In Situ Synchrotron Study of B19 Phase Formation in an Intermetallic γ‐TiAl Alloy

A multitude of phases exists in the binary Ti–Al phase diagram and even greater numbers are formed in structural TiAl alloys, which contain additional alloying elements to improve their properties. In the current study, a Ti–45 Al–3 Mo–0.1 B (in at%) alloy was investigated with respect to the phases occurring in chemical non‐equilibrium. In situ high‐energy X‐ray diffraction experiments enabled to identify a transient phase to be of the B19 type and to determine its temperatures of formation and dissolution.     

In Situ Characterization of a Nb and Mo Containing γ‐TiAl Based Alloy Using Neutron Diffraction and High‐Temperature Microscopy

In recent times, novel titanium aluminides containing the bcc β‐phase at high temperatures are being developed for improved hot‐working capabilities, however, predictions of the phase diagrams are merely uncertain. Here we present in‐situ neutron studies, which are particularly sensitive to the atomic disorder in the ordered phases. Complementary laser scanning confocal microscopy is employed for in‐situ microstructural investigations.     

Processing and Applications of Intermetallic γ‐TiAl‐Based Alloys

Development and processing of high‐temperature materials is the key to technological advancements in engineering areas where materials have to meet extreme requirements. Examples for such areas are the aerospace and spacecraft industry or the automotive industry. New structural materials have to be “stronger, stiffer, hotter, and lighter” to withstand the extremely demanding conditions in the next generation of aircraft engines, space vehicles, and automotive engines. Intermetallic γ‐TiAl‐based alloys show a great potential to fulfill these demands.     

元素粉末法制备TiAl金属间化合物的研究进展

元素粉末冶金因具有成本低、制备的合金组织均匀细小等优点而受到广泛关注。简要介绍了元素粉末法制备TiAl合金的研究进展,主要从反应机理、致密化行为和力学性能等方面进行综述。研究表明,Ti与Al元素的反应由扩散控制,借助TiAl3和TiAl2等中间相最终得到Ti3Al和TiAl相共存的反应产物。在高Nb–TiAl合金的Ti–Al–Nb三元系中,Nb元素主要通过形成中间产物——Nb–Al化合物最终均匀分布在基体相中。从原料和工艺两个角度总结了元素粉末法制备TiAl合金过程中影响致密化的因素,介绍了提高元素粉末法制备TiAl合金的热加工和力学性能的方法,总结了近年来元素粉末法制备TiAl合金的力学性能研究成果。目前来看,元素粉末法制备的TiAl合金力学性能已达到变形合金的水平。     

Oxidation protective coatings for γ‐TiAl – recent trends

Engine designers show continued interest in γ‐TiAl based titanium aluminides as light–weight structural materials to be used at moderately elevated temperatures. Although alloy development has made significant progress in terms of mechanical properties and environmental resistance, protective coatings have been developed that help to extend the lifetime of these alloys significantly. The major challenge of coating development is to prevent the formation of fast growing titania. Furthermore, changes of coating chemistries at high temperatures have to be considered in order to avoid rapid degradation of the coatings due to interdiffusion between substrate and coating. The paper describes recent work of the authors on different coatings produced by means of magnetron sputter technique. Thin ceramic Ti‐Al‐Cr‐Y‐N layers tested at 900 °C exhibited poor oxidation resistance. In contrast, intermetallic Ti‐Al‐Cr, Si‐based and aluminum rich Ti‐Al coatings were tested at exposure temperatures up to 950 °C for 1000h resulting in reasonable and partially excellent oxidation behaviour.     

Improving long term oxidation protection for γ‐TiAl substrates

In previous work, a thermal spray multilayer system consisting of Zirconia (ZrO₂) and MCrAlY top coat showed promising results regarding the oxidation behavior of the Gamma Titanium Aluminides substrates tested, which encouraged further research activities. Diffusion of substrate material was successfully inhibited by a ceramic Zirconia coating. A building up of a dense and stable oxide layer could be achieved by additional application of an MCrAlY top coat, leading to improved oxidation resistance and thus showing feasibility. In this work the main focus for development was put on enhancing adhesion and lowering residual stresses of the coatings in order to allow long term and cyclic testing without delamination taking place. Being a very brittle material, Gamma Titanium Aluminides require special surface treatment to enable roughening which is crucial for a strong mechanical bond between substrate and coating. Alternatives to conventional grit blasting as a standard preparation method were investigated. These were micro‐abrasive blasting and blasting at elevated temperature (≈300–550°C) to allow a more ductile behavior. The paper will highlight the implications by means of these measures and will also show the present development status of the multilayer system.     

In Situ Study of γ‐TiAl Lamellae Formation in Supersaturated α2‐Ti3Al Grains

In situ heating transmission electron microscopy (TEM) was used to investigate the initial stage of γ‐TiAl lamellae formation in an intermetallic Ti–45Al–7.5Nb alloy (in at.%). The material was heat treated and quenched in a non‐equilibrium state to consist mainly of supersaturated, ordered α₂‐Ti₃Al grains. Subsequently, specimens were annealed inside a TEM up to 750 °C. The in situ TEM study revealed that ultra‐fine γ‐TiAl laths precipitate in the α₂‐matrix at ≈730 °C which exhibit the classical Blackburn orientation relationship, i.e. (0001)α₂//(111)γ and [$11{\bar {2}}0$ ]α₂//<110]γ. The microstructural development observed in the in situ TEM experiment is compared to results from conventional ex situ TEM studies. In order to investigate the precipitation behavior of the γ‐phase with a complementary method, in situ high energy X‐ray diffraction experiments were performed which confirmed the finding that γ‐laths start to precipitate at ≈730 °C from the supersaturated α₂‐matrix.     

Realization of a Titanium Spinal Implant with a Gradient in Porosity by 2‐Component‐Metal Injection Moulding

2‐Component‐Metal Injection Moulding (2‐C‐MIM) is a technique derived from plastics industry which has been adapted to metal powders. In the present work, the production of titanium implants with a gradient in porosity was demonstrated by using this technology, starting from titanium feedstocks with and without space holder particles (NaCl, 350–500 µm). Binder systems specially tailored for the application were developed. Compared to established production routes, the net‐shape fabrication of titanium implants by 2‐C‐MIM promises a significant reduction of cost if aiming at large scale production. The feasibility study was accompanied by a detailed characterisation of each production step of 2‐C‐MIM process including influence of MIM processing on mechanical properties.