Metal Injection Moulding Using Intermetallic γ‐TiAl Alloy Powder

Argon gas atomized γ‐TiAl alloy powder of high purity has been used for metal injection moulding. In order to keep the pick‐up of impurities as low as possible most of the process steps were done under inert gas or high vacuum. The binder system used had been especially developed for titanium alloys. In the sintered structures the nitrogen and carbon levels are low, oxygen scatters over a wide range. The porosity of ∼4 % after sintering could be reduced to ∼0.4 % by additional hot isostatic pressing. Tensile tests at room temperature indicate promising properties of σ_0.2 = 409 MPa, UTS = 433 MPa and ϵ_pl = 0.6 %.     

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.     

Enhanced Grain Refinement Through Deformation Induced α Precipitation in Hot Working of α + β Titanium Alloy

This study reports a novel forging process to fabricate bulk fine‐grained (grain size ≈ 1 µm) Ti–6Al–4V alloy, in which temperatures near the β transus (T_β) and strain rates around 0.15 s^?1 are used for the deformation. The formation of fine‐grained microstructure is mainly result from the deformation‐induced precipitation of α grains from the β matrix.     

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.     

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.     

Influence of γ‐α′‐Phase Transformation in Metastable Austenitic Steels on the Mechanical Behavior During Tensile and Fatigue Loading at Ambient and Lower Temperatures

The present investigation is concerned with the three metastable austenitic steels AISI 304 (X5CrNi1810), 321 (X6CrNiTi1810), and 348 (X10CrNiNb189). In the temperature range ?60 °C ≤ T ≤ 25 °C tensile and fatigue tests were performed to characterize the mechanical and phase transformation behavior using stress‐elongation, stress–strain hysteresis, and magnetic measurements. The mechanical properties are significantly influenced by the temperature dependent deformation induced phase transformation from austenite to α′‐martensite which are combined with pronounced hardening processes. Furthermore microhardness measurements after fracture could be correlated with the results of the fatigue tests.     

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.