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Growth Techniques
Published in Alan Owens, Semiconductor Radiation Detectors, 2019
The CZ and LEC techniques have several limitations, which mainly relate to contamination issues and dislocation defect densities (usually expressed in terms of the Etch Pit Density or EPD14). Contamination usually arises from the encapsulate (usually boron when using B2O3) and/or the crucible (which can be fabricated from any of a number of materials, typically quartz, graphite, glassy carbon, BN and AlN). Crystal defects arise largely from temperature gradients across the melt. While these can be minimized using multi-zone heaters or careful heat-shield design, a reduction in gradients also raises the temperature at the crystal surface, which can diminish the compositional stability from the dissociation of volatile elements such as phosphorus or arsenic. Dissociation can be reduced by the introduction of an ambient of the volatile into the space above the melt. For LEC, growth dissociation can also occur from the crystal surface left exposed above the B2O3 encapsulate layer leading to twinning production. Crystal twinning emanating from the neck of the pulled crystal (see Figs. 4.7(c) and (d)) or the cone of the crucible can also be a problem. Twinning occurs when two separate crystals share some of the same crystal lattice points resulting in an intergrowth of two different crystals joined by a so-called twin boundary. Effective methods to reduce the formation of twinning are an optimization of the cone angle at the crystal shoulder and the application of a magnetic field to suppress temperature variations in the melt during growth.
Hot Rolling System Design for Advanced High Strength Steels (AHSSs)
Published in Jingwei Zhao, Zhengyi Jiang, Rolling of Advanced High Strength Steels, 2017
TWIP steels—these steels contain 18–30% Mn (along with Al, Si, Cr, C and N) and therefore they are mainly austenitic at room temperature and do not need any special heat treatment to stabilize the austenite. The steels deform by various mechanisms such as (i) austenite (fcc) → ε (hcp) martensite phase transformation, or (ii) mechanical twinning or (iii) dislocation guide which causes rapid hardening of the steels. The Mn content has been found to have significant effect on stacking fault energy (SFE) which in turn affects the operating hardening mechanism of these steels (Allain et al. 2004)—for example in Fe+3Al+3Si steels for a Mn content of: 22% Mn, the stacking fault energy (SFE) was found to be 15 ± 3 mj/m2,25% Mn, SFE was21 ± 3 mj/m228% Mn, the SFE was determined as39 ±5 mj/m2 (Pierce et al. 2014).
Examining, Analyzing, Interpreting, and Understanding the Fracture Resistance of High Entropy Alloys
Published in T.S. Srivatsan, Manoj Gupta, High Entropy Alloys, 2020
Twinning itself is an effective means to intensify the ductility of metals and alloys [112–119]. The formation of hierarchical twin architectures in HEAs by the inter-junctions of different twin systems further enhances the ductility through creating three-dimensional pathways and permitting facile cross-slips of dislocations among twin boundaries [62]. In the meantime, the twin–twin interactions and obstruction of dislocation motions by twins compromise ductility, but encourage strengthening. The synergistic operations of these micro-deformation processes are the attributed cause of the high fracture resistance of the CrCoNi MEA.
Subsurface defect evolution and crystal-structure transformation of single-crystal copper in nanoscale combined machining
Published in Philosophical Magazine, 2021
Haiyan Li, Zihao Shao, Ruicheng Feng, Yongnian Qi, Qin Wu, Chunli Lei
As illustrated in Figure 4(f), the V-shaped twins are formed under the tool. In general, deformation twins in FCC metals are generated by the Shockley partial dislocations, which have the same Burgers vector on successive planes [32]. Figure 3 shows that the number of HCP atoms tends to be stable with the formation of the V-shaped twins, which is due to the simultaneous occurrence of dislocation slip and deformation twinning during the machining process. In addition, twins can enhance the single crystal copper strength, indicating that twin boundaries are effective dislocation slip barriers. At the same time, it is also found that twins can enhance the strain rate sensitivity and the strain hardening rate, resulting in increased ductility [33]. As a result, twinning can serve as a mechanism to simultaneously increase both the ductility and strength.
