Ductile-brittle Transition Temperature Research Articles

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It is shown that the brittleness of a single-component covalent crystal (diamond, Si, Ge) is due to the “duplicity” of the paired potential of interatomic interaction for elastic (reversible) and for plastic (irreversible) deformation. This leads to the fact that the specific surface energy during plastic deformation of a covalent crystal is more than two times less than the specific surface energy during elastic deformation. Therefore, with a small deformation of a covalent crystal, it is energetically more advantageous to create a surface by irreversible breaking than by reversible elastic stretching. It is indicated that the brittle-ductile transition in a single-component covalent crystal is accompanied by metallization of covalent bonds on the surface. It is shown that the brittle-ductile transition temperature (TBDT) for single-component covalent crystals under static load has an upper limit: TBDT/Tm < 0.45, where Tm is the melting temperature.

"Toughness and Ductile Brittle Transition Temperature of Different Mineral Filler Reinforced TPOS Composites"

"Toughness and Ductile Brittle Transition Temperature of Different Mineral Filler Reinforced TPOS Composites"

"Toughness and Ductile Brittle Transition Temperature of Different Mineral Filler Reinforced TPOS Composites"

"Toughness and Ductile Brittle Transition Temperature of Different Mineral Filler Reinforced TPOS Composites"

Insights into the Impact Behavior and Deformation Substructure Evolution of the N‐Bearing QN1803 and 304 Stainless Steels

The impact behaviors of N‐bearing QN1803 and 304 stainless steels are studied using instrumented Charpy impact test in the range of −192–100 °C. The fracture and deformation substructures are systematically investigated by scanning electron microscopy and transmission electron microscopy. The results show that QN1803 presents ductile–brittle transition temperature (DBTT) and 304 shows ductile fracture. With increasing temperature, the stacking fault energy (SFE) of QN1803 and 304 increases, but the former is always lower than the latter. QN1803 presents low impact toughness with low‐density dislocation at −192 °C. At high temperature, QN1803 and 304 show good impact toughness and the deformation substructures are dominated by high‐density dislocations and deformation twinning.

Effect of Double-Quenching on the Hardness and Toughness of a Wear-Resistant Steel

Martensitic/bainitic wear-resistant steels are widely used in civilian industry, where a good combination of strength and toughness is required. In the present study, a double-quenching process was applied and compared to the conventional single-quenching process. The microhardness and ductile–brittle transition temperature were measured, and the microstructure was characterized with scanning electron microscopy and electron backscatter diffraction (EBSD) technique. It was found that the double-quenching process refined the prior austenite grain size by 43% and simultaneously improved the toughness and hardness. The ductile-to-brittle transition temperature was decreased from −77 °C to −90 °C, and the hardness was increased by 8%. Based on the EBSD data, a detailed analysis of the grain boundary distribution was performed using a recently developed machine learning model. Unlike what was found in previous studies, for the studied wear-resistant steel, the refinement of the prior austenite grain did not increase the block boundary density while increasing the high-angle packet boundary density. As a result, the total density of the high-angle grain boundaries in the double-quenched specimen was not improved compared to the single-quenched specimen. Further inspection suggested that it is the prior austenite grain boundaries and high-angle packet boundaries that contribute to the hardness and toughness, and the key factors that determine their effectiveness are the high misorientation angle between the {110} slip planes and the high slip transmission factor.

Impact toughness of a 12% Cr martensitic steel: Conventional heat treatment vs. thermo-mechanical processing

The impact toughness together with determination of the ductile–brittle transition temperature for the 12 %Cr steel with the low N and high B contents subjected to conventional heat treatment and thermo-mechanical processing have been investigated. The thermo-mechanical processing includes the interim hot forging and long-term annealing at 1050 °C with the aim to dissolve the delta-ferrite and coarse W-rich particles, as well as to decrease the particle number density of M23C6 carbides along the lath boundaries and to precipitate V-rich MX phase. The temperature dependence of Charpy energy curves including upper and lower shelf energy is found to be shifted in a side of higher energies. The ductile–brittle transition temperature decreases from 70 °C to (45–50)°C.

