Study Of Elinvar Property In Iron Doped Manganese-copper Alloy And Generalized Rubber Like Behavior

The abnormal properties of materials are always of great interest to investigators, due to their potential applications in special occasions in various fields. My thesis focuses on two abnormal properties, Elinvar effect and rubber-like behavior. A century ago, Guillaume discovered and developed two kinds of alloys with properties independent of temperature: Invar and Elinvar alloys, and thus he was awarded the Nobel Prize for his extraordinary discoveries. In order to reveal the underlying mechanism of Invar/Elinvar effects, Weiss proposed a theoretical framework that coexistence of two kinds of magnetic states in the alloy, named by high spin state and low spin state, leads to the unusual temperature dependence of properties. Within the following decades, a lot of theoretical effort has been expended to clarify different magnetic states and how their affect lattice parameter and modulus. However, the exact physical picture is so far not clarified, especially on mesoscale. One purpose of this thesis is to study anomalous temperature dependence of modulus in Fe doped MnCu alloys by experimental procedures and phase-field simulation. Most accepted mechanism for "rubber-like behavior"(RLB) is the theory of "symmetry-conformed short range order"(SC-SRO), which is, we think, relative weak power of explaining such effect in disordered alloy systems. For this reason, a phase field model considering RLB in disordered system was carried out to pursue a different explanation from SC-SRO. Investigation results on such two abnormal properties are summarized as follows.1. For doped Mn-Cu alloys with doping different Fe concentration dynamic mechanical analysis (DMA) results showed normal modulus-temperature dependence in alloys with low Fe doping(Mn80Fe15Cu5) and high doping(Mn60Fe35Cu5) from-150℃ to 150℃. The former is dominated by collinear antiferromagnetic (AF) structure and the latter is dominated by noncollinear AF structure. For the alloy with medium Fe doping, the modulus increases anomalously upon heating, which is speculated to be relevant to coexistence of collinear & noncollinear AF structure within the sample. According to our simulation result, we proposed a "dynamical AF domain size effect"(DAFDS), that is, collinear AF domains have high spin and their sizes increase (decrease) upon cooling (heating) accompanying with the shrink (expansion) of domain walls with low spin between them upon cooling (heating). As a result, modulus abnormally and continuously changes because of relatively lower modulus of high spin collinear AF structure. Moreover, the necessary condition and sufficiaent condition of modulus anomaly is proposed. The former is coexistence of more than one variant of high spin domains and the latter is the increase of density and width of low spin domain walls caused by doping. The phenomena of normal and abnormal modulus in Mn-Cu alloys with doping can be successfully explained on mesoscale2. According to the mechanism for modulus anomaly mentioned above, it is hardly for an single phase alloy to maintain Elinvar effect over a wide temperature range. Therefore, in Mn7oFe25Cu5 alloy, a dual-phase structure was designed by optimizing the heat treatment. One phase is β phase, in charge of normal modulus behavior, the other is y phase, in charge of abnormal modulus behavior. In this way, this dual-phase alloy exhibit almost constant modulus from -150 to 150℃, with modulus fluctuation lower than 2%, that is to say, an Elinvar behavior with a temperature expand of 300K.3. Martensite variants’evolution under stress along different directions ([100], [110] & [111]) was studied using a phase field model. Evolution upon loading exhibits different path ways with different loading direction:If the stress is parallel to [100], the detwinning microstructure will include two variants and the critical stress for this transition to complete is the smallest; If the stress was applied along [110] direction, single variant was observed at the end of evolution, which relies on the motion of twin boundaries of the single martensite variant, and the critical stress is higher than that along [100] direction; While for the stress along [111] direction, the total system will turn to be complete parent through the motion of martensite/parent phase interface, which is similar to the reverse martensitic transformation. It has largest critical stress is the largest amongst these three cases. Strain recovery upon unloading of stress along [100] and [110] is mediated by rearrangement of martensite variants, this pseudoelastic behavior is different from the well-known forward and reverse martensitic transition, and pseudoelastic behavior based on martenstie variants rearrangement is defined as "rubber-like behavior".4. By energy analysis of the simulation results, the driving force for the RLB is the elastic energy stored by the disappearance of one single variant in three variants; strain with longstanding stress in the simulation tent to be more stable upon unloading due to disappearance of the single variant, unfavorable for RLB. Because there is no such SC-SRO mechanism considered in the simulation, although the conclusion is consistent with SC-SRO mechanism, in which shrink and growth of one martensite variant is the origin of the pseudoelastic behavior, is the intrinsic origin of RLB accompanying with stress-aging effect.