FEA TURE FOCUS: EMERGING MA TERI- Metallic glasses Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/120/06/72/6381561/me-1998-jun4.pdf by guest on 30 September 2021 bulk up A new formulation for metallic glass, now being vacuum-die-cast into golf clubs, will improve the production of this high-performance material. By Steven Ashley, Associate Editor
HENEVER A NEW high-strength material is developed these days, its first real W world application always seems to be in golf clubs. Vitreloy, a metallic-glass alloy of zirconium and beryllium from Amorphous Technologies International (AT I) in Laguna Niquel, Calif., is no exception. The novel noncrystalline material has been used since last year in 3-millimeter-thick faceplate inserts for high-end golf club heads sold by Mi ZLlno Sports, Maruman Golf, and Bridgestone Sports-the three major Japanese golf-equipment suppliers. In the meantime, ATI has established its own company, Liquidmetal Golf, to market "fully amorphous" driver, iron, and putter designs. T lLis new metal offers high strength- to-weight ratios Liquidmetal golf-club heads are fabricated from an amorphous metal alloy (on the bottom) that has excellent rebound and vibration-absorbing properties. and hardness, extreme springiness and rebound characteristics, and good acoustic-dam.pening properties. aries, which can serve as points of weakness. According to company representatives, it could become In the past, producing the high- performance alloy in "the next big thing in golf clubs." bulk has proven difficult. Unlike conventional metals, Developed at the California Institute of Technology in which are usually cooled slowly until they fully solidify, Pasadena, the metal alloy known as Vitreloy-or Liquid metallic glasses must be cooled very rapidly and very uni metal when used in golf clubs-is approximately formly to fi-eeze their random atonLic pattern in place be two-thirds zirconium, one-fifth beryllium, and the re fore crystallization occurs due to the nucleation and mainder split among copper, titanium, and nickel. Con growth of crystal grains. Four decades ago, when applica ventional metals have a crystalline structure in which tions of t1Li s phenomenon were first being explored, the the atoms are lined up in neat, orderly arrays; the bulk only way to e},.'tract heat fast enough to n1.aintain the metal's of these standard metals typically consists of small re random state was to keep the metastable material very gions of aligned atoms, called grains, and the boundaries thin through special techniques such as splat cooling, in between them. Within the metallic glass, however, the which droplets of molten metal of quick-frozen on a cold five types of atoms are packed together in a somewhat surface. Continuolls amorphous metal ribbons less than random fash ion, similar to that of a liquid. Research 0.1 millimeter thick could also be formed, at a cooling indicates that the prized physical properties of metallic rate of 1 millionoC per second, by pouring molten metal glasses arise in large part from their la ck of grain bound- onto a cold, spinning wheel.
72 J UNE 1998 MEC IIANI C AL ENGINEERING In the early 1970s, for example, AlliedSignal Inc. in ized we had to go to a very complicated material," said Morristown, N.J. , marketed a metallic-glass ribbon called Johnson the Ruben and Donna Mettler Professor of Metglas for use in high-efficiency electrical transformers. Engineering and Applied Science at Caltech. "We The ferrous ribbon's lack of crystal defects translated into needed to have four or more atoms with different desirable magnetic properties. The increased fabrication chemical characteristi cs before the liquid gets sufficiently costs due the ribbon-winding fabrication operation and frustrated in its efforts to crystallize." The five-element other issues stalled the transformers' progress. mixture glassifi es as needed only over a very narrow Beyond wear-resistant coatings, metallic glass found lit range of compositions. tle application in the meantime. In 1991 , AT! commer cialized its first successful product: Armacore, a hard GOLF ANYONE? amorphous coating that protects drill pipe as it moves Soon after this method was developed, AT! negotiated an
through oil-well casings. exclusive worldwide license for the Caltech material. Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/120/06/72/6381561/me-1998-jun4.pdf by guest on 30 September 2021 "Right from the start, our focus was on golf because the BULK FABRICATION combination of properties [Vitreloy] offered made it an Over the past decade, methods have been developed to overwhelming choice for a golf material," said Michael produce n1.etallic glasses in bulk. A leader in this field is Tenhover, ATI's vice president of product development Akihisa Inoue's research group at Tohoku University in and chief technical officer of Liquidm etal Golf. For exam Sendai, Japan, which has developed an array of bulk ple, Vitreloy has an almost ideal density for golf irons and metallic-glass systems based on zirconium, magnesium, putters-6.1 grams per cubic centimeter, which lies in be aluminum, and iron. Early approaches to bulk fabrica tween the specific gravity of stainless steel and titanium. tion were mostly empirical in nature, but researchers "The weight of a golf club is fixed by the rules," Ten gradually began to understand that the correct choice hover said, "so their overall shape is dominated by the of elemental constituents would lead to amorphous material density. Furthermore, the amorphous m etal's metals amenable to cooling rates as slow as 1°C to high strength-to-weight ratio allows the mass to be dis 100°C per second. These slower cooling rates mean that tributed differently within the club." The combination large parts can be fabricated. Furthermore, many of of the right density and its high yield strength of275,000 these metallic glasses remain stable against crystalliza pounds per square inch would contribute to the club's tion when heated to temperatures somewhat above their unique appearance and performance. glass-transition temperatures. The company first had to determine which manufac According to Todd Hufnagel, who studies metallic turing process could be used to make the shapes neces glasses as an assistant professor of materials science at sary for golf clubs. At the time, the Caltech researchers Johns Hopkins University in Baltimore, "one of the gen were making 1- to 2-gram samples. Casting a club head eral guiding principles to designing alloys that form bulk is difficult, said Johnson, who noted that the melt is or metallic glasses is to pick elements with dramatic differ ders of magnitude more viscous than most molten metals. ences in size, which leads to a complicated structure" "It's more like a thermosetting polymer; it doesn't flow that crystallizes less easily; a beryllium atom, for example, ·very well." AT! has since made arrangements with two is much smaller than a zirco well-known metal-casting nium atom. Another effec companies-Howmet Metal tive step is "to look for alloy Mold in Whitehall, Mich., compositions with deep which is part of Howmet eutecti cs," he said, "which Specialty Products; and form liquids that are stable Hitchener Manufacturing. to relatively low tempera Co. in Milford, N.H.