Joseph D. Rigney

Loading rate and test temperature effects on fracture of in situ niobium silicide-niobium composites

Arc cast, extruded, and heat-treatedin situ composites of niobium suicide (Nb5Si3) intermetallic with niobium phases (primary—Nbp and secondary—Nbs) exhibited high fracture resistance in comparison to monolithic Nb5Si3. In toughness tests conducted at 298 K and slow applied loading rates, the fracture process proceeded by the microcracking of the Nb5Si3 and plastic deformation of the Nbp and Nbs phases, producing resistance-curve behavior and toughnesses of 28 MPa√m with damage zone lengths less than 500μm. The effects of changes in the Nbp yield strength and fracture behavior on the measured toughnesses were investigated by varying the loading rates during fracture tests at both 77 and 298 K. Quantitative fractography was utilized to completely characterize each fracture surface created at 298 K in order to determine the type of fracture mode (i.e., dimpled, cleavage) exhibited by the Nbp. Specimens tested at either higher loading rates or lower test temperatures consistently exhibited a greater amount of cleavage fracture in the Nbp, while the Nbs, always remained ductile. However, the fracture toughness values determined from experiments spanning six orders of magnitude in loading rate at 298 and 77 K exhibited little variation, even under conditions when the majority of Nbp phases failed by cleavage at 77 K. The changes in fracture mode with increasing loading rate and/or decreasing test temperature and their effects on fracture toughness are rationalized by comparison to existing theoretical models.

Processing and properties of Nb5Si3 and tough Nb5Si3/Nb laminates

Abstract Both mechanical alloying (MA) and reactive sintering (RS) techniques were successfully used to produce Nb 5 Si 3 . The homogeneity of the mechanically prealloyed powders and that of the NbSi powder blend before RS significantly affected the microstructures produced after hot-press consolidation. Model laminates of mechanically alloyed Nb 5 Si 3 and nominally pure niobium were prepared via vacuum hot pressing. Room temperature toughness tests were conducted inside a scanning electron microscope equipped with a deformation stage to evaluate the effect of the niobium reinforcement on the composite fracture behavior. Significant toughness increases were obtained in the laminates, while the niobium exhibited both cleavage and ductile fracture. These results are discussed in light of recent work on ductile phase toughening of brittle materials. It is also shown that the RS process may offer an alternative approach, other than in situ and arc melting and casting processes, for producing both monolithic and ductile-phase reinforced Nb 5 Si 3 .

Fracture toughness of monolithic nickel aluminide intermetallics

Abstract The fructure toughness of several nickel aluminide intermetallics have been determined in accordance with standard testing techniques. The intermetallics tested, Ni3Al (24 at.% Al), Ni3Al + 0.2 at.% B and NiAl (45 at.% Al), were produced by conventional vacuum hot-pressing techniques while processing conditions were varied to produce systematic changes in grain size. The toughness obtained and details of the fracture behavior were distinctly different for the materials studied. Ni3Al exhibited an initiation toughness of MPa m 1 2 , whire the boron-doped material had a toughness exceeding 30 MPa m 1 2 . NiAl, on the contrary, demonstrated toughness values of about 5 MPa m 1 2 . This differences are discussed in the light of deformation and fracture mechanisms operating at the crack tip in the cases described. In situ fracture monitoring and post-failure analyses are utilized in support of the discussion.

Effects of reinforcement size and distribution on fracture toughness of composite nickel aluminide intermetallics

Abstract The effects of particulate size and distribution on the fracture toughness of nickel aluminide composites based on Ni 3 Al, Ni 3 Al + B and NiAl with additions of 10 vol.% TiB 2 reinforcement were determined. Composites were fabricated by conventional vacuum hot-pressing consolidation of blends of pre-alloyed matrix powder and TiB 2 platelet reinforcement. It is shown that reinforcement additions produced a decrease in the toughness of Ni 3 Al and Ni 3 Al + B and an increase in the toughness of NiAl while the range of reinforcement distributions and sizes tested at present did not produce a significant change in the measured composite toughness values. The mechanisms responsible for such behavior are discussed with the aid of in-situ fracture studies.

Chemical stability of titanium diboride reinforcement in nickel aluminide matrices

Chemical stability of titanium diboride (TiB2) reinforcement in NiAl (45 at% Al) and Ni3Al (24 at% Al) matrices has been theoretically and experimentally investigated. Calculations were made using thermodynamic properties of the systems to predict behaviour at temperatures between 1173 and 1573 K. Experimental investigation of hot-press consolidated TiB2 particulate/prealloyed matrix powder blends were conducted using energy dispersive X-ray analysis, X-ray diffraction analysis, Auger electron spectroscopy and transmission electron microscopy. The theoretical and experimental analyses suggest that TiB2 is chemically stable in both matrices up to 1573 K, however, TiB2 was found to be less active in NiAl than in Ni3Al due to lower nickel activity in NiAl.

