High-temperature structural intermetallics

We describe four criteria for the selection of alloying elements capable of producing castable, precipitation-strengthened Al alloys with high-temperature stability and strength: these alloying elements must (i) be capable of forming a suitable strengthening phase, (ii) show low solid solubility in Al, (iii) low diffusivity in Al, and (iv) retain the ability for the alloy to be conventionally solidified. With regard to criterion (i), we consider those systems forming Al3M trialuminide compounds with a cubic L12 crystal structure, which are chemically and structurally analogous to Ni3Al in the Ni-based superalloys. Eight elements, clustered in the same region of the periodic table, fulfill criterion (i): the first Group 3 transition metal (Sc), the three Group 4 transition metals (Ti, Zr, Hf) and the four latest lanthanide elements (Er, Tm, Yb, Lu). Based on a review of the existing literature, these elements are assessed in terms of criteria (ii) and (iii), which satisfy the need for a dispersion...

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Criteria for developing castable, creep-resistant aluminum-based alloys - A review

Advanced materials generally require novel joining techniques. Developments in new materials research should be conducted hand in hand with work on weldability and joining capacity aspects. Sound joint quality for any new material has always been considered a milestone in a research and development scheme for a new material, particularly in terms of widespread applications. Better understanding of the microstructure-mechanical properties relationships of the bonded or welded joints will feed back to the materials development activities both in conventional and new materials areas. The two joining processes diffusion bonding and laser welding are considered in this literature review, since these processes are capable of joining a wide range of materials of interest in the aerospace industry, as well as in many other industrial applications, and offer remarkable advantages over conventional fusion welding processes. Of particular interest is the ability to join the more difficult aerospace alloys wi...

Progress in joining of advanced materials

Oxidation protection of γ-TiAl-based alloys – A review

Mechanical behaviour of Al3Ti intermetallic and L12 phases on its basis

It has been found that Mn or Cr can substitute for Al in Al 3 Ti and change the crystal structure from tetragonal DO 22 to cubic L1 2 . The new phases have been produced with negligible porosity and second phases, and have compositions near Al 67 Mn 8 Ti 25 and Al 67 Cr 8 Ti 25 . As with other cubic forms of Al 3 Ti, the ternary addition can range from bout 5 to 15 at. pct. while still maintaining the cubic phase. These new cubic phases have significantly reduced hardnesses and greatly improved crack resistance compared to binary Al 3 Ti, as well as other cubic forms based on alloying with Fe, Ni, or Cu. In addition to appreciable ductility in compression, the new phases show some tensile ductility

New cubic phases formed by alloying Al3Ti with Mn and Cr

Observations on the systematic alloying of Al3Ti with fourth period elements to yield cubic phases

Gravimetric, SEM, and XRD data are presented which document the significant improvement obtainable in the oxidation resistance of Al3Ti-containing alloys through additions of Cr. The L1(2) Al(67)Cr(8)Ti25 alloy exhibited excellent cyclic oxidation resistance at 1473 K, with the primary oxide formed being the ideally protective alpha-Al2O3. The Al(67)Mn(8)Ti(25) alloy also tested for comparison exhibited poor cyclic oxidation resistance, with substantial occurrence of TiO2 in the protective scales. Catastrophic oxidation was also encountered in the quaternary alloy Al(67)Mn(8)Ti(22)V(3).

Oxidation behavior of cubic phases formed by alloying Al3Ti with Cr and Mn

The recent discovery of cubic L12 trialuminides formed by alloying normally tetragonal Al3Ti with Cr, Mn and Co, when combined with previous work with Fe, Ni, Cu and Zn as cubic stabilizing elements, makes it possible to examine changes in properties and structure as a function of the stabilizing element. in particular, the improvement in mechanical properties with the decreasing atomic number of the stabilizing element, Zn to Cr, can be related to decreased bond strengths as shown by changes in the lattice constant and elastic moduli. As expected, there are corresponding changes in the nature of the dislocations carrying the plastic deformation. Recent attempts to understand the fracture behavior of the cubic forms are discussed. Determinations of the cyclic oxidation resistance and the coefficient of thermal expansion for the alloys with best mechanical properties, Al67Cr8Ti25 and Al67Mn8Ti25, are also presented.

J.P. Nic

Papers

New cubic phases formed by alloying Al3Ti with Mn and Cr

Observations on the systematic alloying of Al3Ti with fourth period elements to yield cubic phases

Oxidation behavior of cubic phases formed by alloying Al3Ti with Cr and Mn

Alloying of Al3Ti to Form Cubic Phases

Temperature dependence of the compressive strength of cubic phases formed by alloying Al3Ti with Mn And Cr