Materials Science and Technology

The goal of this paper is to provide a review of the interrelationship between grain boundary structure, chemistry, and failure. The main thrust of the paper is that one must consider structure and chemistry in order to assess various failure mechanisms and that one must also address these issues in high angle, asymmetric grain boundaries. One of the key themes of the paper is that the boundary is best considered as a chain of molecular or structural units and that, during such processes as fracture or sliding, the ability of the boundary to sample many of these units determines whether or not a material will ultimately fail. The ease of sampling different boundary structures will depend on the chemical composition of the boundary.

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 dissolution are discussed. It is shown that the present results can produce useful information on alloy design and thermomechanical processing via microstructural control.

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 wider region in comparison with that produced by unidirectional compression. Multidirectional deformation can be applied to obtain ultrafine grain structure for thick products of steels.

Discontinuously reinforced metal matrix composites are susceptible to reinforcement particle cracking, which reduces tensile elongation. However, many of the particles experience multiple cracking episodes, which would seem unfavourable in a matrix filled with similar particles. Simulations of an aluminium matrix composite reinforced with 9 vol.-SiC particles have been performed using a non-linear axisymmetric finite element model. Reinforcement particles with and without defects were cracked in the simulation in order to study the subsequent behaviour of the composite. Particle defects were found to decrease the impact of reinforcement particle cracking on the stress in the surrounding matrix. Multiple cracking of reinforcement particles was shown to be feasible because of the load bearing ability of fractured particles.

The microstructural evolution of a spheroidised Fe-0.85C wt- steel during heavy cold rolling and post-aging was studied, with the emphasis on cementite. A new morphology of cementite, wavy thin bands nearly parallel to the rolling plane, was found in the specimens subjected to rolling reductions >90. Slip on 001theta planes was observed in the deformed cementite particles. By X-ray diffraction, it was found that the initial spheroidal cementite particles dissolve partially in a solid solution of ferrite by deformation, in a manner similar to that of the lamellar cementite of pearlite. Greater strain age hardening was observed for the spheroidised steel than for a similarly treated pearlitic Fe-0.85C steel. The strain age hardening peak was observed at 573 K for the spheroidite structure, which is 150 K higher than was observed for the pearlite structure. The activation energy for age hardening was estimated to be 122 kJ mol1, which is attributed to carbon atom diffusion from cementite to dislocations in ferrite.

The texture of two transformation induced plasticity steels has been studied by means of crystallographic orientation mapping. Texture measurements were carried out on ferrite, bainite, and austenite. The polygonal ferrite and the bainite texture, both bcc, could be distinguished based on the image quality parameter of the electron backscattering diffraction measurement. Both bcc textures were very similar, the main difference being the more pronounced 111alpha ND ND=normal direction and 110alpha RD RD=rolling direction fibre textures in the polygonal ferrite. The fcc texture was a strong gamma deformation texture, characterised by the beta fibre. The presence of the alpha fibre confirmed the strong 110gamma ND direction, which was previously detected by means of X ray diffraction XRD. The measured fcc and bcc textures were used to calculate orientation distribution function transformations according the Bain, Kurdjumov-Sachs, and Nishiyama-Wasserman orientation relationships. The predicted cube component {001}gamma 100gamma, which was missing in the measured texture, of gammaret indicates a variant selection for the gammaalphaB transformation. In addition it was shown that crystallographic orientation mapping could be used to make reliable phase fraction determinations, which were previously based on the light optical microscopy of colour etched specimens. This also proves that XRD determination of gammaret is flawed owing to the strong texturing of all phases present in the microstructure.

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 because more time was available for ferrite recovery. Accelerated cooling during deformation followed by air cooling to room temperature led to a uniform microstructure consisting of very fine ferrite grains and fine spherical carbides located in the grain boundaries regions. Air cooling after isothermal testing led to carbide bands and a larger ferrite grain size.

