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The microstructures of a new Ni-Co-base disc superalloy, TMW-4M3, before and after the creep test at 725 °C/630 MPa have been systematically investigated by transmission electron microscopy (TEM). The crept microstructures were marked as three different deformation stages (I, II and III) corresponding to the gradually increased strain. At stage I, stacking fault (SF) shearing was the main deformation mechanism. The SF was extrinsic and lay on {111} plane. However, deformation microtwinning became the dominant mode at stage II and III. The average spacing of deformation twins decreased from 109 ± 15 nm at stage II to 76 ± 12 nm at stage III, whereas the twin thickness did not change significantly. The influence of stacking fault energy (SFE) of γ matrix on the deformation mechanism is discussed. It is suggested that lower SFE in TMW-4M3 is partly responsible for the enhanced creep resistance. [ABSTRACT FROM AUTHOR]

Dynamic strain aging (DSA) in a newly developed Ni–Co superalloy, TMW-2, was examined at temperatures ranging from room temperature to 600°C and strain rates between 3×10−2 and 8×10−5 s−1. The alloy exhibited both normal and inverse DSA effects at the temperatures and strain rates tested. Transmission electron microscopy evidence suggests that the normal behavior is associated with conventional DSA due to dynamic dislocation–solute interaction, while the inverse behavior may be related to the occurrence of stacking faults. [ABSTRACT FROM AUTHOR]

Abstract: In order to improve the high-temperature mechanical properties of NiAl/Cr(Mo) alloys, the effects of Hf additions on microstructure and mechanical properties were systemically examined. Two directionally solidified alloys with composition of Ni-32Al-28Cr-(6−x)Mo-xHf (x =0.2 and 0.5 at.%, respectively), named as 0.2Hf and 0.5Hf hereafter, were prepared. The Hf additions disturbed the cellular structure. The 0.2Hf alloy consisted of dendritic structure, while the 0.5Hf alloy had an intercellular structure. In the 0.5Hf alloy, the Ni2AlHf and Ni16Hf6Si7 precipitates were also confirmed. The high-temperature strength and brittle-to-ductile transition temperature (BDTT) increased with increasing of Hf additions, due to the different strengthening mechanism. In contrast, the ductility and creep resistance decreased with increasing of Hf because of the disturbance of cellular structure. [Copyright &y& Elsevier]

Abstract: The microstructure of a newly developed Co-base superalloy with enhanced high-temperature strength has been investigated using transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP) techniques. It mainly consists of a typical γ/γ′ (FCC/L12) structure and a plate-shaped AB3-type intermetallic compound with hexagonal structure (a∼5.1Å and c∼12.5Å). γ′ is formed with a bimodal distribution and fine γ′ has a cuboidal morphology. Cr and Co are enriched in the γ phase, while Ti, Al and Ni are enriched in the γ′ phase. W and Mo are more or less uniformly distributed in both γ and γ′. Chemical composition analysis by 3DAP suggests that the plate-shaped phase has a higher Ti and lower Al content compared to that of γ′ phase, and the concentration of Ti, Co and Ni has a periodic variation along c-axis with a period of 12.5Å in the plate-shaped phase. [Copyright &y& Elsevier]

Abstract: A novel plate-shaped ω phase formed in a Ti–30Nb–3Pd (wt.%) shape memory alloy during quenching in addition to the formation of α″ martensite, retained β and fine-particle ω phases. The ω and β phases were fully transformed to α″ with a (110)α″ twin structure during room temperature deformation. [Copyright &y& Elsevier]

Abstract: The microstructures of U720Li disc superalloy have been investigated by transmission electron microscopy (TEM) before and after creep test at 725°C/630MPa. The evolution of the crept microstructures was marked as three different stages (I, II and III) corresponding to gradually increased strain 0.1%, 5% and 27%, respectively. At stage I, dislocations bypassed secondary γ′ via Orowan loops. At stage II, partial dislocations started to shear secondary γ′, leaving stacking fault (SF) behind and microtwins formed in part of grains. At stage III, grain boundary sliding occurred due to very large strain and increased effective stress. The results indicated that the creep mechanisms of U720Li at 725°C/630MPa evolved with gradually increased strain. Orowan looping process combining dislocation slip and climb and partial dislocations shearing precipitates were the main creep mechanisms. It is suggested that decreasing the interparticle spacing of secondary γ′, strengthening secondary γ′ and decreasing stacking fault energy (SFE) of γ matrix may be effective methods to improve the creep property at relatively higher temperatures. [Copyright &y& Elsevier]

