Materials Science and Technology

There is clear evidence that creep damage in power plant steels is associated with grain boundary precipitates. These particles provide favourable nucleation sites for creep damage such as grain boundary cavities and microcracks. Monte Carlo based grain boundary precipitation kinetics is combined with continuum creep damage mechanics (CDM) to model both the microstructural evolution and creep behaviour in power plant metals. It is found that grain boundary precipitates, such as M₂₃C₆ in most Cr containing ferritic steels, are harmful to the creep properties of the material, in line with experimental observations. It is also found that to improve the creep behaviour of the material, means should be found either to increase the proportion of MX type particles, such as VN, or to decrease or remove the larger grain boundary precipitates, such as M₂₃C₆. Hafnium has been ion implanted into thin foils of a 9 wt-%Cr ferritic steel to study the effect of hafnium on the grain boundary precipitation kinetics. It is found that the implantation of hafnium to the steel completely prohibits the formation of the common grain boundary M₂₃C₆ particles. Instead, two new types of precipitates are formed. One is hafnium carbide, which is an MX type precipitate, and is very small in size and has a much higher volume fraction as compared with the volume fraction of VN in conventional power plant ferritic steels. The other is Cr- and V-rich nitride of formula M₂N. CDM modelling shows that implantation of hafnium can markedly improve the creep property of the material. In addition, the replacement of M₂₃C₆ with hafnium carbide increases the concentration of Cr in the matrix and is expected to improve the intergranular corrosion resistance of the material.

Polycrystalline nickel based superalloys are prone to grain boundary attack by atmospheric oxygen either in the form of time dependent intergranular cracking during dwell time within a low cycle fatigue loading spectrum, known as hold time cracking, or in the form of intercrystalline oxidation at higher temperatures. In the case of hold time cracking of IN718 it has been shown that the crack propagation velocity is determined by local microstructure and environmental conditions, reaching values up to 10 μm s⁻¹ under four-point bending conditions at 650°C in air. The governing mechanism for this kind of time dependent quasi-brittle intergranular failure has been recognised to be ‘dynamic embrittlement’, i.e. diffusion of the embrittling element into the elastic stress field ahead of the crack tip, followed by stepwise decohesion. In a very similar way to intercrystalline oxidation, this damage mechanism seems to depend on the local microstructure. Assuming that oxygen grain boundary diffusivity is particularly slow for special coincident site lattice (CSL) grain boundaries, bending and oxidation experiments were carried out using specimens that underwent successive steps of deformation and annealling, i.e. grain boundary engineering. It has been shown that an increase in the fraction of special CSL grain boundaries yields a higher resistance to both intercrystalline oxidation and hold time cracking by dynamic embrittlement.

Phosphorus and molybdenum segregation to grain boundaries in a commercial grade A533B steel subjected to a variety of heat treatments has been examined using a field emission gun scanning transmission electron microscope (FEGSTEM) with energy dispersive X-ray micro-analysis. The results indicate that P and Mo concentrations at prior austenite grain boundaries increase with aging time. This follows the prediction of McLean's equilibrium segregation model, when modified to take account of the interaction energy between phosphorus and molybdenum.

The grain microstructure evolution in the course of two dimensional (2D) grain growth is considered in greater detail, taking into account the influence of grain boundary triple junctions. It is shown that there are two limiting regimes of grain growth in polycrystals: the first one is associated with the situation when the kinetics of grain growth are controlled by the motion of grain boundaries, while the second one is defined by the motion of grain boundary triple junctions, i.e. when the mobility of triple junctions determines the kinetics of grain growth. A generalised theory of 2D grain growth including a limited triple junction mobility is presented. The theoretical predictions are compared with results of computer simulations by a virtual vertex model. We introduce a new branch of grain boundary engineering, namely, grain boundary junction engineering that utilises junction properties for microstructure control.

Grain boundaries resist the propagation of cleavage cracks in polycrystalline materials, and 3D geometrical models have been used to predict the accommodation required at a grain boundary as a crack propagates from grain to grain. This paper describes how focused ion beam (FIB) microscopy, which provides topographic and crystallographic contrast imaging and allows ion milling to be undertaken at selected areas of interest, can be used to investigate these local fracture events. Results of low temperature fracture of polycrystalline bcc Fe–3%Si and hcp zinc are presented. The interactions between these results and the geometrical modelling are briefly discussed.

The crystallography of brittle fracture and deformation twinning in ferritic steels is difficult to study experimentally, because of its three-dimensional aspects. The present paper reports the development of methodologies to study the phenomenon via customisation of various electron backscatter diffraction and SEM routes. It is shown that both direct (from the fracture surface) and indirect (from an adjacent polished side) measurements yield valuable information on crystallographic aspects of the fracture processes. Specifically, brittle fracture in three ferritic steels is studied: a C–Mn weld metal, a low alloy Mn–Mo–Ni steel similar to grade A533 and an ultralow carbon (0·002 wt-%C, 0·058 wt-%P) steel plate. The main conclusions resulting from development of the experimental techniques are that cleavage fracture occurs only on {001} planes, and that intergranular accommodation is present at the fracture surface. Further observations suggest that a cleavage side crack, initially deflected by a deformation twin, eventually blunts at the intersection of two deformation twins. This provides a mechanism for limiting the mean length of microcracks during brittle fracture.

