S. Lesz

Influence of Nb Microaddition on a Microstructure of Low-Alloyed Steels with Increased Manganese Content

The present study is a first step of a project to obtain thermo-mechanically processed fine-grained increased Mn content TRIP steels with large fractions of retained austenite. Two 0.17C-3Mn-1.6Al-0.2Si-0.2Mo steels with and without Nb microaddition were produced in a vacuum induction furnace. The influence of Nb microaddition on a macrostructure, a grain size and hot-working behavior were examined. The steels are characterized by a slight macrosegregation of Al in the as-cast state, minimized for a Nb-microalloyed steel. After hot forging refined bainitic-martensitic structures with large fractions of γ phase obtained. The steel microalloyed with Nb has finer granules of retained austenite at comparable fractions of this phase. The force-energetic parameters of hot-working were determined in an uniaxial hot-compression test at temperatures of 1150 and 950°C and strain rates from 0.1 to 10s-1. The Gleeble 3800 thermomechanical simulator was used. The hot-working behaviour of the investigated steels is challenging because of higher flow stresses and εmax strains compared to conventional TRIP steels with lower Mn contents.

Influence of Copper Addition on Glass Forming Ability, Thermal Stability, Structure and Magnetic Properties of Fe-Co-Based BMGs

The aim of the paper was investigation of the effect of Cu addition on glass forming ability (GFA), thermal stability, structure and magnetic properties of Fe-Co-based bulk metallic glasses (BMGs). The raw materials used in this experiment for the production of BMGs were pure Fe, Co, Cu and industrial Fe-B, Fe-Si, Fe-Nb ferroalloy. Investigations were carried out on BMGs in rods shaped with square section with side of 1.5mm. The structure of the investigated BMGs in rod form is amorphous. The addition of small amounts of Cu is effective in changing GFA and magnetic properties. The melting temperature - Tm remained almost constant for both investigated alloy. Two alloy compositions are at or very close to the eutectics, what according to ref. [1] should guarantee the best metallic glass-forming alloys. The investigated alloys have good soft magnetic properties. The successful synthesis of the Fe36.00Co36.00B19.00Si5Nb4 and Fe35.75Co35.75B18.90Si5Nb4Cu0.6 alloys with high GFA and good soft magnetic properties by using starting industrial alloys are encouraging for the future industry applications.

Crack initiation and fracture features of Fe–Co–B–Si–Nb bulk metallic glass during compression

The aim of the paper was investigation crack initiation and fracture features developed during compression of Fe-based bulk metallic glass (BMG). These Fe-based BMG has received great attention as a new class of structural material due to an excellent properties (e.g. high strength and high elasticity) and low costs. However, the poor ductility and brittle fracture exhibited in BMGs limit their structural application. At room temperature, BMGs fails catastrophically without appreciable plastic deformation under tension and only very limited plastic deformation is observed under compression or bending. Hence a well understanding of the crack initiation and fracture morphology of Fe-based BMGs after compression is of much importance for designing high performance BMGs. The raw materials used in this experiment for the production of BMGs were pure Fe, Co, Nb metals and nonmetallic elements: Si, B. The Fe–Co–B–Si–Nb alloy was cast as rods with three different diameters. The structure of the investigated BMGs rod is amorphous. The measurement of mechanical properties (Young modulus - E, compressive stress - ?c, elastic strain - ?, unitary elastic strain energy – Uu) were made in compression test. Compression test indicates the rods of Fe-based alloy to exhibit high mechanical strength. The development of crack initiation and fracture morphology after compression of Fe-based BMG were examined with scanning electron microscope (SEM). Fracture morphology of rods has been different on the cross section. Two characteristic features of the compressive fracture morphologies of BMGs were observed. One is the smooth region. Another typical feature of the compressive fracture morphology of BMGs is the vein pattern. The veins on the compressive fracture surface have an obvious direction as result of initial displace of sample along shear bands. This direction follows the direction of the displacement of a material. The formation of veins on the compressive fracture surface is closely related to the shear fracture mechanism. The results of these studies may improve the understanding on the fracture features and mechanisms of BMGs and may provide instructions on future design for ductile BMGs with high resistance for fracture.