Observation of thermally etched grain boundaries with the FIB/TEM technique

Thermal etching is a method which is able to reveal and characterize grain boundaries, twins or dislocation structures and determine parameters such as grain boundary energies, surface diffusivities or study phase transformations in steels, intermetallics or ceramic materials. This method relies on the preferential transfer of matter away from grain boundaries on a polished sample during heating at high temperatures in an inert/vacuum atmosphere. The evaporation/diffusion of atoms at high temperatures results in the formation of grooves at the intersections of the planes of grain/twin boundaries with the polished surface. This work describes how the combined use of Focussed Ion Beam and Transmission Electron Microscopy can be used to characterize not only the grooves and their profile with the surface, but also the grain boundary line below the groove, this method being complementary to the commonly used scanning probe techniques

Tensile behavior of normalized low carbon Nb-microalloyed steel in the presence of rare earth elements

In this study, attempts have been made to study the effects of RE on the tensile behavior of a low carbon Nb-microalloyed steel which confronts yield point phenomenon during its tensile test. The results of the tensile tests and microstructural examinations presented in this research showed that the upper and lower yield points increase by the RE addition which was mainly attributed to the indirect effects of RE on the Nb-precipitation manner and refinement of the microstructure. The results also demonstrated that the flow behavior (oscillation in stress level) during the Lüders strain zone is considerably different for the base and RE-added steels. It was found that the RE-added steel undergoes a uniform propagation of the Lüders bands while the base steel showed a distinct non-uninform Lüders strain with rough fluctuations of stress level within this area. This could be due to the uniformity in distribution of nanoprecipitates and solute, e.g. C, atoms in the RE-added steels. Moreover, It was observed that the Lüders strain increases in the presence of RE which could be probably attributed to the finer ferrite grains and the change in nanoprecipitation behavior caused by RE addition. A significant increase in total elongation of the RE-added steel was also observed.The authors from the University of Tehran gratefully acknowledge the financial support provided by the Office of International Affairs and the Office of Research Affairs, College of Engineering, for the project number 8107009.6.34. The authors from CENIM-CSIC would like to acknowledge the financial support from Comunidad de Madrid through DIMMAT- CM_S2013/MIT-2775 Project. The authors are grateful to the Phase Transformations and Microscopy labs from CENIM-CSIC. Mr. Javier Vara Miñambres from the Phase Transformations lab (CENIM-CSIC) is gratefully acknowledged for his continuous experimental support. The authors also acknowledge ICTS-Centro Nacional de Microscopía Electrónica (CNME) and Mr. Esteban Urones Garrote for the experimental supports in TEM lab.Peer Reviewe

EFFECTS OF MO ADDITION ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF CAST MICROALLOYED STEEL

In industry, the cost of production is an important factor and it is preferred to use conventional and low cost procedures for producing the parts. Heat treatment cycles and alloying additions are the key factors affecting the microstructure and mechanical properties of the cast steels. In this study an attempt was made to evaluate the influence of minor Mo addition on the microstructure and mechanical properties of conventionally heat treated cast micro-alloyed steels. The results of Jominy and dilatometry tests and also microstructural examinations revealed that Mo could effectively increase the hardenability of the investigated steel and change the microstructure features of the air-cooled samples. Acicular microstructure was the consequence of increasing the hardenability in Mo-added steel. Besides, it was found that Mo could greatly affect the isothermal bainitic transformation and higher fraction of martensite after cooling (from isothermal temperature) was due to the Mo addition. The results of impact test indicated that the microstructure obtained in air-cooled Mo-added steel led to better impact toughness (28J) in comparison with the base steel (23J). Moreover, Mo-added steel possessed higher hardness (291HV), yield (524MPa) and tensile (1108MPa) strengths compared to the base one

Demonstration of elemental partitioning during austenite formation in low-carbon aluminium alloyed steel

This work investigates the influence of aluminium, in solid solution, on austenite formation in a lowcarbon aluminium alloyed (0.48 wt. %) steel during continuous heating. A thin section across an untransformed ferrite and austenite interface was prepared for transmission electron microscopy by focused ion beam milling. Microstructural characterization using imaging and elemental analysis demonstrates that aluminium partitions from austenite to ferrite during very slow heating conditions, stabilizing this latter phase and shifting the final transformation temperature for austenite formation (Ac3)Peer reviewe

Effects of mo addition on the microstructure and mechanical properties of cast microalloyed steel

In industry’ the cost of production is an important factor and it is preferred to use conventional and low cost procedures for producing the parts. Heat treatment cycles and alloying additions are the key factors affecting the microstructure and mechanical properties of the cast steels. In this study an attempt was made to evaluate the influence of minor Mo addition on the microstructure and mechanical properties of conventionally heat treated cast micro­alloyed steels. The results of Jominy and dilatometry tests and also microstructural examinations revealed that Mo could effectively increase the hardenability of the investigated steel and change the microstructure features of the air- cooled samples Acicular microstructure was the consequence of increasing the hardenahility in Mo-added steel. Besides, it was found that Mo could greatly affect the isothermal hainitic transformation and higher fraction of martensite after cooling (from isothermal temperature) was due to the Mo addition The results of impact test indicated that the microstructure obtained in air-cooled Mo-added steel led to better impact toughness (28J) in comparison with the base steel (25J). Moreover. Mo-added steel possessed higher hardness (291HV), yield (524MPa) and tensile (11OSMPa) strengths compared to the base one.Peer Reviewe

Evolution of Pearlite Microstructure in Low-Carbon Cast Microalloyed Steel Due to the Addition of La and Ce

The effects of rare earth elements (RE) addition on the pearlite microstructure in low-carbon microalloyed steels have been investigated under two heat treatment conditions: (1) a normalizing treatment (as a conventional heat treatment used industrially to obtain the final mechanical properties of such steels), and (2) an isothermal treatment at 650 °C. This research reports the following effects due to the addition of RE: (i) refinement of the nodule and colony size of pearlite along with the ferrite grain size in the normalized condition, without a significant change in the volume fraction of pearlite. This microstructural refinement observed at room temperature is a consequence of the refinement of cast and austenitic microstructures formed during cooling in the presence of RE; (ii) the interlamellar spacing of pearlite isothermally transformed at 650 °C, as observed by SEM and TEM, is effectively reduced in the RE-added steel. This is likely due to two different effects combined: (i) direct influence of RE on atom carbon diffusion; and (ii) pearlite growth being boundary diffusion controlled by RE partitioning.The authors from the University of Tehran gratefully acknowledge the financial support provided by the Office of International Affairs and the Office of Research Affairs, College of Engineering, for the project number 8107009.6.34. The authors from CENIM-CSIC would like to acknowledge the financial support from Comunidad de Madrid through DIMMAT-CM_S2013/MIT-2775 Project.Peer Reviewe