This work presents some results concerning the crystallization of Ni based alloys containing various amounts of boron. We will describe the crystalline phases and show that the nucleation mechanisms can be very different when the boron content is changed.

The engineering light metals, especially aluminium and magnesium, exhibit limitations in alloying behaviour and performance that identify them as particular targets for development via rapid solidification. The present contribution reviews recent work in the author's group aimed at advancing understanding of both alloying and processing, including:

(1) parametric mapping as a guide to extension of phase fields

(2) prediction and application of observed characterisation of binary and ternary extended solid solutions

(3) optimisation and control of conditions during rapid solidification and consolidation,

An investigation was performed to study the origin and stability of microstructures in rapidly solidified aluminum alloys. Al-Ni and Al-Fe base alloys were rapidly solidified by means of laser surface melting and melt spinning techniques. Microstructures were studied using optical and transmission electron microscopy. The effect of microstructure on mechanical properties was also studied using microhardness measurements. The origin of the observed microstructural constituents will be explained in terms of features of the metastable phase diagram. The effect of ternary additions on stability will also be considered.

Rapidly solidified Al containing up to to 15 wt.% Mn was prepared by melt spinning. The alloys were examined by TEM and X-ray diffraction in the as-spun condition and after annealing at 450°C. Four precipitate phases were detected, and their growth kinetics were correlated to subgrain structures in the Al matrix.

A rationale for rapid solidification processing of aluminum-lanthanide alloys is given, and preliminary results of microstructural studies on an aluminum-neodymium alloy are presented. Three distinct types of microstructure are seen in the as-cast rapidly solidified material, which are still evident after heat treatment at 350°C for 1 hour. Microhardness measurements indicate each microstructure has a different response to isochronal heat treatments.

Submicron powders, amorphous films and melt spun ribbons of various Al-Ge alloys have been analyzed to determine the relative roles of undercooling and cooling rate in the production of non-equilibrium structures. All analyses were performed in transmission electron microscopes equipped with energy dispersive x-ray spectrometers. The submicron powders, produced by electro-hydrodynamic atomization, were analyzed in their as-received condition and then annealed and/or melted using the electron beam as a local heating source. Once molten, the liquid droplets were undercooled at different cooling rates by varying the rate of beam obstruction. In this manner, a number of different microstructures were produced. These included metastable crystalline phases and mixed amorphous/crystalline structures. By combining this technique with a microscope heating stage, it was possible to carry out controlled dynamic undercooling experiments and determine phase selection as a function of undercooling and composition. The amorphous films were rapidly heated with the electron beam in the microscope and metastable as well as stable phases were produced. The results of these complementary analyses will be compared and discussed with reference to current models and theories of rapid solidification.

The titanium alloys Ti-6A1-4V and Ti-11.5Mo-6.0Zr-4.5Sn (Beta III) were produced at cooling rates of 103°C/sec and 105°C/sec using the Rotating Electrode Process (REP), and the Electron Beam Splat Quenching (EBSQ) process, respectively. Material from both alloys examined in the as-produced, heat treated, and vacuum hot pressed (VHP) conditions exhibited an order of magnitude decrease in beta grain size with increasing cooling rate (~135 μm and 7 μm in the Ti-6A1-4V alloy; ~40 μm and 2–4 μm in the Beta III alloy), and a difference in aging response between REP powder and EBSQ flakes. After heat treatment the Ti-6A1-4V developed an alpha morphology which was lenticular in REP powder and equiaxed in EBSQ flakes. Three possible mechanisms are proposed to explain this change involving an increased dislocation density and a tendency for grain boundary allotriomorphic alpha in the EBSQ case. Heat treatment of the Ti-11.5Mo-6.0Zr-4.5Sn materials showed a change in the location of alpha precipitation from interdendritic to grain boundary in going from REP to EBSQ. It is suggested that this change may relate to the decreased solute redistribution which occurs in the more rapidly cooled material.

