Jia-Jun Wang

Determination of Martensite Start Temperature in Engineering Steels Part I. Empirical Relations Describing the Effect of Steel Chemistry

The dependency of the martensite start (ms) temperature upon composition of engineering steels has been examined by analyzing the results predicted by an artificial neural network (ANN) model and thermodynamic data. Two new formulas, the simple linear and binary interaction ones, have been statistically derived and applied to predict the is temperature in an Fe-C-Si-Mn-Cr-Mo system. It is shown that the separation of the influence of interactions from that of individual alloying elements is successful since most of the statistical results are reasonable and thus have been physically interpreted. The thermodynamic calculations show that the alloying elements have similar influence upon the is and A 3 temperatures. The apparent effect of carbon depends largely on C-X interactions. C-Mn and C-Mo interactions weaken the effect of carbon while that of C-Si interaction intensifies the role of C. This is supported by phenomenological results and has been physically interpreted. The interactions between substitutional alloying elements have also significant influence upon the is temperature. The Si-Mn interaction strongly increases the Ms while Si-Mo interaction significantly decreases the is. So far, there is no proper physical explanation for this though supportive evidence has been obtained from phenomenological results. in and Mo have the weakest apparent interaction, that is, their influence can be simply added up. Moreover, a semi-physical model has been built to predict the is temperature from a critical temperature, which can be calculated thermodynamically. It shows that the semi-physical method gives a satisfactory prediction of is with a standard error of 15.3°C. Evaluation of nine common empirical methods indicates that the Kung and Rayment (KR) formula gives the best predicting results amongst them.

Formation of bainite in ferrous and nonferrous alloys through sympathetic nucleation and ledgewise growth mechanism

The subunits constituting a bainitic sheaf in an Fe-C-Cr-Si alloy were discovered by scanning tunneling microscopy (STM) to consist of sub-subunits, and sub-subunits were also composed of sub-sub-subunits. Detailed investigation shows that a bainitic relief is composed of many smaller reliefs, which correspond to a different structure of bainite,i.e., subunits, sub-subunits, and sub-sub-subunits. It is determined by STM that the surface relief arising from the formation of bainite in an Fe-C-Cr alloy istent shaped rather than an invariant plane strain (IPS) type of surface relief. Careful observation shows that the relief obtained from a sub-sub-subunit is also tent shaped. It is discovered by STM that an α1 plate,i.e., bainite formed in Cu-Zn-Al alloys, is composed of subunits. This is also demonstrated by transmission electron microscopy (TEM). The preceding results indicate that bainitic plates in Cu-Zn-Al alloys and bainitic subunits in steels are not the smallest structural units. Based on the preceding results on the ultrafine structure and the nature of surface relief accompanying bainite, it is proposed that the bainitic structure forms through a sympathetic nucleation and ledgewise growth (SNLG) mechanism. This article shows that the SNLG mechanism can be successfully applied to interpret the complicated structure of bainite.

Enhanced low field magnetoresistance of Fe3O4 nanosphere compact

Unusually large low field magnetoresistance (LFMR), ∼10%, at 300K has been observed with the sample of monodispersed Fe3O4 magnetite nanospheres, ∼200nm, compactly cold pressed and sintered at 800°C. A detailed analysis on the transport and magnetic measurements indicates that the electron conduction is dominated by the spin-dependent scattering or tunneling at the grain boundaries. At low temperatures, 140 and 100K near the Verwey transition, ∼115K, the LFMR (below 2kOe) does not show any sign of dependence on the transition and does not follow the variation of magnetization to reach the saturation region either. On the other hand, at 300K, the MR saturates fast with the magnetization below 2kOe. This temperature dependent property in LFMR is very likely attributed to the scattering or tunneling of the conduction electron passing through the grain boundary layer with spin disordered state.

Aspect ratio of bainite in steels

The aspect ratios of bainitic plates formed in 10 experimental alloys have been statistically analyzed. The results, based on the measurements of 129 individual plates, show that the aspect ratio of bainite is less than 0.25, and generally less than 0.1, with the most frequent value around 0.025 and a theoretical minimum value of 0.008. Alloying elements exert different influence on the aspect ratio. Both C and Si significantly decrease while Cr slightly decreases the aspect ratio. However, Mn clearly increases the aspect ratio. Attempts have been made to interpret the influence of alloying elements on the aspect ratio, and the variation in the aspect ratio has been related to the energy increase due to the balancing between the elastic and interfacial energies.

