This invention relates to amorphous metallic alloys, commonly referred to metallic glasses, which are formed by solidification of alloy melts by cooling the alloy to a temperature below its glass transition temperature before appreciable homogeneous nucleation and crystallization has occurred.
There has been appreciable interest in recent years in the formation of metallic alloys that are amorphous or glassy at low temperatures. Ordinary metals and alloys crystallize when cooled from the liquid phase. It has been found, however, that some metals and alloys can be undercooled and remain as an extremely viscous liquid phase or glass at ambient temperatures when cooled sufficiently rapidly. Cooling rates in the order of 10.sup.4 to 10.sup.6 K/sec are typically required.
To achieve such rapid cooling rates, a very thin layer (e.g., less than 100 micrometers) or small droplets of molten metal are brought into contact with a conductive substrate maintained at near ambient temperature. The small dimension of the amorphous material is a consequence of the need to extract heat at a sufficient rate to suppress crystallization. Thus, previously developed amorphous alloys have only been available as thin ribbons or sheets or as powders. Such ribbons, sheets or powders may be made by melt-spinning onto a cooled substrate, thin layer casting on a cooled substrate moving past a narrow nozzle, or as "splat quenching" of droplets between cooled substrates.
Appreciable efforts have been directed to finding amorphous alloys with greater resistance to crystallization so that less restrictive cooling rates can be utilized. If crystallization can be suppressed at lower cooling rates, thicker bodies of amorphous alloys can be produced.
The formation of amorphous metallic alloys always faces the difficult tendency of the undercooled alloy melt to crystallize. Crystallization occurs by a process of nucleation and growth of crystals. Generally speaking, an undercooled liquid crystallizes rapidly. To form an amorphous solid alloy, one must melt the parent material and cool the liquid from the melting temperature T.sub.m to below the glass transition temperature T.sub.g, without the occurrence of crystallization.
FIG. 1 illustrates schematically a diagram of temperature plotted against time on a logarithmic scale. A melting temperature T.sub.m and a glass transition temperature T.sub.g, are indicated. An exemplary curve a indicates the onset of crystallization as a function of time and temperature. In order to create an amorphous solid material, the alloy must be cooled from above the melting temperature through the glass transition temperature without intersecting the nose of the crystallization curve. This crystallization curve a represents schematically the onset of crystallization on some of the earliest alloys from which metallic glasses were formed. Cooling rates in excess of 10.sup.5 and usually in the order of 10.sup.6 have typically been required.
A second curve b in FIG. 1 indicates a crystallization curve for subsequently developed metallic glasses. The required cooling rates for forming amorphous alloys have been decreased one or two, or even three, orders of magnitude, a rather significant decrease. A third crystallization curve c indicates schematically the order of magnitude of the additional improvements made in practice of this invention. The nose of the crystallization curve has been shifted two or more orders of magnitude toward longer times. Cooling rates of less than 10.sup.3 K/s and preferably less than 10.sup.2 K/s are achieved. Amorphous alloys have been obtained with cooling rates as low as two or three K/s.
The formation of an amorphous alloy is only part of the problem. It is desirable to form net shape components and three dimensional objects of appreciable dimensions from the amorphous materials. To process and form an amorphous alloy or to consolidate amorphous powder to a three dimensional object with good mechanical integrity requires that the alloy be deformable. Amorphous alloys undergo substantial homogeneous deformation under applied stress only when heated near or above the glass transition temperature. Again, crystallization is generally observed to occur rapidly in this temperature range.
Thus, referring again to FIG. 1, if an alloy once formed as an amorphous solid is reheated above the glass transition temperature, a very short interval may exist before the alloy encounters the crystallization curve. With the first amorphous alloys produced, the crystallization curve a would be encountered in milliseconds and mechanical forming above the glass transition temperature is essentially infeasible. Even with improved alloys, the time available for processing is still in the order of fractions of seconds or a few seconds.
FIG. 2 is a schematic diagram of temperature and viscosity on a logarithmic scale for amorphous alloys as undercooled liquids between the melting temperature and glass transition temperature. The glass transition temperature is typically considered to be a temperature where the viscosity of the alloy is in the order of 10.sup.12 poise. A liquid alloy, on the other hand, may have a viscosity of less than one poise (ambient temperature water has a viscosity of about one centipoise).
As can be seen from the schematic illustration of FIG. 2, the viscosity of the amorphous alloy decreases gradually at low temperatures, then changes rapidly above the glass transition temperature. An increase of temperature as little as 5.degree. C. can reduce viscosity an order of magnitude. It is desirable to reduce the viscosity of an amorphous alloy as low as 10.sup.5 poise to make deformation feasible at low applied forces. This means appreciable heating above the glass transition temperature. The processing time for an amorphous alloy (i.e., the elapsed time from heating above the glass transition temperature to intersection with the crystallization curve of FIG. 1) is preferably in the order of several seconds or more, so that there is ample time to heat, manipulate, process and cool the alloy before appreciable crystallization occurs. Thus, for good formability, it is desirable that the crystallization curve be shifted to the right, i.e., toward longer times.
The resistance of a metallic glass to crystallization can be related to the cooling rate required to form the glass upon cooling from the melt. This is an indication of the stability of the amorphous phase upon heating above the glass transition temperature during processing. It is desirable that the cooling rate required to suppress crystallization be in the order of from 1 K/s to 10.sup.3 K/s or even less. As the critical cooling rate decreases, greater times are available for processing and larger cross sections of parts can be fabricated. Further, such alloys can be heated substantially above the glass transition temperature without crystallizing during time scales suitable for industrial processing.
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