Strain relaxation and it’s mechanism of single Si1-xGex/Si epilayer structures grown on Si(00l) substrate

The degree of strain relaxation via a formation of misfit dislocation has been calculated in single Si_1-xGe_x/Si (001) epilayers grown at 640°C, by gas-source molecular beam epitaxy. The scale of strain relief via misfit dislocation was below 22% for Ge content x≤0.15. However, in the case of growth temperature of 550°C. the degree of strain relaxation in 270 nra single epilayer structures was 9% via misfit dislocation formation with negligible relaxation by surface undulation. As growth temperature decreased from 640°C to 550°C with higher Ge content (x=0.22), the mode of strain relaxation appears to have changed from one of misfit dislocation and surface undulation mechanism to that of predominant misfit dislocation formation.     

Ti-6.5Al-2Zr-1Mo-1V钛合金低温应力松弛及组织演变

在500、550和600 ℃及不同初始应力下对Ti-6.5Al-2Zr-1Mo-1V钛合金进行了应力松弛实验。基于经典的Maxwell指数衰减函数,得到了应力松弛极限。提出了利用松弛稳定系数（C_S）和松弛速率系数（C_R）来描述Ti-6.5Al-2Zr-1Mo-1V合金的松弛特性,有利于制定残余应力消减工艺。根据Norton和Arrhenius方程计算了应力指数。通过应力指数和显微组织分析,阐明了应力松弛机理。在不同初始应力下,500 ℃时,位错的攀移和扩散主导了应力松弛过程;550 ℃时,位错滑移在应力松弛过程中起主要作用;600 ℃时,位错滑移、边界滑移和晶粒旋转控制着松弛过程。     

Recent progress in thin film epitaxy across the misfit scale (2011 Acta Gold Medal Paper)

This paper discusses recent progress in thin film epitaxy across the misfit scale through the paradigm of domain matching epitaxy (DME). This epitaxy across the misfit scale is critical for integrating multifunctionality on a chip and creating smart structures for next-generation solid-state devices. There are three sources of strains that are cumulative at the growth temperature, and the relaxation process starts during the growth process. Upon cooling, unrelaxed lattice, thermal and defect strains give rise to net residual strains. In large misfit (ε ? 10%) systems, where lattice misfit strain is predominant, it can be relaxed completely, and then only thermal and defect strains remain upon cooling. In low misfit systems, all three sources contribute to the residual strain upon cooling, as result of incomplete lattice relaxation. The predominant strain relaxation mechanism in thin films is by nucleation of dislocations at the free surface, as the nucleation energy in the bulk is considerably higher. At the free surface, the activation barrier for dislocation nucleation is considerably lower at the steps. Since the step formation energy is lower under a compressive stress compared with tensile stress, it reduces nucleation energy under compressive stress and lowers the critical thickness compared with tensile stresses in thin films. Once the dislocation nucleates, it propagates or glides to the interface to relieve the strain. However, if lattice frictional stress in the film is high, most dislocations may not reach the interface, depending upon the growth temperature and rate. Thus, these two key steps, dislocation nucleation and propagation, play a critical role in the thin film relaxation process. Once the dislocations reach the interface, the atomic structure of the dislocation at the heterointerfaces determines its electronic properties, specifically trapping and recombination characteristics. It is found that the atomic structure of the dislocation is determined by the interplay between strain and chemical free energies. Thus, the dislocations (representing missing or extra planes) play a critical role in the relaxation of thin film heterostructures. This paper focuses on epitaxy across the misfit scale, based upon matching of integral multiples of lattice planes. If the misfit falls between the integral multiples, it is accommodated by the principle of domain variation, where domains alternate to accommodate the misfit. Details of epitaxy from low misfit (～4%) in Ge/Si) to large misfit (～22%) in TiN/Si are shown. In III-nitride/sapphire and II-oxide/sapphire systems, this paper deals with polar orientations, where misfit is uniform in the basal plane, and non-polar orientations, where misfit varies over an order of magnitude in the film plane. It is shown that the DME paradigm is key to the integration of thin film heterostructures across the misfit scale and other complex systems such as vanadium oxide and PZT systems on Si(1 0 0) substrates for the integration of functionalities on a computer chip. Finally, it is shown that the formation of epitaxial and self-assembled nanodots on Si(1 0 0) provides a critical advance, with tremendous implications for information and data storage and related nanomagnetics applications.     

