Volume 105, Issue 8

The high temperature behavior of three iron-based alloys rich in chromium and containing HfC carbides was studied in comparison with two ternary Fe – Cr – C alloys. All these alloys, contain 25 wt.% Cr, 0.25 or 0.50 wt.% C, and additionally 3.7 to 5.6 wt.% Hf for three of them. All alloys were produced in a foundry. They were exposed to air at 1 200 °C for 46 h. Their microstructures in the as-cast state and after exposure to high temperature were characterized using X-ray diffraction, electron microscopy and image analysis. The alloys were also subjected to indentation tests in their as-cast states and in their aged states. The obtained microstructures are composed of a dendritic bcc iron-based matrix and of HfC carbides. These carbides are of two types: script-like eutectic carbides mixed with matrix, and compact pre-eutectic carbides. Some chromium carbides were additionally obtained in two of the three Hf-containing alloys. These microstructures have roughly evolved during the high temperature exposure: variations of the surface fraction of the chromium carbides and increase in the HfC surface fractions. Consequently, the hardness was more (ternary alloys) or less (Hf-containing alloys) decreased. The morphologies of the carbides evolved: coalescence of the chromium carbides, coalescence and fragmentation of the HfC carbides. The behavior of these alloys in oxidation at high temperature is perfectible. It is to be improved to allow the use of such alloys at high temperature.

Nanostructured microstructures consisting of a mixture of very thin plates of bainitic ferrite separated by C-enriched austenite are the main characteristics of the so called NANOBAIN steel family. This paper shows the theoretical approach followed in the design of a new, industrially viable, generation of NANOBAIN steels, a process in which industrial demands such as simpler chemical compositions, faster transformation kinetics and sufficient hardenability have also been considered. The microstructural and mechanical characterization of the bainitic microstructures obtained by isothermal transformation at different temperatures, come to confirm and to validate the theoretical approach used.

This paper presents the experimental results of the impact toughness and hardness of welds deposited on H13 hot work tool steel. To obtain the optimum combination of high hardness and high toughness, multipass gas tungsten arc welding was used to melt four different commercially available filler materials suitable for repair welding of HWTS. Special geometry of the welding sample is presented in this work. The focus is also on the technique of the preparation of the V-shaped notch in order to precisely evaluate the impact toughness. Two different preheating temperatures and two different post weld heat treatments were applied to the samples in order to obtain the effect of heat treatment on the impact toughness and hardness. Data show that weld deposited with filler material C on the base of the hot work tool steel and alloyed with vanadium exhibits high impact toughness in combination with high hardness in the as-welded condition and also after tempering.

Artificial neural network (ANN) and dynamic material model (DMM) are considered to be powerful methods to characterize the flow behaviour of metallic materials. The aim of this study is to analyze the performance of these two methods in the characterization of flow behaviour and deformation stability of medium-carbon microalloyed steel. Flow curves obtained from hot compression tests have been used to describe the flow behaviour of the studied steel using an ANN model. Good correlation between experimental and predicted data was observed. To characterize the deformation stability of the studied steel, experimental processing maps are generated using DMM. Finally, in order to verify the accuracy of ANN results, processing maps based on the DMM have been developed using ANN predicted data. It has been found that these maps agree closely with those obtained using experimental data.

The flow behavior, mechanical and physical properties of alloy Fe-29 Ni-17 Co were studied using hot compression and hot rolling tests over the temperature range of 900 °C to1 200 °C and strain rates of 0.001 s −1 to 1 s −1 . At temperatures below 1 100 °C, the flow curves were similar to those of dynamic recovery; whereas, at higher temperatures, faint peaks appeared on the flow curves. A modified power-law constitutive equation was used to model the dependences of flow stress on temperature and strain rate in different deformation regimes. The Kocks–Mecking equation was used to model the flow curves and to measure the mean flow stress of the material in hot rolling. The samples hot rolled at 1 100 – 1 200 °C to a reduction of 30 – 40 % showed the best combination of ductility and strength and low thermal expansion coefficient. Microstructures of the hot rolled samples indicated that hot rolling at 1 100 – 1 200 °C to reduction of 30 – 40 % leads to complete dynamic recrystallization.

Thixo-joining as a method of joining dissimilar materials was introduced recently. In this method, specimens in a semi-solid condition are brought into contact with each other under compression. As a result, diffusion bonding takes place between the specimens. In this research, two kinds of tool steels, AISI D2 and AISI M2, which are difficult to join using conventional methods, were joined together through the thixo-joining process. While specimens were being heated in a furnace with argon-controlled atmosphere through the direct partial remelting method, a uniaxial constant compressive load was applied to them. The Microstructure and mechanical properties of the joint were analyzed using scanning electron microscopy, shear, and hardness tests. The results showed a joint with good quality regarding the microstructure and mechanical properties. Moreover, energy dispersive spectroscopy revealed the diffusion of elements during the joining process. The fracture surface of the joint was also analyzed and characterized as intergranular.