Fundamental insights on ductile to brittle transition phenomenon in ferritic steel

Ductile to brittle transition (DBT) is a well-known phenomenon responsible for deteriorating the low temperature fracture toughness of ferritic steels. It is unambiguous to state that the transition phenomenon is a direct consequence of the strong temperature and strain rate sensitivity of flow stress, which in turn is regulated by the thermally activated motion of screw dislocation at the atomic scale. Due to very high Peierls stress, screw dislocation motion is accompanied by the nucleation and propagation of kink-pairs along the dislocation line. Thus, the activation energy required for kink-pair nucleation is an essential ingredient for correctly predicting the flow stress sensitivity and hence the ductile to brittle transition temperature (DBTT) of ferritic steel. However, experimental determination of kink-pair activation energy is rather tedious and there exists a strong discrepancy between atomistic simulation results (using Density Functional Theory and empirical potentials). Therefore, the present review is aimed at understanding the mechanism of screw dislocation motion in BCC Fe lattice and comparing the results obtained for kink-pair activation energy through experiments and simulations. The effect of different alloying elements on the overall flow stress sensitivity of Fe is also discussed with reference to the interaction of screw dislocation with solute atoms or clusters. Finally, the importance of screw dislocation on predicting the DBTT is explored to identify the present knowledge gaps and the future research directions.

Toughness and Ductile Brittle Transition Temperature of Different Mineral Filler Reinforced TPOs Composites

Toughness and Ductile Brittle Transition Temperature of Different Mineral Filler Reinforced TPOs Composites

Thermo-Mechanical Processing as Method Decreasing Delta-Ferrite and Improving the Impact Toughness of the Novel 12% Cr Steels with Low N and High B Contents.

The universal thermo-mechanical processing including the interim long-term annealing together with forging for three 12% Cr martensitic steels with different alloying. This thermo-mechanical processing remarkably increases the impact toughness of these steels in wide temperature ranges and reduces the ductile-brittle transition temperature by 10-20 K. There is a 25 °C impact toughness of all 12% Cr steels subjected to the thermo-mechanical processing exceeds 60 J cm-2. Such an increment in impact toughness is accompanied with the significant changes in the structures of all 12% Cr steels with different alloying. The common feature for all 12% Cr steels subjected to the thermo-mechanical processing is found to be a noticeable decrease in delta-ferrite amount. In the steels containing Ta, the decrease in the mean size of prior austenite grains by 20-26% was revealed. For the 12% Cr steels with ultra-low N content, the thermo-mechanical processing provides the changes in the dispersion of M23C6 carbides and MX carbonitrides.

Study of the brittleness in covalent crystals

Based on the previously proposed model of paired covalent bond, the reasons for the brittleness for covalent crystals have been studied. It is shown that the brittleness of a covalent crystal is due to the “duplicity” of the paired interatomic interaction potential for elastic (reversible) and plastic (irreversible) deformation. This leads to the fact that the specific surface energy during plastic deformation of a covalent crystal is more than two times less than the specific surface energy during elastic deformation. Therefore, with a small deformation of a covalent crystal, it is energetically more advantageous to create a surface by breaking than by elastic stretching it. It is shown that the brittle-ductile transition temperature for single-component covalent crystals has an upper limit: TBDT/Tm < 0.45, where Tm is the melting temperature.

Microstructure Evolution of Heat-Affected Zone in Submerged Arc Welding and Laser Hybrid Welding of 690 MPa High Strength Steel and its Relationship with Ductile–Brittle Transition Temperature

Microstructure Evolution of Heat-Affected Zone in Submerged Arc Welding and Laser Hybrid Welding of 690 MPa High Strength Steel and its Relationship with Ductile–Brittle Transition Temperature

Cryogenic-Mechanical Properties and Applications of Multiple-Basis-Element Alloys

Multiple-basis-element (MBE) alloy was defined as the entropy of mixing over 1R (R is the gas constant, 8.31 J/k), and contains at least three principal elements, each one at over 5%. Thus, MBE alloys can include high-entropy alloys (HEAs), medium-entropy alloys (MEAs), amorphous alloys, and some martensite stainless steels, which have been reported to possess excellent cryogenic properties. This paper reviews the progress of the cryogenic-mechanical properties and applications of MBE alloys. It has been concluded that, with the increase of entropy, the ductile-brittle-transition temperatures (DBTT) can be decreased to the liquid helium temperature (4.2 K). In summary, the cryogenic toughness of MBE alloys can be greatly enhanced by entropy adjustments, which is beneficial to their application at low temperatures.

Fracture features of impact samples of low-activation ferritic-martensitic steel EK-181 after high-temperature thermomechanical treatment

A comparative fractographic investigation of fracture in the temperature range from −186 to 100°C of the Charpy impact samples is performed for the reactor low-activation ferritic-martensitic steel EK-181 after its high-temperature thermomechanical treatment (HTMT) and traditional heat treatment (THT). The mechanisms of steel fracture are revealed depending on the impact test temperature and treatment mode. On the upper and lower shelves of the impact toughness temperature curve, the steel fractures by the mechanism of transcrystalline ductile dimple fracture and transcrystalline quasi-cleavage, respectively. In the intermediate region (in ductile-brittle transition area), fracture occurs by a mixed mechanism. The transition temperature to the brittle state is determined, at which the proportion of ductile and brittle fractures is the same (after THT it is −3°C; after HTMT it is −14°C). It is established that HTMT significantly changes the type of fracture of the impact samples in comparison with THT. The microstructure formed during HTMT with hot deformation of austenite leads to the appearance of a crack-arrester type of delamination during the impact tests in the cold brittleness region, favoring an increase in the fracture toughness of the steel at a lower ductile-brittle transition temperature.