Mechanism of induced nucleation of misfit dislocations in the Ge-on-Si(0 0 1) system and its role in the formation of the core structure of edge misfit dislocations

Ge-on-Si(0 0 1) films are grown by molecular beam epitaxy via a three-step epitaxial growth method (Ge/Ge seed/GeSi buffer/Si(0 0 1)). The dislocation structure of the Ge/GeSi buffer interface is studied by high-resolution electron microscopy. Misfit dislocations on the interface are edge dislocations and are aligned regularly with a period of 9–10 nm. A variety of atomic structures of the dislocation core is observed, known in the literature as dissociated or asymmetric Lomer edge dislocations. The assumption that atomic structures of various degrees of complexity are intermediate states in the formation of a perfect edge misfit dislocation in the course of plastic relaxation of a stressed film is justified. A model is proposed which explains the intermediate states in terms of statistical variation of the nucleation site of the complementary 60° dislocation which forms, together with the primary dislocation, a Lomer dislocation at the interface.     

The Effect of Molybdenum Substrate Oxidation on Molybdenum Splat Formation

Disk splats are usually observed when the deposition temperature exceeds the transition temperature, whereas thick oxide layer will reduce the adhesion resulting from high deposition temperature. In present study, single molybdenum splats were deposited onto polished molybdenum substrates with different preheating processes to clarify the effect of surface oxidation on the splat formation. Three substrate samples experienced three different preheating processes in an argon atmosphere. Two samples were preheated to 350 and 550 °C, and another sample was cooled to 350 °C after it was preheated to 550 °C. The chemistry and compositions of substrate surface were examined by XPS. The cross sections of splats were prepared by focus ion beam (FIB) and then characterized by SEM. Nearly disk-shaped splat with small fingers in the periphery was observed on the sample preheated to 350 °C. A perfect disk-shape splat was deposited at 550 °C. With the sample on the substrate preheated to 350 °C (cooling down from 550 °C), flower-shaped splat exhibited a central core and discrete periphery detached by some voids. The results of peeling off splats by carbon tape and the morphology of FIB sampled cross sections indicated that no effective bonding formed at the splat–substrate interface for the substrate ever heated to 550 °C, due to the increasing content of MoO₃ on the preheated molybdenum surface.     

Temperature and grain size dependence of superplasticity in a Zn−0.3wt.%Al alloy

The superplastic deformation behavior of quasi-single phase Zn-0.3 wt. %Al was investigated. A series of load relaxation and tensile tests was conducted at various temperatures ranging from RT (20 °C) to 200 °C. The recently proposed internal variable theory of structural superplasticity was applied. The flow curves obtained from load relaxation tests were shown to consist of contributions from interface sliding (IS) and accommodating plastic deformation. In the case of quasi-single phase Zn-0.3 wt.% Al alloy with an average agrain size of 1 μm, the IS behavior could be described as a viscous flow process characterized by a power index of M_g=0.5. A large elongation of about 1400% was obtained at room temperature and the strain rate sensitivity parameter was about 0.4. Although relatively large-grained (10 μm) single phase alloy showed a high value of strain rate sensitivity comparable to that of fine-grained alloy at very low strain rate range, IS was not expected from the analysis based on the internal variable theory of structural superplasticity at room temperature. As the temperature increased above 100 °C, however, the contribution from IS was observed at a very low strain rate range. A high elongation of ∼400% was obtained in a specimen of 10-μm-grain-size at 200 °C under a strain rate of 2×10⁻⁴/sec. Jointly appointed at Center for Advanced Aerospace Materials (CAAM)     

镍基单晶高温合金在不同条件下的蠕变性能和组织演化(英文)

研究[001]取向的镍基单晶高温合金在不同测试条件下的蠕变性能,采用扫描电镜和透射电镜研究合金蠕变断裂后的γ′相、TCP相和位错组织演化特征。结果表明:合金具有良好的蠕变性能,蠕变曲线显示出两种不同的蠕变变形特征。在(760°C,600 MPa)、(850°C,550 MPa)条件下,蠕变第一阶段较长;在(980°C,250 MPa)、(1070°C,140 MPa)和(1100°C,120 MPa)条件下,蠕变第一阶段很短。蠕变断裂后,在(760°C,600 MPa)条件下γ′相形态变化不大;在(850°C,550 MPa)条件下γ′相已经合并长大;在(980°C,250 MPa)条件下基体γ被γ′相包围;在(1070°C,140 MPa)条件下基体γ不再连续;在(1100°C,120 MPa)条件下基体γ厚度进一步增加。在(760°C,600MPa)、(850°C,550 MPa)和(980°C,250 MPa)条件下合金无TCP相析出,而在(1070°C,140 MPa)和(1100°C,120MPa)条件下有针状TCP相析出。在低温高应力下,变形特征为位错包括层错的剪切机制;在高温低应力下为位错绕过机制,并在γ/γ′相界面形成位错网。     

