Band 110, Heft 11

The strain-rate sensitivity exponent m and activation volume υ ∗ are often used to characterize the strain-rate sensitivity of strength behavior in metals and alloys. Complications can arise when the m and υ ∗ values become indeterminate, due to factors such as an inherent scatter in the mechanical property data. The study of commercial Ti-alloy wires is considered wherein to overcome this limitation, the formulation of the Kocks–Mecking (K–M) model is modified to provide a parameter c b that characterizes the microstructural scale responsible for the observed plasticity and work hardening behavior. The softening factor c b is found to be independent of strain-rate for the Ti-alloy wires of this study. It is proposed that c b !can offer a versatile and complementary computation to the activation volume υ ∗ since its formulation includes the yield and ultimate strength values along with the plastic strain. For the tensile testing of Ti-alloy wires, a low c b -value of 14 is calculated for Ti-6Al-4V that is consistent with >10 % plasticity during work hardening whereas a high c b -value of 135 for Ti-6Al-7Nb corresponds with <4 % plasticity.

The aim of this investigation is to study the comparative cyclic deformation behavior of SA333 Gr-6 steel at two different temperatures. Strain-controlled low cycle fatigue experiments were carried out at 1 × 10 –3 s –1 strain rate at room temperature and 300 °C. Cyclic hardening of the steel was observed at both the temperatures. However, greater cyclic hardening was observed at 300 °C for all the strain amplitudes. Non-Masing behavior was observed at both the temperatures. Greater deviation from Masing behavior was observed at 300 °C due to generation of more dislocation activities. Higher dislocation density and sub-cell structure formation is associated with the hardening characteristic of the material. Lower fatigue lives were observed for all strain amplitudes at 300 °C in comparison to room temperature. This is due to the deleterious effect of dynamic strain aging, which is activated at 300 °C. The fracture surface reveals that with an increase in strain amplitude the number of crack initiation sites increases and the fatigue region decreases, which leads to a reduction in fatigue life.

The effects of nickel and aluminum on the precipitation of copper-rich precipitates in Fe – Cu ferritic alloy were investigated by combining experiments and first-principles calculations. With the addition of Ni and Al, the Fe – Cu – M ( M  = Ni or Al) alloy reaches the peak ageing condition earlier than the Fe – Cu binary system during ageing at 500 °C. The transmission electron microscopy results indicate that the average sizes of the copper-rich particles in the Fe – Cu – Ni and Fe – Cu – Al alloys are smaller than that in the Fe – Cu alloy, while the density numbers of the former are larger than the latter. The interactions of the copper atom with nickel and aluminum atoms are all attractive in the first nearest neighbor shell. These attractions increase the local concentration of nickel and aluminum around copper and promote the clustering of solutes, consequently increasing the nucleation rate of Cu-rich precipitates.

The influence of Mn on the microstructure and mechanical properties of as-cast and as-solution Mg–Mn alloys was investigated in the present work. X-ray diffraction patterns revealed that the dominant phases of the experimental alloys were the α-Mg matrix and Mn precipitates. The average grain sizes of the as-cast Mg–Mn alloys decreased monotonically with increasing Mn content, although Mn addition showed a negative grain refinement effect on the as-cast Mg alloys. Increasing the Mn content enhanced the tensile yield strength of the as-cast alloys from 17.6 to 35.1 MPa. The improved mechanical properties are mainly attributed to precipitation hardening; a large number of Mn precipitates were located within the matrix and along the grain boundaries. Meanwhile, the lower tensile strength and Vickers hardness of the as-solution alloys is explained by the coarsened microstructure and poor solid-solution strengthening behaviour.

The effects of extrusion and T6 heat treatment on the microstructure of AZ91-Pr alloy and its wear properties under different applied loads were investigated. The extrusion refined the grains by dynamic recrystallization and grain breakage. The T6 heat treatment resulted in a more uniform distribution of β-Mg 17 Al 12 . Compared with the as-cast alloy, the hardness of the extruded alloy was increased by 18.8 %, and the T6 heat-treated alloy was increased by 58.4 %. The wear rate and friction coefficient of the three alloys increased with load. Under the same conditions, the values of the alloy after T6 treatment are the lowest and the values of the as-cast alloy are the highest. Abrasion and delamination were the wear mechanisms at the load of 30 N, and delamination and oxidation were the wear mechanisms at the load of 60 N. The wear mechanisms at 90 N load were plastic deformation and delamination.

Aluminum alloy 2017, as one of the aluminum alloys which are used in aircraft structures, is a precipitation-hardened and anodized material. It was heat treated to the natural aging condition (T4). An electrolyte containing sulfuric acid and chromic acid (SCA) was used for anodization treatment of the AA 2017 T4 alloy. The present work investigates the effect of anodized film of 7 μm in thickness on hardness, mechanical strength and fatigue behavior of the alloy. The results showed that the ultimate strength, yield strength and hardness of the heat treated alloy increased by 96, 98.5 and 7.6 % respectively as compared with that of the untreated one. Similar results showed that the hardness of the anodized specimen increased by 12.5 % with a slight decrease by 1.9 and 4.6 % in tensile ultimate and yield strengths respectively as compared with that of the unanodized one. The anodized film affects the fatigue behavior under the rotating–bending fatigue test of R  = –1. The effect of anodized film on fatigue lifetime at low and high stress levels is discussed.

