Twin Density Research Articles

A high-entropy alloy syntactic foam with exceptional cryogenic and dynamic properties

Metal foams are in great demand in extreme service environments with cryogenic temperatures and high strain rates due to their unique properties. However, the reduced ductility and impact toughness under such conditions limit their applications. In this work, we tested the quasi-static and dynamic compression properties of CoCrFeMnNi high entropy alloy syntactic foam at liquid nitrogen temperature (−196 °C). We found that it exhibits ultra-high strengths and energy absorption capacity, especially a superior resistance to embrittlement at cryogenic temperature. Microstructural characterizations reveal that the foam matrix has a high twinning activity and shear localization propensity in cryogenic environments. Dense deformation twins and multiple shear bands intersected, forming a weave-like microstructure that can disperse the deformation and benefits energy absorption. Deformation twins can also strengthen the matrix and delay the crack nucleation and growth. While under dynamic loading, an FCC to HCP phase transformation was activated, forming a nano-laminated dual-phase (NLDP) FCC/HCP structure in the matrix, leading to further strengthening and toughening. Deformation twinning and HCP phase transformation act as additional plastic deformation mechanisms along with stacking faults and shear bands to offset the shortcomings caused by limited dislocation movements at low temperatures and high strain rate dynamic loading, enabling the high strength and energy absorption of the syntactic foam.

Crystallographic orientation dependence of dynamic deformation behaviours in additively manufactured stainless steel

This study reveals the dynamic compressive deformation anisotropy among 316L stainless steel fabricated via wire and arc additive manufacturing process(WAAM). Cylindrical samples extracted from as-built WAAM 316L plate with 35°, 45°, 55°and 90° from scanning direction, were referred to as <111>, <110>, <112> and <100> crystallographic orientations hereinafter, which had been confirmed through backscattered diffraction. Cylindrical samples were subjected to dynamic tests by a split Hopkinson pressure bar. The corresponding mechanical behaviors and deformed microstructures are associated. There are approximately 10% higher dynamic flow stresses from the <111> samples than other samples during dynamic tests at strain rates less than 3500 s−1, which can be attributed to lower density of thinner deformation twins. The lower tendency towards deformation twinning in <111> samples arises from the increase of effective stacking fault energy. WAAM 316L presents an anisotropy of resistance to adiabatic shearing, of which the order is < 100>, <110>, <112> and <111>. Two adiabatic shear bands(ASB) in <111> sample present distinct orientation characters, where the sub-grains in the first ASB show <111> and <112>-fiber while that in the subsequent one take <110>-fiber. This micro-texture discrepancy from two ASBs could be contributed to the precondition change of macro texture in the matrix which give birth to.

Gradient microstructure and prominent performance of wire-arc directed energy deposited magnesium alloy via laser shock peening

Wire-arc directed energy deposition (DED) has attracted significant interest for the fabrication of large-sized, lightweight Mg-alloy components. However, these components generally exhibit poor mechanical properties and limited corrosion resistance owing to their inherent residual stress and non-equilibrium microstructures. Herein, laser shock peening (LSP) was adopted to successfully modify the stress state and microstructure of AZ31 Mg-alloy fabricated using wire-arc DED. The influence of LSP on the residual stress, mechanical properties, electrochemical behaviour, and microstructural evolution was systematically investigated. The experimental results indicate that, compared with the as-built specimen, the performance of the LSP-treated specimen was notable, with a ≈63.8% decrease in the corrosion current density and ≈30% and ≈13% decreases in the yield strength (YS) and ultimate tensile strength, respectively. The enhanced corrosion resistance can be attributed to the LSP-induced compressive residual stress, nanograins, and nanoparticles. Nanocrystallisation, particle refinement, dense mechanical twins (MTs), and planar dislocation arrays (PDAs) jointly contributed to the enhancement of the YS. The LSP-induced nanocrystallisation was rationalized by the accumulation of PDAs, the intersection of multiple nano-MTs, and the transformation of nano-MTs blocks into sub-grains and then into nanograins owing to continuous dynamic recrystallisation. The particle refinement mechanism involved dislocation proliferation and the development of dislocation slip bands, which eventually led to fragmentation and separation. Therefore, this study introduces a LSP post-treatment technology for the residual stress regulation, microstructural modification, and performance enhancement of Mg alloys fabricated using wire-arc DED. Based on the ability of LSP to tailor the microstructure and performance of Mg alloys, a novel method of wire-arc DED with online LSP treatment is proposed. This method can achieve in-situ surface strengthening and the integrated formation of large-sized components with complex geometries.

