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Locci, I. E.

Publications and source records attributed to Locci, I. E..

28 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Effect of oxidation on the mechanical properties of a NbAl3 alloy at intermediate temperatures

The effect of environment on the mechanical properties of an Nb-67Al-7Cr-0.25W-0.5Y alloy was investigated experimentally in the temperature range 800-1200 K. It is found that the severity of environmental attack in the alloy is determined by both matrix plasticity and oxidation kinetics. The former determines the ability of the matrix to accommodate the localized stresses generated during deformation and oxidation, while the latter governs the rate of formation of a protective oxide scale. The environmental degradation of the alloy can thus be reduced or eliminated by increasing atomic mobility.

Raj, S. V.↗
NASA NTRS1992

Deformation behavior of a Ni-30Al-20Fe-0.05Zr intermetallic alloy in the temperature range 300 to 1300 K

The deformation properties of an extruded Ni-30Al-20Fe-0.05Zr (at. pct) alloy in the temperature range 300-1300 K were investigated under initial tensile strain rates that varied between 10 exp -6 and 10 exp -3/sec and in constant load compression creep between 1073 and 1300 K. Three deformation regimes were observed: region I, occurring between 400 and 673 K, which consisted of an athermal regime of less than 0.3 percent tensile ductility; region II, between 673 and 1073, where exponential creep was dominant; and region III, between 1073 and 1300 K, where a significant improvement in tensile ductility was observed.

Raj, S. V.↗
NASA NTRS1992

Correlation of deformation mechanisms with the tensile and compressive behavior of NiAl and NiAl(Zr) intermetallic alloys

To identify the mechanisms controlling strength and ductility in powder-extruded NiAl and NiAl + 0.05 at. pct Zr, tensile and compressive testing was performed from 300 to 1300 K for several grain sizes. Grain size refinement significantly increased yield stress in both alloys and, in some cases, slightly lowered the ductile-to-brittle transition temperature (DBTT), although no room-temperature tensile ductility was observed even in the finest grain size specimens. The small Zr addition increased the DBTT and changed the low-temperature fracture mode from intergranular in NiAl to a combination of intergranular and transgranular in the Zr-doped alloy. Scanning electron microscopy of compression specimens deformed at room temperature revealed the presence of grain-boundary cracks in both alloys. These cracks were due to the incompatibility of strain in the polycrystalline material, owing to the lack of five independent slip systems. The tendency to form grain-boundary cracks, in addition to the low fracture stress of these alloys, contributed to the lack of tensile ductility at low temperatures.

Bowman, R. R.↗
NASA NTRS1991

The Mechanical Properties of a Ni-30Al-20Fe-0.05Zr Intermetallic Alloy in the Temperature Range 300-1200 K

Tensile tests were conducted on an extruded Ni-30(at. pct)Al-20Fe-0.05Zr intermetallic alloy in the temperature range 300-1200 K, and the microstructures were characterized by optical, scanning, and transmission electron microscopy. The alloy did not exhibit any room temperature ductility and failed by transgranular cleavage at a stress of about 710 MPa. This mode of fracture was observed at and below 873 K with the total ductility being less than 2.5 percent. However, the failure mode changed to dimpled fracture, triple point cracking, and rupture above 873 K. Scanning electron microscopy of the fracture surfaces revealed that transgranular cleavage failure had always originated at preexisting defects. In these cases, the fracture stress exhibited a good correlation with the defect size, in accordance with linear elastic fracture mechanics, where the critical stress intensity factor was estimated to be about 4 MPa sq rt m.

Raj, S. V.↗
NASA NTRS1991

On improving the fracture toughness of a NiAl-based alloy by mechanical alloying

Mechanical alloying (MA) has been used to process the NiAl-based alloy Ni-35Al-20Fe, such that a fine-grain (about 2 microns) microstructure is obtained through the addition of 2 vol pct Y2O3 particles. When compared to a conventionally processed, coarse-grained (about 28 microns) Ni-35-20 alloy without the Y2O3 particles, the MA alloy exhibits two to three times higher fracture toughness values, despite a 50-percent increase in yield strength. Room-temperature K(O) values as high as 34 MPa sq rt m are observed, accompanied by a yield strength in excess of 1100 MPa. Fractography confirms a change in fracture characteristics of the fine-grained MA alloy.

Kostrubanic, J.↗
NASA NTRS1991

Microstructure and mechanical properties of a single crystal NiAl alloy with Zr or Hf rich G-phase precipitates

The possibility of producing NiAl reinforced with the G-phase (Ni16X6Si7), where X is Zr or Hf, has been investigated. The microstructure of these NiAl alloys have been characterized in the as-cast and annealed conditions. The G-phases are present as fine cuboidal precipitates (10 to 40 nm) and have lattice parameters almost four times that of NiAl. They are coherent with the matrix and fairly resistant to coarsening during annealing heat treatments. Segregation and nonuniform precipitate distribution observed in as-cast materials were eliminated by homogenization at temperatures near 1600 K. Slow cooling from these temperatures resulted in large plate shaped precipitates, denuded zones, and a loss of coherency in some of the large particles. Faster cooling produced a homogeneous fine distribution of cuboidal G-phase particles in the matrix. Preliminary mechanical properties for the Zr-doped alloy are presented and compared to binary single crystal NiAl. The presence of these precipitates appears to have an important strengthening effect at temperatures not less than 1000 K compared to binary NiAl single crystals.

Locci, I. E.↗
NASA NTRS1989

The influence of annealing in the ferrite-plus-austenite phase field on the stability of vanadium carbide precipitates

The effect of rapid excursions into the ferrite-plus-austenite two-phase field on V4C3 precipitates formed by tempering in the ferrite phases was investigated. Heat treatments were first performed to produce a starting microstructure of fine vanadium carbide particles precipitated in a ferrite matrix, and the microstructure was then subjected to various short-time heat treatment cycles that transformed part of the matrix to austenite. TEM was used to determine the effects of the matrix change on the size, morphology, and distribution of the vanadium carbide particles.

Locci, I. E.↗
NASA NTRS1989

Processing and microstructure of melt spun NiAl alloys

The influence of various melt spinning parameters and the effect of consolidation on the microstructure of melt spun NiAl and NiAl + W alloys have been examined by optical and electron microscopy techniques. It was found that the addition of 0.5 at. pct W to NiAl results in a fine dispersion of W particles after melt spinning which effectively controls grain growth during annealing treatments or consolidation at temperatures between 1523 and 1723 K. Increased wheel speeds are effective at reducing both the ribbon thickness and grain size, such that proper choice of both composition and casting parameters can produce structures with grain sizes as small as 2 microns. Finally, fabrication of continuous fiber-reinforced composites which used pulverized ribbon as the matrix material was demonstrated.

Locci, I. E.↗
NASA NTRS1988

Analysis of microalloy precipitate reversion in steels

The influence of the ferrite to austenite allotropic transformation on the stability of MXn precipitates in an iron matrix is studied. In the MX phase, M is a group IVb or Vb transition metal, such as niobium, titanium, or vanadium. X is carbon or nitrogen and n is in the range of 0.75-1.0. The application of the present model to the case of vanadium carbide reversion in a microalloyed steel is discussed.

Michal, G. M.↗