Materials Science Forum Vol. 1016

Abstract: When steel is nitrided, a compound layer mainly composed of iron nitrides, ε-Fe_2~3N and the γ’-Fe₄N phase, is formed on the steel surface. It is an extremely important industrial issue to clarify factors governing the formation of the compound layer during nitriding and to establish unified views on the mechanism of compound layer formation. Therefore, in order to clarify the effect of change in carbon concentration on the growth of the ε phase and the γ’ phase in the compound layer on nitrided steel, we evaluated the change over time in the concentration of the alloy elements in the surface layer, and the phases of the compound layer on nitrided steels containing various amount of carbon in the matrix. The results were that the change over time in the carbon concentration in the compound layer was mainly responsible for the change over time in the phases of the compound layer. Furthermore, it was discovered that the change over time in the carbon concentration distribution occurred because both increasing of carbon from the matrix to the compound layer, and decreasing of carbon from the surface of compound layer to the atmosphere. That caused the gradient change of chemical potential of carbon in the through-thickness direction of compound layer, and the phases of the compound layer were changed with the treatment time.

Abstract: Development of materials used in the power industry for the production of USC boilers poses new challenges. The introduction of new alloying agents intended at obtaining the best possible mechanical properties, including creep resistance, affects the fabricability of new steel grades. All new materials have to undergo a lot of tests, particularly as regards bending and welding processes, with the aim of enabling the development of technologies ensuring failure-free production and assembly of boiler components. Martensitic steels containing 9% Cr, used in the production of steam superheaters shall have good creep resistance and, at the same time, low oxidation resistance at a temperature above 600°C. In turn, steels with a 12% Cr content, for example, VM12-SHC or X20CrMoV12-1 are characterized by significantly higher oxidation resistance but have lower strength at higher temperatures, which translates to their limited application in the production of modern USC and A-USC boilers.X20CrMoV12-1 was withdrawn from most of the power plants across Europe and VM12-SHC was supposed to replace it, but unfortunately, it failed in regards of creep properties. To fulfill the gap a new creep strength-enhanced ferritic steel for service in supercritical and ultra-supercritical boiler applications was developed by Tenaris and named Thor™115 (Tenaris High Oxidation Resistance). This publication covers the experience obtained during first steps of fabrication which includes cold bending and TIG welding of homogenous joints.

Abstract: The work investigates refractory metals (bulk W, W produced via plasma spraying, W-1% La₂O₃ and Mo) of interest as plasma facing materials in future nuclear fusion reactors. They have been irradiated by a single Nd:YAG laser pulse to simulate the effects of transient thermal loads of high energy occurring in a tokamak under operative conditions and then examined by SEM observations. In all the materials the laser pulse induces a crater in the central area of laser spot surrounded by a ridge due to movement of molten metal while in a more external area a network of cracks is observed. Diameter and depth of the crater, ablated volume and morphological features of the surrounding area exhibit differences depending on the specific metal, its physical and microstructural characteristics which affect vaporization, melting and heat propagation from the irradiated spot.

Abstract: An integral computer model/program AusEvol Pro was developed to describe the evolution of steel microstructure during thermomechanical processing (hot rolling, forging), as well as subsequent heat treatment (normalization, tempering), and to evaluate the final mechanical properties (yield stress, tensile stress, elongation), hardness and impact toughness. The program implements a set of physically based models that allow quantitative description of all significant processes of steel structure formation with account of the effects of chemical composition both during thermomechanical processing and heat treatment. Calculations of the final mechanical properties are carried out using the developed models that take into account all physically meaningful contributions. The models created are verified both on the extensive database of our own experimental studies and on reliable data from literature for steels of various chemical compositions.

Abstract: Shape memory alloys (SMAs) find use in myriad medical and engineering applications. In these applications, the functional characteristics of the materials are capitalized on. SMAs are used repeatedly over a long period of time in service. With continued usage degradation occurs in their functional properties, leading to a change in recovery strain, recovery stress, phase transformation temperatures and hysteresis. The change in the functional characteristics of the alloys is known as functional fatigue. Functional fatigue affects the performance of the alloys with the alloys losing their intended functionality. This problem is to be addressed if the alloys are to be used effectively and efficiently throughout their lifespan. It is especially important when using the alloys within the human body, where such degradation can affect the performance of the biomedical devices and, in turn, human health and life. Till date not too many researchers have explored this area in greater detail. In order thereforeto better understand this behavior, in the present study, an Ni₅₀Ti_44.7Cu_5.3 alloy wire with a d=1.43 mm and a l=100 mm was cycled (10,000) under constant stress (55 MPa) between its transformation temperatures, which were determined by DSC (without load). The effect of cycling on the shape memory properties (strain recovery, hysteresis, and transformation temperatures) after a specified number of cycles at regular intervals are considered. The results show that there is considerable difference in the properties obtained and are interpreted and discussed in detail in the paper.

