Showing papers in "Materials Science and Technology in 2001"

Residual stress. Part 1 – Measurement techniques

Abstract: Residual stress is that which remains in a body that is stationary and at equilibrium with its surroundings. It can be very detrimental to the performance of a material or the life of a component. Alternatively, beneficial residual stresses can be introduced deliberately. Residual stresses are more difficult to predict than the in-service stresses on which they superimpose. For this reason, it is important to have reliable methods for the measurement of these stresses and to understand the level of information they can provide. In this paper, which is the first part of a two part overview, the effect of residual stresses on fatigue lifetimes and structural integrity are first summarised, followed by the definition and measurement of residual stresses. Different types of stress are characterised according to the characteristic length scale over which they self-equilibrate. By comparing this length to the gauge volume of each technique, the capability of a range of techniques is assessed. In the sec...

Residual stress. Part 2 – Nature and origins

Abstract: Residual stress is that which remains in a body that is stationary and at equilibrium with its surroundings. It can be detrimental when it reduces the tolerance of the material to an externally applied force, as is the case with welded joints. On the other hand, it can be exploited to design materials or components which are resistant to damage, toughened glass being a good example. This paper, the second part of a two part overview, the first part having been devoted to measurement techniques, examines the nature and origins of residual stresses across a range of scales. This extends from the long range residual stress fields in engineering components and welded structures, through the interphase stresses present in composites and coatings, to the microscale interactions of phase transformations with local stresses.

Precipitation in creep resistant austenitic stainless steels

Abstract: Austenitic stainless steels have for some time been used as superheater tubes in the electricity generation industries in harsh environments with temperatures as high as 650°C at pressures of some 200 atm; they are expected to provide reliable service for 30 years or more. Their detailed mechanical properties are dependent on the stability of the microstructure, particularly the formation, dissolution, and coarsening of precipitates. Although the precipitation processes have been studied extensively, there remain important discrepancies. It is known that small changes in the chemical composition or thermomechanical processing can profoundly influence the evolution of the microstructure. This review focuses on precipitation in creep resistant austenitic stainless steels, in particular wrought heat resistant grades containing niobium and titanium additions. Conventional alloys such as 18–8 and 16–10 are included together with the new NF709 (20–25) and other recent variants. Precipitates forming in a...

Design of novel high strength bainitic steels: Part 1

Abstract: Mixed microstructures consisting of fine plates of upper bainitic ferrite separated by thin films of stable retained austenite have seen many applications in recent years. There may also be some martensite present, although carbides are avoided by the judicious use of silicon as an alloying element. The essential principles governing the optimisation of such microstructures are well established, particularly that large regions of unstable high carbon retained austenite must be avoided. With careful design, impressive combinations of strength and toughness have been reported for high silicon bainitic steels. The aim of the present work was to ascertain how far these concepts could be extended to achieve unprecedented combinations of strength and toughness in bulk samples subjected to continuous cooling transformation, consistent with certain hardenability and processing requirements. Thus, this paper (part 1 of a two part study) deals with the design, using phase transformation theory, of a series ...

Design of novel high strength bainitic steels: Part 2

Abstract: A combination of thermodynamic, kinetic, and mechanical property models and physical metallurgy principles was used in part 1 of this study to propose a number of alloys which exploit the carbide free bainitic microstructure at its theoretical best. These alloys have been manufactured and the present paper (part 2) reports the results of metallographic characterisation and mechanical tests. The proposed steels are found to have an unprecedented combination of strength and toughness for bainitic microstructures, matching even the maraging steels which are at least thirty times more expensive. The work confirms the alloy design procedures explained in part 1.

Effect of compositional variations on characteristics of coarse intermetallic particles in overaged 7000 aluminium alloys

Abstract: To provide an understanding of how compositional variations affect the characteristics of coarse intermetallic particles in 7000 aluminium alloys, three Al-Zn-Mg-Cu-Zr alloy plates with different zinc, magnesium, and copper contents have been studied by optical microscopy based image analysis, scanning electron microscopy SEM, energy dispersive X-ray spectroscopy EDS, and differential scanning calorimetry DSC. These coarse intermetallic particles are detrimental, especially to the toughness of the alloy. Experimental observations have been interpreted successfully on the basis of the phase diagram and the temperature dependent S phase solvus, derived in turn on the basis of the regular solution model. The temperature dependent S phase solvus indicates that some compositions in the composition windows of 7050 and 7x75 type alloys will give rise to the detrimental S phase, which cannot be dissolved during solution treatment. Also, the T phase has been analysed and conditions for its formation and di...

