Showing papers on "Hardening (metallurgy) published in 2003"

Fully reversible, dislocation-based compressive deformation of Ti3SiC2 to 1 GPa.

Abstract: Dislocation-based deformation in crystalline solids is almost always plastic. Here we show that polycrystalline samples of Ti3SiC2 loaded cyclically at room temperature, in compression, to stresses up to 1 GPa, fully recover on the removal of the load, while dissipating about 25% (0.7 MJ x m(-3)) of the mechanical energy. The stress-strain curves outline fully reversible, rate-independent, closed hysteresis loops that are strongly influenced by grain size, with the energy dissipated being significantly larger in the coarse-grained material. At temperatures greater than 1,000 degrees C, the loops are open, the response is strain-rate dependent, and cyclic hardening is observed. This hitherto unreported phenomenon is attributed to the reversible formation and annihilation of incipient kink bands at room-temperature deformation. At higher temperatures, the incipient kink bands dissociate and coalesce to form regular irreversible kink bands. The loss factor for Ti3SiC2 is higher than most woods, and comparable to polypropylene and nylon. The technological implications of having a stiff, lightweight machinable ceramic that can dissipate up to 25% of the mechanical energy per cycle are discussed.

On the Precipitation-Hardening Behavior of the Al-Mg-Si-Cu Alloy AA6111

Abstract: The precipitation-hardening behavior of aluminum alloy AA6111 during artificial aging and the influence of prior natural aging on the aging behavior were investigated. The evolution of microstructure was studied using quantitative transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The evolution of the relative volume fraction of precipitates for the solution-treated alloy was determined using isothermal calorimetry and a new analysis based on the DSC technique. Quantitative TEM was also used to obtain the rate of precipitation of microscopically resolvable phases during aging at 180 °C. Three types of precipitates, i.e., unresolved Guinier-Preston (GP) zones, β″, and Q′, were found to form during aging at 180 °C. The evolution of yield strength was related to the evolution of microstructure. It was found that the high hardening rate during artificial aging for the solution-treated alloy is due to the rapid precipitation of the β″ phase. Natural aging prior to artificial aging was found to decrease the rate of precipitation of β″. The slow hardening rate for the naturally aged alloy was attributed to the slower nucleation and growth of β″ phase.

Surface Hardening of Steels: Understanding the Basics

Abstract: Surface Hardening of Steels: Understanding the Basics is a practical selection guide to help engineers and technicians choose the most efficient surface hardening techniques that offer consistent and repeatable results. Emphasis is placed on characteristics such as processing temperature, case/coating thickness, bond strength, and hardness level obtained. The advantages and limitations of the various thermochemical, thermal, and coating/surface modification technologies are compared. Economic concerns and health and safety considerations are also addressed. Recent developments in the understanding of the relationships between microstructure and fatigue and wear performance are reviewed, as are more recently introduced surface hardening processes such as vacuum-related technologies, laser processing, CVD/PVD, and ion implantation. Methods for evaluating hardness patterns and depths of hardness for quality control and failure analysis are described. The book also reviews methods for measuring and controlling case depth, residual stresses, and distortion. Metallurgical comparisons are made between those processes that offer rapid heating and rapid cooling (self quenching) characteristics for example, induction hardening and conventional furnace hardening. While all of the surface engineering methods discussed enhance wear resistance, some such as electroless nickel plating, carbide salt-bath deposition, and chrome platingualso offer resistance to corrosion and oxidation. Wear and corrosion data are provided to demonstrate the benefits of each process. Contents: Process Selection Guide Gas Carburizing Vacuum and Plasma Carburizing Pack and Liquid Carburizing Carbonitriding Nitriding Nitrocarburizing Boriding (Boronizing) Thermal Diffusion (TD) Process Surface Hardening by Applied Energy Surface Hardening by Coating or Surface Modification Appendices: The Iron-Carbon Phase Diagram, Hardness Conversion Tables, Austenitizing Temperatures for Steels Index.

Discrete dislocation analysis of size effects in thin films

Abstract: A discrete dislocation plasticity analysis of plastic deformation in metal thin films caused by thermal stress is carried out. The calculations use a two-dimensional plane-strain formulation with only edge dislocations. Single crystal films with a specified set of slip systems are considered. The film-substrate system is subjected to a prescribed temperature history and a boundary value problem is formulated and solved for the evolution of the stress field and for the evolution of the dislocations structure in the film. A hard boundary layer forms at the interface between the film and the substrate, which does not scale with the film thickness and thus gives rise to a size effect. It is found that a reduction in the rate of dislocation nucleation can occur abruptly, which gives rise to a two-stage hardening behavior.

