G. B. Schaffer

Displacement reactions during mechanical alloying

The occurrence of simple solid-state displacement reactions during mechanical alloying has been investigated. The reduction of cupric oxide to pure copper metal by a variety of metallic reducing agents was studied, and the powders were examined by X-ray diffractometry and electron microscopy. When milled with a liquid process control agent, the reaction progressed gradually with time, whereas an unstable combustion reaction occurred when no such control agent was employed. A minimum adiabatic temperature of 1300 K is necessary for combustion to occur in these systems. The reaction enthalpy is an important factor in determining the precombustion period. The as-milled powders consisted of finely divided, nanometer-sized crystallites with an extremely high defect density. It is proposed that the increased reactivity of the system arises through the unique conditions prevailing during mechanical alloying.

Reduction of metal oxides by mechanical alloying

The chemical reduction of metal oxides by mechanical alloying with a strong reducing element has been investigated. Using x‐ray diffraction to follow the reaction it was found that the mechanical alloying of CuO and Ca using toluene as a processing lubricant resulted in the formation of Cu. The mechanical alloying of CuO and ZnO together with Ca resulted in the formation of β’ brass.

Liquid phase sintering of aluminium alloys

The principle that alloys are designed to accommodate the manufacture of goods made from them as much as the properties required of them in service has not been widely applied to pressed and sintered P/M aluminium alloys. Most commercial alloys made from mixed elemental blends are identical to standard wrought alloys. Alternatively, alloys can be designed systematically using the phase diagram characteristics of ideal liquid phase sintering systems. This requires consideration of the solubilities of the alloying elements in aluminium, the melting points of the elements, the eutectics they form with aluminium and the nature of the liquid phase. The relative diffusivities are also important. Here we show that Al-Sn, which closely follows these ideal characteristics, has a much stronger sintering response than either Al-Cu or Al-Zn, both of which have at least one non-ideal characteristic

On the kinetics of mechanical alloying

The kinetics of solid-state displacement reactions during mechanical alloying have been investigated. The effects of charge ratio and ball size on the progress of the reaction between CuO and Fe have been evaluated from measurements of ignition temperature, combustion time, and crystallite size. The reaction kinetics are shown to increase with charge ratio. This is rationalized in terms of the effect of charge ratio on the number of ball/particle collisions. Ball size influences reaction kinetics through both the particle collision frequency and collision energy.

An age hardening model for Al–7Si–Mg casting alloys

Yield strength (YS) ageing curves have been modelled for A356 and A357 aluminium casting alloys below the solvus temperature of the main hardening precipitate. Predictions are based on the Shercliff and Ashby methodology (Acta Metall. Mater. 38 (1990) 1789) for wrought alloys. Differences between strengthening in wrought and cast Al–Si–Mg alloys are considered. A Brinell hardness to YS conversion incorporating strain hardening has been established to enable YS ageing curves to be predicted with reduced experimental effort.

The effect of solubility and particle size on liquid phase sintering

Three binary systems were examined: Al-Sn, Al-Zn, and Al-Cu. Starting additive powders were either <45 {micro}m in size or 125--150 {micro}m. The aluminium powder was air atomized, passed through a 100 mesh screen and had an average particle size (d{sub 50}) of 60{micro}m. The use of fine additive powders in systems having a substantial transient aspect results in a greatly reduced quantity of liquid phase formed during sintering. Conversely, the use of coarse powders increases the amount of liquid phase that forms and prolongs its existence during sintering. This occurs in systems having appreciable solid solubility of the additive in the base and/or exhibiting preferential diffusive flow from the additive to the base. Coarser powders enhance sintering in such systems. Where there is no mutual solid solubility, particle size is unimportant in liquid development.

The effect of trace elements on the sintering of Al-Cu alloys

Trace additions of Sn, In, Bi, Sb and Pb have been used to activate the liquid phase sintering of an Al-4Cu-0.15Mg alloy. Additions of as little as 0.05 wt% (similar to 0.01 at.%) increases the sintered density from 88 to 92% of the theoretical density. The elements which aid sintering have both high vacancy binding energies and high diffusivities in Al. It is suggested that the trace element diffuses into the Al, and forms trace element-vacancy clusters. This reduces the diffusivity of the Cu in the Al matrix, delaying Cu dissolution therefore causing the liquid to persist for longer times. This enhances sintering and therefore densification

The effect of trace elements on the sintering of an Al-Zn-Mg-Cu alloy

Trace elements can have a significant effect on the processing and properties of aluminium alloys, including sintered alloys. As little as 0.07 wt% (100 ppm) lead, tin or indium promotes sintering in an Al-Zn-Mg-Cu alloy produced from mixed elemental powders. This is a liquid phase sintering system and thin liquid films form uniformly throughout the alloy in the presence of the trace elements, but liquid pools develop in their absence. Analytical transmission electron microscopy indicates that the trace elements are confined to the interparticle and grain boundary regions. The sintering enhancement is attributed to the segregation of the microalloying addition to the liquid-vapour interface. Because the microalloying elements have a low surface tension, they lower the effective surface tension of the liquid. This reduces the wetting angle and extends the spreading of the liquid through the matrix. An improvement in sintering results

On the use of trace additions of Sn to enhance sintered 2xxx series Al powder alloys

The mechanical properties of a typical sintered aluminium alloy (Al-4.4Cu-0.8Si-0.5Mg) have been improved by the simultaneous use of trace additions of Sn, high sintering temperatures and modified heat treatments. Tin increases densification, but the Sn concentration is limited to less than or equal to 0.1wt% because incipient melting occurs during solution treatment at higher Sn levels. A sintering temperature of 620 degrees C increases the liquid volume over that formed at the conventional 590 degrees C sintering temperature. However, the higher sintering temperature results in the formation of an embrittling phase which can be eliminated if solution treatment is incorporated into the sintering cycle (a modified TS heat treatment). These conditions produce a tensile strength of 375 MPa, an increase of nearly 20% over the unmodified alloy

On development of sintered 7xxx series aluminium alloys

AbstractThe sintering characteristics and the tensile properties of the 7000 series Al–Zn–Mg–Cu alloys, fabricated using elemental powders in a conventional press and sinter powder metallurgy process, are examined. Microalloying with 100 ppm of lead or tin enhances the sintering response of these alloys significantly, with a corresponding increase in the tensile strength. The system has aspects of both transient and supersolidus liquid phase sintering. Zinc melts and eutectic liquids form during heating to the sintering temperature but these liquid phases are absorbed by the aluminium on further heating. Sintering above the solidus induces the formation of additional liquid. Because it has aspects of both transient and supersolidus liquid phase sintering, the system is extremely sensitive to process variables, including particle size, sintering temperature, and heating rate, but insensitive to green density. When the alloy composition and the process variables are optimised, tensile strengths in excess of...