Frederick G. Yost

Thermal Expansion and Elastic Properties of High Gold-Tin Alloys

The Au-Sn eutectic alloy, composed of the ζ and δ phases, has been used for many years in the sealing of hermetic parts. Recently, it was shown that the ζ phase undergoes an ordering transformation to ζ′. Since this transformation may affect hermeticity, measurements of density, linear thermal expansion coefficient, elastic moduli, and Poisson’s ratio were made on ζ-phase, eutectic, and δ-phase specimens. Measurements were made over a temperature range that includes the ζ → ζ′ transformation. Abrupt changes in these thermophysical properties, near the phase transformation temperature, are noted for ζ and eutectic specimens but not for the δ specimen.

High-temperature crystallization behaviour of amorphous Fe80B20

Samples of 0.003 in. round Fe80B20 amorphous wires were annealed in vacuo for 1 sec to 8 h periods at 780° C and the crystallinity induced in these wires from this heat treatment was studied through X-ray diffraction and field-ion microscopy. X-ray diffraction studies indicate that complete crystallinity is produced following 1 sec anneal at 780° C. However, the initial product is a primitive-tetragonal Fe3B phase unlike the body-centred tetragonal Fe3B observed in low-temperature isothermal transformation studies with this alloy. The Fe3B phase is seen to persist in the diffraction patterns for annealing durations of up to 15 min. Upon annealing for periods of up to 1 h, an intermediate three-phase structure consisting of α-Fe, Fe3B, and Fe2B is seen to result with a gradual decrease in the Fe3B phase corresponding to longer annealing durations. Anneals of more than 1 h at 780° C are seen to result in the disappearance of the Fe3B phase producing a two-phase microstructure consisting of α-Fe(b c c) and Fe2B (b c t). Field-ion-microscopy with a pure neon imaging gas at 78 K likewise indicates that existence of a three-stage phase structural change during the isothermal anneals, and the atomic arrangement of the various species are quite readily discernible because of the different symmetries contained in the three distinct phases.

On the Definition of Microhardness

Microhardness testing can be a very useful tool for studying modern materials, but is plagued by well-known experimental difficulties. Reasons for the unusual behavior of hardness data at very low loads are explored by Monte Carlo simulation. These simulations bear remarkable resemblance to the results of actual hardness experiments. The limit of hardness as load or indentation depth tends to zero is shown to depend on experimental error rather than upon intrinsic material properties. The large scatter of hardness data at very low loads is insured by the accepted definition of hardness. A new definition of hardness is suggested which eliminates much of this scatter and possesses a limit as indentation depth approaches zero. Some simple calculations are used to show the utility of this new approach to hardness testing.

Characterization of crystallization in Metglas 2826 MB alloy

The crystallization behaviour of the Metglas 2826 MB alloy (Fe40Ni38Mo4B18) has been studied using resistance measurements and X-ray diffraction techniques. Three annealing sequences were used to follow the process. Samples were annealed isothermally (a) at 780° C in a vacuum of 2×10−5 torr for times in the range 1 sec to 4 h, (b) for 2 h in an argon atmosphere at temperatures where the resistance curve indicated phase changes to occur, and (c) for 300 h in 100 torr of helium at 400, 600, 700 and 850° C. From these annealing sequences it was found that the alloy did not crystallize below 410° C and followed a crystallization process of: amorphous Fe40Ni38Mo4B18 → FexNi23−xB6 (cubic)+glassy matrix → FexNi23−xB6+(Fe, Ni) (FCC) → (Fe, Ni)3B(bct). This series of transformations was followed for Sequences (a) and (c) above, but was slightly different for Sequence (b). An orthorhombic (Fe, Ni)3 B phase was found in the samples annealed in a vacuum of 2×10−5 torr.

Soldering to gold films

Reliable solder joints can be made on gold metallised microcircuits using lead-indium solders providing certain important conditions are understood and carefully controlled. This paper reviews the three fundamental concepts of scavenging, wetting, and ageing, which are relevant when soldering to gold films.