Formation of delta-ferrite and its effect on impact toughness of GTAW weld metals in a 12 wt-% Cr steel

This research focused on the formation of the δ-phase and the corresponding effect on the impact toughness in a GTAW weld metal of a 12%Cr F/M steel. Two kinds of δ-phase were identified: one located at the prior austenitic grain boundaries (PAGBs) (called δPAGB) and the other was in the inter-dendritic region (called δinter). The δinter implied that the peritectic transition that occurred at the final stage of solidification in the DSC test, was suppressed in the welding process. A preheating treatment at 95°C decreased optimally the δ-phase fraction and thereby decrease the ductile-brittle transition temperature (DBTT). However, excessive preheating at higher temperatures enhanced the fraction of the δinter by promoting the micro-segregation and thus increased the DBTT.

The Microstructure, Tensile and Impact Properties of Low-Activation Ferritic-Martensitic Steel EK-181 after High-Temperature Thermomechanical Treatment

In this work, we study the effect of high-temperature thermomechanical treatment (HTMT) with deformation in the austenite region on the microstructure, tensile properties, impact toughness, and fracture features of advanced low-activation 12% chromium ferritic-martensitic reactor steel EK-181. HTMT more significantly modifies the steel structural-phase state than the traditional heat treatment (THT). As a result of HTMT, the hierarchically organized structure of steel is refined. The forming grains and subgrains are elongated in the rolling direction and flattened in the rolling plane (so-called pancake structure) and have a high density of dislocations pinned by stable nanosized particles of the MX type. This microstructure provides a simultaneous increase, relative to THT, in the yield strength and impact toughness of steel EK-181 and does not practically change its ductile-brittle transition temperature. The most important reasons for the increase in impact toughness are a decrease in the effective grain size of steel (martensite blocks and ferrite grains) and the appearance of a crack-arrester type delamination perpendicular to the main crack propagation direction. This causes branching of the main crack and an increase in the absorbed impact energy.

Crystallographic study on deformed bainite structure of ultra-high strength steel and its relationship with strength and ductile-brittle transition temperature

• Variation of strength and DBTT in thickness direction of medium plate is elaborated. • The surface can obtain higher strength and lower DBTT than the core. • Surface can weaken variant selection and refine crystallographic packet and block. • Dominant Bain group forms in the core, resulting in low density of grain boundaries. • Driving force of bainite transformation is the guarantee to weaken variant selection. The significant contribution of deformed bainitic structure to the strength and ductile-brittle transition temperature (DBTT) of ultra-high strength steel was investigated from the aspect of crystallography in order to elucidate the mechanism responsible for the variation in strength and toughness between surface and core. The surface has yield strength of 860 MPa, tensile strength of 913 MPa and DBTT of −110 °C. However, lower strength (yield and tensile strength: 721 and 835 MP) and higher DBTT (−84 °C) appeared in the core, although the ductility changed little. Visualization and quantification of crystallographic structures show that weakening of variant selection occurs on the surface, inducing the staggered configuration of different Bain and CP (closed-packed plane) groups, and then promoting the formation of high density of various grain boundaries conducive to both of toughness and strength. The insufficient transformation degree inferred from the retained austenite leads to the strengthening of variant selection in the core, resulting in the formation of dominant Bain group and reduction of various grain boundary density.

Effects of laser printing parameters on molten pool formation, microstructure evolution and mechanical properties of laser directed energy deposition of difficult-to-process tungsten heavy-alloy

Laser directed energy deposition (LDED) additive manufacturing is promising in the fabrication of high-performance W-based heavy-alloys which are normally difficult-to-process due to their unique nature such as the extremely high melting point and high ductile-brittle transition temperature. In this work, a novel 95 W heavy alloy modified by nanosized Ni and Fe particles was manufactured by LDED process. The effects of laser power on the formation mechanisms of molten pool were investigated. It was found that a relatively high laser power could promote the complete melting of W powder and formation of regular molten pool morphology. The microstructures of laser-processed W heavy alloy showed columnar grains, cellular grains and equiaxed grains from bottom to top within the molten pool along the building direction. The three-dimensional finite element simulation revealed the temperature gradient ( G ), cooling rate ( C ) and solidification velocity ( S ) within the molten pool were different, which was responsible for the microstructural development. The bulk-form 95 W heavy-alloy part free of defects was produced at the laser power of 1400 W. The optimally processed 95 W heavy-alloy by LDED showed a high average nanohardness of 8.6 GPa and a sound wear performance with a low coefficient of friction (COF) of 0.5. The tensile tests also revealed the achievement of a combination of an excellent tensile strength of 541 MPa and an elongation of 0.99%. This work demonstrated that LDED was able to manufacture high-performance tungsten heavy-alloys in bulk form with outstanding mechanical properties.