Investigation of surface relief accompanying Widmanstatten ferrite formation by scanning tunneling microscopy

There are two types of surface relief associated with phase transformation, i.e., N-like type (Invariant Plane Strain -- IPS) and tent-shaped type (non-IPS). Although many scientists investigated the surface relief accompanying Widmanstaetten ferrite (WF) formation by light optical microscopy (LOM), controversy still exists on its formation mechanism. Up to now, most research concerning surface relief has been done by LOM, scanning electron microscopy (SEM) and transmission electron microscopy (replica), which either do not have enough vertical or lateral resolution. Scanning tunneling microscopy (STM) invented in the 1980s, which has high vertical resolution (0.01 nm) and lateral resolution (0.1 nm), has proved to be very suitable to investigate the surface relief quantitatively.

Formation mechanism of bainitic ferrite and carbide

Superledges on the broad faces of bainitic ferrite plates have been observed with transmission electron microscope (TEM). The observed superledges, ranging from less than 1 to 24 nm in height, are imaged in three dimension by way of tilt operation under TEM. Also, an array of smaller superledges are observed to pile up in front of a secondary phase. Pileup of superledges in front of a barrier is indicative of the mobility of an individual superledge. The precipitation of carbide associated with bainitic ferrite is also studied. It is observed that a carbide of a wedgelike shape may exist in front of a superledge with its tip(i.e., thinner end) penetrating the austenite and its root (the other end) originating at α:γ boundary. This condition indicates that the observed carbides may nucleate at the austenite side of α:γ phase boundary and grow toward austenite matrix.

Modelling the continuous cooling transformation diagram of engineering steels using neural networks: Part II: Microstructure and hardness

Abstract A neural network model for the calculation of the phase regions of the continuous cooling transformation (CCT) diagram of engineering steels has been developed. The model is based on experimental CCT diagrams of 459 low-alloy steels, and calculates the CCT diagram as a function of composition and austenitisation temperature. In considering the composition, 9 alloying elements are taken into account. The model reproduces the original diagrams rather accurately, with deviations that are not larger than the average experimental inaccuracy of the experimental diagrams. Therefore, it can be considered an adequate alternative to the experimental determination of the CCT diagram of a certain steel within the composition range used. The effects of alloying elements can be quantified, either individually or in combination, with the model. Nonlinear composition dependencies are observed.

Application of atomic-force microscopy to metallography

Atomic-force microscopy (AFM) was applied to study the surface undulations (reliefs) resulting from the martensitic transitions in Fe-Ni-C and Cu-Zn-Al alloys. Both the morphology of the surface undulations determined by a profile line across the surface and the height of the undulation could be measured directly and easily with AFM. The results show that AFM can be a powerful tool in metallography.

Use of scanning tunneling microscopy in metallography

Abstract Scanning tunneling microscopy was used for the first time to study the bainitic microstructure of an austempered Fe-1.0C-4.0Cr-2.0Si alloy when certain experimental procedures were taken to minimize surface absorption and oxidation. Specimen preparation methods and observation procedures required for the study of easily oxidized materials by scanning tunneling microscopy, and the interpretation of the scanning tunneling image, including microstructural contrast, phase morphologies, etc., are discussed. The results demonstrate that scanning tunneling microscopy, which is used mainly in studying special materials, such as pure gold, single crystal silicon, etc., is another powerful metallographic tool.

Study of ledge structures of bainites in the Cu-base 27.1Zn-3.6Al alloy

Abstract The fine structure of bainite ( α 1 platelets) in the Cu-27.1Zn-3.6Al alloy was studied by means of transmission electron microscopy. It was found that no stacking faults appear during the earliest growth stage and there exist three-dimensional ledges on the broad faces and (or) at the edges of the α 1 -plates at the different formation temperatures. The results demonstrate that the growth of bainites is governed by a diffusion-controlled ledgewise mechanism.