H. Saage

Formation of ODS L12–(Al,Cr)3Ti by mechanical alloying

Abstract Recently, it has been shown that the addition of a certain amount of ternary alloying elements like Cr transforms the tetragonal D022-structure of Al3Ti to the cubic L12-structure with higher crystal symmetry. However, this attempt of overcoming its brittleness at ambient temperature results in relatively poor creep strength at high temperatures. Therefore, the methods of mechanical alloying and high energy ball milling were used to prepare the ternary (Al,Cr)3Ti L12-intermetallic compound additionally strengthened by small volume fractions of incoherent Y2O3 dispersoids. The development of the microstructure and the phase formation were investigated by X-ray diffraction and transmission electron microscopy (TEM). First results on consolidated bulk samples are presented, indicating the possibility of producing oxide dispersion strengthened (ODS) intermetallics from mechanically milled powders.

Molybdenum alloys for high temperature applications in air

Abstract Molybdenum base silicide alloys exhibit promising oxidation resistance in addition to the inherent high temperature strength of refractory metals. However, alloys with sufficient oxidation resistance are effectively brittle up to temperatures above 816°C (1500°F). Recent progress in alloy and process development, utilising a PM manufacturing route with mechanical alloying as a crucial step, has allowed significant improvement of both oxidation resistance and mechanical properties via micro-alloying additions including nano-dispersed second phase oxide particles.

Creep strength of a binary Al62Ti38 alloy

Abstract Al-rich Ti – Al alloys, as compared to Ti-rich -TiAl-based alloys, offer an additional reduction in density of 20 %, better oxidation resistance and sufficient strength at high temperatures. High temperature creep of a binary Al62Ti28 alloy was studied in compression in the temperature range between 1 173 K and 1 323 K in air. It is shown that the alloy exhibits quite reasonable creep resistance at 1 173 K, especially in view of its low density of around 3.8 g cm– 3. Stress exponents calculated as the slope n = log (strain rate)/ log (stress) = 4 were found to be relatively constant for the temperature and stress regime investigated. This indicates that dislocation climb may be the rate controlling creep mechanism. The values of the activation energies for creep for the as-cast and the annealed Al62Ti38 material coincides well with those found in the literature for interdiffusion of Al in -TiAl.

Recent advances in the development of mechanically alloyed Mo silicide alloys

We review the current development status of Mo-Si-B alloys consisting of Mo solid solution and the intermetallic phases Mo3Si and Mo5SiB2 which could take advantage of the beneficial oxidation resistance of the silicide phases and of the outstanding mechanical properties of molybdenum. For adequate low temperature toughness a continuous Mo solid solution matrix should be established in the microstructure. Besides, wrought processing of such alloys at elevated temperatures requires the presence of an ultra-fine grained (UFG) microstructure. Both the prerequisites can be fulfilled using mechanical alloying (MA) as the crucial processing step which even yields nanostructured supersaturated powders after milling. However, values for the ductile-to-brittle transition temperature (DBTT) close to room temperature are unlikely due to grain boundary embrittlement by Si segregation. The possibility of reducing this segregation tendency by various micro-alloying additions will be demonstrated. Finally, the high temperature deformation behaviour of these UFG materials will be comparatively assessed against state-of-the-art Nickelbase single-crystalline superalloys.

High Temperature Deformation Behavior of a Mechanically Alloyed Mo Silicide Alloy

A 3-phase Mo-Si-B alloy consisting of Mo solid solution and the intermetallic phases Mo3Si and Mo5SiB2 (T2) was manufactured employing mechanical alloying (MA) as the crucial processing step. After consolidation via cold compaction, sintering in hydrogen atmosphere and final hot isostatic pressing (HIP) at 1500°C, one obtains an ultra-fine microstructure with a nearly continuous Mo(ss) matrix and the sizes of all phases being in the 1 micron range. Tensile tests were carried out in vacuum at initial strain rates ranging from 10-4 to 10-2 s-1 and the temperature varied between n1200 an 1400 °C. With a stress exponent of about 2 and the activation energy being close to that of Mo-self diffusion the material exhibits superplasticity at temperatures as low as 1300°C and tensile strain to failures up to 400%, thus, making sound wrought processing on industrial-scale facilities at temperatures typical for refractory metals and alloys feasible. To enhance creep resistance at high temperatures the alloys were annealed at 1700°C for 10h for a coarsening of the microstructure. While, still, the average sizes of all phases were below 10 microns, a considerable reduction in minimum creep rate was noted. This finding also demonstrates the extraordinary high thermal stability of this 3-phase Mo-silicide alloy.