-to tures." This kind of new un develop commercial pro derstanding has opened the cessing routes for net-shape opportunity to make true forming and fabricate 'the metallic glasses in volume. club heads. In 1992, Caltech researchers In general, Tenhover said, Atakan Peker and William solid Vitreloy is formed di L. Johnson figured out how rectly from the hardening of make Vitreloy in bulk quan a cooled liquid. Each cross tities. The team, which was section of a part needs to funded by the U.S. Depart have a chunk of material ment of Energy and NASA, with a certain critical cool was trying to develop a new ing rate to keep the amor aerospace material. To slow phous state. "At least one di the metal's critical cooling mension of the part should rate to approximately 1°C to Liquidmetal putters have a "soft" feel when striking the ball, which the be less than 3 to 4 inches," 100°C per second, "we real- makers claim provides players with improved touch on putting greens. he said. "Ultimately, you're
ME HANI CA L ENGINEERING JUNE 1998 73 limited by the heat-transfer rate, the speed at which you serio us applications. Under a contract from the U.S. can get heat out of the bulk of the material." The largest Army Research Office, for example, AT! researchers are part made ofVitreloy weighs about 30 pounds. working to develop manufacturing-process technology While Hitchener uses an undisclosed processing route, for metallic-glass tank-annor penetrator rounds to re Howmet engineers adapted their proprietary metal mold place the current depleted uranium penetrators, which casting process for titanium to Vitreloy, said Don Larson, are suspected of biological toxicity. Another area of corporate program manager and metallurgical engineer. commercial interest is a highly biocompatible, nonaller "We use a vacuum die casting in which we inject the melt genic form of the glassy material that would be suitable [at 1,400°F] under high pressure in a high vacuum, forcing for medical components such as prosthetic implants and the material into the die cavity. It's like an automotive die surgical instruments. casting unit, except that the controlled atmosphere means Recent fatigue and fracture-toughness tests ofVitreloy, we have to melt smaller charges individually, which leads however, seem to cast some doubt on the new material's to a much slower cycle time. prospects for future use in tougher, industrial-type appli Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/120/06/72/6381561/me-1998-jun4.pdf by guest on 30 September 2021 "There's no mushy zone during solidification," Larson cations that require long-term performance beyond that added, "which makes shrinkage porosity less likely, as long demanded by sporting goods. Work conducted by as you keep the pressure on." Vitreloy is susceptible to Robert 0. Ritchie, professor of materials science at the 1- to 2-percent thermal shrinkage. The material "welds University of California, Berkeley, and head of the nicely to titanium or stainless steel using electron-beam or Structural Materials Department at Lawrence Berkeley laser-welding systems." It also takes a ex- tremely highly polished surface. 500
ON AND OFF THE LINKS Liquidmetal The density and strength-to-weight ratio 400 alloys 0 are not the only reasons golfers should be '§ excited about the Liquidmetal clubs, ac :E M" 300 Cl E cOl'ding to Tenhover. The low-modulus 'ID 3 material's "lower vibrational response B ~ c..
3 less shot dispersion on off-center hits. Density (g/cm ) The new clubs do not hit like rubber, however. The material's excellent rebound characteristics mean that less energy is The elevated strength-to-weight ratios exhibited by Liquidmetal alloys make the metallic glasses absorbed by the club head at impact, so promiSing for a range of high-performance applications. more energy is transferred to the ball, which translates to a longer carry. According to company Laboratory, confirmed that the glassy metal has high literature, steel club heads transfer about 60 percent of the fracture toughness, but he also concluded that "any re input energy to the ball and titanium tran:sfers 70 percent, crystallization of the material ca uses a dramatic drop in whereas the metallic glass transfers 99 percent. toughness." This could mean that applications at temper The new Liquidmetal clubs will be assembled in San atures above the glass-transition temperature are limited, Diego. The clubs, which are now being reviewed by U.S. at least regarding duration. Golf Association, are not cheap: Liquidmetal Golf has Furthermore, standard stress-strain fatigue tests show already placed its fully amorphous putters (with steel or that Vitreloy has an extremely low resistance to crack graphite shafts) on the market with a price tag of about initiation-the propagation of a crack once it has $400, Tenhover said. Irons with graphite shafts costing formed. "If this alloy does start to yield or fracture, it about $2,700 a set and $500 drivers with graphite shafts goes [fails] quickly," Ritchie said. should be available in August. Ritchie added that he and Caltech's Johnson are consid Sports enthusiasts may also soon find bulk metallic ering producing a more robust in situ composite by adding glasses like Vitreloy in other high-end sporting goods "a ductile crystalline phase" to the Vitreloy material to such as tennis rackets, baseball bats, bicycle frames, serve as "a crack-stopper, which would improve its damage hunting bows, and even edged tools such as axes. ATl tolerance." Of course, this alteration "could take away management stated that high-performance sporting some of the material's desirable properties," he noted. goods is only a first step in the market; the new family Nevertheless, these potential limitations should have little of materials could also be promising for other, more- effect on the performance of Liquidmetal golf clubs . •
74 J UNE 1998 M EC H AN IC AL ENG I NE ER I NG