Probe M?ssbauer spectroscopy of the evolution of mechanically alloyed Mo92O8(57Fe) system upon heat treatment

X-ray diffraction and M?gsbauer spectroscopy have been used to study the multistage character of the process of recovery to equilibrium in the course of isochronous (1 h) annealings (300?C1300°C) of a mechanically alloyed nanocrystalline Mo₉₂O₈ system with 1 at % M?gsbauer isotope ⁵⁷Fe. Three stages of the recovery to the equilibrium state have been established: at 300?C700°C, the stage of structural relaxation; at 700?C1100°C, the stage of normal grain growth and formation of a dislocation structure; and at 1100?C1300°C, the stage of the formation of a composite Mo₉₉ ⁵⁷Fe₁/MoO₂.     

Evolution of interfacial dislocation networks during long term thermal aging in Ni-based single crystal superalloy DD5

Interfacial dislocations found in single crystal superalloys after long term thermal aging have an important effect on mechanical properties. Long term thermal aging tests for DD5 single crystal superalloy were carried out at 1,100 ℃ for 20, 100, 200, 500 and 1000 h, and then cooled by air. The effect of long term thermal aging on the dislocation networks at the γ/γ' interfaces was investigated by FE-SEM. Results showed that during the long term thermal aging at 1,100 ℃, misfit dislocations formed firstly and then reorientation in the(001) interfacial planes occurred. Different types of square or rectangular dislocation network form by dislocation reaction. Square dislocation networks consisting of four groups of dislocations can transform into octagonal dislocation networks, and then form another square dislocation network by dislocation reaction. Rectangular dislocation networks can also transform into hexagonal dislocation networks. The interfacial dislocation networks promote the γ' phase rafting process. The dislocation networks spacings become smaller and smaller, leading to the effective lattice misfit increasing from-0.10% to-0.32%.     

Strain conservation in implantation-doped GeSi layers on Si(100)

Metastable pseudomorphic GeSi layers grown by chemical vapor deposition or by molecular beam epitaxy on Si(100) substrates were implanted at room temperature. The implantations were performed with 90 keV As ions to a dose of 1 x 10¹³ cm^-2 for Ge_0.08Si_0.92 layers and 70 keV BF₂ ions to a dose of 3x 10¹³ cm^-2 for Ge_0.06Si_0.94 layers. The samples were subsequently annealed for short 10–40 s durations in a lamp furnace with a nitrogen ambient, or for a long 30 min period in a vacuum tube furnace. For Ge_0.08Si_0.92 samples annealed for a 30 min-long duration at 700°C, the dopant activation can only reach 50% without introducing significant strain relaxation, whereas samples annealed for short 40 s periods (at 850°C) can achieve more than 90% activation without a loss of strain. For Ge_0.06Si_0.94 samples annealed for either 40 s or 30 min at 800°C, full electrical activation of the boron is exhibited in the GeSi epilayer without losing their strain. However, when annealed at 900°C, the strain in both implanted and unimplanted layers is partly relaxed after 30 min, whereas it is not visibly relaxed after 40 s.     

Effect of temperature,strain rate and grain size on the mechanical response of Ti3SiC2 in tension

Tensile, relaxation and cycling loading–unloading tests indicate that the mechanical response of Ti₃SiC₂ has a strong dependence on temperature and strain rate, but a weak dependence on grain size. Loading at low temperatures, and/or high strain rates, results in elastic and anelastic deformation, followed by brittle fracture. Anelastic deformation in this regime can be attributed to the easy glide of dislocation into pileups during loading, and their run back during unloading. At high temperatures (≈1100–1200°C), and/or low (<10⁻⁵ s⁻¹) strain rates, the response is plastic. The resulting strain is elastic, anelastic and plastic. Even at 1200°C, intense stress-relaxation processes are observed, and a sizable fraction (≈13%) of the strain is anelastic. At intermediate temperatures and strain rates (transition regime) the mechanical response is controlled by simultaneous damage formation (microcracking) and localized plastic deformation. Combining the results obtained in this work with previous results, viz. tensile creep and strain transient dip tests, a deformation map that takes into account temperature, grain size and strain rate is defined.     