Equilibrium pseudobinary Al–Mg2Si phase diagram

Abstract: Preliminary experiments and phase diagram calculations were conducted to determine the equilibrium phase diagram of the Al–Mg2Si pseudobinary section. It was found that there is a narrow ternary phase field of Al+Mg2Si+liquid in the diagram. At the pseudoeutectic composition of Al–13.9 wt-%Mg2Si, a pseudoeutectic reaction takes place between the temperatures of 583.5 and 594°C. The solubility of Mg2Si in Al at 583.5°C is calculated as 1.91 wt-%.

Biomedical materials for new millennium: perspective on the future

Abstract: Millions of orthopaedic prostheses made of bioinert materials have been implanted with an excellent 15 year survivability of 75–85%. Improved metal alloys, special polymers, and medical grade ceramics are the basis for this success, which has enhanced the quality of life for millions of patients. However, an increasing percentage of our aging population require greater than 30 years survivability of the devices. It is proposed that to satisfy this growing need for very long term orthopaedic repair a paradigm shift is needed; a shift in emphasis from the replacement of tissues to the regeneration of tissues. Such a shift from a materials and mechanics approach to tissue repair requires an increase in the understanding and utilisation of biologically approaches. Two new biologically orientated alternatives in biomaterials for orthopaedics in the new millennium are discussed: tissue regeneration, where in situ repair is initiated in the host tissue, and tissue engineering, where repair is initiated i...

Axisymmetric compression test and hot working properties of alloys

Abstract: The paper investigates the use of the hot axisymmetric compression test for the determination of the hot working properties of alloys. Qualitative analysis shows that stress determinations from the test are subject to systematic errors. These arise from frictional effects and deformation heating. The errors are affected by the conditions of the test and the effects of these parameters have been investigated using a fully coupled finite element procedure. Specimen geometry, specimen volume, friction, temperature, strain rate, and strain have been investigated. Procedures outlining methods of error calculation for general testing conditions are given. Methods of using the data to correct stress–strain curves, to validate such curves and to optimise testing conditions are discussed.

Thermal stability of electrodeposited nanocrystalline nickel and iron - nickel alloys

Abstract: The thermal stability of electrodeposited nanocrystalline nickel and iron–nickel alloys has been studied using TEM, X-ray diffraction, and atom probe analysis. All of the as deposited materials were purely fcc and had grain sizes of 10–20 nm. Heat treatment of nanocrystalline nickel in the range 190–320°C resulted in abnormal grain growth with an activation energy of 122±15 kJ mol-1. Abnormal grain growth in Fe–50 at.-%Ni was only observed at 400°C but not at 220 or 300°C, where grain growth was very slow. In Fe–33 at.-%Ni, room temperature aging resulted in the formation of large grained areas (∼1 µm), some of which transformed to bcc. In heat treated nickel specimens, some evidence of sulphur and carbon enrichment was found at grain boundaries.

Ultrahigh strength hot rolled microalloyed steels: microstructural aspects of development

Abstract: Ultrahigh strength hot rolled microalloyed steels of yield strength 690 and 760 MPa with ferrite–bainite microstructure have been developed. Impact toughness of ∼135 J at -40°C and a ductile–brittle transition temperature of less than approximately -70°C have been obtained in steels of gauge ∼3 mm. The attractive strength–toughness combination was achieved by applying microalloying concepts and controlled rolling in an interactive manner. Alloy design aspects are qualitatively described in relation to the contributions of solute, grain size, precipitates, and microstructural constituents.

Characterisation of microstructure and texture in friction stir welded joints of 5754 and 5182 aluminium alloy sheets

Abstract: The microstructure and texture of friction stir welded joints of hot mill stock 5754 and 5182 aluminium alloy sheets were examined after a commercial joining process. The as received sheet was cut into two pieces, one piece was cold rolled with 20 reduction in thickness, whereas the other was subsequently aged at 230C for 100 h after the 20 prestrain. These two pieces with different thermal pretreatments were joined by friction stir welding FSW. On FSW, a uniform fine grained microstructure in the through thickness of the sheet having a width corresponding to the diameter of tool shoulder was observed in both the 5754 and 5182 joints. Moreover, the microhardness profile did not reveal a softened heat affected zone between the weld and the base metal. An X-ray pole figure study showed that the 5754 and 5182 weld zones were not highly textured and both comprised of similar texture components. The calculated average sheet formability of the weld zones predicts a more isotropic behaviour than that for...