Effects of controlling grain growth mode on microstructures and properties of Cu foil

By means of direct current electrodeposition, Cu foil with columnar grain growth and equiaxed grain growth with different average grain sizes were prepared. Electron backscatter diffraction (EBSD), X-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscope (TEM) were employed to elucidate the correlation between the microstructures and properties of Cu foil after a certain relaxation time under different grain growth methods. A remarkable preferential ratio of {220} orientation was exhibited in the columnar-grown copper. However, as the copper layer transitions to equiaxed grain growth, the superiority altered to {200} crystal orientation. This variability in grain orientation makes the copper layer profile of columnar crystal growth higher than that of equiaxed crystal formation. The twin boundary density in copper foil increases as the grain growth mode changes from columnar to equiaxed, and grain refinement is beneficial to increase the twin boundary density in the samples. And columnar crystal copper and equiaxed crystal copper show different twin characteristics. There are a few single-stage twin steps in columnar crystal copper and more multi-stage twin steps in equiaxed crystal copper. With an increase of twin boundary density, both the tensile strength and the ductility increase remarkably.

Nanotwin stability in alloyed copper under ambient and cryo-temperature dependent deformation states

Nanometer size grains, solid solution strengthening, low temperatures, and a high density of nanotwins can provide significant strengthening to a material. Nevertheless, under loading the microstructure may evolve altering the effect of grain size, twin density, etc. on this strengthening. In some cases, these effects can be coupled as in detwinning, which is a mechanism for cryogenic grain growth. In this work, a series of copper nanotwinned columnar grains are solid solution alloyed with aluminum and subject to ambient and cryogenic (−196 °C) environments during mechanical indentation, with the influence of alloying on the detwinning evolution studied. Under indentation, at a lower aluminum solute content (2 at.%), the columnar grains bent corresponding to a loss of twins. At a higher aluminum solute content (8 at.%), the grains shear and retain the nanotwins. Atomistic-scale simulations reveal an increase in the incoherent twin boundary velocity with decreasing temperature, but this boundary velocity is reduced with increasing alloy solute content. The coupling of solid solution strengthening and nanotwin stabilization is then used to explain the microstructural stability as a function of indentation at different temperature conditions.

Strain Rate-Dependent Anomalous Work Hardening of a Single-Crystalline CoNi-Base Superalloy

The deformation mechanisms operating in superalloys depend on different parameters such as composition, temperature and deformation rate. So far, the transition from shearing by APB-coupled dislocations to shearing under the formation of stacking faults has been studied exclusively as a function of temperature but not as a function of the strain rate. Therefore, interrupted compression tests with strain rates between 10–3 s−1 and 10–5 s−1 were performed on the single-crystalline CoNi-base superalloy ERBOCo-4 at a temperature of 850 °C. The evolution of the defect structures has been analyzed by conventional transmission electron microscopy. A change of the deformation mechanism from APB-coupled dislocation shearing to stacking fault shearing is found to depend on the strain rate. At lower strain rates, an increased stacking fault density is associated with a higher yield strength and higher work hardening rates at the early stages of plastic deformation due to a very high stacking fault density. After approximately 2.0 pct plastic strain, the stress reaches a plateau and decreases subsequently, which is associated with the formation and thickening of twins as shown by high-resolution scanning transmission electron microscopy. At higher strain rates, the work hardening rate is significantly lower in the early deformation stage. The role of segregation to planar defects and the influence of local phase transformations (LPT) at SESFs is further discussed in reference to the influence of the strain rate. The segregation of W as an η stabilizing element is found to be crucial for the formation of a local phase transformation in ERBOCo-4. At higher strain rates the phase transformation is hindered by insufficient W segregation, resulting in a higher twin density.

Effect of hydrogen on the deformation mechanisms of metastable 304 austenitic stainless steel

The influence of hydrogen on the phase transformation, deformation twins, and dislocation density of metastable stainless steel 304 was investigated in this work. Hydrogen charging resulted in the formation of large amount ά-martensite, great density of deformation twins, and high dislocation density, compared with an uncharged state at the same plastic deformation. These led to a considerable loss of ductility by 60% in metastable austenitic stainless steel under the presence of hydrogen. ά-martensite and twins were preferred site for crack initiation and propagation, resulting in the increase of hydrogen embrittlement (HE) susceptibility.