The secondary processing of low carbon steel strip produced by twin roll casting was investigated to examine its effect on microstructural development and mechanical properties. The as cast microstructure is predominantly acicular ferrite with regions of bainitepearlite and polygonal ferrite. Deformation at temperatures below Ar1 produces a heterogeneous microstructure with regions of moderately deformed acicular ferrite adjacent to highly deformed regions containing shear bands. Cold rolled and warm rolled steels show similar behaviour to conventional hot band in that dynamic recovery during warm rolling results in sluggish recrystallisation and produces a coarse final grain size. However, the initial as cast microstructure recrystallises at a slower rate than conventional hot band and produces a weaker recrystallisation texture. This can be attributed to the heterogeneous microstructure of the as cast strip such that, after rolling, nucleation occurs within shear bands and more ill defined sites, which results in nucleation of randomly oriented grains thereby producing a weak final texture. It was found that austenitising the as cast strip followed by rolling in the vicinity of Ar3 produces a uniform distribution of equiaxed, ultrafine ferrite UFF grains throughout the thickness of the strip. The production of UFF by twin roll casting and subsequent rolling represents a simple processing route for the production of fine grained low carbon sheet steel products.

Submerged arc weld materials have been employed in a study of the effects of manganese and carbon on phosphorus segregation and intergranular embrittlement. The equilibrium grain boundary segregation behaviour of these elements during aging has been studied in two different boiler shell weld materials, which differ mainly in the manganese concentration and operating temperature during service. The materials have seen temperatures above 300C during operation of the boiler in service, at which temperature thermally induced segregation and embrittlement occurs. A new co-segregation model has been compared with the existing site competition model. The microstructure shows fine and coarse grained regions. The effects of manganese and carbon on the grain boundary segregation of phosphorus have been examined. Thermally induced grain boundary segregation during full service life up to 50 years as a function of temperature is described. To evaluate the free matrix concentration of a given element, equilibrium thermodynamic software was used to allow for the tendency to form precipitates within the alloy matrix. The predicted results reveal the dependence of the grain boundary concentration on temperature and show that manganese and carbon decrease the phosphorus segregation by site competition. The final segregation consists of non-equilibrium and equilibrium segregation, which occur during quenching after welding, post-weld heat treatment, and service. The microstructure has been investigated by optical microscopy and transmission electron microscopy to show carbide formation at the grain boundaries and intergranular precipitation of MnS. Preliminary analysis of the grain boundary has been made and the results compared with theoretical segregation predictions.

The effects of elements such as Ni, Mo, Cr, Cu, and V on the wear behaviour of high manganese steels have been reported extensively. Most researchers agree on the influence of many of the elements but disagree on the effect of Cr. A study has been conducted to evaluate the effect of 1.7 and 2.3 wt-Cr on high manganese steel when subjected to various wear conditions: impact loading, abrasion, combined impact-abrasion, and combined abrasion-corrosion. The study has revealed that adding Cr to high manganese steel resulted in an increase in hardness and hardenability, and a decrease in toughness. The effect on wear resistance was found to depend on the wear conditions. Chromium alloyed high manganese steels showed superior wear resistance compared with plain Hadfield steels where corrosion, abrasion, and combined impact-abrasion conditions prevailed. Such conditions required a high surface hardness. Plain Hadfield steel showed superior wear resistance in conditions where pure impact wear is encountered. Such conditions required an increase in toughness rather than surface hardness.

Instrumented impact testing was used to investigate the effects of aging on the impact deformation and fracture of different weldments of an AISI type 316L stainless steel plate and a superduplex stainless steel plate. Aging at 800C of metal inert gas MIG and laser beam LB weldments of the 316L stainless steel plate produced a precipitation of sigma phase, which was more intense in the MIG weldments. Correspondingly, the MIG weldments showed a noticeable decrease in impact energy and in impact yield and maximum loads as the aging time was increased. The LB weldments, on the other hand, showed an initial decrease in absorbed energy followed by a noticeable increase, because of the appearance of delaminations during the fracture event. Aging at 900C of the parent metal and of LB and plasma arc PA weldments of the superduplex stainless steel plate produced a precipitation of sigma phase which was much more intense in the parent metal. Both the parent metal and the weldments showed a noticeable decrease in impact energy and in impact yield and maximum loads as the aging time was increased. These weldments were found to be more sensitive to the precipitation of sigma phase than the austenitic stainless steel weldments.

