Sunil C. Jha

Carbide-dispersion-strengthened B2 NiAl

Abstract The microstructure and high temperature deformation behavior of a rapidly solidified and hot-extruded NiAl-4%HfC alloy are described. Rapid solidification processing led to the incorporation of 20–50 nm sized HfC precipitates in a B2-ordered NiAl matrix. The carbide precipitates were found to be stable and resistant to coarsening at temperatures as high as 1500 K. The as-extruded microstructure consisted of a recrystallized grain structure, with grain size in the range 5–8 μm. Transmission electron microscopy on the as-extruded alloy revealed the presence of subgrains, and a dislocation substructure stabilized by the carbide precipitates. Departure side pinning of matrix dislocations at the carbide precipitates was observed in the as-extruded material. The flow characteristics of NiAl-4%HfC alloy were investigated by constant-velocity compression testing at 1200 and 1300 K, with strain rates ranging from 10 −4 to 10 −7 s −1 . It was observed that carbide-containing NiAl continuously work hardened at all temperatures and strain rates investigated. The flow stress-strain rate data indicated the presence of a threshold stress for the creep deformation of carbide-dispersion-strengthened NiAl intermetallic, and departure side pinning of dislocations by the carbide precipitates was also observed in the deformed sample.

THE PROCESSING AND EVALUATION OF CLAD METALS

Clad metals are a specific form of composites in which the materials are arranged in a layered structure. Cold-roll bonding techniques are employed to produce more than 20,000 tonnes of clad metal laminates each year in the United States. This article is an introductory description of the processing steps in cold-roll bonding, the nature of the bond created in this process, and the methods used to evaluate the bond’s strength.

1000-1300 K SLOW STRAIN RATE PROPERTIES OF NIAL CONTAINING DISPERSED TIB2 AND HFB2

Abstract Rapid solidification technology was used to produce NiAl containing dispersions of fine TiB 2 or HfB 2 particles in an effort to improve the elevated temperature strength of this intermetallic. As-melt spun ribbons of NiAl2TiB 2 were pulverized, and the resulting powders were densified by extrusion at 1420 K with a 16:1 reduction ratio, while NiAl2HfB 2 was initiall;y consolidated by hot isostatic pressing at 1505 K and 207 MPa for 4 h followed by forging 65% at 1535 K. Both materials were compression tested in air between 1000 and 1300 K under constant velocity conditions at nominal strain rates ranging from 2 × 10 −3 to 2 × 10 −7 s −1 . NiAl2HfB 2 displayed steady state behavior under all test conditions while NiAl2TiB 2 exhibited diffuse yielding at 1000 K and generally slow, continuous work hardening at the higher temperatures. Analysis of the flow stress-strain rate data indicated that both materials behaved normally, and deformation could be described by temperature compensated power laws. HfB 2 was found to be a more effective strengthening addition than TiB 2 .

Influence of grain size on the creep behavior of HfC-dispersed NiAl

Abstract Rapid solidification technology has been utilized to produce a NiAl-4(wt.%)HfC composite containing about 0.3 vol.% HfC as dispersed 50 nm particles. Study of the 1300 K compressive creep properties demonstrated that the initial, small grain size microstructure was unstable under slow strain rate deformation conditions. The grain growth which occurred during testing led to considerable strengthening. Subsequent measurements of the creep properties of the coarse grained specimens revealed that this strength was achieved by a large increase in the activation energy for deformation without any change in the stress exponent. Based on this work it is concluded that large grain microstructures will be required for optimum elevated temperature creep properties in dispersed NiAl.

High-Temperature deformation of B2 NiAl-base alloys

The high-temperature deformation behavior of three rapidly solidified and processed NiAl-base alloys-NiAl, NiAl containing 2 pct TiB2, and NiAl containing 4 pct HfC—have been studied and their microstructural and textural changes during deformation characterized. Compression tests were conducted at 1300 and 1447 K at strain rates ranging from 10-6 to 10-2 s-1. HfC-containing material showed dispersion strengthening as well as some degree of grain refinement over NiAl, while TiB-2 dispersoid-containing material showed grain refinement as well as secondary recrystallization and did not improve high-temperature strength. Hot-pack rolling was also performed to develop thin sheet materials (1.27-mm thick) from these alloys. Without dispersoids, NiAl rolled easily at 1223 K and showed low flow stress and good ductility during-the hot-rolling operation. Rolling of dispersoid-containing alloys was difficult due to strain localization and edge-cracking effects, resulting partly from the high flow stress at the higher strain rate during the rolling operation. Sheet rolling initially produced a (111)< 112) texture, which eventually broke into multiple-texture components with severe deformation.

