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# Metallic Glasses Bulk UpPUBLIC ACCESS

A New Formulation for Metallic Glass, Now Being Vacuum-Die-Cast into Golf Clubs, will Improve the Production of this High-Performance Material in Bulk Form.

[+] Author Notes

Associate Editor

Mechanical Engineering 120(06), 72-74 (Jun 01, 1998) (2 pages) doi:10.1115/1.1998-JUN-4

## Abstract

Vitreloy, a metallic–glass alloy of zirconium and beryllium from Amorphous Technologies International (ATI) in Laguna Niquel, CA, is extensively being used in golf clubs. Liquidmetal golf-club heads are fabricated from an amorphous metal alloy that has excellent rebound and vibration-absorbing properties. Research indicates that the prized physical properties of metallic glasses arise in large part from their lack of grain boundaries, which can serve as points of weakness. Liquidmetal putters have a ‘soft’ feel when striking the ball, which the makers claim provides players with improved touch on putting greens. The elevated strength-to-weight ratios exhibited by Liquidmetal alloys make the metallic glasses promising for a range of high-performance applications. Sports enthusiasts may also soon find bulk metallic glasses like Vitreloy in other high-end sporting goods such as tennis rackets, baseball bats, bicycle frames, hunting bows, and even edged tools such as axes. ATl management stated that high-performance sporting goods are only a first step in the market; the new family of materials could also be promising for other, more serious applications.

## Article

Whenever a new high-strength material is developed these days, its first real-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-zuno 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. This new metal offers high strength- to-weight ratios and hardness, extreme springiness and rebound characteristics, and good acoustic-dampening properties. According to company representatives, it could become "the next big thing in golf clubs."

Developed at the California Institute of Technology in Pasadena, the metal alloy known as Vitreloy-or Liquidmetal when used in golf clubs-is approximately two-thirds zirconium, one-fifth beryllium, and the remainder split among copper, titanium, and nickel. Conventional metals have a crystalline structure in which the atoms are lined up in neat, orderly arrays; the bulk of these standard metals typically consists of small regions of aligned atoms, called grains, and the boundaries between them. Within the metallic glass, however, the five types of atoms are packed together in a somewhat random fashion, similar to that of a liquid. Research indicates that the prized physical properties of metallic glasses arise in large part from their lack of grain boundaries, which can serve as points of weakness.

In the past, producing the high- performance alloy in bulk has proven difficult. Unlike conventional metals, which are usually cooled slowly until they fully solidify, metallic glasses must be cooled very rapidly and very uniformly to freeze their random atomic pattern in place before crystallization occurs due to the nucleation and growth of crystal grains. Four decades ago, when applications of t1Lis phenomenon were first being explored, the only way to e},.'tract heat fast enough to n1.aintain the metal's random state was to keep the metastable material very thin through special techniques such as splat cooling, in which droplets of molten metal of quick-frozen on a cold surface. Continuous amorphous metal ribbons less than 0.1 millimeter thick could also be formed, at a cooling rate of 1 million°C per second, by pouring molten metal onto a cold, spinning wheel.

In the early 1970s, for example, AlliedSignal Inc. in Morristown, N.J., marketed a metallic-glass ribbon called Metglas for use in high-efficiency electrical transformers. The ferrous ribbon's lack of crystal defects translated into desirable magnetic properties. The increased fabrication costs due the ribbon-winding fabrication operation and other issues stalled the transformers' progress.

Beyond wear-resistant coatings, metallic glass found little application in the meantime. In 1991, ATI commercialized its first successful product: Armacore, a hard amorphous coating that protects drill pipe as it moves through oil-well casings.

Liquidmetal golf-club heads are fabricated from an amorphous metal alloy (on the bottom) that has excellent rebound and vibration-absorbing properties.

## Bulk Fabrication

Over the past decade, methods have been developed to produce metallic glasses in bulk. A leader in this field is Akihisa Inoue's research group at Tohoku University in Sendai, Japan, which has developed an array of bulk metallic-glass systems based on zirconium, magnesium, aluminum, and iron. Early approaches to bulk fabrication were mostly empirical in nature, but researchers gradually began to understand that the correct choice of elemental constituents would lead to amorphous metals amenable to cooling rates as slow as 1°C to 100°C per second. These slower cooling rates mean that large parts can be fabricated. Furthermore, many of these metallic glasses remain stable against crystallization when heated to temperatures somewhat above their glass-transition temperatures.

According to Todd Hufnagel, who studies metallic glasses as an assistant professor of materials science at Johns Hopkins University in Baltimore, "one of the general guiding principles to designing alloys that form bulk metallic glasses is to pick elements with dramatic differences in size, which leads to a complicated structure" that crystallizes less easily; a beryllium atom, for example, is much smaller than a zirconium atom. Another effective step is "to look for alloy compositions with deep eutectics," he said, "which form liquids that are stable to relatively low temperatures." This kind of new understanding has opened the opportunity to make true metallic glasses in volume.

In 1992, Caltech researchers Atakan Peker and William L. Johnson figured out how make Vitreloy in bulk quantities. The team, which was funded by the U.S. Department of Energy and NASA, was trying to develop a new aerospace material. To slow the metal's critical cooling rate to approximately 1°C to 100°C per second, "we realized we had to go to a very complicated material," said Johnson the Ruben and Donna Mettler Professor of Engineering and Applied Science at Caltech. "We needed to have four or more atoms with different chemical characteristics before the liquid gets sufficiently frustrated in its efforts to crystallize." The five-element mixture glassifies as needed only over a very narrow range of compositions.