Layer growth in Au-Pb/In solder joints

The solid state reaction between a Pb-In solder alloy and thin film Au has been investigated at ten aging temperatures ranging from 70 to 170°C. Also, bulk Au-solder samples were aged at 150°C for metallographic analysis. No significant difference was found between the aging behavior of thin and bulk Au specimens. A thin single phase layer of Au9In4 was found adjacent to Au while a thick two-phase layer of AuIn2 and Pb was found between Au9In4 and solder. The Pb phase was shown to have considerable mobility and able to ripen at room temperature. Peculiar planar interface instabilities and voids in the Au-Au9In4 interface were found. Although the total layer thicknessvs aging time data could be closely fitted with a power law relationship, it was shown that a linear relationship also fits well and is consistent with accepted metallurgical concepts. An activation energy of 0.61 eV was found by regression analysis of the intermetallic growth kinetics.

Annealing behaviour of amorphous Fe80B20 on continuous heating

Resistance measurements during direct heating of Fe80B20 amorphous alloys indicate phase changes occur at 395, 500, 720 and 840° C. Samples heated to these temperatures, and maintained for five minutes in a neutral atmosphere, show that a hardness maximum occurs at the crystallization temperature of 395° C and that annealing at 500° C produces a material with the same hardness. Above 500° C the microhardness is seen to drop below that of the amorphous alloy. Saturation magnetization measurements show a steady increase following each anneal, up to a temperature of 720° C, and the rate of increase is seen to drop in the range of 720 to 840° C. X-ray diffraction studies show that only a small fraction of the matrix is crystallized following the anneal at 395° C and the transformed phases are α-Fe and Fe3B. Following annealing at 500° C, an increased proportion of α-Fe and Fe3B are observed with complete crystallinity while samples heattreated at 720° C are seen to consist of a three-phase mixture of α-Fe, Fe23B6 and Fe2B. Annealing at 840° C is seen to produce an equilibrium phase mixture of α-Fe and Fe2B phases. Only in the sample annealed at 395° C is a fraction of the amorphous phase seen to persist, indicating that a 5 min anneal is not sufficient, at this temperature, to induce complete crystallization. These structural features are corroborated by field ion microscope analyses, made at liquid nitrogen temperature in a medium of pure neon, and scanning electron microscopy, and are also consistent with our earlier study involving the isothermal annealing, for various times, of Fe80B20 alloy at 780° C.

On the use of Ti50Be40Zr10 as a spring material

A useful figure-of-merit for spring material candidates is derived and computed for a select group of iron and titanium alloys. Two commercially available amorphous alloys are shown to have considerably more attractive figures-of-merit than do several crystalline alloys. Since springs left in a stressed condition must resist stress relaxation, tests of this type were performed on an amorphous titanium alloy. The results indicate that relaxation kinetics depend upon initial heat treatment but are independent of initial applied stress. This behaviour is shown to be consistent with hyperbolic flow in the limit of low stress. A simple least-squares analysis of the relaxation kinetics yields an activation energy of 67 kJ mol−1. When used for springs, this alloy should be heat treated.

The Au-In-Pb system:The AuIn2-In-Pb portion

The Au-In-Pb system was investigated in the AuIn-In-Pb portion of the system. Pb-AuIn and Pb-AuIn2 quasibinary systems were found:the first, consisting of a eutectic and a monotectic reaction and the second, a eutectic with an inflection in the liquidus. The chief source of information was differential thermal analyses and verification by metallography, X-ray diffraction and microprobe analyses. Four isothermal reactions were found as follows: 1) L1 ⇌ L2 + AuIn2 + AuIn at 433°Celsius 2) L1 ⇌ Pb + AuIn2 + AuIn at 316°Celsius 3) Pb + L ⇌ α + AuIn2 at 172°Celsius 4) α1 + L ⇌ AuIn2 + In at 159°Celsius. Verification of these events and the isopleth from 50-50 wt pct Pb-In, a typical solder, to ≈ 47 wt pct Au are given.

The triple line in reactive wetting

The classical method for specifying the conditions of capillary equilibrium has been used to describe the wetting line in a simple reactive system. The results, being Young-Neumann equations for the contact angles, are compared to suggestions made by Landry et al. It was shown that under specific circumstances the suggestions made by Landry et al. agree with the classical approach. A discussion was also presented regarding the complexity of real capillary microstructures and their very simplistic theoretical description. An approach that considers microstructural defects and can distinguish metastable and true equilibrium states in complex systems is needed.