The initiation and propagation of a crack has been investigated in a 110110 oriented Ni3Al alloy single crystal by in situ TEM and using tension deformation at room temperature. The result has shown that the macropropagating direction of the crack was parallel to the tensile axis and the crack followed a zigzag path. Trace analysis indicated that the slip on 111 and 111 were activated during the crack propagating process. Calculations show that for this orientation of Ni3Al crystal, the stress concentration arising from the dislocation pile-up decided the choice of secondary slip systems and the macrodirection of the crack.

A high temperature fatigue crack growth study on an experimental disc alloy of approximate composition Ni-14.75Cr-14-19Co-4.75Mo-3Al-3.75Ti-1.75Ta-0.7Hf-0.06Zr-0.02C-0.0175B wt- has been undertaken. Comparison of constant load fatigue crack growth tests conducted in air and a vacuum at 725C and in air at room temperature indicate that an oxidising environment has a major influence on crack growth rates over a wide range of applied stress intensity range. In particular it contributes to enhanced embrittlement of grain boundary regions, promoting an early transition to intergranular failure along with a concomitant increase in growth rate. Constant stress intensity factor range tests at high R ratio showed that a decrease in frequency at 725C in both air and a vacuum caused an increase in the crack growth rate per cycle due to time dependent crack growth. This was most significant in air at 725C rather than in vacuo, although in both instances low frequency tests were accompanied by wholly intergranular crack growth. At lower R ratios the influence of time dependent processes is less pronounced, especially in a vacuum. For the purposes of prediction a straightforward linear summation model using sustained load crack growth results combined with high frequency growth rates was found to model reasonably the influence of frequency on crack growth rates for the air tests at 725C. This can be rationalised by the observation that in air intergranular failure proceeds by linking of prior cracked or embrittled grains some distance ahead of the crack tip. At lower values of stress intensity range a slight under prediction of growth rates was evident, but improvements could be made through the use of triangular waveform data. For the vacuum tests, the linear summation model was consistently found to over predict growth rates due to the transitory nature of the sustained load crack growth rates under repeated loading and was not generally deemed suitable. Under these conditions damage occurs close to the crack tip and direct interaction between time dependent and time independent mechanisms will occur. This is not taken into account by a summation approach and more accurate modelling of damage formation in the varying strain fields ahead of the crack tip is required to predict this.

Decreasing the cycle time significantly for forming the commercially available superplastic aluminium alloy 5083 has been achieved. Forming results and conditions are compared with previous relevant works which are actually scarce. A circular cup having a depthdiameter ratio of 1:2 can be formed in 70 s. This ratio requires flat sheet to be stretched in area by up to three times, which should be large enough when dealing with actual industrial sheet forming. On average, the thickness is decreased by two-thirds; in fact, the thickness distribution is not uniform and the gradient is concentrated at the wall of the cup. The location of minimum thickness in rapid forming is different from that in conventional forming. Disregarding the traditional approach, the pressure-time profile employed in this work was not restricted to yield the so called optimum strain rate, which is usually low. Following the same processing profile, but proceeding in stages of partial forming, a series of progressive forming configurations was obtained in order to analyse the strain rate path leading to the successful rapid forming. For a specimen processed at 500C, the maximum volume fraction of cavities is 4 existing at the location of minimum thickness.

A TiAl alloy of Ti-47Al-2Cr-2Nb at.- was used to study phase transformation and microstructure control by heat treatment and a glow discharge spectrometer was applied to analyse the compositional variation from the specimen surface to the interior. It was found that Al tended to evaporate at elevated temperatures. As a result, Al was significantly depleted and a quite different microstructure developed in the outer layer compared with that in the interior. Results also showed that the outer layer appeared thicker with increasing heating temperature, time, and cooling rate, and with changing the environment from vacuum to air. Since the evaporation of Al leads to microstructure variation and perhaps property deterioration as well, it is suggested that this effect should be considered for alloy design and processing.