On mechanical twinning in tetragonal lattice
Published in Philosophical Magazine, 2023
Martin Zelený, Andriy Ostapovets, Lucius Fridrich, Petr Šesták, Martin Heczko, Tomáš Kruml
Two crystals are in a position of twins if they have (i) the same lattice, (ii) common crystallographic (twinning) plane and (iii) there is an operation of symmetry by which one crystal can be transformed into the other. This operation of symmetry can be either reflection (Type I twins) or rotation about an axis lying in the twinning plane (Type II twins). Compound twins fulfil both criteria (e.g. twins in cubic crystals). Twins can be formed either at high temperature when atoms can diffuse easily (growth twins) or at low homologous temperature as a result of stresses in the crystal (mechanical or deformation twins). The terms ‘mechanical’ twinning and ‘deformation’ twinning are synonyms that both appear in the literature frequently [1]. Mechanical twinning is a common mode of plastic deformation of crystals. The experimental observations indicate that mechanical twinning can occur in many (maybe all) types of crystal lattices at suitable conditions [2]. There is also wide agreement in the literature that mechanical twinning is a stress-mediated (not deformation-mediated) mechanism [1]. Therefore, the term mechanical twinning is used in this paper. Mechanical twinning thus occurs when local shear stress, i.e. twinning stress, is large enough for a twin to nucleate and propagate. It means that the twinning is frequently observed in deformation under high stress, i.e. at low homologous temperatures, high strain rates or in situations where dislocation slip is difficult or the number of slip systems is limited. The notoriously cited examples of such materials are hcp crystals [3–7]. Mechanical twinning has been reported also for fcc metals and related alloys, which exhibit low stacking fault energy (SFE) [2, 8–12]. Twinning also plays a crucial role in excellent mechanical behaviour of austenitic steels [13, 14] and medium or high entropy alloys [15–18]. The same twinning modes as in fcc-like structures are active in materials exhibiting tetragonally distorted fcc-like structures [19–23].
Understanding raft formation and precipitate shearing during double minimum creep in a γ′-strengthened single crystalline Co-base superalloy
Published in Philosophical Magazine, 2021
F. Xue, C. H. Zenk, L. P. Freund, S. Neumeier, M. Göken
Under sufficient stress, the a/3[] leading superpartial dislocation can enter the γ′ precipitate, leaving a SISF in its wake and the a/6[] trailing partial dislocation remains at the γ/γ′ interface. Note that a dislocation reaction involving a/2 < >{111} dislocations from different slip systems should be active as well since SFs with at least two slip systems are evident. In comparison with Ni-base superalloys, the precipitate shearing is markedly different from cutting via coupled a/2 < >{111} dislocations occurring in the HTLS regime [19,20,24], but analogous to SF shearing in the early creep stages in the LTHS regime [21,22]. This result is in agreement with the findings that the γ′ precipitates in Co-base superalloys exhibit a lower resistance against shearing than in Ni-base superalloys [12,49]. However, cutting happens only infrequently and seems negligible until the end of stage III, as mentioned before. It becomes only a significant deformation mode in stage IV. In addition to SF formation, twinning on the nanometer and micrometer scales is activated in stage IV. Twinning is usually observed upon low-temperature deformation, but a few polycrystalline (PX) and SX Ni-base superalloys also exhibit twinning after creep at 650–850°C. Twinning typically decreases the creep resistance and may change the fracture mode from ductile to brittle [50]. The mechanism was recently also observed in a multicomponent PX Co-base superalloy CoWAlloy2 with 32 at.% Ni during tensile creep at 750°C / 530 MPa [10]. However, twinning was not reported in Co-base superalloys in the HTLS regime. One possible reason for twinning observed here can be related to the tension/compression asymmetry of twin formation in the L12 structure. A preferential twinning normally under compressive stresses with [001] stress axis is also known from some SX Ni-base superalloys which lead to an asymmetry of their creep behavior in tension and compression [51,52]. As explained by Kakehi [51], compressive stresses favor twin formation, because, in that way, the shear is not hindered by unfavorable nearest neighbor relationships. Such a tension/compression asymmetry of creep involving twinning under compressive creep but not under tensile creep at 850°C and 400 MPa has also been reported in a Co-base superalloy ERBOCo-1 with 32 at.% Ni [53]. Even though the composition and creep condition of ERBOCo-1 are different from the present work, this study indicates that twinning might be also a result of a tension/compression asymmetry.