Modes of strain accommodation in Cu-Nb multilayered thin film on indentation and cyclic shear

Two-phase layered thin films with a high density of semi-coherent interfaces exhibit excellent mechanical properties and thermal stability. In this study, a magnetron-sputtered Cu-Nb dual-layered thin film (∼500 nm for Cu and ∼150 nm for Nb) having an amorphous interface between Cu and Nb with a high density of aligned growth twins in Cu is subjected to severe surface deformation. The material is loaded using indentation and cyclic shear under tribological testing. The strain accommodation in the subsurface microstructure after deformation varies based on the local structure and deformation mode. Grain refinement and crack formations in the stressed region of the Nb layer and localized crystallization of the amorphous interface are observed after indentation and scratch testing. Pronounced detwinning of growth twins in the Cu layer under the cyclic shear strain leaves large dislocations sites and loops which are observed both by high-resolution transmission electron microscopy and experiment-guided molecular dynamic (MD) simulations. Our simulations provided insights into understanding the pathway for the detwinning process under cyclic shear loading.

The effects of strain rate and strain on the deformation-induced grain refinement process and hardness evolution of a polycrystalline cobalt

In this study, the effects of strain rate and strain on the evolution of microstructure and hardness of a polycrystalline cobalt subjected to high-speed impact and quasi-static compression, with the strain rate of ∼ 103 s−1 and 3 × 10−3 s−1, respectively, were investigated. During high-speed impact, the high strain rate promoted the activation of various types of twinning: 101̅2, 101̅1, 112̅1 and 112̅2 twinning, as well as the 101̅1−101̅2 double twinning. Meanwhile, the temperature rise during high-speed impact activated the incomplete dynamic recrystallization. Deformation twinning, deformation-induced martensitic phase transformation and dynamic recrystallization contributed to the grain refinement. During quasi-static compression, dislocation slip and twinning dominated the deformation. Only 101̅2 twins with different orientations were triggered. The number of twins increased with the increase of strain, but was less than that in the high speed compressed sample. The high speed compressed samples exhibited higher hardness values compared with the quasi-statically compressed samples. The more pronounced hardening effect mainly accounted for the higher density of twins produced during high-speed compression. Although the recrystallization during high-speed compression may lead to strain softening, such softening effect was overwhelmed by the strengthening effect of grain refinement.

Quasicrystal phase evolution and mechanical properties of Mg-3.5Zn-0.6Gd alloy processed by high-pressure torsion

To explore the effect of high-pressure torsion (HPT) on the icosahedral quasicrystal phase (I-phase), the microstructure evolution and mechanical property of extruded Mg-3.5Zn-0.6Gd alloy after HPT were investigated in detail. The results showed that a large density of twins and dislocation were generated during HPT, which effectively promoted the microstructure refinement. The block-shaped I-phase was broken into chain structure and gradually evolved into granular I-phase with the increasing strain. The block and granular I-phase through HPT exhibited a more evenly distribution. After HPT for 7 turns, the average grain size was refined to round about 89 nm and microhardness reached to a maximum value about 119 HV. The 2-fold symmetry of icosahedral quasicrystal phase (I-phase) was unveiled by TEM. According to the HRTEM and IFFT, the [11 2‾ 0]Mg//[2-fold]I-phase and (0002)Mg//(2-fold)I-phase can be identified in the Mg-3.5Zn-0.6Gd alloy. The dislocation and defect with an angle nearly 135° were also observed after HPT for 9 turns. With the further increase of strain, the microhardness appeared to decrease due to the dissolution of granular I-phase. The modified strength mechanism was constructed to distinguish the contribution of grain boundary, dislocation and precipitates.