An investigation of microstructural development and structure-property relationships in a transient liquid phase bonded Ti-48Al-2Cr-2Nb at.- alloy abbreviated here to 48-2-2 is presented. The use of copper containing interlayers is examined. The primary focus of the paper is wide gap composite interlayers consisting of copper as a liquid former, plus a TiAl alloy binary TiAl or 48-2-2 as a non-melting diffusional sink. However, bonds employing commercial purity copper foils of thickness 5 and 50 m are also examined. Bond microstructures are characterised by edge-on transmission electron microscopy, plus scanning electron microscopy and light microscopy. Mechanical properties are determined through the use of room temperature shear and bend testing, supplemented by both room and elevated temperature tensile testing. The formation of copper containing intermetallics at the bond line and the formation of gamma-TiAl by both epitaxial growth and non-epitaxial precipitation are discussed. The microstructure of the bonds is compared with that of the parent 48-2-2 material. The influence of both the main bond line microstructure and the fillets on the mechanical properties of transient liquid phase bonded 48-2-2 is determined.

The properties of intermetallic compounds are sensitive to alloy composition and interstitial element content determined by the melting process. Induction skull melting is one of the best methods for melting reactive alloys. For induction heating under vacuum, melt temperature control is problematic. A numerical model for simulating temperature field during induction skull melting has been developed using the direct finite difference method. Factors such as water cooling boundaries, electromagnetic stirring meniscus, and power distribution in the charge were analysed. The chargecrucible interface was taken as a radiation boundary before melting and as a combined radiation and conduction boundary after melting. The free surface was taken as a radiation boundary. The relationship between the height of electromagnetic stirring meniscus and charge weight and melting power was reduced based on the conservation of mass. Based on the conservation of energy, the distribution density of induction current was ascertained. With the program, the relationship between melting power, charge weight, and melt temperature was established. During induction skull melting of gamma-TiAl based alloys, the melt temperature was measured carefully. The theoretical and experimental results were found to be in agreement.

The reaction layer thickness at the Al2O3AgCuTi interface has been measured with different bonding times at a fixed bonding temperature. The effect that the change in reaction layer thickness had on the shear strength of a Al2O3AgCuTiTi-6Al-4V brazed joint was analysed using SEM. The relationship between reaction layer thickness and joint strength has been discussed with the aim of providing a theory for guiding industrial manufacture and increasing the joint strength.

This study presents observations made pertaining to the slurry wear behaviour of a zinc based alloy and its composite containing dispersed alumina particles. The influence of varying the sand concentration in the medium and the traversal distance on the response of the specimens has also been investigated. Wear rate increased initially with traversal sliding distance, attained a peak, and then decreased thereafter at longer traversal distances. This trend was much more pronounced when tests were conducted in a liquid only medium than in liquid with sand. The presence of suspended sand particles in the test environment led to a considerably reduced wear rate of the specimens when compared with that of the liquid only medium. Furthermore, intermediate sand content caused the maximum wear rate within the slurries, although it was substantially less than that caused by the liquid only medium. A comparison of the wear response of the composite and the matrix alloy suggested a mixed trend in the liquid only medium and the slurry with 60 wt- sand. The composite exhibited a lower wear rate than the matrix alloy when tested in slurries with 20 and 40 wt- suspended sand particles. The observed wear response of the specimens has been discussed in terms of specific characteristics, such as susceptibility to corrosion and hardness of various phases, and the interfacial effects in the specimens. The changing nature of the medium, such as the corrosivity and impingingabrading efficiency, has also been discussed. Analysis of the affected surfaces and subsurface regions of typical specimens by SEM enabled the understanding of the operating wear mechanisms under specific test conditions and thereby it was possible to substantiate the observed wear characteristics of the specimens.

This paper discusses the microstructure and mechanical properties of a Co based alloy coating, produced by CO2 laser cladding technique, for steam turbine applications. The role of the laser beam interaction time, the so called dwell time, has been investigated. It has been realised that the beam interaction time has a significant effect on the solidification and microstructure development process. The results indicated that the laser cladded zone exhibited a fine microstructure and a higher hardness after a short interaction time compared with a coarse structure and lower hardness after a long interaction time. Since the dilution of the base material into the cladding was significantly low after a short interaction time, the clad could retain a significant amount of strength after a long exposure time, which is a unique feature for the turbine industries.

