Distribution of misorientations and grain boundary planes in grain boundary engineered brass

Abstract

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.     

Analytical model linking growth restriction to solute interaction in model Al based ternary systems

Abstract

An analytical model is constructed to determine the effect of solute–solute interactions on the growth restriction parameter Q. Relevant to analysing the as cast grain size in alloys inoculated with potent nuclei, the value of Q describes the effect of solute partitioning on the growth rates of solid in the liquid. The model is constructed for highly simplified aluminium based ternary systems with two solutes of identical partitioning behaviour. The regular solution coefficient between the two solutes is varied, and its effect on Q is determined. The sign of the change of Q is dependent on the partition coefficient of the solutes: four distinct regimes are found, for k<½, ½<K<1, 1<k<2, and k>2.     

Relative anisotropy of structures and ultrasound attenuation response between laboratory casting in permanent mould (vacuum induction melted) and casting processed through electroslag refining

Abstract

In the present study, ingots of 'modified 15CDV6' (0·3C–CrMoV) steel were laboratory melted and cast by the vacuum induction melting–electroslag refining (VIM–ESR) and VIM (water cooled steel mould) routes. The ingots have been evaluated for their relative anisotropy differences by ultrasound attenuation (UA) studies along respective radial and axial directions. The expected sources of UA are crystallographic texture and the presence of defects. These defects include inclusions, grain boundaries, segregation and microporosity. Apart from microporosity, the presence of defects has been characterised along the respective casting directions. Although both castings/ingots had gone through successive phase transformations after solidification (liquid→δferrite→austenite→ferrite), the electroslag refined (ESR) material had significantly more anisotropy in crystallographic texture, but insignificant anisotropy in inclusion distribution, as compared with the vacuum induction melted (VIM) material. The grain boundary character distributions and long range compositional fluctuations were similar and nearly isotropic between the two ingots. The ESR material, in general, was denser, which may indicate a lower presence of microporosity. Taking grain size differences into account, the UA estimated anisotropy was more for ESR material, a response clearly dominated by the anisotropy of the crystallographic texture. At a higher frequency, and correspondingly finer resolution for defects, the anisotropy owing to defects played a more dominant role, but even then the highest frequency of 2 MHz was not adequate to highlight a dominance of defect anisotropy.     

Structural stability and non-catalytic nucleation inhibition effect of Si–Zr–B mould coating on undercooled superalloy melt

Abstract

The investment moulding technique was first adopted to prepare a SiO₂–ZrO₂–B₂O₃ (Si–Zr–B) substrate layer on the inner surface of the mould, by employing SiO₂ glass dust and ZrO₂ powder, SiO₂–ZrO₂ sol, and analytical grade H₃BO₃ as refractory material, binder, and softening agent, respectively. Then using sol–gel processing, seven layers of Si–Zr–B film of the same formula as the aforementioned Si–Zr–B substrate layer were compounded with the substrate layer step by step. After glassing treatment at 850°C for 60 min, this film transformed into a glass lined coating. It was shown from X-ray diffraction analysis that, after holding it at a temperature of 1500°C for 30 min, the amount of crystallinity in the Si–Zr–B coating was about 1–3% (vol.-%). Finally, the undercooling experiment showed that a large undercooling (up to 140 K) was achieved in a DD3 (Ni–Cr–Mo–Al–Ti–Co–W) single crystal superalloy melt in this coated mould. So it is concluded that a Si–Zr–B coating has got a good structural stability at high temperature and provides ideal non-catalytic nucleation inhibition for an undercooled superalloy.     

Study of vibration welding mechanism

Abstract

The study on the vertical and horizontal spot vibration welding of Inconel 690 alloy was carried out to observe the dendrite morphologies and estimate the temperature gradient G and growth rate R under different vibration conditions. The purpose is to further understand the mechanism of microstructure changes under vibration. Based on different temperature distributions along vertical and horizontal directions in the centre of a melting pool, it is found that vertical and horizontal vibrations induce the divergence of the nucleates site and grain growth rate then affect the grain morphologies. Vertical vibration welding creates a coarse dendrite structure with sturdy secondary and tertiary dendrite arms, and the X-ray diffraction (XRD) profile of this structure shows a strong (200) peak. Horizontal vibration welding results in grain refinement and a relatively disordered structure, which is reflected by its low XRD intensity. The study shows that vibration affects the weld structure by improving nucleates and changing growth rate.     

Effects of Chromia Coatings on the High-Temperature Behavior of F17Ti Stainless Steel in Air: Analytical Studies of the Effect of Rare-Earth-Element Oxides

The MOCVD deposition of neodymium oxide and/orchromium oxide provided beneficial effects both onisothermal- and cyclic high-temperature behavior ofcommercial F17Ti stainless steel. Fracture crosssections provided information about the morphology ofthe oxide scales formed on bare steel and coatedspecimens. The chromia scales developed small equiaxedgrains on the Nd₂O₃-coated samplesand columnar grains on the uncoated ones. Neo dymium segregatedwithin a surface layer composed ofMn_1.5Cr_1.5O₄ spineloxide. A complex phase (close to the structure ofCeTi₂₁O₃₈) was identified in thiszone. It could act as a source of neodymium ions, which couldsegregate to the grain boundaries of the chromia scale.Polished cross sections associated with X-ray mappingstudies confirmed the scale structure and the location of the rare-earth element in the outer part ofthe oxide layer.     

Effect of powder size on microstructure and mechanical properties of Ti(C,N) based cermets

Abstract

Effect of the particle size of TiC and TiN on the microstructure and mechanical properties of Ti(C,N) based cermets has been evaluated. Ti(C,N)–WC–Co cermets made from four groups of mixed raw powders of different sizes were manufactured by vacuum sintering. The microstructure and composition were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectrum (EDX). The result shows that the four samples have the typical microstructures of 'black core/grey rim'. The mechanical properties of the cermet manufactured from submicron TiC and nano TiN are the best among the four samples.     

Possibility of grain size prediction in AA5754 aluminium ingots using neural networks

Abstract

The approach to the grain size prediction in AA5754 Al alloy ingots based on artificial neural networks (ANN) has been used in the present study. The ANN has been trained on data that was measured in the real industrial conditions during the process of direct chill Al ingots casting. A very complex relation between the numerous casting parameters and the microstructure of the ingots justifies the application of neural networks, which are known for mapping complex and non-linear systems. A feed forward ANN model with the resilient back-propagation learning algorithm and weight decay regularisation has been developed to relate the grain size to casting rate, meniscus level, casting temperature, water flow for the metal mould cooling and speed of wire for master alloy addition. The results obtained from the ANN are found to be consistent with the theoretical researches and experience from the foundry.     

Effect of modified AA4043 filler on partially melted zone cracking of Al-alloy gas tungsten arc welds

Abstract

The effect of grain refiners (zirconium, Tibor and scandium) added to the fusion zone through AA4043 filler on the partially melted zone (PMZ) cracking in gas tungsten arc (GTA) welded AA6061 alloy was investigated. Welds of AA6061 in thermal tempers of artificially aged condition (T6) were made with continuous current and pulsed current techniques. Varestraint testing was carried out to study PMZ cracking in welds, and optical SEM examination performed to evaluate it. Addition of grain refiners to the fusion zone improved the PMZ cracking resistance very significantly. Pulsed current technique was also found to improve the resistance to PMZ cracking, as a result of the possible reduction in the strain in the PMZ owing to the ductile fine grained fusion zone. Severe PMZ cracking in the welds of AA4043 filler without grain refiners was attributed to a greater amount of silicon rich eutectic at the grain boundaries of the PMZ.