Partially Melted Zone Research Articles

Effect of liquation on the tensile properties of cast Mg–9Al–1Zn Alloy fusion welds

This paper aims at understanding the role of liquation and solidification phenomena on the tensile properties of arc welded cast AZ91 magnesium alloy. Owing to its refined microstructure, the fusion zone exhibited improved ductility/strength compared to the base metal. It is demonstrated that the partially melted zone (PMZ) which is characterised by constitutional liquation of eutectic β-Mg17Al12 phase is the weakest link in the weldment. The reduced strength and ductility associated with the liquation phenomena are due to the increased volume fraction and thickness of the eutectic β-Mg17Al12 in the PMZ as well as the formation of liquation micro-cracks in a network of brittle intermetallic compounds. Controlling the liquation phenomena is critical for producing sound and reliable cast AZ91 arc welds.

Role of interfacial microstructure on mechanical properties of cold metal transfer welded dissimilar A6061-T6 and A6082-T6 joints

A6061-T6 and A6082-T6 dissimilar aluminium alloys are welded using CMT (Cold metal transfer) process and the interface microstructure of the individual alloys is correlated with mechanical properties. Microstructures indicate that dendrites next to the PMZ (Partially melted zone) of A6082-T6 side are finer than the dendrites next to the PMZ of A6061-T6 side. Liquation at grain boundaries and within the grains is clearly visible in the PMZ of A6061-T6 interface, whereas the PMZ of A6082-T6 interface does not reveal liquation phenomenon at grain boundaries and grain interiors. Among the interface regions, the A6082-T6 side shows superior mechanical properties as compared to the A6061-T6 side.

Simulation and experimental studies on process parameters, microstructure and mechanical properties of selective laser melting of stainless steel 316L

SLM process parameters such as laser power, scanning speed, layer thickness and hatch spacing play a crucial role in the quality of the processed product. In order to find the parameters suitable for SLM process, a finite element model was established to simulate the distribution of temperature field of selective laser melting (SLM) stainless steel 316L (SS316L) parts. After finite element simulation, the optimum factor levels for SLM processing SS316L were as follows: laser power of 100–200 W, scanning speed of 500–1500 mm/s, hatch spacing of 0.04–0.12 mm, and layer thickness of 0.03–0.06 mm. Then, based on a series of experiments, different laser energy densities were divided into three regions corresponding to three melting mechanisms: the partial melting zone (37.88–75.76 J/mm3), the full melting zone (75.76–151.52 J/mm3), and the over-melting zone (larger than 151.52 J/mm3). The surface roughness, density, microstructure and mechanical properties of the SLM SS316L were strongly dependent on the laser energy density. The results showed that an excellent SS316L part with a relative density higher than 99.66% and good mechanical properties could be obtained at a laser energy density of 119.05 J/mm3 with a laser powder of 200 W, a scanning speed of 700 mm/s, a hatch spacing of 0.08 mm and a layer thickness of 0.03 mm.

A Study on the 2060-T8/2099-T83 Aluminum-Lithium Alloys T-Joints Welded by Double-Sided Laser Beam Welding

The mechanical properties of 2060 and 2099 T-joints welded by double-sided laser beam welding were analyzed in this experiment. Various mechanical tests were performed to determine the transverse tensile properties of the region perpendicular to the weld, the tensile properties of the skin and stringer, the compressive properties of the region parallel to the weld seam, and the fatigue properties perpendicular to the weld seam. The T-joint was found to consist of the base metal, partial melting zone, equiaxed zone (EQZ), columnar crystal zone, and cellular dendritic zone from the base metal to the center of the weld seam. The EQZ was proved to be the weak area during mechanical properties tests of the T-joint. Moreover, the welding penetration had a great influence on the transverse tensile properties and weak areas were located in the EQZ near the lower fusion line. The EQZ near the upper fusion line was the weak area in the process of compression. In addition, severe porosities and cracks greatly influenced the mechanical properties of the joint. The fatigue property was closely related to the deformation angle; the fatigue source was located at the weld toe, and the crack propagation was along the base metal.

