Microstructural evolution in silicon alloyed γ-Ti–Al alloys after thermomechanical processing

Abstract

The microstructures of silicon alloyed γ-Ti–Al alloys containing silicide particles have been studied after thermomechanical treatments to investigate microstructural evolution. Important parameters including temperature, forging strain, and sequence of thermomechanical treatments were systematically studied. Isothermal forging below the eutectoid temperature resulted in inhomogeneous dynamic recrystallisation with fine equiaxed grains in recrystallised areas and residual α₂ + γ lamellae elsewhere. Eutectic silicides play an important role in destruction of the as cast structure by promoting dynamic recrystallisation during deformation and static recrystallisation on subsequent annealing. There is evidence that silicon, in solution, also enhances recrystallisation. The presence of fine silicides produced by precipitation in the solid state restricts the size of grains produced by both dynamic and static recrystallisation. Silicon also alters significantly the phase equilibrium between the α and γ phases.     

Effect of refinement of grains and icosahedral phase on hot compressive deformation and processing maps of Mg-Zn-Y magnesium alloys with different volume fractions of icosahedral phase

The effect of the volume fraction of I-phase on the hot compressive behavior and processing maps of the extruded Mg-Zn-Y alloys was examined, and the obtained results were compared with those of the cast alloys in a previous work. The average grain sizes, fractions of dynamically recrystallized (DRXed) grains, and sizes of DRXed grains of the extruded alloys after compressive deformation were significantly smaller, higher and smaller, respectively, than those of the cast alloys after compressive deformation under the same experimental conditions. This was because the microstructures of the extruded alloys, having much more grain boundaries and more refined I-phase particles than the cast alloys, provided a larger number of nucleation sites for dynamic recrystallization than those of the cast alloys. The constitutive equations for high-temperature deformation of the extruded and cast alloys could be derived using the same activation energy for plastic flow, which was close to the activation energy for lattice diffusion in magnesium. Compared with the cast alloys, the onset of the power law breakdown (PLB) occurred at larger Zener-Holloman (Z) parameter values in the extruded alloys. This was because the extruded alloys had finer initial grain sizes and higher fractions of finer DRXed grains compared to the cast alloys, such that the onset of PLB caused by creation of excessive concentrations of deformation-induced vacancies was delayed to a higher strain rate and a lower temperature. The flow-stress difference between the extruded alloys and the cast alloys could be attributed to the difference in the fraction of DRXed grains. According to the processing maps, the extruded alloys exhibited higher power dissipation efficiency and flow stability than the cast alloys. This agreed with the microstructural observations.     

Stochastic modelling of dendritic grain structures

Abstract

A three-dimensional cellular automaton–finite element model has been developed over the past five years for the prediction of macrostructures formed in casting. The present article briefly summarises the growth algorithm of the model. Applications are then given for the prediction of the grain structure formed in a directionally solidified turbine blade produced by investment casting, a continuously cast cylinder, and a near net shape steel sheet obtained by twin roll strip casting.     

Forging of Mg–3Al–1Zn–1Ca alloy prepared by high-frequency electromagnetic casting

The 1 wt.%Ca–AZ31 alloy produced by electromagnetic casting (EMC) in presence of electromagnetic stirring (EMS) was extruded and then subjected to the closed-die forging to make a pulley for automobile application. Effective dynamic recrystallization (DRX) took place during the forging process, leading to formation of fully recrystallized grains with the average size of 3–4 μm. High-forging ability and high degree of grain refinement achieved during the forging were attributed to the novel microstructure of the cast composed of small and equiaxed grains with the average size of 50 μm and thin layer (Al, Mg)₂ Ca phase at grain boundaries, which would provide more nucleation sites and a faster rate of recrystallization during deformation by forging as compared to that of the conventionally processed cast composed of large size grains and thick layer (Al, Mg)₂ Ca phase. The forged pulley exhibited the ultimate tensile strength of 273–286 MPa with tensile elongations of 30%. The present result demonstrates a possibility that EMC + EMS techniques can be used in producing magnesium feed stocks with high-forging ability.     