Low-temperature impact toughness and deformation mechanism of CT20 titanium alloy

The impact toughness and deformation mechanism of near-α-type low-temperature titanium alloy CT20 at different temperatures (20 °C, 0 °C, −50 °C, −100 °C and − 196 °C) are studied. The results show that the impact toughness is as high as 90 J/cm 2 at 20 °C, and does not decrease obviously at 0 °C, −50 °C and − 100 °C, but brittle fracture occurs at −196 °C (29 J/cm 2 ). The tortuosity of the crack path decreases with decreasing temperature. At 20 °C, the grain boundary and α colony interface with high-angle grain boundary deflect the crack propagation direction effectively, forming a tortuous crack path, while crack tends to cut through α lamellae, forming a relatively straight path at cryogenic temperature. The lamellar α undergoes kink deformation at 20 °C, showing severe plastic deformation characteristics. In addition, deformation twins and geometrically necessary dislocations (GNDs) are observed near and far from the fracture surface, indicating that the plastic zone is large. As the temperature decreases, the twin density near the crack path tends to increase. However, few twins are seen in the region far from the fracture surface, and GND densities are low at −196 °C, implying that the plastic zone is small. In a word, the synergistic effect of tortuous crack path, dislocation slip and deformation twinning enable CT20 to achieve high impact toughness at 20 °C. At −196 °C, the plastic deformation capacity of the microstructure decreased significantly, resulting in the formation of a small plastic zone and brittle fracture. • α colony interface deflects crack propagation direction effectively at 20 °C, while crack tends to cut through α lamellae at cryogenic temperature. • The synergistic effect of tortuous crack path, dislocation slip and twinning enables CT20 to achieve the highest impact toughness at 20 °C. • The coordinated deformation of dislocation slip and twins keeps the impact toughness at a high level in the temperature range of 0 °C ~ -100 °C. • The plastic deformation ability decreases significantly, resulting in the formation of a small plastic zone and brittle fracture at −196 °C.

Enhancement of mechanical properties of commercially pure titanium by shock induced gradient microstructure with martensitic transformation

In the present work, explosion hardening (EH) technique was used to produce gradient microstructure with martensitic transformation in commercially pure titanium (CP Ti) to improve its mechanical properties. After EH treatment, phase structure, grain size distribution, dislocation structure and twin structure in CP Ti were characterized. Subsequently, microhardness tests, tensile/compressive tests and three-point bending tests were conducted to investigate mechanical properties of EH treated CP Ti along different depth positions from EH treated surface. Experimental results show that gradient microstructure in terms of grain size, α-ω martensitic transformation, texture intensity, twin density, dislocation density and hierarchical nanotwins, are observed in EH treated CP Ti. According to microstructure characteristics, four layers including nanostructure surface layer, refined structure layer, deformed coarse grained layer and coarse grained matrix are divided from EH treated surface to matrix. Microhardness tests show a gradient hardness layer with a maximum hardness increase of 83% within 3.5 mm from EH treated surface. Tensile and compressive test results show that the most significant strengthening exists within 2 mm depth from EH treated surface with a maximum tensile yield stress increase of 71% and the total EH affected depth is more than 25 mm. In addition, the gradient microstructure is affected by the number of EH treatments. For example, bending yield stress increases by 41.3% in 1-EH treated specimens and 64.8% in 2-EH treated specimens. Combining microstructure characterization and mechanical properties testing, shock-induced strengthening mechanism of CP Ti is analyzed: for positions close to EH treated surface, formation of α-ω martensitic transformation and grain refinement is responsible for the significant strengthening phenomenon; whereas, for positions far away from EH treated surface, the moderate strengthening phenomenon is due to the generation of high-density dislocations/twins.

Nanotwinning-induced pseudoplastic deformation in boron carbide under low temperature

Nanotwinned metals exhibit enhanced strength due to the twin-boundary-dislocation interaction, but nanotwins may play a different role in strong, yet brittle ceramics due to the absence of mobile dislocations under low temperatures. Here, we carry out molecular dynamics (MD) simulations using a machine-learning force field to illustrate how nanoscale twins influence the mechanical properties and deformation mechanisms of superhard boron carbide (B4C). Surprisingly, in contrast to brittle failure in single-crystal B4C, we identify an abnormal pseudoplastic deformation behavior in nanotwinned B4C (nt-B4C) which arises from successive breakage of stretched inter-icosahedral B-C bonds. This pseudoplastic behavior shows a strong dependence on temperature and twin density, and it becomes more prominent as temperature decreases and twin density increases. This abnormal anti-correlation between pseudoplasticity and temperature mainly arises from enhanced atomic shear strain induced by thermodynamic vibrations at high temperatures. Moreover, a nanotwinning-induced softening mechanism is observed in nt-B4C, since amorphization preferentially nucleates near twin boundaries due to stress concentration. This work demonstrates that introducing high-density nanotwins into B4C is effective to enhance its low-temperature ductility.