Investigation on the process parameters of TIG-welded aluminum alloy through mechanical and microstructural characterization

Multi-pass TIG welding was conducted on plates (15 × 300 × 180 mm3) of aluminum alloy Al-5083 that usually serves as the component material in structural applications such as cryogenics and chemical processing industries. Porosity formation and solidification cracking are the most common defects when TIG welding Al-5083 alloy, which is sensitive to the welding heat input. In the experiment, the heat input was varied from 0.89 kJ/mm to 5 kJ/mm designed by the combination of welding torch travel speed and welding current. Tensile, micro-Vicker hardness and Charpy impact tests were executed to witness the impetus response of heat input on the mechanical properties of the joints. Radiographic inspection was performed to assess the joint’s quality and welding defects. The results show that all the specimens displayed inferior mechanical properties as compared to the base alloy. It was established that porosity was progressively abridged by the increase of heat input. The results also clinched that the use of medium heat input (1–2 kJ/mm) offered the best mechanical properties by eradicating welding defects, in which only about 18.26% of strength was lost. The yield strength of all the welded specimens remained unaffected indicated no influence of heat input. Partially melted zone (PMZ) width also affected by heat input, which became widened with the increase of heat input. The grain size of PMZ was found to be coarser than the respective grain size in the fusion zone. Charpy impact testing revealed that the absorbed energy by low heat input specimen (welded at high speed) was greater than that of high heat input (welded at low speed) because of low porosity and the formation of equiaxed grains which induce better impact toughness. Cryogenic (−196 °C) impact testing was also performed and the results corroborate that impact properties under the cryogenic environment revealed no appreciable change after welding at designated heat input. Finally, Macro and micro fractured surfaces of tensile and impact specimens were analyzed using Stereo and Scanning Electron Microscopy (SEM), which have supported the experimental findings.

Microstructures and mechanical properties of TIG welded Al-Mg2Si alloy joints

Abstract In this paper, the microstructures and mechanical properties of tungsten inert gas (TIG) welded joints were studied. Three zones were formed in these joints: a heat-affected zone (HAZ), a partial melting zone (PMZ) and a fusion zone (FZ). In the FZ, the microstructure consisted of only α-Al and eutectic Mg2Si. The α-Al segregation phenomenon occurred when the welding current increased from 125 A to 135 A. The low-angle grain boundaries (LABs) found in the base material (BM) transformed into high-angle grain boundaries (HABs) in the FZ. The texture of the BM was concentrated in three specific preferred orientations, whereas the texture was eliminated in the FZ. The hardness in the FZ was 45 % higher than that in the BM. The eutectic Mg2Si phase in the welded joints became fine and dense, and the needle-like β″ phase in the BM was replaced by a high density of fine precipitates, which led to an increase in hardness. The ultimate tensile strength (UTS) and elongation of the joints welded at 125 A were lower than those of the BM and the joints welded at 135 A. Furthermore, no cracks were generated in the joints after multiple thermal shocks.

Geophysical characterization of the Northwest Geysers geothermal field, California

The Clear Lake Volcanic Field in northern California is the youngest and northern-most part of a long-lived volcanic system that has produced recent (~10 ka) eruptions. Adjacent to the Clear Lake Volcanic Field is the world's largest energy producing geothermal field, The Geysers. The hottest part of The Geyser's geothermal field is in the northwest where temperatures reach ~400 °C at 3 km depth. Low permeability, high thermal gradients, and low steam saturation prescribed development of an enhanced geothermal system (EGS) in the Northwest Geysers to increase energy producing capacity. Though the Northwest Geysers is known to be the hottest part of the field, geophysical methods have failed to adequately image any inferred heat source. This project aims to image the heat source of the Northwest Geysers using newly collected gravity and magnetotelluric (MT) measurements. Gravity data were jointly modeled with existing magnetic data along a two-dimensional profile aligned with an existing geologic cross-section. The key feature of the potential field model is a low-density, low-susceptibility body at 5 km depth (bmsl) under the EGS. MT data were modeled in three-dimensions to characterize subsurface resistivity structure, where the upper 3 km of the resistivity model agrees well with existing data. Lithologic and steam saturation are estimated from modeled resistivity values using existing geophysical data. Below 3 km depth (bmsl), the resistivity model images a possible young intrusion under the EGS. A possible zone of partial melt (<5%) below 7 km depth (bmsl) in the northwestern part of the field is also imaged which extends northeast towards the main part of the Clear Lake Volcanic Field.