High temperature flow behavior and deformation mechanism of Nb-22.5Cr-2.5Mo alloy

The high temperature compression is performed by isothermal compression tests using Gleeble 3500 thermo-mechanical simulator at a constant strain rate for the hot-pressed Nb-22.5Cr-2.5Mo alloy. The deformation temperature ranges from 800 to 1200 °C and the strain rate ranges from 0.001 to 0.1 s−1. The high temperature flow behavior and deformation mechanism of the alloy are investigated based on the compression test data and TEM observation of the deformed microstructure. The results show that the ductile–brittle transition temperature of the alloy is about 800 °C, and the flow stress is sensitive to the deformation temperature and strain rate, in which the plastic deformation ability increases obviously with the increase of deformation temperature and the decrease of strain rate. Besides, the hyperbolic sine Arrhenius model with stress exponent of 3.131 and apparent activation energy of 402 kJ/mol is established for the peak flow stress. The value of apparent activation energy is higher than that of Nb metal (246 kJ/mol), which implies the existence of Laves phase NbCr2 makes the deformation of the alloy more difficult. Moreover, it is found that the deformation mechanisms in the Nbss matrix are mainly DRV controlled by the glide and climb of the dislocations and DRX formed by bulging nucleation mechanism, while the deformation mechanisms in the Laves phase NbCr2 particles are mainly twinning and synchroshear of the Shockley partial dislocation.

Influence of precipitate and grain sizes on the brittle-to-ductile transition in Fe–Al–V bcc-L21 ferritic superalloys

The limiting factors to achieving a wide application of bcc-superalloys are the high brittle-to-ductile transition temperatures (BDTT). The understanding of the mechanisms controlling the BDTT and how to optimise the microstructure in polycrystalline bcc-superalloys remains a concern today.In the present work, the influence of grain and coherent precipitates sizes on strength and brittle-to-ductile transition temperature (BDTT) are studied in a Fe78Al10V12 (A2+L21) ferritic superalloy, toward application in high-efficiency power plants. Additionally, the A2 matrix behaviour was evaluated in a derived single-phase-bcc Fe84Al8V8 alloy. Thermal ageing and coarsening treatments were applied to produce samples with different precipitate and grain sizes. Tensile tests were carried out at different temperatures and strain rates to assess the variation of the yield stress. Charpy impact tests were used to measure the BDTT in both alloys, which was substantially reduced with grain size refinement, and precipitate coarsening. It was found that the increase in cleavage stress by precipitation strengthening follows the same behaviour that the increase in yield stress for coherent strengthened bcc-superalloys. Integration into a physical-based model, which identified a novel interplay with cleavage stress, provides enhanced BDTT predictive capability for ferritic superalloys.

Phase Continuity, Brittle to Ductile Transition Temperature, and Creep Behavior of a Eutectic Mo–20Si–52.8Ti Alloy

Mo–Si–Ti alloys, like eutectic Mo–20Si–52.8Ti (at%), have previously been intensely investigated, owing to their excellent oxidation and creep resistance. To better understand high‐temperature mechanical behavior, a holistic assessment of microstructural features is necessary. Correspondingly, 3D‐focused ion beam tomography is carried out in Mo–20Si–52.8Ti. The results indicate a severely interconnected network of Mo solid solution (MoSS) and intermetallic (Ti,Mo)5Si3. Both phases retain similar network connectivity, lamellar sizes, etc. The brittle to ductile transition temperature (BDTT) is then determined through a series of bending tests and interpreted using the microstructural information. The BDTT is found to be ≈1100–1150 °C, different from Mo–9Si–8B with a continuous MoSS network. The BDTT is an immediate consequence of the continuous network of both MoSS and (Ti,Mo)5Si3. The MoSS network is instrumental in crack trapping and bridging, indicating that the present phase distribution maximized the mechanical performance over (Ti,Mo)5Si3. Having determined the network microstructure and BDTT, tensile creep behavior is evaluated and compared to previously published compressive creep results. The results show consistency in terms of strain rate, stress exponent, and microstructural features indicating a reliably good creep resistance for the network microstructure of Mo–20Si–52.8Ti regardless of loading direction.