Vacancy hardening in single-crystal TiNx(001) layers

Abstract: We investigate the effect of N vacancies on the mechanical properties of epitaxial δ-TiNx(001) layers with x=0.67–1.0. The relaxed lattice parameter increases linearly with x in good agreement with ab initio density functional calculations, indicating that deviations from stoichiometry are entirely due to anion vacancies. Hardness values increase continuously, while the elastic modulus decreases with increasing N-vacancy concentration. We attribute the observed vacancy hardening to a reduced dislocation mobility arising from an increase in the rate-limiting activation energy for cation migration.

Grain boundary versus transgranular ductile failure

Abstract: The competition between intergranular and intragranular fracture is investigated using a bilayer damage model, which incorporates the relevant microstructural features of aluminium alloys with precipitate free zones (PFZ) nearby the grain boundary. One layer represents the grain behaviour: due to precipitation, it presents a high yield stress and low hardening exponent. The other layer represents the PFZ which has the behaviour of a solid solution: it is much softer but with a much higher strain hardening capacity. In both layers, void growth and coalescence is modelled using an enhanced Gurson-type model incorporating the effects of the void aspect ratio and of the relative void spacing. The effects on the ductility (i) of the flow properties of each zone, (ii) of the relative thickness of the PFZ, and (iii) of the particles spacing and volume fraction in the PFZ are elucidated. Comparisons are made with experimental data. Based on the previous analysis, qualitative understanding of trends in the fracture toughness of aluminium alloys can be gained in order to provide a link with the thermal treatment process. (C) 2003 Elsevier Science Ltd. All rights reserved.

Grain-size effects on tensile behavior of nickel and AISI 316L stainless steel

Abstract: The aim of this work is to provide experimental results to understand the grain-size effects on tensile hardening of fcc polycrystalline materials. The contribution of grain size on hardening rate is discussed in terms of backstress (X) and effective-stress (Σ ef) evolutions in the different hardening stages. Based on this stress partition, the origin of the classical Hall-Petch relationship is clarified at the different levels of microstructural heterogeneities. If the backstresses and effective stresses verified the Hall-Petch formulation, however, the effective stress is less dependent on grain size than the backstress. The grain-size effect on short-range internal stresses (effective stress) is well explained in terms of a mean path length using classical dislocation modeling. The backstress dependence on grain size seems to be mainly the result of intergranular plastic-strain incompatibilities in relation with the formation of a grain-boundary layer in stage I. In others stages (higher plastic strain), the interactions between intergranular and intragranular long-range internal stresses have been illustrated. The degree of these interactions remains unclear.

A physically based model for TRIP-aided carbon steels behaviour

Abstract: A physically based model for TRIP carbon steels is developed suitable to predict the macroscopic behaviour of multi-constituent aggregates. It includes the effects of phase composition and morphology on flow stress and strain hardening. In a first part, a detailed description of the stress-assisted and strain-induced martensitic transformation kinetics is given based on a generalised form of the Olson–Cohen model. The appearance of the much harder martensitic phase during plastic straining gives rise to a strong hardening of the retained austenite islands. The matrix behaviour is described using a model previously developed for ferritic–martensitic steels. A quite simple but accurate homogenisation approach is used to determine the TRIP steel behaviour. The predicted evolution of strain-induced martensite volume fraction, flow stress and incremental work hardening is in good agreement with experimental data and illustrates the critical importance of the retained austenite stability on the formability of TRIP steels.

Hardness trends in micron scale indentation

Abstract: A continuum theory for elastic-plastic solids that accounts for the size-dependence of strain hardening is employed to analyze trends in the indentation hardness test. Strain gradient plasticity theory incorporates an elevation of flow stress when non-uniform plastic deformations occur at the micron scale. Extensive experimental data exists for size-dependence of indentation hardness in the micron range for conical (pyramidal) indenters, and recent data delineates trends for spherical indenters. Deformation induced by rigid conical and spherical indenters is analyzed in two ways: by exploiting an approximation based on spherically symmetric void expansion and by finite element computations. Trends are presented for hardness as a function of the most important variables in the indentation test, including the size of the indent relative to the material length parameters, the strain hardening exponent, the ratio of initial yield stress to Young's modulus, and the geometry of the indenter. The theory rationalizes seemingly different trends for conical and spherical indenters and accurately simulates hardness data presented recently for iridium, a low yield strain/high hardening material. The dominant role of one of the material length parameters is revealed, and it is suggested that the indentation test may the best means of measuring this parameter.