CUVAR-a new controlled expansion, high conductivity material for electronic thermal management

The increasing level of integration and operating speed of integrated circuit chips is leading to larger chip sizes with higher power dissipation. Electronic packages incorporating such devices require low cost and highly efficient thermal management materials for reliable performance of the device. W/Cu and Mo/Cu materials are often used as heat sinks or substrates for direct bonding the Si devices. While W and Mo based materials possess the required thermal expansion and thermal conductivity for electronic packages, they are difficult to machine and fabricate into intricate shapes. This report describes a new thermal management material, called CUVAR, which possesses low thermal expansion and high thermal conductivity. CUVAR is an extruded composite of copper and Invar powders, where Invar restrains thermal expansion while copper provides thermal conductivity. Most importantly, CUVAR is easily machined, enabling high speed and efficient fabrication of components. It is readily plated with copper, nickel, silver or gold for joining and finishing. CUVAR is an alternative to low thermal expansion materials such as Kovar and Alloy 42 with a much higher thermal conductivity. CUVAR can also be a replacement for the conventional W and Mo based thermal management materials with the advantages of low cost and excellent fabricability for high volume applications.

Producing titanium aluminide foil from plasma-sprayed preforms

High-strength titanium alloy and titanium aluminide foils are required for fabricating composite structures and honeycombs for advanced aircraft engines and airframes. Titanium aluminide alloys possess limited workability, which results in significant yield loss when these materials are produced by the conventional ingot metallurgy route. This article describes the use of induction plasma spray technology to fabricate foil preforms of a titanium alloy and a titanium aluminide. These plasma-sprayed preforms were converted into 100% dense wrought titanium aluminide foil by a roll-consolidation process. The microstructure and mechanical properties of titanium aluminide foil produced from plasma-sprayed preforms were virtually identical to those of conventional ingot metallurgy foil. The plasma-spray plus roll-consolidation route may lead to the production of titanium aluminide foil as continuous coil, which would improve process efficiency and yield high-quality titanium aluminide foil at low cost.

Does a threshold stress for creep exist in HfC-dispersed NiAl?

Recently it was proposed (Jha et al., 1989; Whittenberger et al., 1990) on the basis of constant velocity testing at 1300 K that dispersion strengthened NiAl composites containing about 4 wt pct HfC possess threshold stresses for creep. Further, 1300 K compression testing has been conducted on NiAl+4HfC, and diametrically opposite behavior has been found: for constant load creep tests a normal power law behavior was observed. However, additional constant velocity testing still indicates that the flow stress is essentially independent of strain rate below 10 exp -6/s. Examination of NiAl+4.3HfC specimens deformed under constant velocity conditions revealed that the original hot extruded small grain structure could be converted to large, elongated grains during testing. Such a transformation appears to be responsible for the apparent threshold stress behavior in HfC dispersed NiAl.

THE MICROSTRUCTURE AND PROPERTIES OF RAPIDLY SOLIDIFIED DISPERSION-STRENGTHENED NIAL

An advanced rapid solidification technology for processing reactive and refractory alloys, utilized to produce large quantities of melt-spun filaments of NiAl, is presented. The melt-spun filaments are pulverized to fine particle sizes, and subsequently consolidated by hot extrusion or hot isostatic pressing. Rapid solidification process gives rise to very fine-grained microstructures. However, exposure to elevated temperature during hot consolidation leads to grain growth. Alloying agents such as borides, carbides, and tungsten can pin the grain boundaries and retard the grain growth. Various alloy compositions are investigated. The eventual goal is to utilize the hot-extruded and forged stock to grow single-crystal NiAl blades for advanced gas-turbine engine applications. Single-crystal NiAl, containing a uniform dispersion of carbide strengthening precipitates, is expected to lead to highly creep-resistant turbine blades, and is of considerable interest to the aerospace propulsion industry.