Effects of alloying elements on plastic deformation of single crystals of MoSi2

Effects of alloying elements on the compression deformation behavior of single crystals of MoSi₂ have been investigated in the temperature range from room temperature to 1500°C. The alloying elements studied include V, Cr, Nb and Al that form a C40 disilicide with Si and W and Re that form a C11_b disilicide with Si. The addition of Al is found to decrease the yield strength of MoSi₂ at all temperatures while the additions of V, Cr and Nb are found to decrease the yield strength at low temperatures (below 800°C) and to increase the yield strength at high temperatures (above 1300°C). In contrast, the additions of W and Re (C11_b formers) are found to increase the yield strength at all temperatures. Of the ternary elements investigated, Al and Nb seem to be the most effective in improving the low-temperature deformability of MoSi₂ while Re and Nb seem to be the most effective in improving the high-temperature strength, judging from the observed yield strength.     

Plastic deformation of single crystals of TiSi2 with the C54 structure

The deformation behavior of single crystals of TiSi₂ with the orthorhombic C54 structure has been investigated as a function of crystal orientation in the temperature range from room temperature to 1400 °C in compression. Plastic flow is possible only when slip along <110> on (001) is operative and no other slip systems are observed in the whole temperature range investigated. While plastic flow is observed above 300 °C for as-grown crystals, the onset temperature for plastic flow is lowered considerably down to room temperature when the crystal is prestrained at 1300 °C and then re-deformed at low temperatures. The critical resolved shear stress (CRSS) for (001)<110> slip decreases with increasing temperature, exhibiting a moderate peak (or plateau) in the temperature range from 800 to 1100 °C. The deformation mechanism of TiSi₂ is discussed in comparison with those reported for other transition-metal disilicides with the C11_b and C40 structures, which are closely related to the C54 structure of TiSi₂.     

Plastic deformation of single crystals of VSi2 and TaSi2 with the C40 structure

The deformation behavior of (0001)110 basal slip in single crystals of VSi₂ and TaSi₂ with the C40 structure has been investigated in the temperature range from room temperature to 1500 °C in compression. Plastic flow is observed only above 400 and 600 °C for VSi₂ and TaSi₂, respectively. The critical resolved shear stress (CRSS) for basal slip decreases with increasing temperature but exhibits a moderate peak around 1100 and 1400 °C for VSi₂ and TaSi₂ respectively. Dislocations with b = 1/3110 gliding on (0001) basal planes both in VSi₂ and in TaSi₂ are observed to dissociate into two identical partials with b = 1/6110 involving a stacking fault. High-resolution transmission electron microscopy of 1/3110 dislocations indicates that basal slip both in VSi₂ and in TaSi₂ occurs through a conventional single shearing mechanism, unlike in isostructural CrSi₂ and Mo(Si,Al)₂ in which basal slip occurs through a synchroshear mechanism.     

Plastic deformation behavior and operative slip systems of ZrSi2 single crystals with C49 type of structure

Plastic deformation behavior of ZrSi₂ single crystals with C49 type of structure was investigated at temperatures up to 1500 °C in comparison with other silicides with C11_b, C40 and C54 types of structures. [1 0 0](0 1 0), [0 0 1](0 1 0), [1 0 1](0 1 0), and [0 0 1](1 0 0) slip systems were determined to operate and their CRSSs monotonously decreased with increasing temperature.     

Creep deformation of single crystals of binary and some ternary MoSi2 with the C11b structure

The creep deformation behavior of binary and some ternary MoSi₂ single crystals with the soft [0 15 1] and hard [001] orientations has been investigated in the temperature range from 1200 to 1400°C in compression. The alloying elements studied include Nb and Al that form a C40 disilicide with Si and W and Re that form a C11_b disilicide with Si. The creep strain rate for the [001] orientation is significantly (2 orders of magnitude) lower than that for the soft [0 15 1] orientation. The creep strain rate for the [0 15 1] orientation is improved by an order magnitude upon alloying with Re and Nb while alloying with W and Al causes a decline in the creep properties of this orientation. The creep strain rate for the [001] orientation is further improved upon alloying with Re, Nb, W and Al with the extent of improvement significantly larger for the Re addition.     

Effect of long-range order on the cyclic deformation behaviour of Ni3Fe single crystals

Effect of ordering on cyclic deformation in disordered and ordered Ni₃Fe single crystals was investigated focusing on stress–strain response and deformation substructure. The cyclic hardening depended strongly on the long range order. The maximum stress in the disordered crystals increased gradually with increasing number of cycles and then reached a saturation, while ordered ones exhibited cyclic softening after an initial strong cyclic hardening. The cyclic hardening at an early stage of fatigue in ordered crystals may be due to APB tubes and debris which were produced by the intersection between primary and secondary slips. Coarse slip bands were observed in fatigued ordered Ni₃Fe single crystals. In the bands, three-dimensional dislocation structure was formed accompanied by a decrease in the degree of order, which was responsible for the cyclic softening.     

Influence of plastic deformation on the velocity and ultrasonic attenuation of copper single crystals

High-purity copper single crystal samples were plastically deformed between 0.02% and 1% in the direction. The absolute ultrasonic velocity and attenuation related to the dislocations in copper single crystal were measured as a function of the plastic deformation. The amplitude-independent Swing Model for dislocation resonance was considered to obtain information about the evolution of the dislocation density (Λ) with the degree of plastic deformation (%). The experimental data were interpreted as resulting from the contribution of two kinds of dislocations that lie in a cell structure. The results show a linear increase with the deformation of the density of dislocations (Λ_I) that lies in the cell walls [Λ_I (cm⁻²)≈1.5×10⁹ %]. The dislocation density inside the cell (Λ_II) is about two orders of magnitude lower than Λ_I.     

