S. Dymek

Deformation mechanisms and ductility of mechanically alloyed NiAl

Abstract An NiAl-based alloy has been produced by mechanical alloying and hot extrusion, resulting in material which is fully dense, with a homogeneous distribution of oxide particles and with a fine grain size of less than 1 μm. Mechanical properties of the mechanically alloyed (MA) NiAl were studied by compression testing from room temperature to 1300 K. At room temperature, the alloy exhibited high yield strength (1380 MPa) and considerable compressive ductility (greater than 11.5%). Transmission electron microscopy of the compressed specimens was carried out. In order to determine the Burgers vectors of slip dislocations a rigorous procedure was followed. The 〈100〉 slip was found to be predominant but strong evidence of 〈110〉 slip was also gathered. The occurrence of the slip vectors satisfies the general requirement for plasticity and contributes to the notable compressive ductility. Cast and hot extruded NiAl has been also investigated for comparison with the MA material. At room temperature, it exhibited a poor ductility (2.3%), low yield strength (400 MPa) and only 〈100〉 slip dislocations were observed. The 〈100〉 slip provides three independent slip systems, an insufficient number for general plasticity. The different behavior of cast and MA NiAl is believed to be a result of distinct textures, 〈111〉 and 〈110〉 respectively, exhibited by these differently processed materials.

Synthesis and characterization of mechanically alloyed Nb3Al-base alloys

Abstract Two Nb–Al alloys containing 18 and 20 at.%Al were successfully processed by mechanical alloying followed by hot pressing. Analysis of X-ray spectra as well as transmission electron microscopy studies revealed the presence of four phases: niobium solid solution (Nb ss ), Nb 3 Al, Nb 2 Al and dispersoids of Al 2 O 3 . The grain size was estimated to be approximately 1 μm. Nb ss grains contained a very high dislocation density while a high density of planar stacking faults was observed in the intermetallic phases. The alloys exhibited only limited compressive ductility at room temperature but were ductile at 1000°C. The mechanism for creep deformation in the examined Nb–Al alloys is postulated to be dislocation creep through diffusion controlled climb. The compressive strength was higher and creep rates were lower in the present materials compared to reference NiAl-based materials processed and consolidated using the same techniques.

Strain hardening mechanisms in a NiMoCr alloy

HAYNES 242 alloy has been recently developed for gas turbine components applications. This age-hardenable alloy, consisting essentially of Ni-25%Mo-8%Cr, utilizes a long-range-ordering reaction to form uniformly sized and distributed, extremely small (on the order of 10nm), ordered particles. Excellent strength and ductility at elevated temperatures, low thermal expansion characteristics and good oxidation resistance of Haynes 242 alloy has encouraged a number of studies designed to characterize its properties. What is lacking is an attempt to understand the fundamentals of the deformation and strengthening mechanisms in this alloy. This on-going research has been undertaken to explore deformation mechanisms in unaged and aged Haynes 242 alloy. The emphasis has been put on the effects of initial precipitation structure on the development of deformation structure and how it controls selected mechanical properties. This paper presents selected results and reports a change in the deformation mode from crystallographic glide in an unaged alloy into twinning in the presence of ordered particles. Deformation twinning in Ni-Mo and Ni-Mo-Cr alloys was reported earlier but was not discussed in detail. This research sheds light on possible origins of particle-induced twinning in alloys strengthened by small ordered particles.

Microstructure and Mechanical Properties of Mechanically Alloyed NiAl

Mechanical alloying (MA) has been used to produce NiAl powders from either elemental or prealloyed constituents. The powders were consolidated by hot extrusion resulting in material which was fully dense, with a grain size around 1 μm and a homogeneous distribution of oxide particles with sizes in the range 10 to 100 nm. TEM observation indicates the presence of a significant dislocation density after consolidation. Mechanical properties have been studied by compression testing from room temperature to 1300 K in air. Yield strengths ranged from 1453 MPa to 32 MPa depending on material and test temperature. Work hardening was observed at all test temperatures for both materials. Substantial ductility was observed even at room temperature where it exceeds 7.5 %. The effects of microstructure on the mechanical properties are discussed.

The Role of Dispersoids in Mechanically Alloyed NiAl

Mechanical alloying followed by hot extrusion has been used to produce fully dense, crack free, very fine grained NiAl-based alloys containing a bimodal distribution of aluminum oxide dispersoids. The unique microstructure provides the materials with high strength and good compressive ductility at ambient and elevated temperatures. The emphasis of the paper is on the importance of the dispersion phase in controlling grain size, texture, deformation mechanisms and ultimately mechanical properties of the mechanically alloyed NiAl-based materials.