Manufacture and microstructural characterisation of bimetallic gas turbine discs

Abstract

A number of model Ni based superalloy bimetallic discs have been manufactured, each having a cast and wrought U720Li hub bonded to a cast IN738LC ring by means of hot isostatic pressing augmented by isothermal forging. The microstructures of both the as hipped and HIP plus forged model discs showed a complex precipitation reaction at the interface between the two alloys. Different types of precipitate were identified at the bond line, which had formed as a consequence of interdiffusion and recrystallisation at the interface during hipping. Residues from a pre-HIP acid cleaning process were also detected. Preliminary forging trials conducted on the model discs found poor control of the deformation and shape of the interface due to temperature and die lubrication issues and resulted in strong recrystallisation of the IN738LC and internal cracking. Subsequent full scale disc manufacture was able to overcome these problems, although further work is continuing in the microstructural and mechanical characterisation of these parts.     

Effects of boron,carbon, and silicon additions on microstructure and properties of a Ti–15Mo based beta titanium alloy

Abstract

Additions of boron, carbon, and silicon have been made to a series of Ti–15Mo based β titanium alloys prepared by plasma arc melting and subjected to various processing and characterisation techniques. The purpose of these additions was to investigate their grain refining effect in the as cast, as forged, and heat treated states. The boron and carbon additions promoted dendritic solidification. Boride and carbide particles were present in the interdendritic regions. However, a fully equiaxed fine grained structure could not be obtained even at the highest levels of addition (1.0 wt-%). These additions resulted in substantial microstructural refinement after forging and the particles stimulated the nucleation of recrystallised grains. Significant retardation of grain growth was observed after solution treatment and attributed to the Zener pinning effects of the boride and carbide particles. Despite the microstructural refinement, the ductility of the alloys containing boron and carbon was severely impaired.     

Extrusion of AI–Zn–Mg–Cu–Zr alloys

Abstract

Four aluminium alloys of different zinc/magnesium ratio have been studied under various extrusion conditions. The alloys were cast in steel book moulds and subjected to initial thermomechanical treatments. Studies were made of hot extrusions and cold hydrostatic extrusions and in each case the changes in the extrusion parameters were analysed. An attempt has been made to explain some of the extrusion defects which appeared in various extruded sections. The extrusion speed was found to be crucial, since sections developed surface cracks at higher speeds. The extrusion speed was also found to vary inversely with the extrusion ratio, with higher speeds at low ratios. A well defined solute–depleted weld zone was observed on each of the four faces of a square tube extruded using a porthole die. Thermal treatment was not found to improve this weak weld zone. Tubes extruded using a floating-mandrel die withstood pressure testing up to 550 MPa.

MST/43     

Application of superplasticity in commercial magnesium alloy for fabrication of structural components

Abstract

An investigation of the superplastic characteristics of magnesium alloys with several grain sizes revealed that grain boundary sliding took place more easily with grain refinement. The required grain size for high strain rate superplastic forming was estimated to be ～2 µm. The required grain structure could be obtained by several procedures, hot extrusion with a high extrusion ratio, severe plastic deformation via equal channel angular extrusion, consolidation of machined chip, and/or powder metallurgy processing of rapidly solidified powders, on a laboratory scale. The processing route of hot extrusion was selected in this study. An experimental study of superplastic press forming was conducted for a commercially extruded ZK60 alloy. The fabricated product did not essentially contain macroscopic defects, i.e. cracks or cavities. From an examination of tensile characteristics, it was found that the post-formed alloy exhibited higher strength and higher ductility compared with some conventional cast magnesium alloys, aluminium alloys, and steels. The experimental results support the possibility of using superplastically formed magnesium to produce structural components.     

Investigation on microstructure and mechanical properties of Ti14 alloy after semisolid deformation

Abstract

This study details the development of microstructure of Ti14 alloy as a function of the forging temperature and forging ratio in semisolid state and influence of resulting microstructure on the mechanical properties. The results reveal that dynamic recrystallisation occurred during semisolid forging, and the grain refinement was attained. Grain size increased in the forging temperature and decreased in the forging ratio. High ultimate tensile strengths and low elongation have been achieved after semisolid forging. The strength decreased with increasing forging temperature, while the ductility increased with increasing forging ratio. The relative contributions of tensile properties were attributed to the varieties of grain size obtained by thixoforging.     

High strength Al2O3p/6061 Al composites: effect of particles,subgrains and precipitates

Abstract

Composites of 6061 Al reinforced by Al₂O₃ particles have been produced by squeeze casting followed by hot extrusion and a T4 precipitation hardening treatment. The tensile properties at room temperature have been determined and analysed based on microstructural parameters. The strength contributions from the matrix, particles, subgrains and precipitates have been estimated individually, and then based on an assumption of linear additivity, the yield stress values of the composites under the extruded and heat treated conditions have been calculated. Good agreement between the calculated and experimental values has been found, illustrating the suitability of the process for the manufacture of strong composites, with a maximum yield stress of 524 MPa obtained for a composite containing 60 vol.-%Al₂O₃.     