Rapidly solidified (RS) Ti alloys containing novel additives were prepared by splat quenching and melt spinning techniques. Microstructures of the as-quenched and heat-treated alloys were studied by electron microscopies. The results show that microstructural refinement and precipitation reaction are universal phenomena in all RS Ti alloys. A significant difference in second phase coarsening was observed between metalloid-origin precipitates and those of rare earth-origin. The precipitates in a Ti-Al-La(Ce) were identified predominantly as rare earth-Al compounds. Exce llent stability for rare earth-origin precipitates was found.

Except for a carbon-containing alloy (700 ° C), age hardening behavior is a universal phenomenon in all RS Ti alloys with additives. A significant strength increase (hardness) in the RS alloy was noted at both room and elevated temperatures.

Titanium-base binary alloys containing eutectoid forming additions such as Fe, Si, W, and Cr were rapidly solidified by pendant drop melt extraction, melt spinning, and “hammer and anvil” techniques. The materials obtained were fibers, ribbons, and splats, respectively. The microstructures of these rapidly solidified materials were analyzed by optical microscopy, SEM, and TEM. The fibers showed fine microstructures near the quenched side, but less homogeneous and coarser microstructures were observed away from this side. Similar variations were observed between the edge and the center of splats. The results of this microstructural characterization are discussed, and a comparison made between microstructures obtained by the different rapid solidification methods used.

Ostwald ripening of precipitates in rapidly solidified Ti- 5A1-2Si (wt.%) has been studied by TEM. Results show that particle coarsening at 700 and 800°C is controlled by volume diffusion. Diffusion coefficients of Si in a-Ti were found to be ˜7.5 × 10−13 cm2/sec at 700°C and ˜1.2 × 10−11 cm2 /sec at 800°C.At these temperatures, microstructural refinement due to rapid quenching had no noticeable effect on the coarsening mechanism for the RS Ti-5A1-2Si alloy.

Rapidly-solidified microstructures were produced in aTi-22 wt% Fe alloy by laser surface melting, melt extraction, melt spinning and splat cooling. Increased cooling rates during solidification promoted increasingly finer beta grain sizes ranging from approximately 75 microns in a laser melt to 0.5 to 2.0 microns for splats. Beta grain morphologiesw ere generally equiaxed, although epitaxially-nucleated columnar grains were observed at the pool-substrate interface for laser melts. Fitting of the equiaxed beta grain size data to a λ0 = Boε−n0 power relationship for homogenous nucleation and isotropic growth resulted in Bo = 3 × 104 and no = 0.62. The refinement of dendrite structure with increasing cooling rate was also documented for laser melts. Dendrite spacing versus cooling rate data were fitted to a similar general power relationship and resulted in B1 = 45 and n1 = 0.34.

Techniques of rapid solidification processing were used to refine a dispersion of rare earth oxides in Ti. The dispersion was produced by laser surface melting and subsequent heat treatment of a Ti-Er alloy. The second phase was identified as the rare earth oxide. The stability of the dispersion was investigated analytically and experimentally and the correlation between the analysis and experiments was shown to be very good.

The microstructure and yield strength of two nickel base superalloys prepared by melt spinning and plasma deposition are compared in the as-solidified condition and after annealing. The results support the interpretation of the yield strengths obtained by tensile testing melt spun ribbon as representative of the values obtained for bulk specimens with equivalent microstructures. The effectiveness of grain size strengthening in a nickel base superalloy is also examined. The Hall-Petch relation appears to be obeyed, with a slope k = .77±.15 MPa−m1/2.

Powders of stoichiometric Ni3Al and Ni3Al containing boron and titanium were produced by rapid solidification rate (RSR) processing. Most particles were found to be spherical; some had skins and some contained holes. Fractured particles were found, indicating possible brittleness. Particle size distributions were log-normal with an arithmetic mean particle size of 23.5μm. Both alloys exhibited a dendritic structure in which compositional differences were present. In addition, stoichiometric Ni3Al contained interdendritic lamellae. X-ray diffractometry indicated that the powders were only partially ordered.