Orientation dependence of the shape memory properties in aged Ni45.3Ti29.7Hf20Pd5 single crystals

The shape memory properties of Ni_45.3Ti_29.7Hf₂₀Pd₅ single crystals aged at 550 °C for 3 h and at 600 °C for 48 h were investigated along the [111], [011] and [−117] orientations in compression. The material was stronger along the [−117] orientation compared to the [111] and [011] orientations based on load-biased thermal cycling experiments. The shape memory properties such as reversible strain, temperature hysteresis, critical stress for stress-induced martensite transformation and Clausius–Clapeyron relations were also strong functions of orientation and aging condition (precipitate characteristics). Shape memory effect with no or negligible irrecoverable strain was observed under stress levels as high as 1000 MPa. After aging at 550 °C for 3 h, the maximum reversible strains were 2.2%, 2.7% and 0.7% along the [111], [011] and [−117] orientations, respectively. Aging at 600 °C for 48 h resulted in maximum reversible strains of 2.3%, 3.2% and 0.9% along the [111], [011] and [−117] orientations, respectively. In both cases, similar levels of transformation strain, as a function of orientation, were observed during superelastic testing. The maximum work output reached 27 J/cm³ in the [011] orientation after aging at 550 °C for 3 h.     

An internal variable approach of orientation dependent deformation mechanism of rolled AZ31 magnesium alloy

Load relaxation behavior of an AZ31 Mg alloy has been studied in relation to temperature and orientation dependence. The rolled plate with 50 mm thickness was first homogenized at 400 °C for 4 h before preparing test specimens in the directions of 0° (RD), 45°, and 90° (ND) from the rolling direction. A series of tensile tests was consequently carried out under a strain rate of 10⁻²/s at room temperature (RT), 100 °C, and 200 °C. The RD specimens were found to deform mainly by dislocation slips without twinning. The 45° and 90° specimens were, on the other hand, found to deform in a combined mode of twinning and dislocation slips. Load relaxation tests were also performed to obtain flow curves in terms of stress and strain rate at the three different temperatures. The flow curves in terms of stress vs. strain rate were also found to consist of a plasticity curve due to dislocation glides in the lower strain rate region and a twin curve in the high strain rate range for the 45° and 90° specimens as prescribed by an internal variable theory. These results were consistent with the tensile test results and were further confirmed by microstructure observations.     

The oxidation of 18/8 type stainless steel in high pressure CO2/2% CO

The oxidation of a variety of 18/8 stainless steels in high pressure CO₂/2% CO has been followed in the temperature range 550–650°C using intermittent weight gain measurement metallography, fractography and scanning electron microscopy. Duplex scales formed at an early stage of oxidation. The weight gain curves were analysed in terms of a two stage process, the initial stage exponentially approaching a slower secondary stage. The primary rate of duplex scale formation did not show the usual temperature dependence. Rather a maximum in the rate was observed at ～600°C. Secondary rates were well established at 650°C and 550°C the secondary rate at 550°C, however, being higher than that at 650°C. Various spinels occurred and the results are explained in terms of the rate controlling process being cation diffusion through different inner spinels.     

Configuration and interaction of misfit dislocations in si1-xgex/si(001) epilayer heterostructures grown by gas-source mbe

Transmission electron microscopy has been employed to investigate dislocation configurations and interactions m Si_1-xGe_x/Si(001) epilayer heterostructures grown by Gas-Source MBE Thick Si_1-xGe_x/Si(001) epilayers had 3-dimensional configuration of misfit dislocations in the entire structure The main arrays of misfit dislocations at each single Si_1-xGe_x/Si(001) interface were along the two <110> directions, being 60° type in character Three-way dislocation configuration, resulting from the interaction between dislocations, was observed at the interface Observation of <001> oriented dislocations having striation lines was new in low misfit (1<%) Si_1-xGe_x/Si(001) epilayer heterostructures     

Cyclic Oxidation Behavior of the Ti–6Al–4V Alloy

The cyclic oxidation behavior of the Ti–6Al–4V alloy has been studied under heating and cooling conditions within a temperature range from 550 to 850 °C in air for up to 12 cycles. The mass changes, phase, surface morphologies, cross-sectional morphologies and element distribution of the oxide scales after cyclic oxidation were investigated using electronic microbalance, X-ray diffractometry, scanning electron microscopy and energy dispersive spectroscopy. The results show that the rate of oxidation was close to zero at 550 °C, obeyed parabolic and linear law at 650 and 850 °C, respectively, while at 750 °C, parabolic—linear law dominated. The double oxide scales formed on surface of the Ti–6Al–4V alloy consisted of an inner layer of TiO₂ and an outer layer of Al₂O₃, and the thickness of oxide scales increased with an increasing oxidation temperature. At 750 and 850 °C, the cyclic oxidation resistance deteriorated owing to the formation of voids, cracks and the spallation of the oxide scales.     

