H.M. Flower

Electron backscattering diffraction study of acicular ferrite, bainite, and martensite steel microstructures

Abstract This study deals with acicular ferrite, bainite, and martensite microstructures observed in three low alloy steels. Electron backscattering diffraction (EBSD) was used to assess crystallographic features of these microstructures. In each area studied by EBSD mapping, ‘crystallographic packets’ defined as clusters of points sharing the same crystallographic orientation were compared with ‘morphological packets’ observed in the corresponding light micrograph. Microtexture studies suggested that acicular ferrite and upper bainite grow with Nishiyama– Wassermann relationships with the parent austenite phase, whereas lower bainite and martensite consist of highly intricate packets having Kurdjumov–Sachs relationships with the parent phase. In all cases three highly misoriented texture components were found within each former austenite grain. Electron backscattering diffraction also gave information about the cleavage and intergranular reverse temper embrittlement fracture mechanisms of these steels. In conclusion, it is shown that EBSD is a powerful tool for studying phase transformation and fracture mechanisms in steels on a microscopic scale.

Phase equilibria and transformations in a Ti-Zr-Si system

The phase equilibria in the titanium-zirconium rich region of the ternary Ti-Zr-Si system have been studied in the temperature range from 1473 to 1323 K, together with microstructures pres-ent in the as-cast state. DTA, microscopy, X-ray diffraction (XRD), and chemical analysis by X-ray energy-dispersive spectroscopy have been employed. Whereas Ti5Si3 nucleates hetero-geneously and grows rapidly in titanium matrices, Ti3Si exhibits very slow precipitation kinetics. It also is displaced by (Ti, Zr)5Si3 and a ternary silicide in ternary and more complex systems, explaining its absence in commercial alloys. Zirconium exhibits substantial solid solubility in Ti5Si3in ternary alloys, while Ti exhibits only limited solubility in Zr2Si, Zr3Si, and Zr5Si3. Additionally, a new silicide of general formula (Ti, Zr)2Si is formed in ternary alloys with a Ti:Zr ratio ranging from 2.26 to 0.68. Isothermal sections for the Ti-Zr-Si system are presented for temperatures of 1473 and 1323 K.

Application of novel technique to examine thermomechanical processing of near β alloy Ti–10V–2Fe–3Al

Abstract A novel specimen design and testing strategy has been exploited to determine the effect of thermomechanical processing on the microstructural development of the near β titanium alloy, Ti–10V–2Fe–3Al. A double truncated cone test geometry was isothermally deformed at near βtransus (∼795°C) temperatures, to obtain microstructural information for a range of strains within a single specimen. A finite element modelling package was employed to produce strain profiles, which readily correspond to the equivalent microstructural profiles of the test specimens. A parametric study of the effects of process (e.g. friction) and material (e.g. strain rate sensitivity) parameters on the strain distributions obtained during the test was investigated. Finite element modelling was conducted to interpolate the friction ring compression test. The effectiveness of this testing strategy is illustrated with a qualitative description of the microstructural evolution with strain, for various strain rates, at the sub-βtransus forging temperature of 760°C.

Development of a high strain rate superplastic Al–Mg–Zr alloy

Abstract For superplastic forming of aluminium to break out of the niche market that it currently occupies, alloys will be required to possess a higher strain rate capability, appropriate in service properties, and a significantly lower price and to be capable of volume production. This paper describes an approach that has been developed in an attempt to address these fundamental requirements. A series of Al–Mg–Zr alloys with increasing levels of zirconium (0–1 wt-%)has been prepared via extrusion consolidation of cast particulate (solidification rate ∼103 K s-1). The superplastic properties of the resultant cold rolled sheet have been evaluated as a function of thermomechanical treatment and zirconium addition. It has been found that increasing the level of zirconium has the twofold effect of improving the superplastic properties of the alloy while significantly decreasing the concomitant flow stress. At present the optimum superplastic behaviour has been obtained at strain rates of 10-2 s-1, with the 1%Zr material exhibiting ductilities in excess of 600%. The manufacturing route produces a bimodal distribution of Al3Zr comprising >1 µm primary particles in combination with nanoscale solid state precipitates. The current postulation is that this high strain rate superplasticity is conferred by a combination of particle stimulated and strain induced recrystallisation.

The Role of Borides in Near‐γ Titanium Aluminides

This paper reviews how boride additions affect the microstructure, strength and creep properties of α 2 + γ titanium aluminides. Important titanium boride structures in near-y alloys include TiB 2 and TiB; their structures and stability ranges are discussed first. Then, influences on matrix microstructure-grain refinement, enhanced microstructural conversion kinetics, and the stabilization of fine lamellar structure-are elaborated upon, for matrix microstructure plays the most important role in determining mechanical properties. The combined effects of reinforcement and microstructure on mechanical behavior are then explored, illustrating how the incorporation of grain refiners, such as titanium borides, facilitates thermomechanical processing and enables the tailoring of microstructure to optimize specific mechanical properties.

Materials Science: A moving oxygen story

Titanium is one of the most versatile materials used in engineering. An innovative way of producing it from titanium dioxide will make the metal cheaper — if the process can be scaled up.

The Development of a High Strain Rate Superplastic Al-Mg-Zr Alloy

In order for superplastic forming of aluminium to break out of the niche market low cost alloys are required that exhibit higher strain rate capability that are capable of volume production. This paper describes an investigation into the feasibility of producing such an alloy. A series of Al-4Mg alloys with 0, 0.25, 0.5, 0.75 & 1 % Zr additions was prepared using a cheap particulate casting route, in an attempt to achieve higher levels of Zr supersaturation than are possible with conventional casting. The particulate was processed into a sheet product via hot extrusion followed by cold rolling and the effect of a number of process variables on the SPF performance of the sheet was investigated. It was found that increasing the Zr content, and manipulation of the thermomechanical processing conditions improved the SPF performance. Ductilities in excess of 600% have been achieved at a strain rate of 0.01 s -1 , together with flow stresses less than 15MPa.

A moving oxygen story

Titanium is one of the most versatile materials used in engineering. An innovative way of producing it from titanium dioxide will make the metal cheaper — if the process can be scaled up.

Continous Fiber Reinforced Aluminum Matrix Composites – Influence of the Alloy Composition on the Mechanical Properties

The use of aluminum as matrix material for coatinuous fiber reinforced composites has been investigated for over 20 years, with varying degree of sticcess: industrial applications of these materials remain very limited. This paper describes the development of a practical method of producing new net shape, selectively reinforced, components with good mechanical properties in both unreinforced and fiber containing regions and relates this development to previous work illustrating the potentials and the limitations of continuous fiber reinforced aluminium based composite materials.

High Strain Rate Superplastic Aluminium Alloys: The Way Forward?

The technical and commercial barriers to the development and successful exploitation of a high strain rate superplastically deformable aluminium alloy for use in the automotive industry are considered in this paper. Batch processing routes, such as mechanical alloying or equal channel angular extrusion, employed to deliver appropriate chemistry and structure, are inherently costly and unlikely to deliver either the quantity or the size of strip required commercially. There is evidence that there is still scope for development of conventional casting and rolling routes, but a particulate casting route combined with roll consolidation offers the prospect of a commercially viable Al-Mg-Zr product. The use of alloying additions, including zirconium, is also discussed and comparative costs are presented: on this basis the use of scandium appears economically prohibitive.