Multicolor Techniques for Identification and Filtering of Burst Signals in Jet Engine Pyrometers

A Defense Advanced Research Projects Agency (DARPA)-funded multicolor pyrometry (MCP) experiment was carried out on a government-provided aircraft engine to study the nature of hot particulate bursts generated from the combustor at certain engine conditions. These bursts of hot particulates lead to intermittent high-voltage signal output from the line-of-sight (LOS) pyrometer that is ultimately detected and used by the onboard digital engine controller (DEC). The investigation used a high-speed MCP system designed to detect bursts and identify their properties. Results of the radiant temperature, multicolor temperature, and apparent emissivity are presented. The results indicated that the apparent emissivity calculated during the signal burst was lower than that of the blade. The root cause for the signal burst was identified as soot particles generated as a by-product of combustion under certain conditions. This conclusion was drawn based on both experimental and simulation results. Technical strategies to separate, reduce, or remove the burst signal are proposed.

Properties of monolithic and composite NiAl processed by hydrostatic extrusion and vacuum hot-pressing

Abstract Nickel aluminide intermetallics and composites have been processed by hydrostatic extrusion or vacuum hot-pressing. Hydrostatic extrusion at both 298 K and 573 K was used to process nominally stoichiometric cast and previously extruded NiAl, while vacuum hot-pressing of prealloyed NiAl (45 at.% Al) powders was additionally used to process NiAl and NiAl+TiB 2 particulate composites. The microstructures, hardnesses, and compressive yield strengths of the processed materials were evaluated. It is shown that NiAl can be hydrostatically extruded at both temperatures, although substantial cracking was observed in the material extruded at 298 K. Hydrostatic extrusion at 573 K leads to significant hardness and strength increases without producing fracturing around the hardness indentations. Reinforcement with TiB 2 in the hot-pressed materials similarly provided an increase in both the compressive strength and hardness, although cracking around the hardness indents was observed. The fracture toughnesses of the hydrostatically extruded cast/extruded NiAl, the vacuum hot-pressed powder processed monolithic NiAl, and the composite NiAl intermetallics were determined in accordance with standard testing techniques. The effects of TiB 2 reinforcement particle size and distribution at 10 vol.% loading on the composite fracture toughness were also characterized. Monolithic NiAl demonstrated toughness values of about 5 MPa√m. Reinforcement additions to NiAl resulted in a toughness increase while the range of reinforcement distributions and sizes tested did not significantly change the measured composite toughness values. Significant increases in toughness and R-curve behavior were obtained in NiAl hydrostatically extruded at 573 K. The fracture behavior of the monolithics and composites is discussed in light of the deformation and fracture mechanisms operating at the crack tips, aided with in situ fracture studies and post-failure analyses.

Effects of dislocation substructure on strength and toughness in polycrystalline NiAl processed via low-temperature hydrostatic extrusion

The effects of changes in the dislocation substructure on the strength and toughness of stoichiometric polycrystalline NiAl have been determined. The dislocation substructure was introduced via thermomechanical processing by hydrostatic extrusion of polycrystalline NiAl at 573 K that was previously cast and conventionally extruded at high temperature (1173 K). Recent texture analyses revealed that the beneficial stress state imposed by hydrostatic extrusion near the ductile-to-brittle transition of NiAl produced a change in texture from the starting material, while complementary transmission electron microscopy and optical metallography revealed that recrystallization did not occur during or after such low-temperature deformation processing. The dislocation substructure retained after hydrostatic extrusion provided an increase in both the compressive strength and the fracture toughness, accompanied by R-curve behaviour and a dramatic change in the fracture morphology from that exhibited by the starting material. Subsequent hot microhardness experiments and high-temperature annealing studies revealed that removal of the dislocation substructure in the NiAl hydrostatically extruded at 573 K occurred after exposure to 973 K, accompanied by a reduction in both the strength and the toughness to values similar to those obtained on polycrystalline NiAl that was cast and conventionally extruded at high (i.e. 1173 K) temperature,

Multi-Color Techniques for Identification and Filtering of Burst Signals in Jet Engine Pyrometers

A DARPA funded Multi-color Pyrometry (MCP) experiment was carried out on a government provided aircraft engine to study the nature of hot particulate bursts generated from the combustor at certain engine conditions. These bursts of hot particulates lead to intermittent high-voltage signal output from the line-of-sight (LOS) pyrometer which is ultimately detected and used by the onboard digital engine controller (DEC). The investigation used a high-speed MCP system designed to detect bursts and identify their properties. Results of the radiant temperature, multi-color temperature and apparent emissivity are presented. The results indicated that the apparent emissivity calculated during the signal burst was lower than that of the blade. The root cause for the signal burst was identified as soot particles generated as by-product of combustion under certain conditions. This conclusion was drawn based on both experimental and simulation results. Technical strategies to separate, reduce or remove the burst signal are proposed.Copyright

In situ fracture monitoring of plasma-sprayed MoSi2-Ta composites

Initial work has shown that the ambient toughness of brittle silicide intermetallics such as MoSi2 have been toughened by incorporating irregular pancake-shaped Ta phases via deposition of powders by plasma-spray processing techniques. The mechanisms of toughness enhancement were observed in situ by conducting fracture toughness tests in an instrumented loading stage situated in a scanning electron microscope. It was observed that increases in toughness were obtained via ductile-phase toughening and the development of bridgedzones. The initiation and peak toughness of the composites varied with testing orientation, with the highest values measured when the crack plane intersected the Ta particles perpendicular to their edge. The shape of the resistance curve, however, was independent of Ta orientation.