Nicholas J. Grant

On the measurements of superplasticity in an Al-Cu alloy

The usual method of measuring the strain rate sensitive ‘m’ values of superplastic materials through differential cross-head speed is found to result in improperly definedm values;m is found to depend strongly on the strain to which the material is subjected, especially at low strains. In this connection, the shape of the log stress-log strain rate curve is examined for the Al-33 wt pct Cu eutectic alloy. The inherent grain growth of the very fine grains which occurs during deformation, and the strain dependence ofm at low strains, are shown to be the causes of the familiarS shape of the log stress-log strain rate curves for the Al-Cu alloy. At high strains (15 to 20 pct and higher) where the stress is no longer importantly strain sensitive, the log stress-log strain rate curve is a straight line of slope near 0.5. The elongation at fracture also does not go through a maximum but continues to increase slowly to the lowest strain rate examined: 10-7 per s.

A mathematical model of the spray deposition process

Spray deposition is a recently developed atomization process desigted to produce high density, bulk metal shapes directly from the melt. The process consists of two basic steps: first, a molten metal stream is atomized using a gas; the spray thus produced is then collected onto a suitably designed substrate. In this paper a mathematical model for the analysis of heat transfer during SD is described. The model is in two parts: the first part calculates the thermal histories of atomized droplets in flight, whereas the second part computes the transient temperature profiles inside the growing preform. More specifically, the mathematical model estimates droplet size distribution, temperatures, fractional solidification and microstructures of the atomized droplets in the spray, and the temperature field and microstructure of the resulting deposit. In contrast atively low (1 to 10 °C/second). The results also indicate that a small fraction of liquid mixed with solid exists at the top of the growing preform during deposition. The tiny pools of liquid may play a role in the formation of the characteristic equiaxed grain microstructure of as deposited preforms. The results of the calculations are very sensitive to the value of the enthalpy of the impinging spray. Therefore, the production of good quality deposits requires accurate control of the heat fluxes during deposition.

New microcrystalline phases in the NbNi and TaNi systems

Abstract Utilizing the splat-cooling technique, Nb-Ni alloys with 33–78 at.% Ni and Ta-Ni alloys with 55–67 at.% Ni were rapidly quenched from the melt. The thin foils or flakes obtained were investigated by X-ray and electron diffraction. The diffraction patterns exhibited only broad maxima instead of relatively sharp crystalline peaks. Computer calculations based on several microcrystalline models were able to account for the features of the diffraction pattern in the region of the first maximum; best agreement was attained by assuming microcrystals of about 150 atoms in a body-centered-cubic W-A2 type configuration (equivalent to spherical microcrystals 15–16 A dia.). The microcrystalline structure persists for 1 hr at 600°C; at higher temperatures it decomposes into the equilibrium phases. In the Nb-Ni decomposition, a crystalline transition phase is first formed. The transformation of the microcrystalline Ta-Ni phase to TaNi2 is regarded as an ordering reaction, combined with particle growth. A crystal-chemical relationship of the new microcrystalline phases to α-Fe may exist.

Spray deposition of metals: A review☆

Abstract The development of spray atomization and collection processes, i.e. Osprey, consolidated spray deposition (CSD) and liquid dynamic compaction (LDC) was motivated by the severe problems of oxide contamination associated with quenched reactive metal powders as well as by the need to minimize the overall number of processing steps from the liquid metal to fully dense product, perhaps in the near-net shape state. So far, these techniques have been successfully applied to numerous aluminum-, magnesium-, nickel- and iron-based alloys, yielding attractive combinations of properties, which include improvements in tensile strength, axial fatigue strength, fatigue crack growth resistance, superplasticity and magnetic properties. Furthermore, experimental and mathematical modeling studies have shown that the aforementioned property improvements depend on the production of a rapidly solidified, microstructurally homogeneous material during spray deposition. In turn, the achievement of such structures depends on the characteristics of the atomized droplets in flight and on impact, i.e. droplet size and distribution, relative proportions of solid and liquid, temperatures, droplet velocities, dendrite arm spacings, degree of undercooling etc. The purpose of this paper is to review the more important findings over the past decade of spray atomization and collection studies.

