Xu Zuyao

Size effect on the Fe nanocrystalline phase transformation

Abstract A thermodynamics for the phase transformation from γ(fcc) to α(bcc) in nanocrystalline (NC) Fe is considered. Gibbs free energies of the interfaces in NC γ- and α-Fe particles were calculated, respectively, by means of a quasiharmonic Debye approximation, yielding a larger increase in the total Gibbs free energy of α-Fe than that of γ-Fe. This is attributed to the difference in their interfacial energies. As a result, the fcc NC Fe can be thermodynamically stable at room temperature when the grain size is sufficiently small. Taking into account the thermodynamic equilibrium condition, the critical grain size for the γ-Fe phase to exist in stable form at 300 K was quantitatively calculated for different excess volumes Δ V , a parameter describing the state of interface based on a dilated crystal model. The assumptions made in the present model and the factors influencing the critical grain size are discussed.

The effect of martensite ordering on shape memory effect in a Copper-Zinc-Aluminium alloy

Abstract The correlation between the shape memory effect (SME) and the degree of ordering in martensite formed through various heat-treatment processes, e.g. ice water quenching, step-quenching and aging etc., has been studied in a Cu-25.83wt. %Zn-3.96wt. %Al alloy, by estimating the spacing difference (Δd) of some pairs of diffracting planes with indices satisfying the relation (( h 1 2 − h 2 2 )/3 = ( k 2 2 − K 1 2 )/ n , which may reflect the degree of ordering in martensite. It is shown that SME dependes monotonically on Δd. The extent of ordering of the DO 3 parent phase, and in turn that of the martensite, increases with the holding duration in the course of step-quenching, which may be the main factor for enhancing the shape recovery rate η. The stabilization of martensite may be associated with the clustering of vacancies resulting in the local distortion of lattice plane and the formation of sessile dislocation, thus lowering the degree of ordering.

Martensitic transformation in Fe-Mn-Si based alloys

The stacking fault energy (SFE) or the stacking fault probability plays an important role in the determination of the critical driving force ΔGγ→e for the martensitic transformation fcc(γ)→hcp(e) in ternary Fe-Mn-Si alloys, ΔGγ→e increases with the content of Mn and decreases with that of Si. Thermodynamical prediction of Ms in ternary Fe-Mn-Si alloys is established. The fcc(γ)→hcp(e) martensitic transformation in Fe-Mn-Si is semi-thermoelastic and the nucleation process does not strongly depend on soft modes. Nucleation of e-martensite is not dominated by a pole mechanism. Based on the phenomenological theory of martensite crystallography, a shuffle on (0001)hcp planes is required when d111≠d0002. The derived principal strains for the Bain distortion are smaller, i.e. more reasonable than the values given by Christian. Alloying elements which strengthen the austenite, lower the SFE of the γ phase and reduce the TγN temperature may be beneficial to the shape memory effect (SME) of Fe-Mn-Si based alloys.

Diffusion of carbon during the formation of low-carbon martensite

The time required for carbon diffusion from martensite to enrich surrounding austenite from 0.27 to 1.04% C during the formation of low-carbon matensite is 10~(-7)s in order of magnitude as calculated. It does prove that the diffusion of carbon atoms can keep pace with the formation of lath martensite. From thermodynamical calculation, it is reasonable to recognize that the precipitation of carbon from martensite results the enrichment of austenite. TEM observation revealed that the quenched structure in a 0.12C-low Ni-Cr steel mainly contains lath martensite and interlath retained austenite, and also twin martensite. The existence of the latter further confirms the occurrence of carbon diffusion to enrich austenite during the martensite formation, and twin martensite forms at the parent phase where carbon enrichment is not very high. The interface of austenite and martensite is somewhat straight. The typical upper bainite (B_Ⅱ), B_Ⅲ type bainite and carbide-free bainite (B_1) can aU appear in the ame steel and there exists superledges at the interface of austenite and bainitie ferrite which is quite different to that of austenite and martensite. In addition, from the kinetics point of view. the growth rate of low-carbon martensite is 3 to 4 order of magnitude greater than that of upper bainite. It is more likely to conclude that the mechanism of the formation of lath martensite is not identical with that of bainite.

