Study On Alloy Design Theory At The Electronic And Atomic Scale For Fe-Cr-Mn-C-B System And Its Application

Fe-Cr-Mn-C metastable austenite matrix cast alloy shows excellent wear-resistance, and Fe-Cr-C-B alloy exhibits high slurry erosion resistance. It is of great significance to study and develop higher performance Fe-Cr-Mn-C-B metastable austenite matrix cast alloy based on before-mentioned alloys. The key theoretical problem of the alloy design lies in the study on the influence of boron on the multielement alloyed austenite system, and the calculation of the multielement alloyed austenite containing boron at the electronic and atomic scale. The occupation, distribution, and solid solubility of boron in austenite, the substitution of boron for carbon, the influence of boron and other elements on austenite, and the role of boron in the martensite transformation induced by friction can be explained by the energy calculation of the alloyed austenite large system.The large cluster of alloyed austenite with slight amount boron was studied by precisely calculating the small cluster containing boron with quantum chemistry ab initio (QCAB) calculation method, and the large cluster with the semi-empirical interatomic pair potential (SEIPP). The local precise calculation for the small cluster containing slight amount element, and the less precise calculation for the large cluster can not only indicate the role of slight amount element, but also realize the calculation of multielement alloy large system at the electronic and atomic scale.Several common computational programs for alloy design at the electronic and atomic scale were reprogrammed and designed. The common QCAB calculation program MQAB-80 was reprogrammed and expanded, renamed as MQAB-2001, in which the basis-sets types, input and output mode of data, process control, and system clock were embedded. Its advantages are simple in structure, shortness, precise, convenient to customize special function, and more flexible than the analogous commercial software. The program SYMMETRY-01, a generator of the operation table on cluster's symmetry, was designed and compiled. Several kinds of clusters' symmetry operation tables can be quickly and precisely generated. The quantum chemistry model potential ab initio (QCMPAB) calculation program MPMQAB-01 was designed based on MQAB-2001. Because of the preciseness and less computing amount (about 1/10-1/100 of the QCAB calculation), MPMQAB-01 will be widely used when the whole basis sets being developed. The molecular dynamics program MD wasreprogrammed and expanded, and the new program becomes WXMD. The Finnis-Sinclair many-body potential and simulated annealing method were added in WXMD. In this program, the potential function is Finnis-Sinclair many body potential, and the force field is Finnis-Sinclair short-range interation and Lennard-Jones long-range attraction. That attractive force on the ith atom is calculated as/=^------p—rtl (rt/>d , rl)=rl-rl , d is the parameter of Finnis-Sinclairpotential). And the balance geometries of several Fe,, clusters were calculated by this method. The WXMD was non-tightly embedded into the MQAB-2001 (which can only do single point calculation), and after reprogrammed and expanded, a new program MQAB-2003 formed, which can calculate both single point, and geometry optimization by fixing the bond angle and symmetry. In MQAB-2003, the WXMD segment provides the original geometry for the QCAB calculation segment, which prepared for the tight combination of the two scale method—the precise QCAB or QCMPAB calculation directly provides potential function for the molecular dynamics.An analytical method of calculating the volume fraction of M7C3 eutectic carbide in Fe-Cr-C system alloy based on the phase diagram was put forward. The volumefraction K =-----^—^-----of M7C3 in several alloys was calculated at the double phased+Pdzone y-Fe+M7C3. The calculated results agree with the experimental data. That method can be easily extended to other double phase zone and the triple phase zone.The QCAB calculation method was for the first time introduced into local precise calculation of the austenite large system with boron and carbon. The results are that the atomic average binding energy (AABE) of the octahedral interstice with boron is 1.6978eV, and the AABE of the octahedral interstice with carbon is 1.3520eV, while the nearest bond lengths are 0.29967nm and 0.25780nm respectively. And each octahedral interstice boron atom reduces the total bond energy 116.91443eV of the austenite Fe cluster, while each carbon atom reduces 0.459142eV. And the influence of boron is 254.6 times as that of carbon on the austenite energy. The potential energy of the double-atom cluster of Fe-Cr-Mn-C-B alloy was calculated by the QCAB calculation method. A fit principle was put forward that the lowest point of the fitted curve of the potential energy should coincide with the minimal value calculated byQCAB calculation. And then the parameters of the SEIPP of Lennard-Jones form were derived. The AABE and balance interatomic distance of iron clusters were calculated by SEIPP method, and the results approximately agree with the experiment. This method can be extended to any double-atom cluster in all alloys. The QCAB calculation method precisely dealing with small cluster was combined with the SEIPP method approximately dealing with large cluster. The interstitial solid solubilities of boron and carbon in austenite were calculated by the combined method. By comparing the results of the interstitial solid solubility of carbon and boron in austenite, the calculated octahedral interstitial solid solubility of boron in austenite is 0.097m7%. And the boron in the octahedral interstice can change the energy of austenite largest, but in the crystal boundary and defects slightly.It can be concluded from the result of SEIPP that boron is easier to bind with iron on the crystal boundary. Thus boron has large probability in the crystal boundary of the solid solution. And because of high-boron-content ratio in boro-carbide, it can be estimated that the boron content in the austenite matrix iron alloy is about 0.05~1.00w/%. With the fixed content of Cr, Mn and C, the influence of boron on the AABE of the austenite cluster was calculated. The results show that the AABE becomes lower with the increasement of boron content. When boron content increased to 0.0427h'/%, the AABE of the cluster is lower 10% than that of the pure iron austenite. And that boron content was defined as the limit content of boron in the cluster. The limit content of boron in austenite will obviously change with the content of other alloy elements. When the AABE becomes lower, the martensite transformation, resulting form both during cooling and induced by friction, becomes easier.The volume fractions of the boro-carbides of M7(C, B)3, M3(C, B) and M23(C, B)6 were calculated by empirical formula. And by the calculating results of austenite with boron at the electronic and atomic scale, it was founded the complete method of the composition and structure design of Fe-Cr-Mn-C-B system metastable austenite matrix wear resisting cast alloy. Based on the alloy design, eight test alloys were founded. And the compositions were exmined, the structures analyzed, and the properties experimented. The results of the experiments of wear resistance show that the more boron in the austenite, the more readily the friction induced martensite transformation takes place in the surface layer, and the higher the wear resistance acquired. The wearresistance is higher than Fe-Cr-Mn-C system alloy. The wear resistance of O.3vf/%B15nY%Cr alloy of Fe-Cr-Mn-C-B system is 4.2 times as that of 25vv/%Cr martensite-matrix high-chromium cast iron, and 7.7 times as that of Mnl3 cast steel. And from surface to inner, the hardening layers due to friction and wear are in turn high-dislocation ribbon-like a[, > dislocation-tangled austenite cellular structure, a[, with micro-twin crystal substructure, and the clustering zone of sm, yt. fault dislocation and dislocation. The wear resistance of the metastable austenite alloy is from the stress induced martensite transformation and the work hardening effect of austenite. The developed Fe-Cr-Mn-C-B system metastable austenite-matrix wear-resisting cast alloy is a type of new wear resisting material.