Structural changes of radial forging die surface during service under thermo-mechanical fatigue

Radial forging is one of the modern open die forging techniques and has a wide application in producing machine parts. During operation at high temperatures, severe temperature change associated with mechanical loads and the resultant wearing of the die surface lead to intense variation in strain on the die surface. Therefore, under this operating condition, thermo-mechanical fatigue (TMF) occurs on the surface of the radial forging die. TMF decreases the life of the die severely. In the present research, different layers were deposited on a 1.2714 steel die by SMAW and GTAW, with a weld wire of UDIMET 520. The microstructure of the radial forging die surface was investigated during welding and service using an optical microscope and scanning electron microscope. The results revealed that, after welding, the structure of the radial forging die surface includes the γ matrix with a homogeneous distribution of fine semi-spherical carbides. The weld structure consisted mostly of columnar dendrites with low grain boundaries. Also, microstructural investigation of the die surface during operation showed that the weld structure of the die surface has remained without any considerable change. Only dendrites were deformed and broken. Moreover, grain boundaries of the dendrites were revealed during service.     

Assessment and modelling of isothermal forging of intermetallic compounds Part 2 – Ti3 Al

Abstract

In this, the second of four papers devoted to the isothermal forging of intermetallic compounds, the Ti₃ Al based alloy Superalpha-2 has been deformed in compression at constant strain rate, over temperature and strain rate ranges of 900–1050°C and 0·0003–0·035 s^-1, respectively. Calculation of the material property parameters suggests that this material deforms via dynamic recrystallisation of the B2 phase. Microstructural examination has shown that isothermal forging generates non-equilibrium microstructures. Deformation modelling has been successful in predicting the forging behaviour. Finally, the possibility of explosive grain growth immediately after forging raises questions as to whether the mechanical properties of this material can usefully be controlled by isothermal forging.     

Aluminium nitride precipitation and grain structure of continuously cast billet corners

Abstract

The highly grain refined corners of continuously cast steel billets and the high levels of associated aluminium nitride precipitation were investigated. From experiments using steels grain refined by the addition of aluminium, it was found that aluminium nitride precipitated during recalescence after the temperature of the billet corners had fallen below the Ar₃ temperature in the mould and secondary cooling zone. The aluminium nitride was found to persist and prevent austenite grain growth during reheating to the straightening temperature and this produced the refined grain structure and high aluminium nitride content of the cooled billet.

MST/1170     

Effect of composition on critical (cold working) strain and recrystallised grain size of dilute Al–Fe alloys

Abstract

Low strain cold deformation and annealing techniques have been applied to establish the effects of composition and heat treatment variables on the recrystallisation behaviour of Al–Fe alloys. The parameters controlling the critical strain required to initiate recrystallisation and the grain size produced by subsequent recrystallisation annealing have been established to be the volume fraction of secondary phases, the eutectic colony size in both as cast and processed material, matrix composition, the initial grain size preceding final cold deformation, the amount of deformation before annealing, and the final annealing temperature. The results derived from the application of the strain annealing test showed that increasing the volume fraction of secondary precipitate phases, the homogenisation and final annealing temperatures, or the eutectic colony size, decreases the critical strain and increases the maximum grain size produced after annealing, but increasing the initial grain size has opposite effects on these parameters.

MST/1348     

Caustic stress corrosion cracking of a spheroidal graphite cast iron: characterisation of ex-service component

Abstract

This paper discusses observations of features suggesting grain boundary embrittlement ahead of stress corrosion crack tips in ex-service cast iron components exposed to strong caustic environments during Bayers process for alumina processing. The cracks and the neighbouring areas in the ex-service specimens were examined using conventional metallography, SEM, the extraction replica technique in the TEM and Auger electron spectroscopy. In all cases, the cracking was initiated at the surface of the steel exposed to the corrosive environment and it appeared that crack nucleation may have been aided by local stress concentrations and/or zones of local residual stress concentration. The fracture mode was almost exclusively intergranular and the crack path followed ferrite grain boundaries. There was clear evidence of a local zone of grain boundary embrittlement extending ahead of the tip of the major cracks examined. The phenomenon was established by investigating the fresh fracture surfaces produced by extending pre-existing cracks under impact loading at liquid nitrogen temperature. Auger electron spectroscopy of the fresh intergranular fracture surface failed to reveal any evidence of local elemental grain boundary segregation that might account for the observed embrittlement. In the absence of evidence of any other embrittling species on the exposed intergranular facets, there arises the possibility of hydrogen being involved in the embrittlement. The paper discusses hydrogen assisted intergranular cracking, as observed in the case of similar materials, to be the possible mechanism.     

