Microstructure And Properties Of Low Density Heat-resistant Fe-based Alloy

The cost of high-temperature iron-based alloy is relatively low,and the high-temperature mechanical properties are excellent.Therefore,it is widely used in aerospace,ship,special vehicles,petrochemical and many other fields.However,the density of iron-based alloys is relatively large and the weight-reducing content is limited,which does not meet the requirements for lightweight equipment in more demanding environments.Therefore,it is urgent to develop a new way to research low-density high-strength heat-resistant iron-based alloys.The addition of a low-density reinforcing phase(such as an intermetallic compound)to an iron-based alloy can significantly reduce the density of the alloy while maintaining good mechanical properties,thereby increasing the specific strength of the ferroalloy.This kind of research idea has been initially verified in domestic and foreign research,and has been highly valued by low density and high mechanical properties of ferroalloy researchers.In this paper,B2-NiAl powder was prepared by mechanical alloying method.Iron powder and NiAl powder with 30wt.%,40wt.%,50wt.%,60wt.%contents were mechanically mixed,respectivly.Then NiAl/Fe alloy bulk samples(?30mm)with different contents of B2 structure NiAl reinforcing phase were prepared by vacuum hot pressing sintering method.Subsequently,the NiAl/Fe alloy with excellent microstructure and properties was selected as the matrix,and different amounts of synthetic diamond and AlN reinforcing phase were added,respectively.The bulk samples are prepared by hot pressing sintering.The morphology composition and phase analysis of the alloy were carried out by scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),X-ray diffraction(XRD)and transmission electron microscopy(TEM).The hardness,density,room temperature compression strength and thermal conductivity at different temperatures of the iron-based alloy were tested.The effects of alloy composition,reinforcement distribution and bonding degree of reinforcing phase and matrix on the properties of the alloy were also analyzed.The main results and conclusions are summarized as follows:(1)The NiAl phase retains B2 structure in the NiAl/Fe bulk alloy prepared by hot press sintering.The hardness,compressive strength and thermal conductivity of NiAl/Fe alloy increase first and then decrease with NiAl content increasing.When the content of NiAl is50wt.%,the density of the iron-based alloy is lower,and the mechanical properties and thermal conductivity are excellent.Compared to pure iron,the density(6.44 g/cm~3)is reduced by14.5%.The room temperature compressive strength reach to 2530 MPa,which is 239.6%higher than that of pure iron;the thermal conductivity at 600?reached to 32.5 W/(m·k).(2)The bonding property between micron-sized diamond and substrate is obviously improved after diamond pretreatment(pickling then heat treatment at 1000?).After pretreatment,the compressive strength of the diamond-reinforced NiAl/Fe alloy is 373.28%higher than that of the untreated diamond-reinforced alloy,reaching 997.2 MPa.With the diamond content increasing,the density of the alloy decreases gradually.When the diamond content is 10%,the density of the iron-based alloy drops to 5.8 g/cm~3.The hardness,compressive strength and thermal conductivity of the alloy increase first and then decrease with the increase of diamond content.The hardness and compressive strength reach the maximum when the diamond content is 10%,which is 44.5HRC and 1543.5 MPa respectively.The thermal conductivity of the alloy reaches the maximum at each temperature when the diamond content is 15wt.%,and the thermal conductivity is 53.2 W/(m·k)at 600?.(3)AlN particles disperse uniformly in the sintered matrix,which prevents dislocation movement and grain growth,thus fining grain and increasing material hardness.When the AlN content is 5%,AlN-reinforced NiAl/Fe alloy with relatively uniform structure can be obtained.The hardness reaches the maximum value of 59.5HRC,and the density decreases to6.14g/cm~3.The compressive strength of the alloy increases first and then decreases with the increase of AlN content,reaching a maximum of 2220.5 MPa when the AlN content is 5%.The thermal conductivity of the alloy increases first and then decreases with AlN content increasing.When the AlN content is 10wt.%,the thermal conductivity of the alloy reaches the maximum,and the value of thermal conductivity is 34.7W/(m·k)at 600?.