Identification of the effective grain size responsible for the ductile to brittle transition temperature for steel with an ultrafine grain size ferrite/cementite microstructure with a bimodal ferrite grain size distribution

This research investigated the mechanism responsible for the ductile to brittle transition temperature for the newly developed steels with a bimodal, ultrafine grain size, ferrite/cementite microstructure (UGF/C), which are produced by caliber warm rolling followed by annealing. The microstructure of the steel was characterised. Charpy impact tests were carried out in the temperature range from 373 K to 4.2 K and the fracture surfaces were analysed. The effective grain size responsible for the ductile-to-brittle transition temperature corresponded to the grain size of the large grain size regions. The mechanism of this phenomenon was attributed to the characteristics of the grain boundaries, as high angle grain boundaries are more effective in impeding cleavage crack propagation. The grain size of the large grain size regions was important in determining the DBTT because these grain boundaries were high angle grain boundaries, whereas the small gain size regions were dominated by the low angle grain boundaries.     

Mechanical behavior and tensile/compressive strength asymmetry of ultrafine structured Ti–Nb–Ni–Co–Al alloys with bi-modal grain size distribution

Bulk ultrafine structured metallic materials with the bi-modal grain size distribution exhibit both high strength and good ductility. Here we show a new family of bi-modal Ti–Nb–Ni–Co–Al alloys. Their microstructure consists of an ultrafine structured eutectic matrix and relatively coarse β-Ti dendrites. Chemical modification of the parent Ti–Nb–Ni–Cu–Al system significantly affected type and volume fraction of matrix phases. In its turn, this influenced the deformation behavior. The unexpected “double yielding” behavior as well as tensile/compressive fracture strength asymmetry of the designed alloys are discussed in detail. For that, the microstructure alterations under compressive and tensile loading was in situ and ex situ analyzed by scanning electron microscopy.     

Processing, microstructure and mechanical properties of nickel particles embedded aluminium matrix composite

Nickel particles were embedded into an Al matrix by friction stir processing (FSP) to produce metal particle reinforced composite. FSP resulted in uniform dispersion of nickel particles with excellent interfacial bonding with the Al matrix and also lead to significant grain refinement of the matrix. The novelty of the process is that the composite was processed in one step without any pretreatment being given to the constituents and no harmful intermetallic formed. The novel feature of the composite is that it shows a three fold increase in the yield strength while appreciable amount of ductility is retained. The hardness also improved significantly. The fracture surface showed a ductile failure mode and also revealed the superior bonding between the particles and the matrix. Electron backscattered diffraction (EBSD) and transmission electron microscopy analysis revealed a dynamically recrystallized equiaxed microstructure. A gradual increase in misorientation from sub-grain to high-angle boundaries is observed from EBSD analysis pointing towards a continuous type dynamic recrystallization mechanism.     

Thermomechanical analysis of thermal shock fracture in the brittle/ductile transition zone. Part I: description of tests

This article describes the experimental part of a project to develop a method for testing thermal shock combined with mechanical loading. The main objectives were to create the conditions for a major shock and to carry out the test in the laboratory using conventional equipment. The work focused on 16 MND 5 steel (A508) used for reactor vessels and on tearing close to the brittle/ductile transition temperature. The article is divided into two main parts:

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The first describes how the properties of the material were determined. These properties were required for numerical simulations and for understanding the fracture mechanisms.

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The second describes the test proposed using a specially-adapted compact specimen and the preliminary test results. It also describes the test potential, particularly the thermal gradients that can be obtained and the possible loading/temperature changes.

     

The influence of beryllium addition on the microstructure and mechanical properties of Al-15%Mg2Si in-situ metal matrix composite

The effects of Beryllium (Be) on the microstructure and mechanical properties of Al-15%Mg₂Si composite were investigated. The results showed that with the increase of Be content, the average size and volume fraction of primary Mg₂Si particles decreases. Pseudo-eutectic Mg₂Si has mainly a fine fibrous structure in both specimens with and without Be. However, in the samples containing Be, the path for crack propagation along cell boundaries is very tortuous. Meanwhile, β_Al-Mg-Be phase based on a β-Al₃Mg₂ phase of the Al-Mg system was observed in specimens containing Be. It was found that with the addition of Be the ultimate tensile strength (UTS) and elongation values improved. A study of the specimen's fracture surfaces via scanning electron microscope revealed that Be increases the number of fine dimples and decohered Mg₂Si particles.     

Investigation on the Weibull parameters identification for local approach application in the ductile to brittle transition regime

Fracture of BCC metals is a temperature dependent process characterized by a temperature transition regime where material failure is the result of the competing action of two different mechanisms of rupture, namely, cleavage and ductile fracture. In this paper, two micromechanics based methodologies, the local approach and a non-linear CDM model, are used to model fracture processes in the temperature transition regime for a low alloy steel. The study revealed that approaching the nihil ductility temperature (NDT), the identification procedure for the material parameters needed in the local approach becomes no longer stable resulting in a non unique set of σ_W and m values. This occurrence can be used to discriminate in the experimental data set those data failed in a non-complete brittle manner. Using m exponent equal to 4, a linear dependency, between σ_W and K_J, also in upper shelf regime, is demonstrated.     