Pure molybdenum is one of the most important refractory metals owing to its high melting point, low thermal expansion coefficient and good thermal conductivity. Molybdenum carbide exhibits superior wear resistance to molybdenum owing to its low friction coefficient and high hardness; therefore, in this study, to endure abrasive conditions, the surface of molybdenum was modified as molybdenum carbide. In accordance with this purpose, pure molybdenum powder was sintered under constant pressure (40 MPa), various temperatures (1 650 °C, 1 700 °C and 1 725 °C) and holding periods (180 s, 360  s and 540 s) using the spark plasma sintering technique. The effects of sintering temperature and holding period on relative density, microhardness, microstructure and wear properties of the specimens were investigated. Moreover, carbide formation on the surface due to carbon diffusion was also investigated. Relative density values decreased with increasing sintering temperature and period, obtaining the highest value of 97.55 % at 1 650 °C and 180 s. A microhardness value of 15.20 GPa was reached on the surface while the maximum value measured at the cross-section was 2.17 GPa, showing the formation of molybdenum carbide structure on the surface of all specimens. Furthermore, microstructural analysis supported the microhardness results and showed a minimum 288-μm thick molybdenum carbide layer on the surface. In addition, a typical eutectic microstructure was observed between molybdenum and molybdenum carbide layers; however, the decrease in temperature decreased the thickness of the eutectic layer and it gradually disappeared from the centre to edge of disc-shaped specimens. Wear resistance of surfaces was improved by decreasing holding period where a non-porous carbide layer and highest microhardness values were achieved. The cross-sectional analysis corresponding to molybdenum base metal proved that the formation of molybdenum carbide layer because of carbon diffusion improved the wear properties of the sample with the low friction coefficient and low wear depth.

The main purpose of this study is to comprehend the effects of cryomilling (CM) time on dry sliding wear behaviour of 10 wt.% TiB 2 reinforced Al-12.6 wt.% Si metal matrix composites (MMCs). The MMCs were synthesised via sequential milling (mechanical alloying + cryomilling) and pressureless sintering. Indentation tests performed at a maximum load of 200 g showed that the Young's modulus and hardness of the composites increased up to 20 min of CM time. The effects of CM time and load on the composites were examined based on their tribological properties. Based on the results, the 4 h mechanical alloyed (MA'd) and 30 min CM'd sample showed the lowest wear rate under the normal load of 1 N, with its wear mechanism being predominantly oxidative. However, for the normal load of 4 N, the lowest wear rate was found in the 4 h MA'd composite, in which the dominant wear mechanism was abrasion.

The enthalpies of mixing in liquid alloys of the ternary Ce–Cu–Sb system were determined over a wide range of compositions by means of isoperibolic calorimetry at temperature 1300 K. Measurements were performed along five sections ( x Cu / x Sb = 0.20/0.80; 0.40/0.60 and 0.60/0.40 for x Ce changed from 0 up to 0.35 and x Cu / x Ce = 0.20/0.80 and 0.60/0.40 for x Sb changed from 0 up to 0.25). The enthalpies of mixing in the ternary system were found to be exothermic and steadily increasing in absolute values from the binary constituent systems Ce–Cu and Cu–Sb towards the Ce–Sb boundary, reaching the minimum value of approximately –120 kJ · mol –1 in the vicinity of the phase CeSb.

The effects of in-situ AlP formation on the solidification process of an Al-20 %Si alloy were studied by examining the change law of the solidification microstructures after the addition of Cu–P alloy to the melt and stirring. During the early stage of stirring, the eutectic Si was clearly refined and Si mainly existed in a point-like shape. During the middle stage of stirring, the typical eutectic microstructure disappeared, and the microstructure consisted of fine primary Si uniformly distributed in the α matrix. Many AlP precipitates with sizes near the critical nucleation radius of primary Si were formed after the Cu–P alloy was dissolved in the Al-20 %Si alloy and stirred. These AlP precipitates became the nuclei of the primary and eutectic Si. The Si-poor layers around the primary Si provided the conditions necessary for the precipitation of the primary α phases when the primary Si grew. For finer primary Si, the distance between the primary Si was reduced, and lower temperature facilitated the formation of a complete α loop or continuous α phases around the primary Si. During the eutectic reaction, the ultrafine AlP caused the eutectic Si phases to nucleate alone and grow; the eutectic α phases grew based on the primary α phases, which resulted in the formation of divorced eutectic microstructures.

Hot isostatic pressing is the most common method to prepare powder metallurgy high speed steel. However, due to the limitation of the steel capsule, it is still a challenge to directly produce complex-shaped high speed steel. Therefore, a gelcasting sintering process is proposed in this study. Complex-shaped high speed steel parts were prepared. First, fine powders (10 μm) were employed to prepare a gelcasting slurry. Then, the slurry was poured into a silicone mould followed by debonding and pressureless sintering. Nearly full densification was achieved in the gelcasting sintered samples. Homogeneous microstructure was observed with fine carbides (1 – 2 μm) evenly distributed in the matrix. Compared to traditional samples prepared by hot isostatic pressing, the bend strength increased from 2 800 MPa to 3 800 MPa. Additionally, the oxygen content of the sintered samples was lower than 100 ppm.

The study proposes internal electromagnetic stirring as a new electromagnetic stirring method. The structural features of the internal electromagnetic stirrer and traditional electromagnetic stirrer have been compared. Further, the effects of the internal electromagnetic stirring on the direct chill casting of a large-sized 2219 aluminum alloy billet have been investigated. The results demonstrate that the internal electromagnetic stirring can refine and homogenize the microstructure more significantly than the traditional electromagnetic stirring; grain sizes of 124 – 135 μm and 137 – 157 μm are obtained using the former and latter methods, respectively. Moreover, compared with the traditional electromagnetic stirring, the internal electromagnetic stirring is more efficient.