| High entropy alloy,with excellent properties,is a new type of alloy developed in recent years and has a wide prospect of application.Most of the as-cast high entropy alloys are dendritic and have some defects,such as segregation,porosity and coarse grains.As an effective means to refine the grain and control the microstructure and properties of the alloy,thermomechanical treatment and strong magnetic field conditions can greatly improve the microstructure and properties of as-cast high entropy alloys and play an active role in the research and application of high entropy alloys.In this thesis,the Al0.3CoCrFeNi high entropy alloy ingots obtained by induction melting was cold-rolled to get 90%and 95%reduction after a homogenization at 1200? for 10 hours.Finally,the cold-rolled samples were annealed at 600-1000? without and with high magnetic field for 1h.The X-ray diffraction(XRD),electron back scattering diffraction pattern(EBSD),hardness test and room temperature tensile test were used to investigate the microstructure and mechnical properties that influenced by annealing temperature(600?,700?,800?,1000?),magnetic direction(MD//RD,MD?RD)and cold-rolled reduction(90%,95%).It has been found that the Al0.3CoCrFeNi high entropy remains single fcc phase after 90%and 95%cold-rolled ruction.The second phase(ordered bcc)begins to precipitate after annealing at above 600?.The greater the amount of cold rolling ruduction,the more obvious the precipitation of the second phase.Moreover,with the increase of the annealing temperature,the second phase increases;and the diffraction peak becomes most obvious(mainly at the(211)diffraction peak)when the temperature reaches 800?.Finally when the annealing temperature rises above 1000?,the second phase redissolves into the matrix phase.The high magnetic field and its applied direction have an important effect on the orientation distribution of the second phase.After annealed at different temperatures,the microstructure of the 90%cold-rolled Al0.3CoCrFeNi alloy changes obviously.And at 600?,the sample is in the recovery state,and the recrystallization in the shear band has appeared;at 700?,the recrystallization is obvious,the low angle boundaries(LABs)in the sample begin to decrease and the high angle boundaries(HABs)begin to increase;At 800?,the recrystallization has basically been completed and some grains begin to grow,the average grain size is 1?m,the HABs increase to 85%;At 1000?,the growth of grain becomes very obvious.The magnetic field promotes the transition from LABs to HABs and the grain growth in the initial stage of recrystallization.The proportion of HABs of 95%cold-rolled annealed samples is higher than 90%cold-rolled annealed samples.The hardness of A10.3CoCrFeNi alloy after homogenization annealing is 132.9HV.And the hardness increases obviously after 90%and 95%cold rolling.During the annealing process,the hardness of the alloy increased at 600? and then decreased gradually.The high magnetic field has a slight effect on the hardness of the alloy.The as-cast Al0.3CoCrFeNi alloy has a very high elongation.And the plasticity decreases obviously while the strength increases 4-5 times after 90%and 95%cold rolling.With the increase of annealing temperature,the strength decreases,the plasticity increases,and the fracture morphology changes from the dissociative feature to the dimple feature.After recrystallied at 800?,the alloy has better balanced strength and plasicity.The strength of the samples annealed at 700? and 800?with magnetic field are lower than those without magnetic field,and the elongation is better.It is found that the textures of 95%cold-rolled annealed alloy are more complex than 90%cold-rolled annealed alloy;in addition,the texture change from Brass to Goss and the texture type becomes more complicated with the annealing temperature increasing.The high magnetic field weakens the texture intensity greatly and makes the texture become more cluttered and diffuse. |
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