Ductility and Compressibility Accommodate High Magma Flux Beneath a Silicic Continental Rift Caldera: Insights From Corbetti Caldera (Ethiopia)

AbstractLarge silicic magma reservoirs preferentially form in the upper crust of extensional continental environments. However, our quantitative understanding of the link between mantle magmatism, silicic reservoirs, and surface deformation during rifting is very limited. Here, we focus on Corbetti, a peralkaline caldera in the densely populated Main Ethiopian Rift, which lies above a focused zone of upper mantle partial melt and has been steadily uplifting at a maximum rate of 6.6±1.2 cm yr−1 for more than 10 yr. Numerical modeling shows that a maximum concomitant residual gravity increase of 9±3 μGal yr−1 by the intrusion of mafic magma at ∼7 km depth into a compressible and inelastic crystal mush best explains the observed deformation and gravity changes. The derived magma mass flux of ∼1011 kg yr−1 is anomalously high and at least 1 order of magnitude greater than the mean long‐term mass eruption rate. This study demonstrates that periodic and high‐rate magmatic rejuvenation of upper‐crustal mush is a significant and rapid contributor to mature continental rifting.

Fate of Forearc Lithosphere at Arc‐Continent Collision Zones: Evidence From Local Earthquake Tomography of the Sunda‐Banda Arc Transition, Indonesia

AbstractA new 3‐D seismic P wave velocity model of the Sunda‐Banda Arc Transition from local earthquake tomography reveals (i) northward subduction of oceanic lithosphere, associated with the convergence of Australia and Sundaland, as a high‐velocity zone extending down to ~200 km depth; (ii) two distinct low‐velocity zones, one immediately above the slab, which is likely a zone of partial melt, and one in the 0–40 km depth range, which is probably a magma chamber associated with active volcanoes above; and (iii) a northerly dipping high‐velocity zone that bisects the two low‐velocity anomalies, which we interpret as an underthrust forearc sliver of continental origin. Based on He3/He4 measurements from volcanic outflows, it is possible that the magma supply is contaminated by interaction with this forearc sliver as it ascends from the melt region below.

Crustal thickness beneath Mt. Merapi and Mt. Merbabu, Central Java, Indonesia, inferred from receiver function analysis

In this study, we analysed 2708 receiver functions (RFs) using data recorded by 53 seismographic stations that surround Mt. Merapi and Mt. Merbabu – two volcanos in Central Java - to map the boundary between Earth's crust and upper mantle. We observe that a number of RFs from this new dataset have complex signals and do not exhibit typical RF characteristics; in particular, where the converted Ps signal from the Moho discontinuity is the clearest and strongest amplitude arrival following the P onset. This effect may be related to complex shallow velocity structure due to the presence of magmatic rocks and sediments. Further analysis of the RF results using the H-κ method suggests that Moho depth varies between 27 and 32 km beneath the array, with no apparent correlation between crustal thickness and surface topography, as one might expect from Airy isostacy. For instance, the Moho is quite shallow beneath Mt. Merapi (up to 27 km depth), despite its elevation of nearly 3 km. This may be a consequence of dynamic support from an active upper mantle coupled with erosion and/or weakening of the lower crust due to the active volcanic plumbing system. To the north of Mt. Merapi, the Moho is deeper (30–31 km depth) below Mt. Merbabu. Vp/Vs ratio estimates from the H-κ method are relatively high (~1.9) beneath the Mt. Merapi and Kendeng Basin area, which may indicate the presence of a zone of hydrous and active partial melting in the underlying crust. Lower Vp/Vs ratios (~1.7) are found beneath Mt. Merbabu, which may be due to its relative lack of volcanic activity compared to Mt. Merapi.