Assessment of creep behaviour of the die-cast cylinder-head alloy AlSi6Cu4-T6

Abstract Mechanical properties of a T6-AlSi6Cu4 cylinder-head alloy are investigated at 4 stress levels up to 545 K through tensile and compressive test to assess creep damage. The effect of over-ageing for 200 h at comparable temperatures was studied through RT hardness tests. Creep behaviour was characterised by a minimum creep rate and an extended steady state was not observed. No threshold stresses were apparent from minimum creep rate vs. stress plots at any of the temperatures studied here. Apparent stress exponents were both temperature and stress dependent and the apparent creep activation energy was 2–3 times the lattice selfdiffusion activation energy for aluminium. The Norton plot was best described by an exponential relation. The total strain to failure exhibited a maximum for a particular applied stress and decreases for both higher and lower applied stress. Creep life is well described by the Monkman– Grant relation at the highest temperature, but exhibits deviations from the idealised behaviour at the lower test temperatures.

High temperature creep behaviour of Al-rich Ti-Al alloys

Compared to Ti-rich γ-TiAl-based alloys Al-rich Ti-Al alloys offer an additional reduction of in density and a better oxidation resistance which are both due to the increased Al content. Polycrystalline material was manufactured by centrifugal casting. Microstructural characterization was carried out employing light-optical, scanning and transmission electron microscopy and XRD analyses. The high temperature creep of two binary alloys, namely Al60Ti40 and Al62Ti38 was comparatively assessed with compression tests at constant true stress in a temperature range between 1173 and 1323 K in air. The alloys were tested in the cast condition (containing various amounts of the metastable phases Al5Ti3 and h-Al2Ti) and after annealing at 1223 K for 200 h which produced (thermodynamically stable) lamellar γ-TiAl + r-Al2Ti microstructures. In general, already the as-cast alloys exhibit a reasonable creep resistance at 1173 K. Compared with Al60Ti40, both, the as-cast and the annealed Al62Ti38 alloy exhibit better creep resistance up to 1323 K which can be rationalized by the reduced lamella spacing. The assessment of creep tests conducted at identical stress levels and varying temperatures yielded apparent activation energies for creep of Q = 430 kJ/mol for the annealed Al60Ti40 alloy and of Q = 383 kJ/mol for the annealed Al62Ti38 material. The latter coincides well with that of Al diffusion in γ-TiAl, whereas the former can be rationalized by the instability of the microstructure containing metastable phases.

On the Orowan stress in intermetallic ODS alloys and its superposition with grain size and solid solution hardening

Abstract The intermetallic trialuminide Al67Ti25Cr8 was strengthened by nano-scaled dispersions of up to 3 vol.% yttrium and aluminum oxide via mechanical milling. Subsequent powder consolidation, heat treatment, and thermo-mechanical treatment yielded grain growth to an increasing degree, resulting in grain sizes ranging from sub-µm level to about 0.5 mm. Consequently, the room-temperature yield strength σ0, as determined by constant compression rate tests, increases due to three different hardening contributions, namely dispersion strengthening σOR due to Orowan bypassing, fine-grain strengthening σHP due to Hall – Petch relation, and, finally, as virtually always present, some solid-solution strengthening σs. The analysis of the particle parameters was done using TEM as well as focused ion beam (FIB) imaging. The latter provides the opportunity to directly determine the mean radius rs of planar intersections with spherical particles and their nearest-neighbour spacing L since it displays a plane sectio...