A small amount of boron addition in rapidly solidified Ni3 Al has been found to yield remarkable improvements in bolh room-temperature strength and ductility. In this study, the microstructure of melt-spun Ni3Al ribbons with various boron modifications ranging from 0 to 6.0 at% was investigated in detail by using transmission electron microscopy. All alloy compositions studied reveal a completely ordered fcc L12 matrix phase, in which polygonized dislocation networks and subgrain boundaries are observed. The boron-free Ni3 Al contains a dispersion of an Al-rich martensitic phase consisting of alternate twins. The boron addition tends to suppress the formation of the martensitic phase, but excessive boron (≥ 2.0 at%) causes the precipitation of M23B6 type borides.

There is a growing national need for development of new improved materials having superior corrosion resistance combined with high mechanical strength. This paper discusses the properties and corrosion behavior of a new nickel-chromium-molybdenum microcrystalline alloy made via rapid solidification powder process, making it suitable for potential applications in corrosive environments. The alloy, called Markomet 1119, has the following composition in weight percent:

Bal Ni, 20.3Cr, 22.8Mo, 2.9Fe, 0.5B, 0.8C.

This is prepared as rapidly solidified powder by the method of a melt-spinning-pulverization process. The powder is consoldated into rods by hot extrusion. The mechanical properties and corrosion tests carried out on Markomet 1119 are reported. The microstructural feature of the alloy as investigated by SEM is also reported.

Ni-based stable and metastable solid solutions with T3 and T4 transition metals have been prepared by rapid liquid quenching. The microhardnesses of these alloys show a substantial increase (by a factor of 4 to 5) that scales with their size mismatch as well as the solute concentration. It was found that the microhardness increase, ΔH, can be fitted well to the theoretical equation: ΔHv = K μ (cη)4/v3 where μ is the solvent shear modulus, c is the solute concentration and η is the size mismatch. The constant K for Ni base solid solutions is ~85.

Rapidly solidified ribbons of Cu-Fe alloys with iron contents up to 20 at.−% have been prepared by melt-spinning. Optical and electron microscopy as well as x-ray and electron diffraction techniques were used to characterize quantitatively the microstructure, i.e., grain size and shape, solubility of iron, lattice parameter, volume fraction and distribution of precipitated iron-particles, etc.

Whereas the free surfaces of melt-spun Cu-Fe ribbons have been found to be very smooth, the contact surfaces usually consist of isolated areas of good thermal contact with small equiaxed grains separated by bands without contact during casting and therefore poor heat transfer. The cross sections of the ribbons generally exhibit a strong anisotropy in their microstructure: very fine crystals adjacent to the contact surface develop into narrow columnar grains, generally significantly elongated and extending across the whole section. The average columnar width of the grains has been found to decrease significantly with increasing iron content. Precipitation of iron not only depends on the iron content but also on the distance from the contact surface.

Ribbons of a Cu-Al-Ni shape memory alloy have been rapidly solidified by planar flow casting. Attempts have been made to vary casting and secondary quench conditions in order to identify the predominant process variable in determining the subsequent β1↔γ1′ transformation temperatures. By varying wheel speed, wheel material and secondary quenching rate it is concluded that the large excess vacancy concentration has most influence on the as cast ribbon, but that after subsequent heat treatment, the grain size dominates.

Rapid solidification technology (RST) has been shown to offer outstanding prospects of processing new cost-effective engineering materials with superior properties. There has as yet been only limited effort in the use of RST to synthesize new and improved copper-base alloys. A detailed investigation has been carried out to develop new coppernickel alloys by RST. These alloys find extensive use as condenser tubes, seamless tubes, etc.

New alloy systems suitable for processing by RST have been identified. These systems are as follows:

a) Cu-Ni-B

b) Cu-Ni-Al-B

c) Cu-Ni-Cr-B

d) Cu-Ni-Al-Cr-B

The preferred composition ranges and maximum attainable hardness for each system are given. It is of interest to note that the maximum hardness values attained for this system are around 500 Kg/mm2 which is much higher than the hardness of the conventional copper-nickel alloys.