The improvement of strength and toughness is an engineering requirement, hence this is one of the main goals in materials science. However an increase in tensile strength of metallic materials is usually associated with a decrease in ductility and fracture toughness. Both properties are contradictory.
In the last few years we performed extensive investigation of the mechanisms controlling the fracture toughness of ultrafine grained and nanocrystalline metals. All these materials exhibit very high strength, however a very large variation in the fracture toughness. Nanocrystalline and ultrafine grained metals exhibit always a more or less pronounced grain shape anisotropy (shape texture) with an alignment of the grains in a certain direction, this results in a pronounced anisotropy in fracture toughness, especially pronounced is this anisotropy in SPD pearitic steels. One may assume that this anisotropy of the fracture toughness is a drawback. However, this anisotropy delivers a microstructural design of materials with ultra-high strength with one low fracture toughness for one loading direction but relatively high fracture toughness for the other loading directions. It will be shown that this design concept permits to generate materials with high fracture toughness even for materials and alloys with yield strength near the theoretical limit.
In this paper is presented the results of research on influence of niobium and initial microstructure on start of plastic flow of a material and Lüders bands formation during cold plastic deformation in niobium microalloyed steel. Research was carried out on hot rolled strips from low carbon steel and three steels with the same base chemical composition but with different amount of niobium: 0.035% Nb, 0.048% Nb, 0.060% Nb. Niobium microalloyed steel strips are rolled with selected parameters of thermomechanical treatment and additionally heat treated with the goal of achieving inhomogeneous microstructure. Static tensile testing was performed on samples simultaneously recorded with thermographic camera. The results of thermographic measurements were analysed with software package Irbis 3 professional. The initial microstructure is tested for all samples. During the start of plastic flow in all microalloyed steels, with fine-grained homogeneous microstructure, inhomogeneous deformations are created. These were not found on samples which do not contain microalloying element niobium. In area of inhomogeneous deformations, Lüders bands are formed and propagate through the deformation zone. Lüders band was not found in microalloyed steels with coarse grain. The tests show that the addition of micro-alloying element niobium affects Lüders bands formation but the content of niobium in steel has no effect on the appearance of the Lüders bands. It was found that the initial microstructure has a great influence on Lüders bands occurrence.
An ever-increasing demand for even more effective products with lower weight, more compact design and extended functionality has led to the use of material combinations with locally adapted properties instead of the mono-materials. The Collaborative Research Centre (CRC) 1153 "Tailored Forming" at Leibniz Universität Hannover addresses this topic with an aim to develop hybrid bulk-metal components on the basis of a new, tailored production process using joined semi-finished products. In contrast to existing manufacturing processes for hybrid bulk-metal components where the joining takes place during the forming process or at the end of the production chain, CRC 1153 uses tailor-made semi-finished products which are joined before the forming process. Thus, high-performance components with locally adapted properties can be manufactured.
This paper focuses on the production of hybrid semi-finished products from a steel-steel (C22.8 and 41Cr4) material combination by means of laser beam welding as well as friction welding. The effects of additional ultrasonic coupling into the weld-pool are presented for laser beam welding. Furthermore, the resulting properties of the joining zone are investigated by uniaxial tensile test at elevated temperatures and metallographic analyses of the respective manufacturing processes.
Metastable beta titanium alloys are the most studied class of titanium alloys nowadays due to their unique mechanical properties and potential use in biomedicine. While they offer high specific strength, toughness and fracture resistance, they are limited by a small processing window and higher costs. Therefore, processing of titanium alloys by powder metallurgy has many advantages such as low production costs due to near net shape processing, composition controlling or reduced machining due to deformation and thermal treatment during preparation.
In this study, a metastable beta Ti-15Mo powder was subjected to cryogenic milling in order to achieve an ultra-fine grained structure of powder particles. The compaction of the powders was provided using spark plasma sintering (SPS). Due to high heating rate and fast compaction by SPS, grain growth can be partially avoided and bulk nano-structured materials can be achieved. The effect of cryogenic milling as well as of the parameters of sintering on the microstructure was thoroughly investigated It was revealed that sintering of the non-milled powder requires higher sintering temperatures for achieving fully dense material than in the case of cryomilled powder.