Alloy design for Al addition on microstructure and mechanical properties of Ni3(Si,Ti) alloy

The effect of Al addition on the microstructure and tensile properties of Ni₃(Si,Ti) alloys with an L1₂ ordered structure, which were fabricated through thermomechanical processing from arc-melted ingots, was investigated. Al was added to a Ni₃(Si,Ti) alloy by using two methods such that Al substituted for (1) only Ti and (2) both Ni and Ti along a Ni₃(Si,Ti)-Ni₃Al pseudo-binary line. In the case of the alloys prepared by the former method, the addition of more than 4 at.% Al resulted in a two-phase microstructure consisting of disordered fcc Ni solid solution dispersions in the L1₂ matrix, while in the case of the alloys prepared by the latter method, the addition of 4 at.% Al retained the L1₂ single-phase microstructure. In the case of the 4 at.% Al-added alloys, the room-temperature tensile properties were similar and independent of the alloying methods, whereas the high-temperature yield stress was higher in the alloys prepared by the latter method than in the case of the alloys prepared by the former method. These results suggest that a single-phase microstructure consisting of an entire L1₂ structure is favorable for obtaining high-temperature tensile properties.     

The Brazilian test: a tool for measuring the toughness of a material and its brittle to ductile transition

Due to their brittleness, assembling of ceramics pieces is generally achieved through brazing but thermal stresses during cooling frequently induce cracking of the material used for brazing. In order to check if such damage is avoidable, it is necessary to characterize the brittle to ductile transition (BDT) of the material. Simple compression is not suited for crack studies, because of mixed loading (mode II + compressive mode I cracking). Another type of test, the cylinder splitting test, known as the Brazilian test, can be carried out by applying compressive forces on two opposite generatrix of a cylinder: this causes a uniform tensile stress on the plane containing the axis of the cylinder and the generatrix, leading to mode I cracking. The advantage of this test is to avoid expensive and random machining of brittle samples. This study shows that the Brazilian test is well adapted for the measurement of toughness and the characterization of the BDT of materials whose room temperature behaviour is brittle (silicides, intermetallics etc.).     

Strength and toughness tradeoffs for an ultrafine-grain size ferrite/cementite steel produced by warm-rolling and annealing

For an ultrafine grain ferrite/cementite (UGF/C) steel, the Charpy impact energy was measured at temperatures from 373 K to 4.2 K, and tensile tests were carried out at temperatures between 323 K and 77 K. For the steel with annealed microstructure, the ductile-to-brittle transition appearance temperature (DBTT) was lower than the Charpy transition temperature (CTT). With increasing annealing time at 873 K, the DBTT and the CTT increased, and the DBTT approached the CTT. The DBTT decreased with decreasing effective grain size. The effective grain size correlated to the grain size of the larger grain size peak in the distribution of grains with {1 0 0} planes. The annealed microstructures had higher yield strength for equivalent toughness (including upper shelf energy, DBTT and CTT) compared to the conventional ferrite/pearlite steel.     

A novel damage variable to characterize evolution of microstructure with plastic deformation for ductile metal materials under tensile loading

Damage and fracture of ductile metal materials are greatly associated with evolution of their microstructure under loading, which meso-dimensionally corresponds to grain deformation as well as nucleation, growth, and coalescence of microvoids in later local deformation. The evolution behavior of microstructure is important to realize ductile damage and fracture mechanism of materials. In the present work, a novel damage variable of shape factor of grains was put forward to quantitatively describe the character of microstructure; its evolution with the plastic deformation was built-up by microanalytical and mechanical experiments for Armco iron and mild steel tensile bars. The evolution rule based on the damage variable of shape factor is possible to be extended into all of ductile metal materials.     

Nanoindentation and SEM/EDX characterization of the geopolymer-to-steel interfacial transition zone for a reactive porcelain enamel coating

The increasing use of alternative cementitious materials such as geopolymers as an environmentally-friendly alternative to traditional cements requires an improved understanding of the interfacial transition zone (ITZ) between the matrix and reinforcing steels. In this study, nanoindentation measurements were spatially coupled to images with scanning electron microscopy and chemical composition using energy dispersive X-ray microanalysis. The study focused on the microstructure and chemical composition of the interfacial transition zone (ITZ) for reinforcing steel embedded in a geopolymer mortar. The ITZ was analyzed for uncoated steel and steel coated with a reactive porcelain enamel that improves bonding and corrosion resistance. Results indicate that a more gradual transition of mechanical properties and chemical composition for the coated steel coupled with improved integration to the mortar correlates to increased bond strength measured in macroscale experiments.     

Ductile to brittle transition of an A508 steel characterized by Charpy impact test: Part I: experimental results

This study is devoted to the ductile-brittle transition behavior of a French A508 Cl3 (16MND5) steel. Due to its importance for the safety assessment of PWR vessels, a full characterization of this steel with Charpy V-notch test in this range of temperature was undertaken. The aim of this study is to provide a wide experimental database and microstructural observations to supply, calibrate and validate models used in a local approach methodology. Mechanical and fracture properties of the steel have been investigated over a wide range of temperatures and strain-rates. Effects of impact velocity on ductile-brittle transition curve, on ductile tearing and on notch temperature rise are presented and discussed. A detailed study of ductile crack initiation and growth in Charpy specimens is also carried out. From fractographic investigations of the microvoids nucleation around carbide second phase particles, a plastic strain threshold for nucleation is determined for this material. A508 Cl3 steels undergo a transition in fracture toughness properties with temperature, due to a change in fracture mode from microvoids coalescence to cleavage fracture. A systematic investigation on the nature and the position of cleavage triggering sites and on any change in the ductile to brittle transition (DBT) range has been carried out. This leads to the conclusion that manganese sulfide inclusions do not play an increasing role with increasing test temperature as recently mentioned in other studies on A508 Cl3 steel with a higher sulfur content. In a companion paper [Tanguy et al., Engng. Fract. Mech., in press], the numerical simulation of the Charpy test in the ductile-brittle transition range using fully coupled local approach to fracture is presented.