Influence of material swelling on surface roughness in diamond turning of single crystals

Abstract: The effect of material swelling on the surface roughness in ultraprecision diamond turning has been investigated. Experimental results from the power spectrum analysis indicate that the profile of the tool marks is distorted by the effect of swelling of the materials being cut. A good correlation exists between the surface roughness and the amount of swelling that has occurred in the machined layer. Radically different surface roughness profiles were obtained when machining aluminium and copper single crystals with the same cutting plane and tool shape. The difference in the machining behaviour could not be accounted for by elastic recovery alone but could be explained by considering the plastic deformation induced in the machined layer.

Tensile stress–strain and work hardening behaviour of 316LN austenitic stainless steel

Abstract: The true stress (σ)–true plastic strain (ɛ) data of a type 316LN austenitic stainless steel tested at nominal strain rates in the range 3×10-5–3×10-3 s-1 and temperatures of 300–1123 K were analysed in terms of flow relationships proposed by Hollomon, Ludwik, Swift, Voce, and Ludwigson. The applicability of the particular flow relationship is discussed in terms of ‘complete’ and ‘applicable’ range fits of the experimental σ–ɛ data. At all strain rates, in the case of the complete range fit, the Ludwigson equation followed the stress–strain data most closely at 300 K, while in the temperature range 523–773 K, the flow behaviour was described equally well by both the Ludwigson and Voce equations. In the temperature range 823–1023 K, the Voce equation described the flow behaviour most accurately in the case of the complete range fit of σ–ɛ data at all strain rates. The analysis of σ–ɛ data employing the Ludwigson equation in the applicable range fit covering low and intermediate strains, and the Holl...

Microstructure and tensile properties of modified 9Cr–1Mo steel (grade 91)

Abstract: The influence of different soaking temperatures in the range 973–1623 K (below Ac 1 to above Ac 4) before oil quenching and tempering, on the microstructure, hardness, grain size, and tensile properties of modified 9Cr–1Mo steel has been studied. This was done in an effort to assess the tensile behaviour of the different microstructures likely to be encountered in the heat affected zone of a fusion welded joint of the steel. The steel developed predominantly martensitic structure after quenching. Soaking of steel in the intercritical temperature range (between Ac 1 and Ac 3) reduced the prior austenitic grain size and hardness. Soaking temperatures above Ac 3 increased the grain size and hardness of the steel until the formation of δ ferrite at temperatures above Ac 4. The δ ferrite formation at soaking temperatures above Ac 4 reduced the grain size and hardness of the steel. The tensile strength of the steel exhibited a minimum for soaking in the intercritical temperature range where the ductilit...

Structural refinement of cast magnesium alloys

Abstract: The structure of cast magnesium alloys (grain size and precipitate morphology and size) affects the properties of the products and the scope for use of the alloys. The structure can be controlled by minor additions of inoculants, which are largely determined on the basis of the composition of the alloy concerned. The present paper reviews the scientific background of structural refinement by inoculation and its application to Mg–Zn, Mg–Al, and Mg–Al–Si alloys.

Formation of {111} fibre texture in recrystallised aluminium sheet

Abstract: The deep drawability of commercial purity aluminium sheets is improved by introducing a (in fcc materials rather unusual) {111} fibre texture in the sheet surface layers. An additional step of warm rolling after the conventional hot and cold rolling leads to the formation of a pronounced shear texture in the sheet surface layers. During the final recrystallisation annealing, the desired {111} texture prevails at the expense of the other shear texture components. The present paper aims to clarify the mechanisms of the formation of {111}∥ND orientations during both warm rolling and recrystallisation. The effect of the {111} surface texture on the plastic anisotropy of the resulting sheets is discussed.

Anisotropic dilatation behaviour during transformation of hot rolled steels showing banded structure

Abstract: Dilatation measurements were carried out on cylindrical specimens taken from the longitudinal and the normal directions of hot rolled C-Mn steel strip, giving rather different results for the heat treatments that led to a banded ferrite-pearlite microstructure. Under those conditions, the standard methods of deriving the fractions transformed from the uniaxial dilatation signal give rise to systematic errors. A model describing the anisotropic dilatation behaviour for such an evolving banded structure is presented. The model reproduces the experimentally observed differences semiquantitatively as a function of the band orientation in the specimen. Calculations indicate that the most adequate description of the anisotropic dilatation behaviour during formation of an evolving banded structure should be based on a time dependent model, which accounts for the phase transformation history. The consequences of the anisotropic behaviour for determination of the fractions transformed are discussed.