Thermodynamics of the bainitic transformation in a CuZnAl alloy

The driving forces for various possible reactions within the bai-nitic transformation temperature range of Cu-40at.-%Zn alloy have been cal-culatd.The results show that because of the occurence of β→β′ordering tran-sition within the temperature range,the driving forces ΔG(?) and ΔG(?)increase inversely with decreasing temperature with ΔG~(?)0 and △G(?)0.Moreover,the equilibrium temperature T_0 of the two phases β′and α′is far below the experimental B_s for alloys of various compositions.Therefore,bainitic transformation in Cu-Zn alloys can only proceed as that of diffusionalreaction β′→β′_1+α.

Electronic structure of FCC phase in Fe−Mn−Si based shape memory alloys and its stability

The relationship between the electronic structure of FCC phase in Fe−Mn−Si alloy and its stability has been studied by using the discrete variational method based on the first principle. The reason why Mn and Si elements have different influences on the stacking fault energy may be related to the electron concentration (e/a). Si reduces the hole number of 3d band while Mn is rather complicated. The binding energy has been calculated and the experimental results that martensite start temperature (Ms) varies with SI and Mn are explained. When the external stress is exerted in three directions, the electronic structure, the total density of states, the energy gap at Fermi energy level(E F) and the energy degeneracy will change into other states. When the different external stresses are exerted in one direction, 3d or 4s orbital occupations of the central atom decrease, the partial density of states seems to be thinner and its peak increases atE F, the bond orbit shrinks in the direction of the external stress and another bond orbit comes out vertically. These lead to an a decrease in the structural stability and an increase In Ms temperature under the extemal stress.The relationship between the electronic structure of FCC phase in Fe−Mn−Si alloy and its stability has been studied by using the discrete variational method based on the first principle. The reason why Mn and Si elements have different influences on the stacking fault energy may be related to the electron concentration (e/a). Si reduces the hole number of 3d band while Mn is rather complicated. The binding energy has been calculated and the experimental results that martensite start temperature (Ms) varies with SI and Mn are explained. When the external stress is exerted in three directions, the electronic structure, the total density of states, the energy gap at Fermi energy level(EF) and the energy degeneracy will change into other states. When the different external stresses are exerted in one direction, 3d or 4s orbital occupations of the central atom decrease, the partial density of states seems to be thinner and its peak increases atEF, the bond orbit shrinks in the direction of the external stress and another bond orbit comes out vertically. These lead to an a decrease in the structural stability and an increase In Ms temperature under the extemal stress.

Self-energy and interaction energy of stacking fault in fcc metals calculated by embedded-atom method

The stacking fault energies of five fcc metals (Cu, Ag, Au, Ni and Al) with various quantivalences have been calculated by embedded-atom method (EAM). It indicated that the stacking fault energy is mainly determined by the metallic bond-energy and the lattice constant. Thus, monovalent fcc metals should have different stacking fault energies, contracy to Attree’s conclusion. The interaction energy between stacking faults one {111} layer apart in a fcc metal is found to be 1/40–1/250 of its self-energy, while it becomes zero when the two stacking faults are two layers apart. The twin energy is just half of the energy of intrinsic stacking fault energy without the consideration of lattice relaxation and the energy of a single intrinsic stacking fault is almost the same as that of extrinsic stacking fault, which are consistent with the results from the calculation of Lennard-Jones force between atoms, but differ from Attree’s result.

Effect of boron on martensitic transformation of the Ni–rich NiAl shape memory alloys

The effects of boron on the martensitic transformation and the ductility of the binary NiAl shape memory alloys have been studied by means of x – ray diffraction, dilatometry and compression test. It is found that the boron increases the Ms temperature and the volume change during the martensitic transformation, but affected the Mf temperature slightly. It is also found that the boron improves the ductility of binary NiAl alloys. The Ni-37.5at% Al – B alloy gives a complete pseudoelastic strain of 2.8% at room temperature, while the Ni-35.7at% Al-B alloy displays the shape memory effect and gives a 92% recovery at a strain of 1.9%.