Study On Solidfication Microstructure And Heat Treatment Of Al-3Fe-xNi Alloys

Rodlike nanoscale Al6Fe metastable phase can be obtained by cooling in iron mould for Al-3Fe alloy.However,it’s easy to transform lath-shaped Al3Fe phase which will reduce the mechanical properties by splitting matrix at higher temperatures,so the development of Al-Fe alloy as a new heat-resistant aluminum alloy is limited.As a transitional element,Ni can form a variety of intermetallic compounds with good thermal stability with Al and Fe,It is of positive significance to improve the heat resistance of Al-Fe alloy.The solidification structure and characteristics of Al-3Fe-0.5Ni and Al-3Fe-Ni alloys under slowly cooling and cooling in iron mould were investigated by means of on-line temperature measurement and microstructure analysis in this paper,and the samples obtained by cooling in iron mould were heat treated at 550℃、625℃、640℃for 2h、6h、24h,respectively.The main conclusions are as follows:(1)Al-3Fe-xNi alloys are slowly cooled in the crucible and obtain the lath-shaped primary Al3Fe phase and Al-Al3Fe-Al9FeNi three phases eutectic structure.With increase of Ni content,the number of fibrous,punctate Al9FeNi phases increase obviously,The eutectic temperatures of Al-3Fe-0.5Ni and Al-3Fe-Ni alloys were measured to be 653.8℃ and 653.3℃,respectively.(2)Al-3Fe-xNi alloys which are cooled in the iron mould mainly contain eutectic structure of Al-Al9)FemNi2-m(0<m<2)which peform rod-like or plate,there are also a few of cluster halos with primary Al3Fe phases at the same time.The α-Al dendrite at the edge occurs stratification phenomenon.The eutectic temperatures of Al-3Fe-0.5Ni and Al-3Fe-Ni alloys were measured to be 638.6℃ and 643.5℃,respectively.(3)The microhardness of Al-3Fe-xNi alloys in the center is higher than that of the 1/2 radius.With increase of Ni content,the microhardness of Al-3Fe-xNi alloys solidification microstructure increase gradually.(4)With increase of heating temperature or holding time,the outline of α-Al dendrite in the edge becomes more blurred,eventually disappear.Eutectic structure between interdendritics gradually transforms into globular Al9FemNi2-m phase and lath-shaped Al3Fe phase.The phenomenon of Ostwald ripening occurs for eutectic Al9FemNi2-m phase of Al-3Fe-xNi alloys in the center and 1/2 radius,and lath-shaped Al3Fe phases are precipitated simultaneously.The precipitated phases have priority to exist in grain boundary and form Precipitation Free Zone(PFZ)when the heating temperature is lower or the holding time is shorter,it is shown that the sensitivity of the PFZ width to the temperature is greater than that of time.When the temperature is higher or the holding time is longer,the precipitation of Al3Fe increases and gradually coarsens up.(5)The change of temperature and time in the heat treatment process all do not change the total phase species of Al-3Fe-xNi alloys.The ratio of Fe and Ni atoms decreases gradually in Al9FemNi2-m phase and redundant Fe atoms generate lath-shaped Al3Fe phase.The number of Al3Fe phases of Al-3Fe-Ni alloy is much less than that of Al-3Fe-0.5Ni alloy in the heat treatment process.The stability of Al9FemNi2-m phases is related to the value of "m" the smaller the value of "m" and the closer to 1,the better the thermal stability.