Influence of Shot Peening on the Isothermal Fatigue Behavior of the Gamma Titanium Aluminide Ti-48Al-2Cr-2Nb at 750 °C

One possibility to improve the fatigue life and strength of metallic materials is shot peening. However, at elevated temperatures, the induced residual stresses may relax. To investigate the influence of shot peening on high-temperature fatigue behavior, isothermal fatigue tests were conducted on shot-peened and untreated samples of gamma TiAl 48-2-2 at 750 °C in air. The shot-peened material was characterized using EBSD, microhardness, and residual stress analyses. Shot peening leads to a significant increase in surface hardness and high compressive residual stresses near the surface. Both effects may have a positive influence on lifetime. However, it also leads to surface notches and tensile residual stresses in the bulk material with a negative impact on cyclic lifetime. During fully reversed uniaxial tension-compression fatigue tests (R = −1) at a stress amplitude of 260 MPa, the positive effects dominate, and the fatigue lifetime increases. At a lower stress amplitude of 230 MPa, the negative effect of internal tensile residual stresses dominates, and the lifetime decreases. Shot peening leads to a transition from surface to volume crack initiation if the surface is not damaged by the shots

The Influence of Aluminizing Process on the Surface Condition and Oxidation Resistance of Ti–45Al–8Nb–0.5(B, C) Alloy

One of the major barriers limiting the suitability of TiAl intermetallic alloys for use in the demanding aircraft and automotive industries is their susceptibility to degradation as a result of oxidation at temperatures exceeding 760 °C. Paper presents the characteristics of resistance to cyclic oxidation at 950 °C for Ti–45Al–8Nb–0.5(B, C) alloy with and without protective coating obtained as a result of aluminizing using out of pack method. The characteristics of surface condition were determined by scanning electron microscope with EDS system, transmission electron microscope, and X-ray diffractometer. The favorable behavior of the Ti–45Al–8Nb–0.5(B, C) alloy with a protective coating under cyclic oxidation conditions is a result of a higher content of Al2O3 in the microstructure of the scale and the presence of Al and Nb-rich phases at the substrate interface, which probably constitue a barrier for oxidation process. The high temperature oxidation test revealed that aluminide coating was responsible for a remarkable improvement in the oxidation resistance

Microstructure and Mechanical Properties of Spark Plasma Sintered and Severely Deformed AA7075 Alloy

In this paper, the microstructure and mechanical properties of AA7075 with a coarse-fine-grained laminated microstructure produced by spark plasma sintering (SPS) and the cyclic extrusion severe deformation (KOBO) technique were investigated. It was found that an inhomogeneous grain microstructure was formed from coarse and fine grains by the SPS process and then was transformed into a coarse-fine-grained laminated microstructure by means of KOBO extrusion at room temperature. The grain refinement during KOBO extrusion resulted in a fine grained laminated microstructure created due to the formation of low-angle grain boundaries (LAGBs), followed by dynamic recrystallization, leading to high-angle grain boundaries (HAGBs). The EBSD analysis results reveal the formation of a deformed and partially recrystallized ultrafine grain microstructure owing to the generation and development of shear bands during KOBO extrusion. The ultimate tensile strength (UTS) of the AA7075 alloy rose after SPS-KOBO severe deformation up to 422 MPa, with high strains of about 33%. The obtained results clearly show that the SPS-KOBO extrusion technique allows a bimodal laminated fine gradient grain microstructure to be obtained due to deformation and dynamic recrystallization, which result in both high strength and good ductility. The new heterogeneous AA7075 alloys obtained by the SPS-KOBO combined techniques demonstrate that microstructural heterogeneities can assist in overcoming the strength–ductility trade-off