Mechanical properties of sintered bulk materials such as microhardness, tension and compaction tests were also studied.
Zirconium based alloys are commonly used as material for fuel claddings in the light water reactors. Claddings act as first metallic barriers against loss of fission products during the nuclear power plant operation, intermittent storage or final dry storage. During the reactor operation, claddings are subjected to different stress levels at high temperatures as well as neutron radiation. This results in their corrosion, hydrogen diffusion, hydrogen embrittlement and creep. Integrity of fuel claddings is always critical issue during reactor operation, loss of coolant accidents and during storage of spent fuel. There exists no experimental standard for testing for fuel claddings in their hoop direction due to tubular geometry and limited wall thickness (approximately 0.6 mm). Hydrogen related damages in fuel claddings [i.e. hydrogen embrittlement, hydride re-orientation and delayed hydride cracking] are potential issues at wet and/or dry storage of spent fuel assemblies as the fuel rods are subjected to different loading and temperature conditions.
In this work, ring compression testing method developed is applied to study hydrogen embrittlement and hydride re-orientation behavior effecting the mechanical integrity of as-received Zr-1%Nb (E110 alloy) fuel claddings. Tests were performed on un-irradiated fuel claddings with varying hydrogen concentration between 0 to 1500 ppm at RT - 320°C. Influence of hydrogen concentration, hydride re-orientation and phenomena of stress induced hydride re-orientation on mechanical behavior of fuel claddings will be reported.
The study shows the influence of stretching rate on the tensile properties for two low carbon steels during cold plastic deformation. Studies were carried out at laboratory conditions at room temperature with a static tensile test, at stretching rates ranging from 5 to 30 mm/min. Influence of the stretching rate on the elongation (A), tensile strength (Rm), yield stress (Re) and the ratio Re/Rm were analysed by MATLAB software. For both steels, yield stress and tensile strength increase with the increase in stretching rate, while the elongation decreases for both steels. The ratio Re/Rm for niobium microalloyed steel remains constant. It can be concluded that both steels are strain rate sensitive, and that only Re/Rm ratio cannot be used to determine the strain rate sensitivity of the steel.
Characterization of the Lüders bands that occur in niobium microalloyed steel is carried out in different points of their formation and propagation. During static tensile testing infrared thermography is simultaneously used to record temperature changes and digital image correlation to determine the strain amounts in specific points of Lüders bands formation and propagation. Temperature changes which present stress changes were taken at the different strain amounts 0.005 mm/mm, 0.013 mm/mm, 0.026 mm/mm and 0.032 mm/mm. Performed static tensile tests showed the influence of different strain rates 1.8•10-3 s-1, 7•10-3 s-1 and 1.85•10-2 s-1 on the stress changes during Lüders bands formation and propagation. In different specific points such as Lüders band front, behind the Lüders band front (deformed area) and in front of the Lüders band front (undeformed area) tests were carried out. The studies revealed maximum stress changes behind Lüders band front and their increase with increasing strain rate. At the highest strain rate in the point of maximum stress behind the Lüders band front the greatest temperature changes was measured.
Magnesium and its alloys are considered as a suitable material for biodegradable implants due to its good mechanical properties and non-toxic corrosion products. The main disadvantage of magnesium is its high corrosion rate. Alloying of magnesium improves mechanical and corrosion properties. One of the considered alloys is WE43. It contains about 4 wt.% of Y and 3 wt. % of rare earth elements, especially Nd. However, the final mechanical and corrosion properties are highly affected by the preparation method. It was investigated that powder metallurgy is a good way to produce a material with enhanced properties. Further improvement of corrosion properties is obtained by coating. Magnesium fluoride coating was studied for its good protective and adhesive properties. By a combination of powder metallurgy and protective coating, the composite material with magnesium fluoride layers around each particle should be prepared. The specific composite material was created and it resulted in improved mechanical properties and significantely better corrosion resistance.