The microstructural stability of Ni nanocrystalline electrodeposits was investigated to verify general principles underlying the suppression of grain growth by microalloying with elements of very low solid solubility. Hf ions at 300 keV energy were implanted in Ni nanocrystalline foils at low (5·8 × 10¹⁵ ions cm⁻²) and high (3·0 × 10¹⁶ ions cm⁻²) doses. Their effects on grain growth at 550°C were studied in situ by transmission electron microscopy at 1·25 MeV and by selected area electron diffraction. Grains roughly doublled in size during implantation, but grain growth during subsequent heat treatment was dose dependent and significantly less than in specimens without implantation. Observation on implanted Ni single crystals revealed clustering and the formation of fine Ni₅Hf precipitates. A possible mechanism of grain growth suppression is discussed.

The present paper reports the application of a five parameter determination of grain boundary types to grain boundary engineered α brass. Approximately 20 000 grains constituted the total sample population, giving rise to more than 77 000 grain boundary line segments. This is the first time that the orientation of a large sample population of grain boundary planes has been measured in a grain boundary engineered material. The most important findings of the investigation were that the distribution of planes showed a prevalence of 〈 110 〉 tilt boundaries, especially asymmetric tilt types, and the presence of 〈 111 〉 twist boundaries. This distribution is a consequence of the low energy of these boundary types. Furthermore, more than three-quarters of boundaries could be considered to be ‘potentially special’. The presence of these boundaries greatly fragmented the grain boundary network. This fragmentation is probably a key factor in the development of superior properties in a grain boundary engineered material.

Most new materials are introduced by selectively comparing their properties against those of steels. Steels set this standard because iron and its alloys have so much potential that new concepts are discovered and implemented with notorious regularity. In this 52nd Hatfield Memorial Lecture, I describe a remarkably beautiful microstructure consisting of slender crystals of ferrite, whose controlling scale compares well with that of carbon nanotubes. The crystals are generated by the partial transformation of austenite, resulting in an extraordinary combination of strength, hardness and toughness. All this is in bulk steel without the use of expensive alloying elements. We now have a strong alloy of iron, which can be used for making items that are large in all three dimensions, which can be made without the need for mechanical processing or rapid cooling and is cheap to produce and apply.

Solid state, diffusion controlled phase transformation kinetics with a moving boundary has been quantified using a fully implicit, fixed grid, finite difference method based on the control volume approach. In a departure from the usual modeling techniques for phase change problems, the region undergoing phase change has also been considered as a control volume. A new equation for the interface flux balance has been obtained that minimises the mass balance error that normally plagues the numerical solution of moving boundary problems. The model has been validated with the calculated phase thickness based on binary equilibrium diagram and available experimental data in the literature for the Cu–Zn system and a good match has been obtained. The results obtained by the present formulation are compared with those obtained from the other models. In addition to the improved accuracy of the prediction because of elimination of the mass balance error, the proposed method has the usual advantages of a fully implicit scheme.

Optical and transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis of bulk extracted precipitate residues were carried out on long term (more than 80 000 h) creep tested (at 1023 K) type 304 austenitic stainless steels with different levels of Ti content to assess the microstructural stability and creep strength. B and Ce were added to the steels to suppress the creep cavitation. Finer Ti(C,N) particles with higher density and narrower size distribution were observed in steels with a higher Ti content, resulting in an increase in the creep rupture strength. However, higher Ti content increased the intergranular precipitation of the σ phase on longer creep exposure, resulting in the increase in creep cavitation and in the decrease in creep rupture strength. The study indicated an optimum level of Ti and {C + (6/7)N} content with the Ti/{C + (6/7)N} ratio close to the stoichiometric value of the Ti(C,N) precipitate particles that should also be close to their solubility limit at the solution heat treatment temperature.

To develop low melting point filler metals for brazing TiNi shape memory alloy (SMA) and stainless steel (SS), a series of Ag–22Cu–Zn–Sn (wt-%) filler metals have been studied. Using differential thermal analysis (DTA) analysis, the melting temperatures of Ag–22Cu–Zn–Sn filler metals were determined. The results show that the increase of zinc and tin contents drastically decreases the solidus and liquidus temperatures of the Ag–22Cu–Zn–Sn filler metals and the melting temperatures of the Ag–22Cu–18Zn–Sn filler metals with 5–8 wt-%tin are < 650°C. Metallographic observations indicate that the increase of zinc and tin in the Ag–22Cu–Zn–Sn filler metals helps the formation of eutectic structure and inhibits the formation of α-Ag and α-Cu solid solutions, but the increase of tin also causes the formation of Ag₃Sn and Cu₄₁Sn₁₁ brittle compounds. The results of mechanical property tests of the laser brazed joints of TiNi SMA and SS show that the proper increase of zinc and tin in Ag–22Cu–Zn–Sn filler metals is favourable for improving the strength of the laser brazed joints of TiNi SMA and SS.

