Specialty metal powders for the 1990s

Specialty Metal Powders for the 1990s F H Froes (Institute for Materials and Advanced Processes (IMAP) University ofIdaho, Moscow, Idaho, USA) , Specialty metal powders produced by rapid solidification, mechanical alloying, thermochemical processing, spray deposition, etc are broadening the horizons of powder metallurgy and allowing the production of new types of materials previously considered unattainable. The following article reviews development trends for these specialty PM materials and their market growth in the 1990s.

For those who read the scientific literature the area of powder metallurgy has been the subject of many interesting articles of apparently 'breakthrough' nature in the past few years . Not only have we seen rapid solidification (RS) offer us new and exciting refinements in microstructure, extension of alloying possibilities, and the ability to fabricate previously non-processable compositions, but also production of previously unobtainable crystalline and amorphous structures including a new state of matter - that of the forbidden five-fold symmetry quasicrystal. Mechanical alloying (MA) has offered dispersion strengthened materials with elevated temperature capability well beyond conventional ingot cast and wrought product, and even greater excursions from equilibrium than rapid solidification. Pushing the horizons even further are nanostructures in which the scale of the crystal grain size means that more than half the atoms present are not located on a crystal lattice; the grain boundary atoms can now account for a greater percentage of the atoms pres ent than those located within the grains. This leads to extremely interesting combinations of physical and mechanical characteristics. Synthesis is now the way to put together materials atom by atom to produce novel property combinations in what has been termed the 'Age of Materials'. No longer can we accept the physical and mechanical properties presented to us by nature. The performance requirements of advanced systems are such that we must 'tailor' or 'engineer' materials to optimise behaviour. \Ve have transitioned from the basic materials age to this era of engineered materials. The techniques already mentioned fall into this new era category. In addition composite concepts give performance capabilities beyond the accepted monolithic norm. Here powder product in combination with reinforcing constituents allow attainment of greatly enhanced behaviour. And here synthesis can include newly evolving fabrication techniques such as spray deposition - where both novel 128

author's prerogative, the rest of this commentary will concentrate on advanced and emerging trends in powder metallurgy materials which are likely to mature only in the mid to late 90's. These are the technologies mentioned in the opening two paragraphs of this article .

RAPID SOLIDIFICATION Rapid solidification, that is a process in which the liquid phase is quickly converted to the solid phase, has received substantial scientific and technological attention in the past thirty years . Avoidance of segregation and microstructural refinement has resulted in relatively mature superalloy (eg Rene 95, IN718) and tool steel industries respectively, with projected growth rates up to 10% (Table 1).

Intensive development work by Allied-Signal on amorphous metallic glasses has led to families of iron base alloys which exhibit superior soft magnetic characteristics. Currently, Allied-Signal arc constructing a 60,000 ton per year capacity plant for this product, with projections that usc of this new material in transformer cores can lead to a $3 billion savings per year in the USA just in reduced transformer losses. Pioneering work by Dolco-Romyand others in rapidly solidified permanent magnets oftho Nd-Fe-B type has led to electric motors of reduced size and weighta boon to the automobile industry, and of great potential in weight-conscious aerospace applications. Magnetic applications are projected to grow at a per year rate of over 50% over the next few years (sec Table 1). Evaluation of the potential of enhancing the

microstructure and the ability to produce ncar net shapes directly are attractions. Powder metallurgy is also very amenable to rate dependant process es such as thermochemical processing (TCP) because of the small dimensions associated with the PM product. In this technique hydrogen is used as a temporary alloying element to enhance the processability and mechanical properties of hydride forming clements such as titanium. However, despite this euphoria conv entional press-and-sinter powder metallurgy continues to be the dominant industry product. The single largest segment of the industry, in $M terms of both volume 1988 1993 and dollars, is the production and usc of 10.0 Metallic Grasses 81.0 iron and steel powders; with the cost Iron-Base Alloys 62.0 100.0 obsessed automobile Superalloys 54.0 76.0 industry being the 2.0 Permanent magnets 20.0 predominant user of Aluminum Alloys' 3.25 8.65 this cost-effective Titanium Alloys 0.2 0.5 product. However Magnesium Alloys 0 0.4 despite the present Niobium Alloys 1.0 domination of the Copper Alloys marketplace by this Ceramics 0.15 2.0 relatively, mundane material the . prosont • Majority structural applications article will not further consider this segment of the PM industry. Rather, exercising an

Average Annual Growth Rate (%)

52 10 7

60 22 20 15 68

TABLE 1 US markets for rapidly solidified materials (Courtesy Dr Thomas Abraham, Business Communications Co Inc, Norwalk, Conn.)

