Abnormal Grain Growth Behavior In FeMnAlNi Superelastic Alloys

FeMnAlNi alloys have a very large superelastic temperature range（-196～240℃）and a very low temperature dependence of the critical stress of martensitic transformation（Clausius-Clapeyron slope is less than 0.2 MPa/℃when compressed,and less than 0.5 MPa/℃）,showing good application prospects in aerospace,space exploration,vibration reduction and earthquake resistance.In order to obtain good superelastic properties in FeMnAlNi alloys,the microstructure must be controlled to be single crystal or columnar crystal.Cyclic heat treatment can stimulate the special"subgrain"abnormal grain growth mechanism in FeMnAlNi alloy to achieve the purpose of increasing the grain size.However,this"subgrain"abnormal grain growth mechanism has not yet been fully elucidated.In this thesis,by observing the microstructure evolution during cyclic heat treatment,the influence of alloy composition adjustment and cyclic heat treatment process on abnormal grain growth of FeMnAlNi alloy is studied,and the internal mechanism of"subgrain"abnormal grain growth is discussed.First of all,this thesis studies the influence of alloy composition adjustment on abnormal grain growth,designs Fe_43.5Mn₃₄Al₁₅Ni_7.5 alloys with three composition gradients of Al,Mn and Cu,and characterizes and analyzes the microstructure evolution during cyclic heat treatment.The results showed that the alloying elements mainly change the subgrain size of FeMnAlNi alloy by affecting the morphology of theγphase.With the coarsening of theγphase morphology,the subgrain size increases,and the subgrain size decreases with the refinement of theγphase morphology.There is no fixed correspondence between the grain size and the abnormally grown grain size.Although the Cu element also refines the subgrains,the final grain size does not increase.Through thermodynamic calculation and analysis,it is found that the composition adjustment has little effect on the driving force provided by the high-angle grain boundary.The different grain boundary mobility in the material is mainly caused by the change of the subgrain energy due to the composition adjustment.The subgrain energy is not only related to the subgrain size,but also related to the subgrain orientation difference.Then,two alloys of FeMnAlNi and FeMnAlNi-3Cu were selected for cyclic heat treatment with different annealing temperature,annealing time,and heating and cooling rate,and the law of microstructure evolution during the process was observed.Both FeMnAlNi and FeMnAlNi-3Cu alloys exhibit larger abnormally grown grain sizes at low annealing temperatures.At lower annealing temperatures,smallerγ-phase morphologies and lower subgrain sizes can be obtained to provide more energy.Reducing the annealing temperature of the cycle heat treatment is beneficial to obtain a larger grain size.The maximum grain size in FeMnAlNi-3Cu alloy reaches about 7mm when the annealing time is 5min.Although the subgrain size decreases slightly with the increase of annealing time,the subgrain area decreases drastically,making the subgrain provide less energy.Therefore,the size of abnormally grown grains decreases after extending the annealing time.With the increase in the temperature rise and fall rate during the cyclic heat treatment,theγphase morphology in FeMnAlNi and FeMnAlNi-3Cu alloys is refined,and the subgrain size is also reduced.However,the change in the temperature rise and fall rate significantly increases the high temperature annealing time,resulting in a higher grain size.The grain size increases slightly under the cooling rate process.Therefore,in FeMnAlNi alloys,the main factor affecting the abnormal growth of crystal grains is the energy density of subgrains.It can be adjusted from three aspects:subgrain size,subgrain angle,and subgrain area.The main method of adjusting the subgrain size is composition adjustment,Such as increasing Al,Cu or reducing Mn element content,the precipitatedγphase is adjusted to be finer,so as to obtain a smaller subgrain size.The main means of adjusting the subgrain area is the adjustment of the cyclic heat treatment process.Lowering the annealing temperature and annealing time can increase the sub-crystal area.Using the above methods to reduce the size of sub-crystals and increase the area of sub-crystals to increase the energy density of sub-crystals is conducive to obtaining larger abnormally grown crystal grain sizes.Finally,the designed FeMnAlNi-3Cu alloy and the FeMnAlNi alloy were subjected to multiple cycles of heat treatment and aging treatment,and a single crystal was obtained in the FeMnAlNi-3Cu alloy and showed a superelasticity of 3.5%.The columnar crystal obtained in the FeMnAlNi alloy and showed a superelasticity of 2.1%.The performance far exceeds the polycrystalline under the same composition,indicating that the cyclic heat treatment can effectively control the grain size of the designed composition alloy and obtain good superelastic properties.