Research On Martensitic Pretransformation/transformation And Low Expansion Characteristics Of FeNiCoTi Alloys

Since the discovery of the Invar effect（low expansion coefficient）,many scholars have never stopped researching the origin of its mechanism.In recent years,it has been suggested that the Invar effect of Fe Ni-based alloys may be related to martensitic transformation,but there is still a lack of experimental evidence.In this thesis,the precipitation of Ni₃Ti particles in FeNiCoTi alloy is used to control the martensitic pre-transformation and phase transformation during the aging process,and the internal connection between pretransformation/transformation and the Invar effect is attempted to be established.In this thesis,Fe₅₄Ni₃₁Co₁₀Ti₅（at.%）alloy sheets were obtained by vacuum inducti on melting,forging and rolling,and then they were solution treated and aged at different temperatures.The thermal expansion,resistance tester,etc.were used to study the mart ensitic pre-transformation,phase transformation,Invar effect and the relationship betwe-en them after aging treatment of Fe₅₄Ni₃₁Co₁₀Ti₅（at.%）alloy in different states（forgin g-g,hot-rolled,cold-rolled）relevance.Research results showed that the martensitic transformation of Fe₅₄Ni₃₁Co₁₀Ti₅（at.%）samples subjected to different heat treatments during the cooling process is divided into three situations:no transformation at all,thermoelastic martensitic transformation,and non-thermoelastic martensitic transformation.Among them,the relationship between the starting temperature of martensitic transformation and the aging temperature was studied for the samples after solution treatment at 900°C and then aging at different temperatures,showing a C curve,which proves that the aging precipitation of Ni₃Ti particles can regulate the martensitic transformation..Secondly,the aging process has an influence on the Invar effect.With the prolonging of the aging time,the thermal expansion coefficient increases,the storage modulus softens,and the resistance decreases abnormally,corresponding to the abnormal phenomenon of martensitic pre-transformation.It was also found that the aging treatment affects the temperature hysteresis of the thermoelastic martensitic transformation of Fe₅₄Ni₃₁Co₁₀Ti₅（at.%）alloys.The Fe₅₄Ni₃₁Co₁₀Ti₅（at.%）alloy with thermoelastic martensitic transformation and large hysteresis was subjected to thermal cycling.The increasing number of cycles caused the increase of M_s and A_s at the same time.The rising speed of A_s was faster than that of M_s,and the hysteresis gradually increased.After 20 cycles,its thermoelastic martensitic transformation disappeared.Calculate the thermal expansion coefficient of the alloy was calculated after the cycle.After the 10th cycle,the thermal expansion coefficient is 3.3×10^-6/℃.At this time,the thermal expansion coefficient is the lowest and the sample has the Invar effect;when the cycle reaches the 15th time,the thermal expansion coefficient is 5.0×10^-6/℃.Continuing the thermal cycle,the thermal expansion coefficient no longer changes greatly,but still has the Invar effect.Preliminary analysis suggests that the change in martensitic hysteresis may be related to the Invar effect of the alloy and the introduction of dislocations during cycling.