Yield Strength (MPa) Ultimate tensile strength (MPa) Elongation (%) Fracture toughness (MPalm) SCC resistance (MPa)

INGOT

RAPIDLY SOLIDIFIED

7075-T73'

CS67-TI-

435

580 615 12 47 310

505

13 32 290

• Plate • Extrusions

TABLE 2 Comparison of eam'entional ingot and rapidly solidified aluminum alloys

MPR February 1990

performance of aerospace alloys such as alumlnum.titanlum, magnesium and niobium began later than the magnetic materials. Hero the thrust is to usc RS to improve characteristics such as strength, fracture ch aracteristics. elevated temperature performance and corrosion behaviour. Led by aluminum alloys improved materials have resulted from RS processing. For example, the performance of 7000 series aluminum alloys produced by conventional ingot processing and RS techniques arc shown in Table 2. The RS ' development of the other base materials mentioned is still in the research and early developmental stage and commercialization is unlikely until the mid to late 90's at the earliest. For the tilanium system it has already been demonstrated that the PM route of ncar net shape production can produce a cost-effective component wilh mechanical properties at least equivalent to those of conventional ingot metallurgy product. However. the extremely conservative aerospace has yet to accept the PM product. Dispersion strengthened RS tilanium alloys. and RS of titanium aluminldos offer some advantages but these are not of a large enough magnitude to stimulate extensive growth ofRS titanium alloys-hence the small growth projected in Table 1. Higher strength-corrosion resistant RS magnesium alloys. and niobium alloys wilh built in elevated temperature oxidation resistance using the RS route also look attractive but again the conservatism of the aerospace industry is likely to lead to acceptance only by the late 90s',

Property Thermal Expansion (10 6IK) Specific Heat at 295K (J/g K) Density (g/cm 3) Elastic Moduli Young's Modulus (GPa) Shear Modulus (GPa) Saturation Magnetization at 4K (emu/g) Susceptibility (10' 4 emu/De g) Fracture Stress (kg/mm 2 ) Superconducting Critical Temp (K) Activation Energyfor Diffusion (eV) Debye Temperature (K)

Glass

Crystal

Cu Pd Fe

31 0.37

18

6

7.5

16 0.24 7.9

Pd Pd Fe Sb Fe-1.8C Al Ag in Cu Cu in Cu Fe

88 32 130 20 6720 3.2 0.51 0.64 345

mature than that on the aluminum system but thoro are indications that the MA technique offers some interesting possibilities in developing novel Ti alloys. A major draw-back with RS elevated temperature titanium alloys is the low «5%) volume fraction of dispersed particles. This could well be rectified using the MA technique allowing temperature capabilities well beyond the current 1100F to be achieved. Replacement of nickel-based superalloys would then result in considerable weight savings in advanced engines; Other possibilities in the titanium system include low density alloys by mechanical alloying with Mg or perhaps Li, and the MA of the' titanium aluminides to produce material with a refined microstructure.

NANOSTRUCTURES

MPR February 1990

Nanocrystal

215 -0.03

123 43 222 -1 560

1.2 2.0 2.04 467

TABLE 3 Properties ofnanoctystalline materials compared witli their crystalline and glassy counterparts

MECHANICAL ALLOYING Mechanical alloying is a process in which powder material is repeatedly worked. w elded, and fractured to produce novel extremely fine microstructures often containing dispcnsions of relatively inert second phase particles. A number of MA iron and nickel based alloys are already in commercial uso in large part because of the increased temperature capability resulting from a fine dispension of second phase particles. More recently the MA process has been applied to low density systems such as aluminum base alloys. Reduced density aluminum alloys have been developed using MA such as the AI-905XL composilion (AI.4Mg-l .3Li-1.2C·0.50 2 ) . This alloy is claimed to be thermally stable, unlike higher Li containing alloys, and is 8% lighter and 15% stiffer than 7075-T73 but otherwise equivalent to the ingot alloy in mechanical properties. Of even greater significance is the boost in elevated temperature capability possible using the MA approach, With an AI-Ti base MA allows development of an ultra-fine dual-phase AI·AI J Ti structure in which the extremely fine scale of the microstructure is not dictated by ph ase diagram considerations but rather by the scale of the Ti dispersed within the AI particles. The elevated temperature properties of MA AI·Ti appear to be superior to RS material with claims that this alloy class could be useful to beyond lOOOF. Work on MA of tilanium base alloys is less

Material

Nanostructures are materials with structural dimensions, generally agrain size, in the range of up to 10's of nanometers (lnm=10-9m). Because of this extremely fine grain size there are now many more atoms located in grain boundary regions than in conventional materials, Fig. 1. These impart novel characteristics such as very high diffusion rates along the 'open' grain boundary regions, and the possibility of alloying with normally insoluble constituents. Powder metallurgy is an obvious fabrication technique for production of nanostructures either using condensation from the vapour phase on a cold finger or 'mechanical alloying method,

possibly even via an amorphous precursor. Examples of how behaviour can be extended in moving from conventional crystalline material to amorphous glassy product and finally to nanocrystalline structures is shown in Table 3. The nanostructure materials could well sec applications by the later 90s ..