Two-phase magnetohydrodynamics: Theory and applications to planetesimal cores

Core freezing and resultant compositional convection are likely important drivers for dynamo activity in large terrestrial bodies like the Earth. The solidification of compositional mixtures, such as iron and sulfur, generates mush zones of partial melt at the freezing front, which can eject chemically buoyant or heavy liquid that then drives convection. For smaller bodies such as planetesimals in the asteroid belt, conditions for generating a dynamo are harder to achieve. Nevertheless, evidence for magnetization of achondrite meteorites is abundant, suggesting that many planetesimal cores were somehow magnetized. As such small bodies cool rapidly under low gravity they likely spend much of their evolution with a large poorly compacted partial melt mushy zone. The magneto-hydrodynamic behavior of a deformable partial melt zone can induce magnetism via separation of solid and liquid phases, and conversely magnetism can impose extra forces on the phase separation in the mush zone. To this end, we have developed a new two-phase magneto-hydrodynamic theory for deformable mushes and slurries. The model includes the standard effects of Lorentz forces, and the competition between magnetic field stretching and diffusion. There are additional effects at the liquid pore or solid grain scale, which involve the interaction between phases, akin to Darcy or Stokes drag; these include Lorentz drag, as well as pore/grain-scale diffusive exchange of magnetism between solid and fluid phases, and field stretching due to relative motion between phases. Magnetic induction by gravitational phase separation is most significant after extensive mixing of liquid and solid phases, such as induced by vigorous mechanical stirring due to, for example, tidal and elliptic instabilities, or impacts with other bodies, all of which are conceivably common in the asteroid belt, especially in the early solar system. Gravitational phase separation following such events can induce significant magnetism in the liquid and solid phases, and much more rapidly than can magnetization by large-scale circulation. Magnetic field variances can be at first orders of magnitude larger than an imposed background field, during initial gravitational phase separation of the well-mixed slurry. As the phases separate toward the top or bottom, the solid phase compacts, the separation velocity decreases, and the magnetic field variance likewise diminishes. However, solitary waves in the compacting region can cause an additional large magnetic induction in the liquid, taking the form of strongly magnetized wave packets that can be trapped in the solid. Thus, phase separation, segregation and compaction potentially induce large magnetic field anomalies. A linear stability analysis for convection in a porous medium (with a rigid matrix) is also explored. Pore and grain scale effects, such as field stretching due to phase separation, are found to enhance the influence of the magnetic field on convective instability.

High Bulk and Shear Attenuation Due to Partial Melt in the Tonga‐Lau Back‐arc Mantle

AbstractSeismic attenuation measures energy loss during seismic wave propagation and quantifies the relaxation of rocks' elastic moduli. Abundant seismic studies have observed shear attenuation, but few have quantified bulk attenuation (bulk modulus relaxation), and these studies controversially locate finite bulk attenuation in different parts of the Earth, from the asthenosphere to the inner core. Here we present conclusive evidence localizing bulk attenuation to a specific region of the uppermost mantle. By analyzing amplitude spectral decay of P and S waves from Tonga earthquakes recorded at local seismic stations, we observe unusually low QP/QS ratios (&lt;1.5) in the Tonga‐Lau mantle wedge. All seismic raypaths with significant path‐average bulk attenuation (Qκ−1 &gt; 0.01 or Qκ &lt; 100) are confined to the region immediately beneath the Lau back‐arc spreading centers west of the Tonga Arc. Tomography results show that the highest bulk attenuation (Qκ−1 = 0.037 ± 0.008 or Qκ = 27.0 ± 0.2) is about 75% of the in situ shear attenuation. The observed high bulk attenuation anomalies coincide with an inferred zone of partial melting, suggesting that the causative mechanism involves either infinitesimal movement of melt through pores or changes in melt fraction in response to seismic waves.

