Comparison of the Cracking Behavior of Powder Metallurgy and Ingot Metallurgy Ti-5Al-5Mo-5V-3Cr Alloys during Hot Deformation

The hot workability of metallic materials is significantly dependent on its ability to form plastic without cracking and fracturing. In this work, the cracking behavior of powder metallurgy (PM) Ti-5Al-5Mo-5V-3Cr (Ti-5553) alloy, consolidated from powder mixture, at a deformation temperature range of 600 °C–850 °C and strain rate of 0.1 s−1–10 s−1, was investigated through isothermal compression tests. The cracking behavior of the as-cast ingot metallurgy (IM) Ti-5553 alloy, at a deformation temperature of 700 °C was also investigated for comparison. Results suggested that the PM Ti-5553 alloy had a better hot workability, with a larger cracking-free processing window, and a lower deformation resistance than the IM counterpart. 45° shear fracture occurred in the PM alloy, compressed at the condition of 600 °C/10 s−1, and edge cracking was observed at the 700 °C/10 s−1. 45° shear fracture was also significant in the IM alloy specimen tested at 700 °C/10 s−1, and all the other IM alloy specimens compressed at 700 °C displayed longitudinal cracking. Moreover, the microscopic cracking observation showed that ductile dimple cracking can be found in the IM alloy, but brittle cleavage fracture was dominant in the cracking surface of PM alloy with a relatively low cracking ductility.

Fracture Analysis of an Aluminum Alloy Gear Pump Housing

The gear pump housing in bulldozer, which was made of cast Al-Si alloy, was found to break in two halves during running. In order to determine fracture reason of the gear pump housing failure analysis in detail were conducted by visual inspection, optical microscope and scanning electron microscope (SEM), chemical analysis, hardness inspection. Visual inspection showed that the crack source has a white semicircle on the interior side of the section and propagated toward the exterior surface of the housing with obvious radial state. Meanwhile, the inner wall has the wear trace in depth of 0.1 mm due to boring phenomenon. SEM observation indicated brittle cleavage fracture mechanism with many casting defects such as dendritic shrinkage and inclusion as well as initiated crack in gear pump housing. The casting defects probably promoted the initiation and propagation of the crack and decreased the loading capacity of the gear to impacting force from the inside shaft. The analysis results show that the failure of gear pump housing mainly resulted from serious boring phenomenon, dendritic shrinkage and the lower material performance.

Preparation and Mechanical Properties of a Bulk Icosahedral Quasicrystalline Ti-Zr-Sc-Ni Alloy

The discovery of the icosahedral quasicrystalline phase (i-phase) in as-cast Ti40.83Zr40.83-xScxNi18.34(x = 0~2.0) alloys is described herein. The effect of Sc on the structure and mechanical properties of the bulk quasicrystalline alloys is investigated. The results show that the phase structure of the as-cast alloys are mainly composed of icosahedral phase accompanied by minor C14 Laves phase (L-phase), and the mechanical properties of the bulk quasicrystalline alloys have been examined at room temperature, the compressive fracture strength first increased and then decreased with increasing x from 0.4 to 2.0, and the highest strength is near 1400 MPa when x =1.2, it was 380 MPa higher than the without Sc alloy. The bulk quasicrystalline alloy exhibits the elastic deformation by the compressive test, and the fracture mode was brittle cleavage fracture.

Study of Tensile Properties and Fracture Mechanisms of Fully Lamellar TiAl Based Alloy

The tensile properties and fracture mechanisms of fully lamellar TiAl based alloy was studied by room and high temperature (400°C) tensile test. The results indicated that: the crack initiated between weak lamellar interfaces．With tension increased, the cracks extended along lamellar direction. Crack propagation resistance was enhanced at colony boundary. Ligaments broke while load increased, and then the main crack and microcrack linked. The fracture process was that the main crack occurred firstly, microcrack appeared and then propagated, the main crack and microcrack were linked，and the alloy fractured finally; The ductility of the alloy at 400°C was superior to that at room temperature; The fracture mechanism of the fully lamellar TiAl based alloy is brittle cleavage fracture.

Powder Metallurgical Processing of Ti6Al4V Alloy

Ti6Al4V powders were produced by Argon gas atomization, the powder fraction ＜ 250μm was hot isostatically pressed (HIP) at 920°C and 140MPa. The properties of pre-alloyed powders and the compact were investigated in this paper. Powder particles are almost perfectly spherical. The microstructure of powder surface is approximate hexagonal cellular structure, the inner structure exhibits cellular αphase and needle-like martensiteα′ phase, these are resulting from the rapid solidification. After HIP, Ti6Al4V alloy has a Widmanstaten microstructure consisting of continuous grain boundary α（GBα）phase and β transformation structure, the grain size of GBα phase is in the range of 5～15μm . The tensile test at room temperature shows that strength of samples is 880MPa, the fracture surface exhibits obvious brittle cleavage fracture features including cleavage facets with river pattern and a few elongate dimples of different sizes and big voids at localized area.

Effect of Heat Treatment on Microstructures and Properties of Mg-9Gd-4Y-0.3Zr Alloy

microstructures, aging hardness, mechanical properties of Mg-9Gd-4Y-0.3Zr alloys were investigated. The microstructure is a typical dendritic structure of as-cast sample, The aging test of extruded samples were carried at a temperature rang of 200-300°C and at a different aging time. The aging peak hardness is about 120HV, tensile strength was tested at temperature 25°C, 200°C, 250°C and 300°C, tensile strengths are 375 Mpa, 364 Mpa, 329 Mpa, 286 Mpa respectively, the maximum elongation is 13.32% at 300°C. The fracture mode is mainly microvoid coalescence fracture combination the brittle cleavage fracture at room temperature, and microvoid coalescence fracture at 200-300°C.