Designing a gradient structure in a Ni-based superalloy to improve fretting fatigue resistance at elevated temperatures through an ultrasonic surface rolling process

In this study, a gradient nanostructured GH4169 superalloy was prepared by ultrasonic surface rolling process (USRP), and the fretting fatigue resistance was investigated at elevated temperatures. After USRP, a high density of deformation twins, stacking faults and dislocations as well as precipitates were observed in the surface of the gradient nanostructured GH4169 superalloy when examined using transmission electron microscopy. Factor separation analysis was carried out to determine the leading factor for the improvement of the fretting fatigue properties of GH4169 at elevated temperatures, and the compressive residual stress was found to be the leading factor. The mechanism was determined to be dislocation movement that can be pinned by deformation twins, stacking faults and precipitates and, thus, is able to stabilize the compressive residual stress at elevated temperatures. As a result, the USRP-treated GH4169 with gradient nanostructure was found to have the highest fretting fatigue resistance at elevated temperatures.

Effect of Si content and annealing temperatures on microstructure, tensile properties of FeCoCrNiMn high entropy alloys

In the present study, FeCoCrNiMnSix (x = 0, 0.1, and 0.2, molar ratio) high-entropy alloys (HEAs) were prepared via arc melting, and subsequently deformed via cold rolling and annealing (CRA). The effects of Si content and annealing temperature on the microstructural evolution and tensile properties of the alloys were systematically investigated. The results indicate that a single-phase FCC structure of coarse-grained FeCoCrNiMnSix HEAs was obtained after homogenized annealing (HOA), and the distribution of each element was uniform. When the molar ratio of Si was increased from 0 to 0.2, the strength of the HOA HEAs increased by nearly 50%, and the elongation did not decrease, largely due to solid solution strengthening and the reduction in stacking fault energy. Conversely, as Si content increased, the recrystallization ratio decreased during annealing. A heterogeneous grain structure composed of recrystallized and residual deformation regions (high density of dislocations and deformation twinning) was obtained by changing the CRA temperature and Si content. A heterogeneous grain structure of CRA HEAs with an annealing temperature of 700 ℃ and Si content of 0.2 exhibits approximately fourfold higher yield strength (733 MPa) compared to that of HOA FeCoCrNiMn HEAs (185.27 MPa). The structure’s tensile strength and ductility were measured at 959 MPa and 29.5%, respectively. The high strength was mainly attributed to the combined effect of multiple strengthening mechanisms. Heterogeneous deformation-induced strain hardening and twinning-induced plasticity provide good ductility.

Generation of fine extension twins in rolled magnesium alloy AZ31B at room temperature by simple shear

A textured AZ31B plate was subject to simple shear along 45° to its normal direction at room temperature, and extension twinning under biaxial stress was investigated by statistical analyses of electron backscatter diffraction maps. Extension twinning is the primary deformation mechanism up to a plastic shear strain of 15% (8.66% equivalent plastic strain), and the most frequent twin thickness remains to be 1.5 μm. The simple shear suppresses twin growth, yet brings sustained twin nucleation. The critical resolved shear stress of extension twinning is determined as 25 MPa and remains virtually constant up to a plastic shear strain of 10%. The present study suggests that by using biaxial stress state that promotes multi-twin nucleation, a microstructure with dense and fine twins can be produced at room temperature in a conventional Mg alloy, without jeopardizing the contribution of twinning to plastic straining.

Effect of rotary swaging on microstructure evolution and adiabatic shear sensitivity of 90W–7Ni–3Fe alloy under dynamic loading

Tungsten heavy alloys (WHAs) are widely used in the preparation of kinetic penetrators. However, the penetrating performance of alloys, measured by the “self-sharpening effect”, is often limited by the low sensitivity of the adiabatic shear band (ASB). To address this issue, the 90W–7Ni–3Fe rods with deformation degrees of 20% and 40% were prepared by rotary swaging (RS) at high strain rates (4000 s−1 and 6000 s−1, respectively). The deformation and ASB behavior of RS-treated alloys were compared to those of undeformed samples. According to the results, RS pretreatment significantly increased the dynamic penetration strength and ASB sensitivity of the alloys. Besides, the characterization of the recrystallized microstructure within ASB and the calculation of the adiabatic temperature rise indicated that microstructure softening could be the primary cause for the initiation of ASB, which was consistent with the rotational dynamic recrystallization (RDR) mechanism. Moreover, the higher strain energy of the 40% RS deformation sample with high densities of dislocations and twins promoted the subgrain rotation and RDR initiation, providing a recrystallization prerequisite for the propagation and proliferation of ASB. The recrystallized twins formed after the completion of the RDR in the 40% RS deformation sample in order to adapt to the reloading impeded the movement of dislocations, which became a stress concentration site promoting microdamage more likely to nucleate and expand, making the ASB more prone to fracture. Therefore, this study is of great significance for the development of self-sharpening WHA kinetic penetrators.