Review of creep behaviour of AZ91 magnesium alloy produced by different technologies

Abstract: The available studies of the creep behaviour of AZ91 alloy produced by die casting, ingot casting, or thixoforming are reviewed in the present paper. Differences in microstructure and creep response are analysed and discussed with a view to providing a unitary, comprehensive description of the creep behaviour of this material. In particular, the minimum creep rate dependence on applied stress is described by means of a modified form of power law, which takes into consideration the effects of grain size and of intragranular precipitates interacting with dislocations. Analysis of the data obtained from the die cast alloy indicates that, in this material, creep is controlled by climb. Based on this observation and on theoretical considerations, as well as on the microstructural similarities, it is concluded that creep is controlled by climb also in thixoformed and ingot alloys. The effect of grain size is then quantified, and the strengthening effect of intragranular precipitates is described by intr...

Modelling strain hardening and steady state deformation of Al–Mg alloys

Abstract: A new approach to the modelling of work hardening during plastic deformation of pure fcc metals has recently been proposed by the present authors. The model is based on a statistical approach to the problem of athermal storage of dislocations. By combining the solution for the dislocation storage problem with models for dynamic recovery of network dislocations and sub-boundary structures, a general internal state variable description is obtained. In the present work this model is further developed by including the effects resulting from grain boundaries, elements in solid solution, and the presence of non-deformable precipitate particles. The result is a work hardening model and associated computer code, capable of providing the stress–strain behaviour for a given solid solution alloy under any combination of constant strain rate and temperature. The model has been applied to the problems of work hardening and flow stress saturation in Al–Mg alloys. It is demonstrated that the model predictions, i...

Grain boundary engineering and the role of the interfacial plane

Abstract: Grain boundary engineering (GBE) involves the use of microstructural design to improve bulk material properties and enhance resistance to intergranular degradation. More specifically, the patented GBE procedure involves the design and control of fcc metallic microstructures using thermomechanical treatments and grain boundary characterisation based on the coincidence site lattice model. The phenomenon of multiple twinning is used to create a ‘twin limited’ microstructure, i.e. a microstructure composed entirely of special grain boundaries and triple junctions that is highly resistant to intergranular degradation. However, the theory behind GBE is not fully developed and therefore further study of the interfacial geometry, including the grain boundary plane and its role in GBE, is required to improve understanding of multiple twinning with the ultimate aim of improving the bulk and intergranular properties of metallic materials. An introduction to GBE is presented, including a number of cases where...

Effect of microstructure on fatigue behaviour of cast Al–7Si–Mg alloy

Abstract: The fatigue behaviour of a cast Al–7Si–Mg alloy, conforming to A356, has been studied. Specimens of this material were tested in both the as cast condition and a solution treated and aged condition. It was observed that the size, number, and position of casting defects influenced the fatigue life very strongly. This marked effect nearly hides that of the heat treatment. Nevertheless, if the analysis is carried out considering only results obtained from sound specimens it is revealed that the heat treatment causes an improvement in the fatigue resistance of the alloy.

In situ stir cast Al–TiB2 composite: processing and mechanical properties

Abstract: Elemental Ti and B powders of stoichiometric composition were mixed and added to molten aluminium. In situ TiB2 particles were formed in the aluminium melt. On casting, an Al–TiB2 composite was produced. Despite the presence of the Al3Ti phase associated with the Al–Ti–B ternary system, the in situ TiB2 particles, with sizes of 1–3 µm formed in the composite was able to yield an improvement of 57% increase in tensile strength, 66% in yield strength and 22% in modulus in an Al–15 vol.-%TiB2 composite. The extent of improvement in these properties depended on the volume fraction in the composite. Fractography showed a texture of dimples seated with hexagonal TiB2 particles indicating retention of high ductility in the composite, despite the fact that the predominance of the coarse Al3Ti in the composite had led to premature rupturing.

Heterogeneous deformation and recrystallisation of iron base oxide dispersion strengthened PM2000 alloy

Abstract: The recrystallisation behaviour of PM2000 oxide dispersion strengthened ferritic alloy has been investigated for tube specimens which had been cold deformed after extrusion. The evolution of the recrystallisation temperature, defined as the minimum temperature at which the specimen begins to recrystallise, has been studied in detail as a function of the level of deformation. The microstructure was characterised using optical and transmission electron microscopy, together with microhardness measurements, and local texture measurements obtained using the electron backscattering pattern technique. The results can be interpreted if it is assumed that any procedure that produces a heterogeneous microstructure, stimulates recrystallisation. In this sense, larger strain gradients lead to more refined and more isotropic grain structures. The way in which these results can be exploited for commercial applications is discussed.

