crystallization and which enhances the crystallization temperature of PP and decreases the spherulites size of PP slightly, DMDBS induces the enhancement of the crystallization temperature of PP and the decrease of spherulites size of PP more greatly. It is concluded that the definitely brittle–ductile transition behavior during the impact process and the great improvement of toughness of the blends are attributed to the sharp decrease of PP spherulites size and their homogeneous distribution obtained by the addition of nucleating agent. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 577–588, 2008

A critical shear displacement criterion for ductile–brittle transition under mixed mode I and II loading

Micromechanisms producing ductile and brittle damage operate in parallel at a crack tip. The dominant mode of failure depends upon which of the two (ductile or brittle) damage parameters first reaches its critical value. This has been shown by a study of ductile–brittle transition behaviour in HY100 steel under mixed mode I and II loading. The transition from ductile to brittle behaviour in HY100 steel was found to be affected by mixed mode I and II ratio (ratio of imposed tensile and shear loading) in a manner such that with increasing shear the transition temperature decreased. In the present paper, a criterion is proposed based on the shear strain ahead of a notch tip, to predict the fracture behaviour at any given temperature and mixed mode ratio.

Microstructure and Mechanical Properties of Ni-Free High Nitrogen Austenite Stainless Steels Fabricated by Mechanical Alloying Method

Particular attention is called to nickel-free or lower nickel austenitic stainless steels containing high nitrogen from the viewpoint of protections against earth resources and Ni allergy for human being. We have made high nitrogen steels (HNS) that were containing 0.7-2.0mass%N and 23Cr-2Mo by mechanically alloying method and the microstructure and mechanical properties have been examined. HNS containing 1.0-1.6mass%N have nearly full austenite phase and the mean grain diameter was about 2-3μm for every HNS. The strength and work hardening rate increased and the elongation decreased with increasing nitrogen content. Especially, HNS containing 1.0mass%N exhibited superior balance of strength-ductile properties, that is, yield strength of 1200MPa and fracture elongation of 36%. The yield point phenomena and ductile-brittle transition behavior can be seen in spite of austenitic stainless steels. The planar dislocation arrays were seen on the deformed structure. These results suggest that the formation of planar dislocation structure, high work hardening rate for high nitrogen steels and temperature dependence of mechanical properties might be caused by the formation of the Mo-N interstitial-substitutional atom couples.

Weibull Statistics as a Basis for Assessment of Ductile-Brittle Transition Behavior

The objective of this paper is to investigate failure characteristics of SA508 carbon steel in ductile-brittle transition temperature region. To achieve this goal, a series of finite element analyses as well as fracture toughness tests are performed for pre-cracked V-notch specimens. An assessment of failure probabilities is, then, carried out employing Weibull stress models with different rank probability options. Finally, a prototype of toughness scale diagram is derived through comparison of estimated fracture toughness data with those for compact tension specimens. The present results can be utilized to found a basis of realistic integrity evaluation on major nuclear components containing defect.