Three‐Dimensional Seismic Attenuation Structure of Central Japan and Deep Sources of Arc Magmatism

AbstractWe estimate the 3‐D P wave attenuation (Qp−1) structure of central Japan to investigate arc magmatism. We first determine the corner frequencies of the earthquakes and then conduct a joint inversion to determine attenuation terms. The 3‐D Qp−1 structure is finally obtained via a tomographic inversion. High‐attenuation areas are located in the uppermost mantle along the volcanic front, which are interpreted as zones of partial melting at 30‐ to 100‐km depth due to the supply of slab‐derived fluids. The subducted Philippine Sea plate acts as a barrier to the upward migration of melt, resulting in the volcanic gap. The distribution of volcanoes is controlled primarily by the geometry of the subducted Philippine Sea plate. The attenuation structures beneath Mount Fuji and Mount Hakone show contrasting melt transport pathways, which are separated at ≤50‐km depth, suggesting that the independent melt supply system results in the distinct activities of these volcanoes.

The Nature of Anomalous Particles (Granules) in Rapidly Quenched PREP Powders: I. A Multiscale Study of the Heating Zone of the Rotating Electrode Used to Obtain the Rapidly Quenched PREP Ni-Based Superalloy Powder

The uniformity of composition of rapidly quenched PREP particles of the powder of high temperature Ni-based superalloys and stainless steels is characterized by an important feature—the occurrence of anomalous particles (granules) with a significantly different content, mainly of microalloying interstitial elements, carbon and boron, as well as active carbide- and boride-forming alloying elements. A detailed multi-scale experimental study of the heating zone of the crater of the Ni-based superalloy electrode after its use to obtain rapidly quenched PREP powder was carried out in order to find the nature and mechanisms of the formation of anomalous granules. Direct nuclear physics methods of activation autoradiography on carbon, track autoradiography on boron, metallography, SEM, EDX, OIM were used. In the electrode crater, the heat-affected zone (HAZ) and the partially melted zone (PMZ) were detected. Intense migration of boron to the electrode surface due to the formation of thermal macrocracks was also revealed. The behavior of carbon is determined by the formation of a thin layer of melt on the surface of the crater. The features of the evolution of the terminal solidification region TSR and incipiently melted regions IMR, the main type of heterogeneity of the composition of the dendritic structure of Ni-based superalloys and stainless steels, are revealed. The interrelation of the evolution of these areas is established, which is a consequence of the thermodynamic principle of the reversibility of the processes of solidification and melting, respectively, in the smelting of an ingot electrode and in the process of subsequent atomization. The analysis of the influence of the behavior of boron, carbon, and the characteristics of the crater structure on the nature and mechanism of the formation of anomalous granules using the PREP method for producing rapidly quenched powder of the Ni-based superalloy is performed.

Characterization of microscopic mechanical performance for welded joint of 9% Ni steel using nanoindentation

Nanoindentation test, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were introduced to investigate the mechanical properties and microstructure of microscopic regions of the nickel-based welded joint. Five indentation series were performed across the weld zone including base metal (BM), coarse grained heat affected zone (CGHAZ), partially melted zone (PMZ), weld metal near the partially melted zone and weld metal center (WM). The mechanical properties (E, H, and n) were then estimated from the load on sample-displacement into surface curves (P-h) data of nanoindentation and SEM analysis results. The results showed that the average nano-hardness value (H) was obviously increased in the CGHAZ. WM has the highest values of elastic modulus (E). BM has the highest plastic rheology stress exponent (n). At the same time, the relationship between the plastic rheology depth and the plastic rheology was also discussed.

The role of the partial melting zone in the nugget growth process of unequal-thickness dissimilar aluminum alloy 2219/5A06 resistance spot welding

The effect of the partial melting zone on nugget growth in aluminum alloys’ resistance spot welding (RSW) process was investigated in this study by exerting same welding schedule on dissimilar 2219/5A06 aluminum alloys’ combination (of unequal-thickness) and commercial pure aluminum 1060 with the same gauge. A two-dimensional symmetrical finite model of unequal thickness RSW of which considering the coupling of the thermal, electrical and mechanical fields was exerted to reveal the dynamic nugget evolution under dissimilar 2219/5A06 aluminum alloys’ combination. Calculation of phase diagram (CALPHAD) technique was used to simulate the dynamic evolution of physical properties regarding the different phase portion at different local temperature. Between the solidus and the liquidus, both 2219 (548–645 °C) and 5A06 (449–626 °C) aluminum alloy exhibits a marked decrease in thermal and electrical conductivity. This leads to nugget shape evolution of dissimilar 2219/5A06 aluminum alloys’ combination (the peak diameter is not located at the workpiece/workpiece interface) differed from that of the commercial pure aluminum ones (the peak diameter is located at the workpiece/workpiece interface). Finally, by comparing the experimental and the calculated data, the model developed in this study provides a more reliable method to predict the size of the load-bearing plane when the effect of the partial melting zone is pronounced.