Effect of process variables on formation of dynamic strain induced ultrafine ferrite during hot torsion testing

Abstract: Ultrafine grain sizes were produced using hot torsion testing of a 0.11C-1.68Mn-0.20Si wt- steel, with ultrafine ferrite <1 m nucleating intragranularly during testing by dynamic strain induced transformation. A systematic study was made of the effect of isothermal deformation temperature, strain level, strain rate, and accelerated cooling during deformation on the formation of ultrafine ferrite by this process. Decreasing the isothermal testing temperature below the Ae3 temperature led to a greater driving force for ferrite nucleation and thus more extensive nucleation during testing; the formation of Widmanstatten ferrite prior to, or early during, deformation imposed a lower temperature limit. Increasing the strain above that where ferrite first began 0.8 at 675C and a strain rate of 3 s1 increased the intragranular nucleation of ferrite. Strain rate appeared to have little effect on the amount of ferrite formed. However, slower strain rates led to extensive polygonisation of the ferrite formed...

Kinetics and dilatometric behaviour of non-isothermal ferrite–austenite transformation

Abstract: A model that describes the ferrite–austenite transformation during continuous heating in Armco iron and three very low carbon, low manganese steels with a fully ferritic initial microstructure is presented. This model allows calculation of the volume fractions of austenite and ferrite during transformation as a function of temperature, and hence knowledge of the austenite formation kinetics under non-isothermal conditions in fully ferritic steels. Moreover, since dilatometric analysis is a technique very often used to study phase transformations in steels, a second model, which describes the dilatometric behaviour of the material and calculates the relative change in length that occurs during the ferrite–austenite transformation, has also been developed. Both kinetics and dilatometric models have been validated by comparison of theoretical and experimental dilatometric heating curves. Predicted and experimental results are in satisfactory agreement.

Formation of uniformly fine grained ferrite structure through multidirectional deformation

Abstract: The grain size of ferrite transformed from deformed and unrecrystallised austenite becomes finer as the plastic strain induced in the austenite increases. A multidirectional deformation has been proposed as a method for introducing a large plastic strain widely in a specimen. The introduced plastic strain was evaluated using an explicit three-dimensional finite element technique, and a spread of the large plastic strain was predicted for unidirectional compression and bidirectional compression, respectively. A multidirection deformation thermomechanical treatment simulator was used to verify the effect of bidirectional compression with regards to the creation of a fine ferrite grain structure, as predicted by the finite element analysis. The microstructure of the unrecrystallised austenite region after unidirectional and bidirectional compressions was investigated for a Fe-0.16C-0.4Si-1.4Mn wt- steel. The uniformly fine grained ferrite structure produced by the bidirectional compression covered a ...

Tensile behaviour of type 304 austenitic stainless steels in hydrogen atmosphere at low temperatures

Abstract: The tensile behaviour of solution annealed type 304L, solution annealed type 304, and solution annealed and sensitised type 304 stainless steels was investigated in hydrogen and helium under a pressure of 1·1 MPa over the temperature range 300–80 K at strain rates ranging from 4·2×10-5 to 4·2×10-2 s-1. For 304L steel, hydrogen environment embrittlement (HEE) increased with decreasing strain rate. For 304L and 304 steels, HEE increased with decreasing temperature, reached a maximum, and then decreased with further decrease in temperature: the decrease was particularly rapid near the minimum temperature for HEE. Sensitisation enhanced the HEE of 304 steel. Above the maximum HEE temperature, the HEE behaviour was similar to the hydrogen embrittlement behaviour of materials in previous studies, but near the minimum temperature for HEE it was different. Three types of hydrogen induced brittle fracture were observed as a result of HEE: transgranular fracture along strain induced martensite laths and twi...

Modelling precipitation of niobium carbide in austenite: multicomponent diffusion, capillarity, and coarsening

Abstract: The growth of niobium carbide in austenite involves the diffusion of both niobium and carbon. These elements diffuse at very different rates. A model is presented for the overall transformation kinetics of niobium carbide precipitation in austenite that takes into account the multicomponent nature of the diffusion process while at the same time allows for the curvature of the transformation front. The inclusion of the curvature (capillarity) effect has, in a numerical scheme, permitted the precipitation and coarsening reactions to be treated in a single model. The model is compared with published experimental data.