The ever increasing utilization of Al – Li alloys for light weight structural applications is limited chiefly due to their lower room temperature formability compared to their Li-free counterparts. In the present work, the effect of 1, 2 and 3 wt.% Li addition to Al – Mg – Si alloy (containing 0.5 wt.% Mg and 0.2 wt.% Si) was studied. Experimental work showed that microstructural features of the second phase particles and mechanical properties changed substantially with Li addition. It was observed that 1 wt.% Li addition resulted in significantly large elongation of over 38 % resembling a super-plastic behavior with decent strength and hardness due to the formation of second phase precipitates uniformly dispersed in α (Al) matrix. Scanning electron microscopy and X-ray diffraction analysis revealed that the addition of Li promotes the formation of second phase precipitates, which strongly affect the mechanical properties of Al – Mg – Si alloy. Quite surprisingly, the alloys containing 2 and 3 wt.% Li showed a much reduced ductility and a much higher hardness after aging.

MgO – TiB 2 nanocomposite was synthesized using mixtures of TiO 2 , H 3 BO 3 and Mg as raw materials via a mechanochemical process. The phase transformation and structural evaluation were investigated by means of X-ray diffractometry, scanning electron microscopy, transmission electron microscopy and differential thermal analysis techniques. Thermodynamics evaluations indicated the reaction was highly exothermic and should be a mechanically induced self-sustaining reaction. According to X-ray analysis, MgO – TiB 2 nanocomposite was formed after 4 h of ball milling. Thermal analysis results revealed that after 3 h milling, the temperature of combustion reaction decreased and all the reactions happened simultaneously. Scanning and transmission electron microscopy observations confirmed the range of particle size was within 100 nm.

New iron-based composites were investigated for applications within the friction materials domain. This study presents the elaboration and the structural and tribological characterization of iron-based friction composites containing iron, copper, graphite and/or nickel, introduced in a powder state. The technology employed for obtaining these materials is via a classical powder metallurgy route. The porosity of the studied materials, the tribological characterization, the roughness of researched composites and their structure are marked out. Results showed that the samples elaborated from the material with 10 wt.% Cu, 7 wt.% graphite, 12 wt.% Ni and the rest iron, compacted at 600 MPa, presented the best tribological behavior. The presence of pearlite, ferrite, nickel-based and copper-based solid solutions and free graphite in composite structures is highlighted.

Al 4 SiC 4 – Al – Si composites were synthesized using a mixture of calcined bauxite, silicon carbide and carbon black as the starting materials. Al 4 SiC 4 began to form at >1 600 °C and Al and Si were identified at 1 800 °C. Al and Si formed only when silicon carbide was in excess of the amount of SiC necessary to form Al 4 SiC 4 . The synthesized powders consisted of irregular plate-like crystals and the surfaces were molten-like. The oxidation characteristics of Al 4 SiC 4 – Al – Si composites as antioxidants were better than the oxidation characteristics of pure Al 4 SiC 4 , since their action took place mainly at 400 – 700 °C (Al – Si) and above 850 °C (Al 4 SiC 4 ).

Electroless nickel coating on poly(ethylene terephthalate) board via a copper-catalyzed process is reported. Poly(ethylene terephthalate) was firstly modified by organic silanes to graft thio groups onto its surface, and then covered with copper particles and clusters in solution with dimethyl amineborane as a reducing reagent. This copper-activated board was dipped into a nickel electroless solution for metallization. An Ni/Cu/poly(ethylene terephthalate) composite was obtained in which the copper interlayer served as both catalyst and interlocker for the formation of the composite. X-ray diffraction and energy dispersive X-ray analysis indicated that the metallic layer was composed of Ni and Cu. Atomic force microscopy revealed that the metal layer was smooth with root-mean-square roughness of 5.98 nm (area: 3 μm × 3 μm). The adhesion of the plated film to the polymer was measured to be 16.3 N cm −1 , higher than those from traditional processes. The Ni/Cu/poly(ethylene terephthalate) composite was conductive and magnetic, and could be used for electromagnetic interference shielding.

Two types of WC – Ni cermet composite coating samples were manufactured using conventional vacuum brazing. One included boron in an Ni – Cr – Si brazing alloy powder sheet, and the other did not. Ball-on-disk wear testing and erosion testing by solid particle attacks were conducted to study their properties. The microstructure of each sample was characterized using scanning electron microscopy. X-ray diffraction patterns were also surveyed and the 3D morphologies of the eroded surface were investigated. Boron addition in the brazing alloy sheet was more effective in improving the abrasive wear resistance than erosion of solid particle impaction.

Crystalline B 4 C was grown on the surface of diamond granules using a solid phase reaction from Ti – B – diamond raw materials. A series of analysis techniques, including X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectrometry, were employed to investigate the as-grown B 4 C grains. The grown B 4 C grains had lengths of several micrometers. A mechanism for B 4 C coating formation on diamond particle is discussed.