| Ferritic based alloys are widely used in the aerospace,marine,automotive and energy industries due to their high strength,high thermal conductivity,low density and low thermal expansion.Body centered cubic（bcc）-based ordered B2-type NiAl precipitates have been employed to strengthen the bcc ferriticα-Fe matrix greatly for their similar lattice parameters closing toα-Fe and the advantage of high volume fraction and small particle size.moreover,nanosized oxide can extend the high-temperature strength limit of ferritic alloys due to its good thermal stability near the melting point.therefore,in the present work,we investigate the effects of alloying elements、fabrication methods and heat treatment measures on the microstructure and properties in the purpose of providing elementary experimental materials for further explore on ferritic alloy.In this paper,the main research processes are as follows:Two alloys with compositions of Fe-5Ni-1Al-2Mn and Fe-10Cr-5Ni-1Al-2Mn（wt.%）alloy were prepared by arc-melting a mixture of metals in a Ti-gettered high-purity argon atmosphere.The heat-treated samples were analyzed by OM,XRD,SEM,EDS,TEM,micro Vickers hardness tester and electrochemical workstation.The alloy is mainly strengthened by NiAl and Ni（Al,Mn）nano-scale precipitation phase.The addition of Cr element can adjust the lattice constant of the ferrite matrix and reduce the lattice mismatch ratio between the matrix and the precipitation phase;The research results indicate that the optimum aging time and hardness of the Cr-free alloy at 550℃are 2 h and 453.4 HV,respectively.The optimum aging time and hardness of the 10 wt.%Cr alloy at 550℃are 0.5 h and 523.2 HV,respectively;The addition of 10 wt.%Cr can increase the corrosion resistance and reduce the corrosion rate of the alloy effectively.The nominal composition of Fe-10Cr-5Ni-1Al-2Mn-0.6Y2O3（wt.%）alloy powder was fabricated via mechanical alloying（MA）using pure Cr powder,Fe-55Cr（wt.%） master alloyed powder and Fe-10Cr-5Ni-1Al-2Mn（wt.%） prealloyed powder as raw materials.The MA powder was consolidated by spark plasma sintering（SPS）.The alloy powder during mechanical alloying and the heat-treated sample are analyzed by means of OM,XRD,SEM,EDS,micro Vickers hardness tester,etc.The three powder mixture was mechanical alloyed at a rotation speed of 300 rpm with the ball/powder weight ratio of 8:1.When the three alloy powders were ball milled for 2h,2 h and 4 h,the mean particle size of the alloy powder reached the maximum value,42.02μm,43.76μm and 67.35μm,respectively;When the milling time reached 60 h,the mean particle size of powder reached the smallest value,9.19μm,9.51μm and 36.87μm,respectively.Continue to extended the ball milling time,the powder would agglomerate,so the best ball milling time was 60 h.As the increasing of ball milling time,the grain size of powder decreased continuously and eventually stabilized.The final grain sizes of the three alloy powders are 9 nm,10.2 nm and 12.4nm,respectively;The lattice distortion of the alloy powder increased with the increasing of milling time,and the lattice distortions value of the three alloy powders were 0.84%,0.9% and 0.73%,respectively.The preparation of supersaturated alloy powder made by pure metal powder and master alloyed powder did no effect on the densification process of SPS;After mechanical alloying process,the densification process of prealloyed powder in the SPS sintering process can be significantly improved,and the density of the alloy can be improved simultaneously.The densities of the three sintered samples were 94.16%,93.57% and 97.33%,respectively.As the aging time prolonged,the microstructure did not change significantly,and the pores in the alloy grew and spheroidized;After solution treatment,the hardness of the alloy reached the largest value,respectively 531.5 HV,526.9 HV and 718.3 HV;The as-aged alloys reached the maximum hardness value at 2 h,4 h and 10 h,respectively,which were 418.3 HV,435 HV and 546.6 HV. |
PDF Full Text Request