Quantitative measurements of strain-induced twinning in Mg, Mg-Al and Mg-Zn solid solutions

The area fraction and number density of twins were determined in pre-polished tensile specimens of cast pure Mg, Mg-Al [0.5, 2 and 9 at.% Al] and Mg-Zn [0.8 and 2.3 at.% Zn] solid solutions, for different applied tensile strains. Profuse twinning developed rapidly in pure Mg, while it occurred very gradually in the Mg-Zn alloys, with Mg-Al showing an intermediate behavior. In the more dilute Mg-Al alloys the area fraction and the number density decreased with the concentration, increasing again for the 9 Al, reaching a level above that of pure Mg at large strains. In Mg-Zn both parameters decreased monotonically with the Zn concentration up to the solubility limit (2.5 at.%). For given alloy and solute content, the number density increased (the twins became smaller) for finer grain sizes in all three materials. It is argued that the availability of nucleating sites created by the microplasticity preceding profuse twinning, which is greater for the alloys due to the delayed onset of twinning, or for fine grained polycrystals due to the increased specific grain boundary area, accounts for the increased number density with the solute concentration or reduced grain size. Solute specific effects are rationalized using concepts of solid solution softening of the prismatic planes in all alloys, and the stronger solid solution hardening due to short range order in the concentrated Mg-Zn alloys.

Twin and dislocation induced grain subdivision and strengthening in laser shock peened Ti

A significantly refined microstructure with high twin and dislocation densities is a common feature in the Ti surface layers subjected to laser shock peening (LSP). In this study, in order to develop a detailed understanding of the different twin variants and accurate estimation of twin and dislocation densities that result from LSP, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were performed at different depths below the peened surfaces of single and triple LSP impacted Ti. To overcome challenges associated with characterization of the nano-sized LSP-induced twins during the conventional EBSD analysis, a novel data mining strategy is developed to identify the characteristic straight twin boundary traces from the band contrast maps. Results show five types of twins, whose total density initially increases and then monotonically decreases with increasing distance from the LSP treated surface. The intersection of the primary and secondary type twins subdivides the grains and leads to grain refinement. In contrast, the dislocation density is highest at the LSP treated surface but decreases monotonically with the increasing depth. A similar trend in the variation of microhardness as a function of the depth indicates that LSP-induced strengthening is dominated by the dislocations and grain refinement. Further analysis reveals that the former's contribution is more pronounced than that of the latter. The understanding developed in this work can be useful for the design of the alloys and the LSP process for improved mechanical performance.

Microstructural analysis of GaN films grown on (1 0 0) MgF2 substrate by 4D nanobeam diffraction and energy-dispersive X-ray spectrometry

The use of highly efficient and solarblind GaN photocathodes as part of multichannel plate UV detectors for applications in astronomy would strongly benefit from the direct growth of GaN on typical window materials with high transmission down to the deep UV range. GaN growth on MgF2 substrates by plasma-assisted molecular beam epitaxy has recently been demonstrated. Here, we report an extensive scanning transmission electron microscopy study of the thin film microstructure for growth at 525 °C and 650 °C on (100) MgF2. These results are systematically supported by X-ray diffraction reciprocal space maps. For both growth temperatures predominant cubic (111), (115) and (110) GaN is found with no preferred nucleation on the substrate and typical grain sizes of 100–200 nm. All observed orientations can be understood as the result of first and second order twins on different {111} planes related to the underlying substrate. The higher growth temperature shows a strongly increased twin density along with a higher surface roughness. Furthermore, grains with cubic (110) GaN growth show a reduced density and a reduced size of about 20 nm. In addition, in-diffusion of Mg and F into the GaN is observed, which is accompanied by the formation of cavities in the MgF2 directly at the interface.