The high-end thermography systems are conceived for application with highest standards in the fields of research and development, non-destructive material testing and active lock-in thermography. Researchers and scientists, who depend on flexible infrared camera technology with maximum sensibility, accuracy, spatial resolution and speed, have a perfect solution available with cooled photon detector systems. Utilizing of such infrared camera is the best way for defect detection inside of metals by lock-in thermography. One of the method, the Inductive thermography induces eddy currents in a specimen which are visualized using an infrared camera. Inhomogeneities in the eddy current distribution indicate material defects. As measurement result, the time lag between inductive excitation and thermal response is evaluated, providing an extremely robust signal, which is insensitive to irregularities or impurities of the component surface. For stimulation, highly accurate industry-proven induction technology is used. Another key component is the appropriate software solution, which needs to be a purpose tool specifically tailored to active thermography. It supports analyses, in which the temperature differences between defective and intact structures are extremely small. It consists of complex evaluation algorithms for gaining conclusive results.
Grain size is a significant parameter that should be controlled during material processing for the reason that it can affect the strength and toughness directly. Laser Ultrasonics is an appropriate method for in-situ measurement of the grain size and distribution during heat treatment e.g. in a Quenching and Deformation Dilatometer.
In this study a Laser Ultrasonic system was used combining a 8 ns width pulsed laser for the ultrasound generation and a specially adapted vibrometer for detection. Several Ti-Al samples were heat treated variously in the Quenching Dilatometer to get different grain sizes but with the same phase ratios.
The true value of grain sizes and distribution were observed via scanning electron microscopy. The frequency dependant attenuation of ultrasonic waves was obtained for various grain sizes. A statistical correlation between the grain size distribution and the frequency dependent ultrasonic attenuation was established.
Cold drawing of steel tubes is the manufacturing process characterized by anisotropic material flow during drawing. In this paper, the microstructural of the E235 steel tube after recrystallization annealing and cold drawing were characterized by optical microscopy and EBSD (Electron BackScatter Diffraction) analysis performed on scanning electron microscope. The input feedstock after recrystallization annealing with dimensions of Ø 25.0 mm × 1.5 mm (O.D × W.T.) was cold drawn in two drawing passes with fixed plug to the final dimensions of Ø 18.0 mm × 0.9 mm (O.D × W.T.). The development of crystallographic texture (both qualitative and quantitative) was analysed on a longitudinal section of the tube. The stress-strain state in the tube material during drawing was calculated by finite element software and the development of the texture depending on the stress-strain state in three different directions using cylindrical coordinates with respect to the drawing direction was evaluated. We have investigated the ratio of individual crystallographic planes depending on stress state during drawn tube. The main objective was to define the development of crystallographic grain orientation from stress- strain state during cold drawn tube.
Creep test to the rupture of both the dissimilar weld joint made of FB2 and F martensitic steels and the base materials was carried out at temperatures ranging from 550°C to 650°C in the stress range from 70 to 220 MPa. Creep rupture strengths of the weld joint and the base materials were evaluated using Larson-Miller parameter. Assessment of microstructure development and changes of hardness was correlated with the creep strength. Critical zones of creep damage were determined. At lower temperatures and higher stresses the weld joint ruptured in the base material of F steel unaffected by welding, while at higher temperatures and lower stresses rupture occurred in the intercritical heated and fine-grained parts of heat affected zone of steel F. During creep at temperatures above 575 °C Laves phase precipitated in all parts of the weld joint and especially in the heat affected zones. Coarse Laves phase particles and their clusters with chromium carbides served as nucleation centres for cavities. As the fine grained heat affected zone of F steel was the softest part of the weld joint, many cavities originated and initiated causing failure of samples.
To elucidate the influence of precipitates on states of hydrogen and hydrogen embrittlement susceptibility of SUS630, solution treatment, sub-aging (410 °C, 1 h), peak-aging (470 °C, 1 h), and over-aging (550 °C, 1 h) specimens were prepared. States of hydrogen were analyzed using thermal desorption analysis (TDA). The hydrogen embrittlement susceptibility was evaluated using tensile test. Specimens were electrochemically precharged with hydrogen and then tensile tests were carried out simultaneously with hydrogen charging under the same conditions as after hydrogen precharging. The susceptibility was evaluated as elongation ratio with/without hydrogen. The solution treatment and sub-aging specimens have a single peak at near 100 °C, whereas peak- and over-aging specimens have two peaks at near 100 °C and 200 °C. The peak near 100 ºC corresponds to hydrogen desorbed from matrix and peak near 200 °C corresponds to hydrogen desorbed from Cu precipitates. Ductility of peak- and over-aging specimens with hydrogen did not decrease, however ductility of solution treatment and sub-aging specimens with hydrogen decreased. These findings indicated that specimens including hydrogen of higher temperature peak associated with Cu precipitates showed less hydrogen embrittlement susceptibility than those including hydrogen of only lower temperature peak.