The microstructure of 8 μm diameter wire produced by the severe deformation of 316L austenitic stainless steel has been examined using TEM and X-ray diffraction. The deformation imparted amounts to a true strain of 6·3. Data from previous studies on strain induced transformation of this steel have been combined with new results to show that true strains >2 are required in order to observe mechanical stabilisation, i.e. the cessation of martensitic transformation when the martensite/austenite interfaces are unable to propagate through the dislocation debris created in the austenite.

Butt joints of 1·6 mm thick sheets of Al–Mg–Si–Cu alloy 6056 in the T4 temper have been produced using a 3 kW Nd:YAG laser with an Al–12 wt-%Si filler wire. The fusion zone consists of a very fine dendritic structure (DAS=5 μm) with a grain size of 100 μm. Its composition is controlled by the ratio between the welding speed and filler wire speed. The interdendritic space is rich in all alloying elements and contains several complex phases. The dendrite interior still contains sufficient solute to enable precipitation hardening during a post-welding heat treatment. The grain boundaries of the base material close to the weld nugget appear to be affected by liquation during the welding process, with spatial extension controlled by the welding speed.

The microstructure of the heat affected zone is characterised by GP zone dissolution. The resulting hardness decrease can be fully recovered by a T6 or T78 post-welding heat treatment. The comparatively lower hardness of the fusion zone can also be improved by precipitation hardening during a post-welding heat treatment.

Aluminium based metal matrix composite containing 5, 10, and 15% by weight of flyash particulates was successfully synthesised using a stir cast method. The microstructure of these stir cast composites exhibited relatively uniform distribution of flyash particles. Mechanical properties such as density, hardness, and tensile strength were investigated. Sliding wear behaviour, slurry erosive wear behaviour, and fog corrosion behaviour of the composite were also investigated in the heat treated condition. The results of wear studies have shown that the resistance to wear increases with increasing percentage of flyash particulates. The addition of flyash particles showed appreciable change in overall density values. The bulk hardness and tensile strength increased with increasing flyash content. The presence of flyash particles appeared to initiate pitting corrosion.

In this study, a 355 nm UV Nd:YAG laser is used to process silicon wafers. In order to obtain microstructures with high aspect ratio, a dual prism optical system is set up to control the cutting linewidth of the UV laser beam. During the laser beam propagation through the prisms, the two prisms are rotated with the same angular velocity, which results in the focal spot of the laser beam moving in a circular path on the silicon substrates. When the laser beam moves relative to the holder (workstation), a laser cutting process can be carried out. With this laser system, the cutting linewidth is controllable ranging from 10 μm to 1 mm by adjusting the initial phase difference in the two prisms. The experimental results show that arbitrary shaped silicon based microstructures with high aspect ratio can be fabricated by this 355 nm UV laser system, and the aspect ratio over 10 can be obtained.

The hot deformation of AZ31 magnesium alloy has been studied by compression testing using a Gleeble 1500 machine at temperatures between 250 and 450°C and at strain rates ranging from 0·005 to 5 s⁻¹. Optical microscopy and transmission electron microscopy (TEM) have been used to observe microstructures of the alloy. The experimental results show that the flow stress behaviour can be described by an exponential law at temperatures below 350°C. At higher temperatures a power law of deformation is valid. The hot deformation activation energy Q derived from the experimental data is 112 kJ mol⁻¹ with a stress exponent n=7. Optical microscopy and TEM observations show that dynamic recrystallisation (DRX) takes place during the deformation process and the formation of new grains occurs by conventional DRX nucleation by bulging. The average DRX grain size D_rex is sensitively dependent on deformation temperature T and strain rate ϵ and is also a function of the Zener–Hollomon parameter Z. The relationship between Z and D_rex has been experimentally constructed using linear regression. In the present work, DRX grains nucleate by bulging of some portions of serrated grain boundaries, accompanied by the formation of twinning.

Previous investigations on the relationship between the mechanical properties and microstructure of metal foams have mainly been related to the compressive response of closed cell aluminium foams. In contrast, little research has been conducted on the tensile properties of open cell metal foams. This investigation examines the effects of changes in foam density, cell size, grain size and strut size on the ultimate tensile strength (UTS) of electrodeposited nickel foams. Preliminary studies were conducted using scanning electron microscope (SEM) to determine the cell size, grain size and strut size of nickel foams. The primary factor controlling the UTS of nickel foams tested appears to be relative density, with increases in relative foam densities from 0·035 to 0·083 accompanying an increase in the UTS from 0·85 to 3·53 MPa. Experimentally obtained UTS values were compared with predicted values obtained from empirical relationships.

This paper addresses the issue of instability of deformation during gas pressure forming of superplastic sheets. With regard to fracture strain, the plastic behaviour of the spherical dome has been described in terms of the local effective stress and the effective strain. These quantities are equated to the uniaxial stress state. The limiting effective thickness strain is obtained utilising the relations between the strain rate sensitivity index and the fracture strain. The results are found to be in good agreement with the measured failure strains.