Grain refinement of DC cast AZ91D Mg alloy by intensive melt shearing

Abstract

Melt conditioning by advanced shear technology (MCAST) is a new process for microstructural refinement of both cast and wrought magnesium alloys. Melt conditioned direct chill (MCDC) casting combines the MCAST process with conventional direct chill (DC) casting. In the present work, melt conditioning has been combined with permanent mould casting to simulate the production of DC cast AZ91D billets and slabs. The results show that the MCDC process can achieve significantly finer grain size and more uniform microstructure than conventional DC process for both billets and slabs. Grain refinement in the MCDC process is due to the fine and well dispersed oxide particles produced after processing in the MCAST unit.     

Direct chill rheocasting (DCRC) of AZ31 Mg alloy

Abstract

A new processing technology, direct chill rheocasting (DCRC), has recently been developed by BCAST at Brunel University, for production of high quality Mg alloy billets and slabs. In the present paper the authors present the DCRC process and experimental results on microstructure and mechanical properties of DCRC billets. Basically, the DCRC process consists of a high quality semisolid slurry supply system, continuously feeding a conventional direct chill caster, to produce billets or slabs. Experimental results show that the DCRC billets have a fine and uniform microstructure throughout the cross section, the average grain size being ～50 μm. Direct extrusion of the DCRC billets was conducted to assess the deformability and mechanical properties after extrusion. The average grain size is <3 μm in the as extruded state. The increased deformability of the DCRC billets has been attributed to the fine and uniform microstructure, while the improved mechanical properties of the extruded product is attributed to the fine grain size.     

Production and properties of rapidly solidified Al-4.5% Cu alloy powder by the rotating-water-atomization process

Rapidly solidified Al-4.5 wt% Cu powder was produced by the rotating-water-atomization process recently developed by the authors. The particles were not spherical but tear-drop shape. The mean particle size of the powder decreased with increasing rotational speed of the drum and with decreasing nozzle diameter. The dendrite arm spacing of the particles produced was in the range 0.6 to 2.8m. The cooling rate was estimated to range from 10³ to 5×10⁵ K sec^–1. Recrystallization of hot extruded powder material after T6 treatment was remarkably suppressed in the central part, while large fully recrystallized grains were obtained in the cast material extruded under the same conditions. Tensile strength of the powdered material extruded in the range 573 to 723 K was slightly higher than the extruded cast material with little loss in elongation. The surface oxidation was not deleterious to elongation but it was very effective in suppressing recrystallization.     

Forging behaviour and properties of metal matrix composites based on mechanically alloyed AI-Mg-Li alloy

Abstract

A range of Al-Mg-Li-C MMCs (25 vol.-% of 3 μm SiC particles) were produced by mechanical alloying and powder processing at Aerospace Metal composites, Farnborough. Upset forging trials were conducted at 350 and 550°C and 0.01 and 0.1 S^-l. No SiC particle cracking or wedge cracking at grain boundary triple points was observed. Forging at 550°C caused some edge cracking and a coarser grain structure. Large billets of varying composition were forged to ～15 mm thick plate at 350°C and 0.0l S-^l or 550°C and 0.l S^-l. Monotonic testing showed the MMCs to exhibit high stiffnesses at moderate strength levels but rather low ductility and fracture toughness. Significant strengthening was found to accrue from dispersion hardening (C content) and solid solution strengthening (Mg and Li solutes). A link was found between lower proof stress and coarser grain structure after forging at 550°C. Fracture toughness K_1C was found to decrease with increasing yield strength which was attributed to higher strain concentrations in the smaller crack tip plastic zone.     

Electroslag crucible melting of age hardening copper–chromium alloy

Abstract

The electroslag crucible melting process, developed on the well established principles of electroslag refining, has been used to cast age hardening Cu–0·5Cr alloys using copper scrap. Initial trials have demonstrated the viability of electroslag crucible melting for producing cast alloys with low impurity content and superior tensile properties. The castings produced by electroslag crucible melting are of high quality in terms of surface finish and casting defects. The cast alloy could be successfully drawn into wire of diameter 3 mm.

MST/3368