To clarify the relationship between dislocation slip and hydrogen for elucidation of hydrogen embrittlement mechanism, the effect of hydrogen on dislocation slip was examined by comparing changes in the stress-strain curve and stress relaxation behavior with/without hydrogen charging. Tempered martensitic steel with the tensile stress of 1480 MPa was used. Hydrogen charging was conducted by the electrolysis charging. Tensile tests were conducted for hydrogen-charged specimens containing hydrogen from 0 to 5.6 mass ppm at strain rates from 1.83×10-5 to 1.83×10-2 s-1. In the stress-strain curve, the elastic modulus did not change, but the upper and lower yield points increased through charged hydrogen. The tensile strength increased and the work hardening rate did not change through charged hydrogen. The yield points and tensile strengths increased with increasing the amount of hydrogen. It suggests that hydrogen suppressed dislocation slip, that is, hydrogen caused hardening. Stress relaxation tests were conducted for hydrogen-charged specimens containing hydrogen from 0 to 4.7 mass ppm. Even though the amount of hydrogen increased, the amount of stress relaxation remained constant. Therefore, hydrogen probably did not promote the increase in dislocation density. These results reveal that hydrogen causes not softening but hardening.
States of hydrogen present in high carbon ferrite (α)-austenite (γ) dual phase steel were analyzed under various hydrogen charging conditions using thermal desorption analysis (TDA). Specimens including 0%γ and 10%γ in α were prepared. The 10%γ specimen consisted of bainitic ferrite and thermally stable retained austenite. When the 10%γ specimen were charged with hydrogen under high fugacity condition, the amount of hydrogen rapidly increased, the end temperature of hydrogen desorption in TDA profile increased, the hydrogen degassing time at 30 ℃ increased, and the trap activation energy increased from about 30 kJ/mol to 47 kJ/mol. This 47 kJ/mol value is excellent agree with the activation energy of hydrogen diffusion in γ phase. These results probably indicated that hydrogen is not absorbed in γ phase but α phase under lower fugacity conditions, i.e., lower amount of hydrogen. In contrast, hydrogen is absorbed in not only α phase but also γ phase under higher fugacity conditions, i.e., higher amount of hydrogen.
Relationship between hydrogen states present in vicinity of fracture surface and hydrogen embrittlement susceptibility for ferrite-martensitic dual phase steels have been investigated. Hydrogen embrittlement susceptibility was evaluated using tensile tests based on fracture strength. The tensile tests at strain rates of 8.33×10-6 s-1 and 1.66×10-6 s-1 were conducted for each specimen containing different amounts of hydrogen. The hydrogen states present in vicinity of fracture surface were analyzed using a thermal desorption analysis just after the specimens were fractured. The results indicated that hydrogen embrittlement susceptibility drastically increased as the amount of hydrogen increased. Additionally, only the specimen fractured at the highest amount of hydrogen (4.5 mass ppm) showed not only the lower-temperature peak but also higher-temperature peak. Hydrogen embrittlement susceptibility also increased as the strain rates decreased, and specimens with lower strain rates also showed the higher-temperature peak. This relationship between hydrogen states present in vicinity of fracture surface and mechanical property indicated that the defects corresponding to higher-temperature peaks probably increase hydrogen embrittlement susceptibility. Since microvoids at the crack tips were observed by SEM only in specimens fractured at lower strain rates, the defects corresponding to higher-temperature peaks probably correspond to the voids.
