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Titanium aluminum intermetallic compound is a possible candidate for a high-temperature structural material, except for a problem of lack of room-temperature ductility. Recently, this problem was found to be overcome possibly by the addition of Mn, but this mechanism has not been fully understood yet. In order to understand the fundamental mechanism of the ductility improvement by Mn addition, microanalyses have been carried out. The results are as follows. Twin structures in a TiAl intermetallic compound in the as-cast state can be climinated by high-temperature annealing, while those in Mn-added TiAl are thermally more stable and exist even after annealing for 86.4 ks at 1273 K. The reason for this thermal stabilization of twin structures is considered to be due to the pinning effect of twin dislocations by Mn addition. The enhancement of twin deformation in TiAl by Mn addition is regarded to be caused by two factors. One is the stabilization of twin partial dislocations, becoming the nucleation sites for twin formation. The other is the decrease in stacking fault energy, which makes twin deformation energetically easier.

Grain boundary segregation of metalloid impurity elements such as phosphorus and antimony is known to induce temper embrittlement of low-alloyed steels. Fracture in temper embrittlement takes place mainly along prior austenite grain boundaries, which are formed in austenite at high temperatures and remain in the martensite or ferrite/pearlite microstructure after phase transformation. Numerous studies on segregation at prior austenite grain boundaries have been performed, [1][2][3][4][5] and these results suggest that metalloid impurity elements are segregated at prior austenite grain boundaries in low-alloyed steels. In order to reveal grain boundary segregation of impurity elements, segregation at grain boundaries in ferritic iron and steel has also been characterized. [6][7][8][9] Besides temper embrittlement of low-alloyed steels, the microstructure formed by austenite decomposition during cooling in carbon steels and low-alloyed steels may also be influenced by grain boundary segregation. Typically, addition of a small amount of boron, which is segregated at grain boundaries, enhances phase transformation from austenite to martensite in steels, that is the hardenability. 10)Since most of steel plates and sheets are produced through continuous cooling, it is of great importance to make clear the role of metalloid elements segregated at grain boundaries relevant to nucleation and growth of ferrite at grain boundaries and sub-boundaries on cooling. This prompts us to study segregation of metalloid elements at grain boundaries in austenite, and its influence on the microstructure of steels. In this work, Auger electron spectroscopy (AES) is used for investigating segregation of phosphorus and boron at prior austenite grain boundaries in manganese steels. Moreover, the microstructure of the samples, which underwent austenite decomposition, is observed. The roles of grain boundary segregation in heterogeneous evolution processes in the microstructure are discussed coupled with experimental results.Ingots of 0.2 %C-0.2 %Si-2 %Mn steels were produced by vacuum-induction melting. The chemical composition of these ingots is listed in Table 1. They were hot-rolled to plates of about 12 mm in thickness. Samples for AES analysis were quenched into water of 273 K after annealing at 1 473 K for 1 200 s, since these samples are relatively easily fractured along at prior austenite grain boundaries. Samples for the microstructure observation were stepwise cooled; they were annealed at 1 173 K for 600 s after annealing 1 473 K for 1 200 s, subsequently annealed at 873 K for 3 600 s and finally quenched into water of 273 K. During annealing at 873 K, austenite in samples was decomposed to ferrite and pearlite.Charpy-type impact tests of samples were performed at 77 K, and the fractured surface of samples was observed by scanning electron microscope (SEM). Samples with phosphorus containing more than 0.03 mass% were fractured along prior austenite grain boundaries. Samples for AES analysis were cut to the shape of 4ϫ4ϫ20 mm 3 w...

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SynopsisAs-hot-rolled, ferrite-martensite dual phase steels of rather simple composition can be produced by the " Dual Phase Rolling (DPR) process" which involves a low finish rolling temperature and a very low coiling temperature. Laboratory DPR experiments have been carried out using CMn steels and those with Cr or Si additions, to examine the effects of alloying and processing factors on the structure and mechanical properties of the processed steels. Major results obtained are as follows:(1) To attain a sufficiently low yield-to-tensile strength ratio, the final finish pass temperature should be at about Ar3 point which varies depending on the composition, so as to bring about early separation of the alpha phase from the gamma phase before cooling starts. The coiling after a rapid cooling should be done at a temperature lower than 200 °C, almost regardless of the steel composition, to suppress auto-tempering of the transformed martensite and aging of the ferrite.(2) Both Cr and Si additions enhance the hardenability of partitioned austenite, allowing a relaxed cooling rate to obtain the martensite phases. However, Cr addition is prone to hinder the early phase separation making the gamma-to-alpha transformation sluggish. Silicon addition accelerates the phase separation, so that a wide range of finishing temperature is available.

The influence of addition of small amounts of boron and nitrogen on the microstructure formed by austenite decomposition in low-alloyed manganese steels was investigated. In order to understand microstructural changes by addition of boron and nitrogen, Auger electron spectroscopy was used for analyzing prior austenite grain boundaries in steels doped with phosphorus, boron and nitrogen. The results by microstructure observation showed that the formation of Widmanstätten ferrite was suppressed by addition of a small amount of boron in the steels, whereas Widmanstätten ferrite appears to be formed again by addition of boron and nitrogen. The Auger spectra showed that small particles of boron nitride were detected on grain boundaries in steel doped with boron and nitrogen, while boron was segregated at grain boundaries in steel with boron. This indicates that segregation of boron at grain boundaries and/or sub-boundaries may suppress the formation of Widmanstätten ferrite, while the formation of boron nitride seems to be ineffective to suppression of the formation of Widmanstätten ferrite in steels doped with boron and nitrogen.

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