Abstract: The characterization of the grain boundaries of Ni–Fe-based superalloy (Alloy 718) after service in a compressor has been performed using the high resolution methods: three-dimensional atom probe and transmission electron microscopy. Atom probe analysis shows a segregation of P (∼2at.%), B (∼2at.%), C (∼0.2at.%) and Mo (∼8at.%) at the grain boundaries. These grain boundaries have a zigzag-like shape which follows the regular arrangement of small γ″ precipitates. [Copyright &y& Elsevier]

Abstract Effects of service temperature on the tensile properties and microstructural evolution of tensile specimens manufactured by commercial pure (CP) titanium subjected to laser shock peening (LSP) were investigated. Special attention was paid to the microstructural features at the top surface close to the fracture zone of the failed LSPed specimens at 20 °C, 150 °C, 250 °C, and 350 °C by means of transmission electron microscopy (TEM) observations. Results indicated that the ultimate tensile strength (UTS) of LSPed specimens gradually decreased, but the area reduction and elongation of LSPed specimens increased with the increment of service temperature. Furthermore, the influence mechanism of service temperature on the microstructural evolution of the LSPed CP titanium consisted of two modes: (a) ultra-high strain rate inhibited the dislocation motion at a lower temperature which was more prone to the occurrence of deformation twinning, and (b) ultra-high strain rate was easier to activate dislocation motion at a higher temperature, making deformation twinning disappear in the plastic deformation layer of CP titanium. Graphical abstract Image 1 Highlights • Increasing temperature is easier to generate necking phenomenon of LSPed CP titanium. • Ultimate tensile strength of LSPed specimens decreased with increasing temperature. • Microstructural features of failed specimens at various temperatures were presented. • Influence mechanism of temperature on microstructure of LSPed specimen was revealed. [ABSTRACT FROM AUTHOR]

Abstract: Micro-structural evolution and grain refinement in ANSI 304 stainless steel subjected to multiple laser shock processing (LSP) impacts were investigated by means of cross-sectional optical microscopy and transmission electron microscopy observations. The plastic strain-induced grain refinement mechanism of the face-centered cubic (fcc) materials with very low stacking fault energy was identified. The micro-structure was obviously refined due to the ultra-high plastic strain induced by multiple LSP impacts. The minimum grain size in the top surface was about 50–200nm. Multidirectional mechanical twin matrix (MT)–MT intersections led to grain subdivision at the top surface during multiple LSP impacts. Furthermore, a novel structure with submicron triangular blocks was found at the top surface subjected to three LSP impacts. The grain refinement process along the depth direction after multiple LSP impacts can be described as follows: (i) formation of planar dislocation arrays (PDAs) and stacking faults along multiple directions due to the pile up of dislocation lines; (ii) formation of submicron triangular blocks (or irregularly shaped blocks) by the intersection of MT–MT (or MT–PDA or PDA–PDA) along multiple directions; (iii) transformation of MTs into subgrain boundaries; (iv) evolution by continuous dynamic recrystallization of subgrain boundaries to refined grain boundaries. The experimental results and analyses indicate that a high strain with an ultra-high strain rate play a crucial role in the grain refinement process of fcc materials subjected to multiple LSP impacts. [ABSTRACT FROM AUTHOR]

Abstract: The plastic deformation behavior and the effects of the impact time on the LY2 aluminum (Al) alloy during multiple laser shock processing (LSP) impacts were investigated. The residual stress in the near-surface region was determined by X-ray diffraction. In addition, the micro-structural features of the hardening layer were characterized by scanning electron microscopy, optical microscopy and transmission electron microscopy. It was found that the micro-structure was obviously refined due to the ultra-high plastic strain induced by multiple LSP impacts. The minimum grain size in the top surface after multiple LSP impacts was about 100–200nm. The grain refinement process after multiple LSP impacts can be described as follows: (i) the formation and development of dislocation lines in original grains; (ii) dislocation tangles (DTs) and the formation of dense dislocation walls (DDWs); (iii) transformation of DTs and DDWs into subgrain boundaries; and (iv) evolution of the continuous dynamic recrystallization in subgrain boundaries to refined grain boundaries. [Copyright &y& Elsevier]