Abstract: Titanium and its alloys, especially Ti-6Al-4V have found application as hip implants due to their mechanical properties, excellent biocompatibility, and corrosion resistance. The use of cementless hip implants has increased over the years as it is thought that this type is more durable compared to cemented hip implants. Cementless hip implants have a porous surface that allows the bone to grow into it and form a strong bone–implant connection. The goal of this study is the use of Finite Element Method simulations to obtain information about how different types of surface topography of a TI-6Al-4V hip implant affect the shear stress, which is used to access the bone-implant connection. Finite Element Analysis is used to analyze the stress distribution in three simple surface modifications in a hip implant under different types of loads. The optimal surface modification out of these three is obtained based on the shear stress distribution, as it is known that lower shear stress promotes bone ingrowth. In this study, we have considered the interaction between cortical bone and implant surface. Material properties and boundary conditions used for the simulations have been adapted from literature.

Abstract: This study aims to optimize the production conditions for forming graphene directly on a quartz substrate, using a carbon 60 (C₆₀) thin film as a solid carbon source. In this experiment, we focused on the relationships between the thickness of the C₆₀ film and the nickel (Ni) catalyst film and the heat treatment conditions. As the thicknesses of the C₆₀ and Ni catalyst films increased, high-crystallinity multi-layered graphene was formed, however the optical transparency of the graphene film decreased. Scanning Electron Microscopy (SEM) observations and Raman scattering spectroscopy showed that after changing the atmosphere of the heat-treatment from an argon (Ar) gas to an Ar+ hydrogen (H₂) gas, the optical transparency of the graphene film was remarkably improved, due to the migration and vaporization of the Ni film, and due to etching of the multi-layered graphene.

Abstract: The modulus of elasticity is an important parameter for an accurate prediction of the springback in sheet metal forming processes. With increasing plastic deformation, this modulus behaves nonlinearly and declines, which leads to an unpredictable springback behavior. The most cited reason for this nonlinearity is the dislocation movement during plastic deformation that especially occurs with multiphase steels. The present contribution investigates the nonlinear unloading behavior and the resulting decrease of the elastic modulus from a differently heat treated DP980 steel. The heat treatments set five different microstructures with martensite volume fractions in the range of 42 to 95 %. By means of the tensile test, a decline of the elastic modulus according to pre-strain was examined by evaluating the chord-modulus during unloading at different strain levels. In addition, a nano-hardness test was performed. It turned out that in all heat treatment conditions, a pronounced decrease in the modulus of elasticity up to 25% from the initial value occurred. With decreasing annealing temperature and lower martensite volume fraction, respectively, the martensite hardness increased, leading to higher hardness differences between the ferrite and the martensite phase in the microstructure. This led to an increase of strain hardening, i.e. to an increased formation of fresh mobile dislocations in the vicinity of the harder martensite phase during plastic deformation. As a result, the modulus of elasticity decreased more sharply. Thus, in the present contribution, an interplay between the martensite volume fraction and its hardness on the decrease of elastic modulus could be clearly manifested.

Abstract: In this study, we investigated the adhesion strength of SiO₂/SiN/TiW/Cu film stacks on silicon by the use of cross-sectional nanoindentation (CSN) technique. The delamination occurred along the SiN/TiW interface as determined by means of SEM and EDX analysis. The critical energy release rate was determined as a quantitative measure of the adhesion strength by application of analytical models as well as Finite Element Method (FEM). Comparative measurements on samples of the same layer composition using the well-established four-point bending (4PB) technique were performed to validate the results of the CSN measurements. FEM was performed to calculate the loading conditions and stress distribution in the samples. The calculations also allowed separating the contribution of plastic and elastic energy in the metallization layers during delamination testing and thereby estimating the value of the interfacial adhesion energy. The experimental results show the good applicability of both the 4PB and CSN method for determining quantitative values of the fracture toughness of thin-film interfaces found in microelectronic components and indicate a good agreement between the two methods.

Abstract: In this study, quasi-static and dynamic compression tests were performed on a ZrCuNiAl bulk metallic glass. The results demonstrated that the ZrCuNiAl bulk metallic glass changed from plastic deformation without strain rate effect to brittle fracture with negative strain rate sensitivity. The fracture surface morphology was related to the strain rate and temperature rise. The modified cooperative-shear model was determined to describe the effect of strain rate and temperature rise on the yield stress of ZrCuNiAl bulk metallic glass.