Precipitation strengthening of the aluminum alloy AA6111

Abstract: There is an increasing use of aluminum alloy sheet in automotive applications due to the desire to decrease vehicle weight. The current study provides a detailed quantitative study on precipitation strengthening in AA6111, which is the alloy of choice in North America for exposed body panels. Transmission electron microscopy was used to characterize the average size, the size distribution, the volume fraction, and the crystal structure of the hardening precipitates for aging at 180 °C (a) directly after solution treatment and (b) following 2 weeks of natural aging. The results indicate that both β″ and Q′ phases co-exist throughout the aging cycle with the relative amount of Q′ being increased both with increased aging time at 180 °C and when natural aging precedes aging at 180 °C. A strengthening model was developed which uses the size distribution and the volume fraction of precipitates as the primary inputs to predict the yield stress. An important feature of this model was that only one fitting parameter was necessary to give very good agreement with the experimental results.

Hardness evolution of electroless nickel-phosphorus deposits with thermal processing

Abstract: Four samples of electroless nickel–phosphorus (EN) deposits coated on mild steel substrate have been analysed for their hardness changes in relation to the deposit phosphorus contents as well as different heating temperatures at isothermal (100–500 °C for 1 h) and linear heating (to 300–600 °C at 20 °C/min) conditions. It was found that the hardness of the EN samples increased with decreasing phosphorus content at as-deposited condition, and could be enhanced by appropriate heating. The results of Vickers microindentation testing showed that the peak hardness of the EN samples could be achieved after heat-treating at 400–450 °C. This is caused by the formation of intermetallic Ni 3 P stable phase at this temperature range, acting as a function of precipitation hardening. The Knoop microindentation testing on the cross sections of the samples indicated variations of the hardness across the depth (distance from the sample surface towards the sample/substrate interface), but for most samples the tendencies of change were not clear. Scanning electron microscopy analysis has shown that the lamellar structure present in the cross sections of the as-deposited EN samples tends to disappear, and agglomeration occurs when the heat-treating temperature is increased. The concept of kinetic strength ( K s ) is adopted to interpret the kinetic energy ( Q ) of increased hardening effects using the Vickers hardness data after the isothermal experiments, but the result has not been satisfactory.

Abrasive wear behaviour of a high carbon steel: effects of microstructure and experimental parameters and correlation with mechanical properties

Abstract: This investigation deals with the observations made towards understanding the role of interlamellar spacing on the high-stress abrasive wear behaviour of a high carbon steel. The samples revealed near-eutectoid (pearlitic) structure. The interlamellar spacing was varied by altering the austenitization temperature. Abrasion tests were conducted over a range of applied load, sliding speed, travel distance and abrasive size. Mechanical properties such as hardness, impact toughness and tensile strength, yield strength and elongation at fracture of the samples were also evaluated. The nature of dependence of abrasive wear rate and the measured mechanical properties on material related factors like interlamellar spacing of the samples has been analyzed. The study indicates that the wear rate does not follow a Hall-Petch relationship with the interlamellar spacing of the samples unlike hardness and yield strength. An analysis of the influence of abrasion test parameters suggested the wear rate to increase sharply with load initially. This was followed by a lower rate of increase or even a reduction in wear rate at higher loads depending on the interlamellar spacing of the samples. Increasing abrasive size caused the wear rate to practically remain unaffected initially. This was followed by a sharp increase in wear rate beyond a critical abrasive size. Increasing speed led to higher wear rates upto a critical sliding speed beyond which the wear rate decreased with a further increase in speed. The varying nature of influence of interlamellar spacing on mechanical properties and interlamellar spacing and abrasion test parameters on the wear response of the samples has been discussed in terms of wear-induced subsurface work hardening/deformation of the specimens, deteriorating cutting efficiency of the abrasive particles, stability of the deformed (transfer) layer in the near vicinity of the wear surface during abrasion and hardening of ferrite in the (eutectoid) cementite–ferrite (pearlite) mixture in the steel prior to testing.