Changes in the substructure of tempered martensitic steel during the application of cyclic elastic stress in the presence of hydrogen have been investigated. Three types of specimens were prepared. The first type was subjected to cyclic elastic stress (strain rate: 3.0×10-4 s-1, the number of cycles: 70000 cycles and stress level: 0.8±0.1 σB) without hydrogen. The second type was subjected to cyclic elastic stress with hydrogen. The third type was annealed at 200 ℃ for 2 h for the second type. These specimens were denoted here as [cycled], [H+cycled] and [H+cycled+annealed] specimens, respectively. Tracer hydrogen was used as a probe for clarifying changes in the substructure of these specimens. After tracer hydrogen charging, hydrogen states present in these specimens were examined using a thermal desorption spectrometry detected from low temperature (L-TDS). tracer hydrogen desorption in the high- temperature range from the [H+cycled] specimen increased compared with that from the [cycled] specimen. Additionally, tracer hydrogen desorption in the high-temperature range from the [H+cycled+annealed] specimen decreased by annealing at 200 ℃ to the level of [cycled] specimen. Therefore, it was inferred that the formation of vacancy type defects was enhanced under the application of cyclic elastic stress in the presence of hydrogen.
The kinds of lattice defects formed beneath fracture surfaces were analyzed using a thermal desorption analysis (TDA) for cold-drawn pearlitic steel fractured under elastic/ plastic region in the presence of hydrogen. The specimen thickness of 0.3 mm made it possible to separate one peak into a low temperature peak and high temperature shoulder. Tensile tests and constant load tests were conducted under various hydrogen charging conditions to prepare for specimens with various elongations and fracture times, respectively. The specimens of 0.3 mm in thickness beneath fracture surfaces were charged with tracer hydrogen to determine the kinds of lattice defects, and then tracer hydrogen desorption profiles were obtained using TDA. For specimens fractured in plastic region, a broad hydrogen desorption continuing to around 180 ℃ appeared compared with a specimen without hydrogen and strain. Since this broad desorption disappeared by aging at 200 ℃, this desorption probably corresponds to the desorption from vacancies. In contrast, for specimens fractured in elastic region, this broad desorption was not appeared. These findings indicate that one of the factors causing hydrogen embrittlement under plastic region is probably vacancies, whereas the factors under elastic region are others.
The nature of hydrogen-related fracture in X80 pipeline steel with stress concentration was investigated through observation of fracture modes at the hydrogen-related crack initiation site and crystallographic analysis of fracture surfaces. The fracture modes and initiations were observed using field emission scanning electron microscope (FE-SEM). The hydrogen-related crack initiation sites of X80 specimens were quasi-cleavage (QC) fracture at the notch tip. In addition, distributions of plastic strain and principal stress near the notch tip were calculated with finite element method (FEM) analysis. Since the plastic strain was the maximum at the notch tip where the initiation of QC fracture was observed, the main factor causing QC fracture is probably the plastic strain. Furthermore, in order to clarify the crystallographic features of hydrogen-related fracture surfaces of X80, a trace analysis of QC fracture surface was carried out using electron backscatter diffraction patterns (EBSD). Since not all points on fracture surface were parallel to {011} slip planes, hydrogen-related fracture consisted of not only {011} slip planes but also various crystal planes due to the cross slip in body-centered-cubic lattice. These findings indicate that the nature of hydrogen-related fracture in X80 is probably associated with plastic strain on various slip planes.
Vanadium carbide (VC) in high strength steels has been divided into two types, i.e. finely formed precipitated VC and coarsely formed undissolved VC. Hydrogen behavior trapped at two types has been investigated using hydrogen desorption analysis (TDA). The specimens including different amount of V (0, 0.10 and 0.15 %V) were prepared. The specimens of 0 %V(900 ℃) and 0.10 %V(900 ℃) were quenched at 900 ℃ and tempered at 600 ℃. The specimen of 0.15 %V-as Q was only quenched at 900 ℃. Hydrogen was absorbed into specimens by immersion in NH4SCN aqueous solution and analyzed using TDA. End temperatures of hydrogen release of 0 %V(900 ℃) and 0.15 %V-as Q specimens were in the range from 76 to 79 ℃, and that of 0.10%V(900 ℃) specimen was 104 ℃. The amounts of hydrogen were 0.10 ppm in 0 %V(900 ℃) specimen, 0.28 ppm in 0.15 %V-as Q specimen and 0.54 ppm in 0.10 %V(900 ℃) specimen. The specimens including precipitated VC had higher end temperature of hydrogen release and more amount of hydrogen than those including undissolved VC and matrix. These findings suggest that precipitated VC in high strength steels has higher trapping capacity of hydrogen than undissolved VC.