Anisotropic plastic deformation behavior of B2-ordered Fe_3Al single crystals at room temperature

The tensile plastic deformation behavior of B2-ordered Fe3Al single crystals at room temperature was systematically investigated. The results show that the mechanical properties are strongly orientation dependent. The plastic elongation of crystals with orientation near [110] is as high as 42%. Slip trace anal ysis shows that although slip planes are found to change among {110}, {11 2} and {123} with the change in orientations, the initial slip planes in a ll c ases are {110}. Five-stage work hardening curve including four linear stages and one parabolic stage is obtained; but not all stages are observed in the a ctual deformation of each crystal. In combination with investigations of disloca tion substructure, it is found that deformations in stage Ⅰ～Ⅲ are corresponding to the motion of two-fold superdislocations. The higher work hardening rate of stage Ⅱ is mainly due to the stronger interactions between primary dislocations and secondary dislocations than those in stage Ⅲ. Deformation in stage Ⅳ involved is not only the motion of two-fold superdislocations but also the slip of dissociated superpartials with APB traps and the formation of APB tube, both of which are attributed to the hardening. Deformation in stage Ⅴ is control led by the cross slip of dissociated superpartials. The dominated softening effect of cross slip reduces the hardening rate and leads to the formation of para bolic stage.     

Effects of plastic deformation on lamellar structure formation in Ti–39 at.% Al single crystals

The effects of plastic deformation on lamellar structure formation in solution-treated Ti–39 at.% Al single crystals were investigated, focusing on the role of dislocations of different slip systems. The dislocations were introduced by indentation on the surfaces of solution-treated single crystals with different crystallographic orientations. Traces of basal and prism slips were observed, depending on the position relative to the indentation. During annealing at α₂ + γ dual-phase temperatures, lamellar structures were formed faster where basal slip had occurred than where prism slip had occurred. After long annealing, the length scale of lamellar structures formed depends on the slip system operated during prior deformation: in the region where only one of either basal or prism slip had occurred the lamellar structure was coarser than in undeformed crystal, while in the region where both basal and prism slips occurred the lamellar structure was finer than those formed in undeformed crystal. The reasons for the differences in lamellar structures are discussed on the basis of the frequencies of stacking fault formation on (0 0 0 1) planes as precursors to γ-precipitates. The results suggest that the cross-slip of dislocations between basal and prism planes, which gives rise to the formation of multiple stacking faults on many parallel (0 0 0 1) planes, is responsible for the refinement of lamellar structures.     

Structure of single crystals of nickel high-temperature alloy after plastic deformation and heating

Methods of electron microscopy, x-ray diffraction analysis, and measurement of the microhardness are used to study the effect of moderate (under the conditions of creep) and intense (shear under pressure) deformation on the structure and hardness of single crystals 〈001〉 of a high-temperature alloy based on nickel. The special features of recrystallization and phase transformations in the alloy after deformation by different methods and heating are described. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 10, pp. 26 – 29, October, 2000. The material of the present paper has been reported at the Bern-shtein Perusal on 27 – 28 October 1999 at the Moscow Institute of Steel and Alloys.     

Ti3SiC2表面渗硅涂层的抗高温氧化性能

采用固体粉末包埋法在Ti3 SiC2 陶瓷上渗硅 ,研究了渗硅涂层的抗高温氧化性能 ,用XRD及SEM /EDS分析了渗硅涂层及其氧化后产物的成分、结构和形貌等。结果表明 :渗硅层主要由TiSi2 和SiC组成 ,在空气中氧化时形成了SiO2 和TiO2 的混合氧化物膜。渗硅样品在 110 0℃和 12 0 0℃下的恒温氧化速率比Ti3 SiC2 降低了 2～3个数量级 ,110 0℃下抗循环氧化性能也优于Ti3 SiC2 。但由于渗硅层中存在裂纹 ,循环过程中当裂纹贯穿整个渗硅层时 ,涂层的氧化速度开始增加 ,其保护作用逐步退化。在 110 0℃下空气中循环氧化时 ,经 40 0次循环后涂层已基本失效。     

Plastic deformation of single crystals of TiSi2

Compression experiments have been performed at high temperatures for single crystals of TiSi₂ with the C54 (oF24) structure. Compression axes chosen are -, b- and c-axes, and intermediate directions between a- and c-axes, and b- and c-axes. Based on the slip line observation and the geometrical consideration, it has been concluded that one of the three slip systems, (001)[110], ( 10)[130] and (0 1)[011], is activated depending on the compression axis; in the former two systems dislocations are assumed to be dissociated into superpartials, i.e. 1/2[1101→1/3[100]1+1/6[130] and 1/2[130]→3 × 1/6[130]. The (001)[110] slip is active at room temperature and the other two slip systems are active only above 1000 K. The thermal-activation analysis of the plastic deformation has shown that the deformation is controlled by the Peierls mechanism for the three slip systems; the total activation enthalpy is 1.5 eV for the (001)[110] slip and 4–5 eV for the ( 10)[130] and (0 1)[011] slips. An asymmetry of the Peierls potential is suggested for the (3 0)[130] slip.