THERMOCHEI\HCAL PROCESSING Thermochemical processing (TCP) is a technique whereby hydrogen is temporarily added to titanium alloys to enhance processability and improve final mechanical properties. The hydrogen is added by annealing in a hydrogen environment, the material is then fabricated or heat-treated with the hydrogen present, and the hydrogen is subsequently removed by a simple vacuum annealing treatment. This technique has been shown to be particularly useful in enhancing fatigue behaviour of titanium alloys such as the Ti·6AI-4V alloy, Because the process involves two diffusion controlled steps it is very amenable to usc with powder product and both ' prealloyed (plasma rotating electrode process) and blended elemental compacts have demonstrated improved fatigue behaviour after TCP. It is anticipated that TCP could sec commercial usc with titanium alloy powders in the 90's, including the titanium aluminidcs, and that application could be extended to other hydride forming clements such as zirconium, hafnium and mobium.

METAL MATRIX COMPOSITES

FIG. 1 Schematic representation of the structure ofan unrelaxetl nanophase material distinguishing between the atoms associated witl: the individual crystal grains (filled circles) and those constituting the interface network [open circles)

Because of their enhanced strength, stiffness and elevated temperature performance metal matrix components (MMC) are attractive materials for aerospace applications despite their often non-optimum fracture behaviour and increased cost over monolithic metals. There are a number of ways of fabricating MMC's including sheet processing. casting approaches and PM methods. Each technique has its own set of advantages and disadvantages which must be carefully weighed before a final decision is made on the method which is most appropriate for a specific system. The PM approach can be divided into approaches which involve solid

129

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FIG. 2 A schematic diagram of the OspreyTM

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FIG. 3 Schematic diagram of a scanning

atomizer developed to cast strip steel state (blending) processing and those in which the fine metal particles are liquid or 'mushy' (at a temperature between the liquidus and solidus) during the initial stage of fabrication.

The latter approach will be discussed UTS VS EI E later under spray deposition methods. 6061-T6 290 255 17 70 The solid state powder processing method can result in significant 6061-T6/20%SiC w 585 440 120 4 enhancement of strength and stiffness 6061-T6/30%SiC w 795 570 2 140 over monolithic metals, for example in the aluminum system (Table 4). The major challenges in this class of TABLE 4 Comparison ofreinforced and monolithic extruded aluminum rod {courtesy Admnced materials is to enhance the fracture Composite Materials Corporation. SiC",silicon carbide related properties such as ductility and whiskers produced from rice husks fracture toughness while producing a using materials such as aluminum, aluminum cost-effective product. However it could well be that as the temperature capability of metal matrix composites and superalloys, polymeric matrix composites is increased that Strip and tubing is also being produced which the MMC's arc restricted to usc in a fairly fits in well with the present trend in the steel industry from capital-intensive processes to narrow temperature range. flexible manufacturing processes. Restriction on strip width with standard spray deposition DIRECf PROCESSING!SPRAY processes, and enhanced control of strip DEPOSITION thickness have been achieved using a 'scanning' atomizer (Fig. 3). In all cases a final Finely divided molten'metal droplets can impinge on a substrate before they completely working step is required to achieve a full density product: for example a 40% reduction solidify allowing direct formation of ncar net in gauge for strip material. shapes rather than having to go through the intermediate step of powder production. One Direct production/spray deposition allows effective usc of material and elimination of such commercial process is the Osprey process various processing steps. As such the process shown schematically in Fig. 2. While this process docs not exhibit solidification rates as should sec increased usc in the energy and high as those discussed previously under rapid .materials conscious 90's. solidification it docs offer two' other advantages: direct production of preform CONCLUDING REMARKS shapes and direct inclusion of reinforcing materials into the parent matrix (termed The conventional press-and-sinter PM co-spray deposition). The latter process gives industry is alive and well. However. exciting the same advantages as the MMC's already , new faceIs of powder metallurgy technology discussed with the additional attraction of are now in various stages of development and these should come into their own as we move ncar net shape production. Billet on disk preforms are being evaluated through the decade of the 90's.

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