Plastic flow and fracture in Pd80Si20 near Tg

Above about 0.7Tg, plastic flow in Pd80Si20 becomes homogeneous in the normal ranges of strain rate of short term laboratory experiments. In this range samples exhibit considerable anelasticity before general yield. Load maxima are observed simultaneously with general yield and are followed soon after by a classical behavior of slanted necking, and a mixture of rupture and fracture accompanied by elongations of about 3% typical of a non-hardening rupturing solid of the given thickness and gauge length. This relatively stable behavior has permitted measurement of the stress exponents m of the strain rate for the material at flow at these temperatures. These exponents decrease to unity as the test temperature approaches Tg. The temperature dependence of the flow stress and its strain rate dependence are in good accord with Argons recent theory for plastic flow in metallic glasses.

The structure and properties of splat-quenched aluminum alloy 2024 containing lithium additions

Abstract High strength aluminum alloys containing lithium additions possess commercially attractive combinations of a high specific strength and a high specific modulus. However, these alloys are difficult to cast and fabricate and they also possess low values of ductility and toughness when processed by conventional ingot-casting methods. In order to realize fully the potential beneficial effects of the lithium additions, the splat-quenching technique was used in this investigation to process aluminum alloy 2024 containing two different levels of lithium. Splat quenching of the alloys resulted in a high degree of microstructural refinement that was preserved through the hot consolidation and heat treatment steps. The addition of each weight per cent of lithium resulted in a 3% reduction in the density and a 6% increase in Youngs modulus. Relative to the ingot-processed 2024 alloy, the lithium-containing alloys possessed superior strength properties at comparable levels of ductility and superior stress-amplitude fatigue life properties. However, the fatigue crack growth behavior and the stress-rupture behavior, especially of the alloy containing the higher level of lithium, were inferior. It is concluded that with proper alloy design the splat-quenching technique can be used successfully to develop AlLi-based alloys with overall improved properties.

Crystallization of an amorphous Cu-Zr alloy

Abstract A Cu-40 at. % Zr alloy was splat cooled into a non-crystalline structure. The crystallization behavior of this alloy was studied by calorimetry, transmission electron microscopy, and hardness testing. Calorimetric studies indicated that crystallization of the initial non-crystalline material occurred by a single exothermic reaction. The heat release was 1260 cal/mole and occurred at 480°C. Evidence for a glass transition at approx. 450°C was also found. Samples previously isothermally annealed below the crystallization temperature exhibited two exothermic reactions when studied calorimetrically. The additional second peak increased in size and shifted to temperatures higher than the first peak with increased annealing. Transmission electron microscopy indicated that crystallization occurred by a nucleation and growth mechanism during isothermal annealing. The crystallites were found to have a fine substructure with a subgrain size of approx. 300–500 A. The crystallization product was single phase. Calorimetry results, direct observations from transmission electron microscopy, and changes in the electron diffraction pattern of annealed samples led to the conclusion that the initial non-crystalline matrix transforms to a second non-crystalline structure which, in turn, transforms to the crystalline phase. Hardness measurements were performed to follow the crystallization process. Results indicated that hardness monitors the transformation of the initial non-crystalline structure with high sensitivity.

The constitution of metastable titanium-rich Ti-Fe alloys:An order-disorder transition

Single phase Ti-Fe alloys with up to 50 at. pct Fe were prepared by splat cooling. Alloys with less than about 35 at. pct Fe consisted of β-titanium solid solution, designated Ti(Fe); alloys with more than 35 at. pct Fe consisted of a partially disordered, off-stoichiometric TiFe solid solution. The degree of order of this phase was determined. A continuous phase transition between the ordered and the disordered metastable phase is possible. The ordered metastable phase fulfills known valence electron concentration criteria for the occurrence of the CsCl structure type.

Shear band induced dilations in metallic glasses

Etude theorique a partir de resultats anterieurs portant sur le changement de volume du a la formation de bandes de cisaillement dans des echantillons cylindriques de verre metallique Pd 77,5 Cu 6 Si 16,5

Property enhancement in Type 316L stainless steel by spray forming

Abstract Type 316L stainless steel was processed by rapid solidification, liquid dynamic compaction (LDC) and subjected to a series of simple thermomechanical treatments (TMT) to produce a fine austenitic grain structure. The as-deposited, fine grained structure was rollable in excess of 80% at liquid nitrogen temperature (77 K) without annealing or edge cracking. High tensile and yield values resulted from the cold worked austenitic structure at 298 K and from martensite formation on rolling at 77 K. Ductility values were on the low side but at useful levels for these high strengths.