Different powder mixing methods were used to prepare Al-5Cu and Cu-10Sn wt- based composites containing nanometre diamond. A new method for obtaining ultrafine Cu, Sn particles and a good distribution of nanometre diamond aggregates was developed. The nanometre diamond aggregates were well dispersed in the Cu-10Sn matrix. However, the distribution of nanometre diamond in the Al-5Cu based composite was unacceptable. Owing to the dispersion of nanometre diamond, Cu-10Sn based composite possessed higher hardness and lower bend strength than Cu-10Sn alloy.

An anodic oxidation treatment method is developed in this paper which is capable of modifying the surfaces of carbon fibres in three-dimensional 3D braided fabrics conveniently and uniformly. The results on the utilisation of this method in the surface treatment of 3D carbon fibre braided fabrics are presented. The effects of anodic oxidation treatment of 3D fabrics on the properties of phenolic resin composites are analysed and discussed. By appropriate treatment, the interfacial shear strength and compressive strength may be improved by 20 and 59.3 respectively. As far as the impact properties of the composites are concerned, the crack initiation energy was increased and the crack propagation energy decreased. Concerning ablative properties of the composites, the linear ablative rate was decreased by 10.7 and the maximum temperature of the underside of the ablative section was reduced by 2.7.

In the present study, a galvanised iron wire mesh structure of two different types was used to reinforce aluminium alloy 1050. The composite was synthesised using conventional casting followed by hot extrusion. Microstructural characterisation of the composite specimens showed good interfacial integrity between the matrix and the reinforcement wire. Thermomechanical analysis demonstrated that the average coefficient of thermal expansion of the composites was decreased below the values predicted by theoretical models. Tensile tests conducted on the composite specimens revealed that the average elastic modulus of the composites exceeded the rule of mixtures predictions. The average values of 0.2 yield strength and ultimate tensile strength of the composites were found to be higher than those of the monolithic aluminium alloy irrespective of a low volume fraction 0.049 of reinforcement. This unusual variation in the properties clearly reflects the uniqueness of using interconnected wires as reinforcement in the metallic matrix. An attempt is made in the present study to rationalise particularly the tensile behaviour, especially the stiffness of the composite material, according to the type of reinforcement preform in the aluminium alloy matrix.

Selected magnesium hydroxide and calcium carbonate fillers were dry coated with stearic acid and calcium, magnesium, and zinc stearates. The course of the coating reaction was monitored using a quantitative diffuse reflectance Fourier transform infrared spectroscopy method and by thermal analysis. The coated fillers were also examined by wide angle X-ray diffraction and by X-ray photoelectron spectroscopy. These techniques were shown to produce complementary information. When the dry coating process was useded, complex reactions occurred. Stearic acid reacted to produce stearate, while the stearates melted and formed coatings, but in some cases they also appeared to form polymorphic metal stearates. Stearic acid produced the best coverage on calcium carbonate, but the poorest on magnesium hydroxide. Of the metal stearates, the best filler coverage was produced when zinc stearate was used, while calcium stearate was the least effective coating.

A new method of identifying and mapping the distribution of both iron oxides and oxyhydroxides using near infrared Raman imaging microscopy NIRIM is reported. This technique offers an important alternative to conventional spectroscopic techniques that provide spatially averaged data. The NIRIM instrument used for these studies combines fibre bundle image compression hardware and multivariate signal processing software to identify and map different iron minerals and corrosion products. The NIRIM images clearly distinguish hematite, magnetite, wustite, goethite, and lepidocrocite microstructures. The first chemical maps of naturally occurring iron minerals and corroded steel surfaces obtained using NIRIM spectral images, classified with the aid of a library of pure compound spectra, are presented.

Three-dimensionally braided carbon fibre reinforced SiC matrix composites have been fabricated and the effects of coating treatment on the mechanical properties have been investigated. It has been found that pyrocarbon coating can improve the strength of the heat treated carbon fibres. When the coating thickness was 0.5 m, the composites had better mechanical properties: a flexural strength of 643 MPa and a fracture toughness of 17.9 MPa m12. The composites also exhibited a toughening fracture mode.