ductility

The aim of this study was to investigate the behavior of hot deformation and occurrence of restoration phenomena during the deformation of AISI H10 hot work tool steel. For this purpose, hot tensile test was performed on the steel in the temperature range of 900-1150 ºC with a temperature interval of 50 ºC and at a constant strain rate of 0.1s-1. The microstructures were examined and the curves of hot flow and ductility were drawn. According to the curves and microstructures, ductility was lower at temperatures of 900 ºC and 950 ºC due to inactivity of repair processes and the presence of carbides. Ductility increased in the temperature range of 1000-1100 ºC due to the occurrence of dynamic recrystallization. Finally, ductility decreased in the temperature of 1150 ºC due to the dissolution of carbide particles and grain growth. The results obtained from hot tensile test and microstructural studies at a constant strain rate of 0.1s-1 revealed that the appropriate temperature range for deformation of AISI H10 hot work tool steel was 1000-1100 ºC.

One of the new methods to improve the mechanical properties of surface layers is the friction stir processing (FSP). If the FSP process is carried out with a consumable material, this new process is known as the friction stir alloying (FSA). Therefore in this research, the Al7075 surface composites by using reinforcing particles (Al2O3) were produced based on this process in accordance with the DOE approach. So, the RSM was selected as the experiment design method and variable factors such as: tool rotational speed, tool feed rate, tool shoulder diameter and size of reinforcing particles were determined as the input variables. The results of ANOVA and regression analysis of experimental data approved the accuracy of regression equations and showed that the tool feed rate, tool shoulder diameter and size of reinforcing particles with linear and second-order effects, affect on the tensile strength and ductility of the composite specimens. Also, if the tool rotational speed is set at 800 rpm, increasing the tool shoulder diameter from 9 mm to 15 mm will increase the tensile strength of the composite specimens by 17.97%. In addition, lowering the tool feed rate from 60 mm/min to 20 mm/min and reducing the size of alumina particles from 50 micron to 20 micron, will increase the ductility of composite specimens by 1.85% and 5.04%, respectively. Finally, by achieving maximum value of desirability function (0.915), the optimal condition of input variables was determined. In addition, the optimal condition has been confirmed by implementing the verification test.

In this research, the microstructures and mechanical properties of DIN 16MnCr5 low alloy steel under ferrite-martensite dual-phase (with various martensite volume fraction) were compared to full martensitic condition. Dual-phase microstructures were developed by inter-critical annealing. For this purpose, the samples were heated at various inter-critical temperatures (from 740 to 840 °C) for 60 minutes and then water quenching to room temperature. Also, to develop full martensitic microstructure, steel samples were immediately quenched in water after austenitization at 900 ˚C for 60 minutes. The microstructure and mechanical properties of the specimens were analyzed by scanning electron microscope equipped with EDS analyzer, X-ray diffraction and tensile and microhardness tests, respectively. Metallographic observations showed that by increasing the inter-critical annealing temperature from 740 to 840 °C, the martensite volume fraction (Vm) increased from 23 to 87%. Comparison of mechanical properties of dual-phase samples showed that by increasing Vm from 23% to 87%, there was an abnormal tensile behavior at Vm=73%, so that the energy absorption capability (product of tensile strength and uniform elongation) for ferrite-martensite dual-phase samples is much higher than to full martensitic samples. This remarkable modification in the mechanical properties of the dual-phase samples is due to the different hardening responses of the ferrite and martensite microconstituents because of the interaction between them. By increasing the Vm from 23 to 87%, the martensite microhardness decreased continuously from 717 to 471 HV10g, while the ferrite microhardness showed a peak value at Vm=73%.

In this research, the fracture behavior and ductile to brittle transition (DBT) phenomenon, as well as the correlation between fracture surface morphologies and ductility/toughness in a Zr-based bulk metallic glass (BMG) is investigated. The amorphous alloy was produced by arc melting pure elements and suction casting into a water-cooled copper mold. Then, the three point bending test was used at two temperatures of 77 and 298 K and displacement rate of 0.2 mm/min. Fracture surfaces were observed through scanning electron microscopy after bending tests. The fracture toughness of samples is determined by measuring the size of fracture surface morphologies, and the brittle and ductile fracture mechanisms were theoretically studied by using the fluid meniscus instability model. Although the Zr-based BMG is nearly ductile at room temperature, at very low temperature (77 K) it becomes more brittle. Results show that the mean fracture toughness changes from ~16 MPa.m1/2 at 298 K to ~3.5 MPa.m1/2 at 77 K. Furthermore, the critical wavelength of meniscus instability (λc) is calculated to be 127 nm for the present alloy. According to the results, if the initial wavelength of meniscus instability (λI) is smaller than the λc, periodic nano-corrugation morphologies can be observed on the fracture surface. On the contrary, if λI is larger than λc, the dimples or vein-like patterns are more likely to be form on the fracture surface.