## Golf Anyone?

Soon after this method was developed, ATI negotiated an exclusive worldwide license for the Caltech material. "Right from the start, our focus was on golf because the combination of properties [Vitreloy] offered made it an overwhelming choice for a golf material," said Michael Tenhover, ATI's vice president of product development and chief technical officer of Liquid metal Golf. For example, Vitreloy has an almost ideal density for golf irons and putters-6.1 grams per cubic centimeter, which lies in between the specific gravity of stainless steel and titanium.

"The weight of a golf club is fixed by the rules," Tenhover said, "so their overall shape is dominated by the material density. Furthermore, the amorphous metal's high strength-to-weight ratio allows the mass to be distributed differently within the club." The combination of the right density and its high yield strength of275,000 pounds per square inch would contribute to the club's unique appearance and performance.

The company first had to determine which manufacturing process could be used to make the shapes necessary for golf clubs. At the time, the Caltech researchers were making 1- to 2-gram samples. Casting a club head is difficult, said Johnson, who noted that the melt is orders of magnitude more viscous than most molten metals. "It's more like a thermosetting polymer; it doesn't flow •very well." AT! has since made arrangements with two well-known metal- casting companies-Howmet Metal Mold in Whitehall, Mich., which is part of Howmet Specialty Products; and Hitchener Manufacturing. Co. in Milford, N.H. to develop commercial processing routes for net-shape forming and fabricate 'the club heads.

In general, Tenhover said, solid Vitreloy is formed directly from the hardening of a cooled liquid. Each cross section of a part needs to have a chunk of material with a certain critical cooling rate to keep the amorphous state. "At least one dimension of the part should be less than 3 to 4 inches," he said. "Ultimately, you're limited by the heat-transfer rate, the speed at which you can get heat out of the bulk of the material." The largest part made of Vitreloy weighs about 30 pounds.

While Hitchener uses an undisclosed processing route, Howmet engineers adapted their proprietary metal mold-casting process for titanium to Vitreloy, said Don Larson, corporate program manager and metallurgical engineer. "We use a vacuum die casting in which we inject the melt [at 1,400°F] under high pressure in a high vacuum, forcing the material into the die cavity. It's like an automotive die-casting unit, except that the controlled atmosphere means we have to melt smaller charges individually, which leads to a much slower cycle time.

"There's no mushy zone during solidification," Larson added, "which makes shrinkage porosity less likely, as long as you keep the pressure on." Vitreloy is susceptible to 1- to 2-percent thermal shrinkage. The material "welds nicely to titanium or stainless steel using electron-beam or laser-welding systems." It also takes a extremely highly polished surface.

Liquidmetal putters have a "soft" feel when striking the ball, which the makers claim provides players with improved touch on putting greens.

## On and Off the Links

The density and strength-to-weight ratio are not the only reasons golfers should be excited about the Liquidmetal clubs, according to Tenhover. The low-modulus material's "lower vibrational response provides a softer, more solid feel" than competing clubs when a golfer strikes the ball. "People say it's like hitting with rubber; basically, the golfer feels only the primary frequency caused by the impact." The clubs are also very forgiving, he added, providing better control and less shot dispersion on off-center hits.

The new clubs do not hit like rubber, however. The material's excellent rebound characteristics mean that less energy is absorbed by the club head at impact, so more energy is transferred to the ball, which translates to a longer carry. According to company literature, steel club heads transfer about 60 percent of the input energy to the ball and titanium transfers 70 percent, whereas the metallic glass transfers 99 percent.

The new Liquidmetal clubs will be assembled in San Diego. The clubs, which are now being reviewed by U.S. Golf Association, are not cheap: Liquidmetal Golf has already placed its fully amorphous putters (with steel or graphite shafts) on the market with a price tag of about $400, Tenhover said. Irons with graphite shafts costing about$2,700 a set and \$500 drivers with graphite shafts should be available in August.

Sports enthusiasts may also soon find bulk metallic glasses like Vitreloy in other high-end sporting goods such as tennis rackets, baseball bats, bicycle frames, hunting bows, and even edged tools such as axes. ATI management stated that high-performance sporting goods is only a first step in the market; the new family of materials could also be promising for other, more- serious applications. Under a contract from the U.S. Army Research Office, for example, ATI researchers are working to develop manufacturing-process technology for metallic-glass tank-armor penetrator rounds to replace the current depleted uranium penetrators, which are suspected of biological toxicity. Another area of commercial interest is a highly biocompatible, nonallergenic form of the glassy material that would be suitable for medical components such as prosthetic implants and surgical instruments.

Recent fatigue and fracture-toughness tests of Vitreloy, however, seem to cast some doubt on the new material's prospects for future use in tougher, industrial-type applications that require long-term performance beyond that demanded by sporting goods. Work conducted by Robert 0. Ritchie, professor of materials science at the University of California, Berkeley, and head of the Structural Materials Department at Lawrence Berkeley Laboratory, confirmed that the glassy metal has high fracture toughness, but he also concluded that "any recrystallization of the material causes a dramatic drop in toughness." This could mean that applications at temperatures above the glass-transition temperature are limited, at least regarding duration.

Furthermore, standard stress-strain fatigue tests show that Vitreloy has an extremely low resistance to crack-initiation- the propagation of a crack once it has formed. "If this alloy does start to yield or fracture, it goes [fails] quickly," Ritchie said.

Ritchie added that he and Caltech's Johnson are considering producing a more robust in situ composite by adding "a ductile crystalline phase" to the Vitreloy material to serve as "a crack-stopper, which would improve its damage tolerance." Of course, this alteration "could take away some of the material's desirable properties," he noted. Nevertheless, these potential limitations should have little effect on the performance of Liquid metal golf clubs.

The elevated strength-to-weight ratios exhibited by Liquidmetal alloys make the metallic glasses promising for a range of high-performance applications.

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