https://digital.library.unt.edu/ark:/67531/metadc886883/m2/1/high_res_d/901160.pdf

Reactive sintering of TiAl–Ti5Si3 in situ composites

A processing technique for the fabrication of layered metal-intermetallic composites is presented, in which a self-propagating, high-temperature synthesis reaction (SHS) was initiated at the interface between dissimilar elemental metal foils. The resultant composite microstructure consisted of a fully dense, well-bonded metal-intermetallic layered composite. In this United States Bureau of Mines study, metal (Fe, Ni, or Ti) foils were reacted with Al foils to produce metal-metal aluminide layered composites. Tensile tests conducted at room temperature revealed that composites could be designed to behave in a high-strength and high-toughness manner by altering the thicknesses of the starting elemental foils. Failure characteristics revealed that the processes that govern ductilevs brittle behavior of the composites occur early in the fracture.

Microstructural and failure characteristics of metal-lntermetallic layered sheet composites

Reactive synthesis of bulk and reinforced aluminides are reviewed. When processed under optimal conditions, reactively-synthesized aluminides display room-temperature mechanical properties as good as, or better than, cast aluminides. Frequently encountered defects resulting from reactive synthesis are: (i) porosity, (ii) incomplete synthesis and (iii) loss of shape due to liquid formation. Using a simple mass balance, the amount of liquid aluminide is calculated as a function of the initial temperature of the reactants. Process maps are discussed for some of the aluminides of nickel and titanium.

Reactive Synthesis of Aluminide Intermetallics

Formation of sheet metal-intermetallic composites by self-propagating high-temperature reactions

Mechanical behavior of submicron-grained γ-TiAl-based alloys at elevated temperatures

Two ternary TiAl-based alloys with chemical compositions of Ti-46.4 at. pct Al-1.4 at. pct Si (Si poor) and Ti-45 at. pct Al-2.7 at. pct Si (Si rich), which were prepared by reaction powder processing, have been investigated. Both alloys consist of the intermetallic compounds y-TiAl, α2-Ti3Al, and ξ-Ti5(Si, Al)3. The microstructure can be described as a duplex structure(i.e., lamellar γ/α2 regions distributed in γ matrix) containing ξ precipitates. The higher Si content leads to a larger amount of ξ precipitates and a finer y grain size in the Si-rich alloy. The tensile properties of both alloys depend on test temperature. At room temperature and 700 °C, the tensile properties of the Si-poor alloy are better than those of the Si-rich alloy. At 900 °C, the opposite is true. Examinations of tensile deformed specimens reveal ξ-Ti5(Si, Al)3 particle debonding and particle cracking at lower test temperatures. At 900 °C, nucleation of voids and microcracks along lamellar grain boundaries and evidence for recovery and dynamic recrystallization were observed. Due to these processes, the alloys can tolerate ξ-Ti5(Si, Al)3 particles at high temperature, where the positive effect of grain refinement on both strength and ductility can be utilized.

Microstructure and high-temperature tensile deformation of tiai(si) alloys made from elemental powders

Microstructures and tensile properties of some γ-TiAl alloys made from elemental powders

Titanium aluminide alloys, based on gamma TiAl, are currently of interest because of potential applications in high performance airframe and gas turbines. Their low densities, high melting temperatures, good elevated temperature strength and modulus retention, and environmental resistance favors them for these applications. However, their practical use are largely limited by their poor workability and ductility at temperatures lower than 700 C. Although the ductility has been improved in two phase TiAl alloys by adding alloying elements such as Cr, Mn, Nb and V, and by microstructural control in recent years, the ability to manufacture them still remains a problem. The reactive powder processing method offers a promising alternative to overcome this problem. This method involves cold-extrusion of an elemental powder mixture and reactive sintering. The as-extruded material can easily be machined or reformed into different shapes, since titanium aluminides are not present at this stage. The reactive sintering is conducted as the last step to form the desired titanium aluminides in the finished products. By this route, the poor workability of titanium aluminides can be avoided. In the present paper, a binary alloy Ti[sub 52]Al[sub 48](TiAl) and a ternary alloy Ti[sub 50]Al[sub 48]Cr[sub 2](TiAlCr), prepared in the same waymore » from elemental powders, are investigated. The tensile tests were carried out at room temperature to 900 C in air. The influence of 2 at.% Cr addition on the tensile properties and fracture behavior of the alloys are reported. An emphasis is placed on the correlation between microstructure and deformation, and fracture behavior of the alloys.« less

Tensile properties and fracture behavior of Ti52Al48 and Ti50Al48Cr2 prepared from elemental powders

Pure Mo was incorporated in TiAl matrixvia two different routes: (1) hot pressing of alternately sandwiched Ti-Al sheets and Mo foils; and (2) coextrusion and heat treatment of Ti-Al green compact and Mo rod. The reaction zone between TiAl and Mo is found to contain two intermetallic phases: β-(Mo,Ti)Al andp-(Mo,Ti)3Al. The β-p boundary is incoherent, whereas the TiAl-β andp-Mo boundaries are semicoherent. The reaction zone grows with increasing heat-treatment time in a parabolic form. The incorporated Mo exhibits lower hardness than the TiAl matrix, implying that ductilizing and toughening of TiAl by introducing Mo as a ductile reinforcement are possible.

Investigation of the reaction zone between TiAl and Mo

The high temperature compression deformation behaviour of a γ based alloy with the composition Ti-49 at.% Al has been studied. In connection with the stress-strain curve established at 900 °C, TEM investigations have shown an increased dislocation density at the stress maximum and evidence for recovery and dynamic recrystallization taking place beyond the stress maximum. In addition, deformation twins, microcracking, destruction of lamellae as well as formation of nano-crystals were observed in strongly deformed specimens

G.-X. Wang

Papers

Microstructure and high-temperature tensile deformation of tiai(si) alloys made from elemental powders

Microstructures and tensile properties of some γ-TiAl alloys made from elemental powders

Tensile properties and fracture behavior of Ti52Al48 and Ti50Al48Cr2 prepared from elemental powders

Investigation of the reaction zone between TiAl and Mo

TEM investigation of high temperature deformation structures in Ti-49 at.% Al