Effect of heat-treatment conditions on structural and phase transformations in a two-phase α + β titanium alloy subjected to thermomechanical treatment

Effect of heat-treatment parameters (heating temperature in a range of 550 to 700°C and holding time from 0.5 to 2 h) on the occurrence of structural and phase transformations in the Ti-6-4 Eli titanium alloy, which was preliminarily subjected to warm rolling at 550°C after quenching from 945°C, has been studied. During the heating of the rolled alloy to 550°C, the formation of recrystallized nanosized grains in the martensitic matrix was found to occur. The increase in the temperature and time of holding favors the activation of recrystallization processes and increase in the size of arising grains to a micron level. It has been found that the decomposition of the α′ martensite can occur at the expense of both the precipitation of nanosized β-phase particles via a heterogeneous mechanism (T _h = 550–600°C) and through the formation of individual β-phase grains between recrystallized α′ particles at the higher heating temperatures (650–700°C). The prevalence of one of these processes, namely, recrystallization or decomposition, during the heat treatment of the deformed alloy determines the character of changes of microhardness characteristics.     

Effect of Silicon on the Steam Oxidation Resistance of a 9%Cr Heat Resistant Steel

The steam oxidation of 9Cr–0.5Mo–1.8W steels containing 0.06 to 0.49%Si was investigated at 500°, 550°, 600°, 650° and 700°C. The steam oxidation rate of the steel decreased with increasing silicon content. The effect of silicon was most remarkable at 700°C. At 500°, 550° and 600°C, the effect was almost the same, and was smaller than that at 700°C. At 700°C, the formation of a protective amorphous-SiO₂ film reduced the oxidation rate considerably. On the other hand, at 600°C or less, silicon dissolved in the Fe–Cr spinel lattice with no evidence of SiO₂. At 650°C, although amorphous SiO₂ was observed, as at 700°C, at the scale–metal interface, the effect of silicon was the least within the test-temperature range. Thus, 650°C was a peculiar temperature for the effect of silicon on the steam oxidation of 9%Cr steels. The relatively small effect of silicon at 650°C is attributed to the formation of metastable FeO.     

Hot deformation and processing maps of as-sintered CNT/Al-Cu composites fabricated by flake powder metallurgy

The deformation behaviors of as-sintered CNT/Al-Cu composites were investigated by isothermal compression tests performed in the temperature range of 300?550 °C and strain rate range of 0.001?10 s^?1 with Gleeble 3500 thermal simulator system. Processing maps based on dynamic material model (DMM) were established at strains of 0.1?0.6, and microstructures before and after hot deformation were characterized by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and high-resolution transmission electron microscopy (HRTEM). The results show that the strain has a significant influence on the processing maps, and the optimum processing domains are at temperatures of 375?425 °C with strain rates of 0.4?10 s^?1 and at 525?550 °C with 0.02?10 s^?1 when the strain is 0.6. An inhomogeneous distribution of large particles, as well as a high density of tangled dislocations, dislocation walls, and some sub-grains appears at low deformation temperatures and strain rates, which correspond to the instability domain. A homogeneous distribution of fine particles and dynamic recrystallization generates when the composites are deformed at 400 and 550 °C under a strain rate of 10 s^?1, which correspond to the stability domains.     

Kinetics and Mechanism of the Oxidation of Molybdenum

The rates of formation of the different oxides on molybdenum in pure oxygen at 1 atm pressure have been determined in the temperature range 500° to 770°C. The rate of vaporization of MoOs is linear with time, and the energy of activation for its vaporization is 53,000 cal per mol below 650°C and 89,600 cal per mol at temperatures above 650°C. The ratio Mo0_{3(vaporizing)}/Mo0_3(surface) increases in a complicated manner with time and temperature. There is a maximum in the total rate of oxidation at 600°C. At temperatures below 600°C, an activation energy of 48,900 cal per mol for the formation of total Mo0₈ on molybdenum has been evaluated. The suboxide Mo0₂ does not increase beyond a very small critical thickness. At temperatures above 725°C, catastrophic oxidation of an autocatalytic nature was encountered. Pronounced pitting of the metal was found to occur in the temperature range 550° to 650°C. Marker movement experiments indicate that the oxides on molybdenum grow almost entirely by diffusion of oxygen anions.