Rate of Magma Supply Beneath Mammoth Mountain, California, Based on Helium Isotopes and CO2 Emissions

AbstractMammoth Mountain, California, has exhibited unrest over the past ~30 years, characterized by seismicity over a broad range of depths, elevated 3He/4He ratios in fumarolic gas, and large‐scale diffuse CO2 emissions. This activity has been attributed to magmatic intrusion, but minimal ground deformation and the presence of a shallow crustal gas reservoir beneath Mammoth Mountain pose a challenge for estimating magma supply rate. Here, we use the record of fumarolic 3He/4He ratios and CO2 emissions to estimate that of the ~5.2 Mt of CO2 released from Mammoth Mountain between 1989 and 2016, 1.6 Mt was associated with active intrusion and degassing of ~0.05–0.07 km3 of basaltic magma. Intrusion at an average rate of ~0.002–0.003 km3/year into a postulated zone of partial melt at ~15‐km depth could occur without detection by local Global Navigation Satellite System stations.

Nano-indentation and in-situ investigations of double-sided laser beam welded 2060-T8/2099-T83 Al-Li alloys T-joints

2060 and 2099 are both newly developed high-strength aluminum-lithium (Al-Li) alloys for fuselage panels of jetliners. In this work, double-sided laser beam welding of 2-mm-thick 2060-T8 and 2099-T83 Al-Li alloys was performed using AA4047 filler wire under optimized parameters. Weld zone microstructure and T-joint mechanical properties were investigated by means of scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), nano-indentation and in-situ tensile test. Experimental results indicated that the equiaxed grain zone (EQZ) featured by dense large-angle (θ＞10°) grain boundaries was observed in the weld zone along the fusion lines. Besides the softening problem found in the partially melted zone (PMZ), local softening phenomenon was detected in the local enrichment region of the EQZ. During in-situ tensile tests, the crack was initiated at the weld toe and then propagated alternately in the PMZ and EQZ's local enrichment regions. Micro-voids induced the localized softening problem and the crack's intergranular expansion in the EQZ. Localized absence of silicon was most likely to cause continuous micro-voids in the EQZ.

Liquation cracking susceptibility of partially melted zone in 304B4 SS multipass weldments

AISI 304B4 stainless steel (SS) containing 1.3 wt% boron is reported to be weldable under moderate restraint forces provided welding parameters are chosen carefully. However, during fabrication of components, extensive cracking issues are being reported. Hence, a study involving assessment of base metal, unmixed zone and partially melted zone (PMZ) of 304B4 steel and its welds made using E309 and boron containing consumables on hot cracking is carried out. In this article, results obtained on cracking susceptibility of PMZ formed in this steel are presented and discussed. In order to evaluate hot ductility behavior of PMZ formed in unmixed zone in a multipass weld, PMZ was first simulated using “Gleeble™” thermo-mechanical simulator. Subsequently, hot ductility tests were conducted on this simulated PMZ. Results of the study showed that liquation cracking susceptibility of PMZ of this steel subjected to repeated thermal cycle is higher than that of 304B4 base material. Microstructural analysis of tested specimens showed that formation of Fe-rich ((Fe,Cr)2B + Cr2B) borides in PMZ lowers ductility recovery temperature by 50 °C which results in an increase in cracking factor by 84% of that of base metal. Reasons for high cracking susceptibility of PMZ formed on fused 304B4 have been deduced.

Thermo‐mechanical interaction of a large impact melt sheet with adjacent target rock, Sudbury impact structure, Canada

AbstractThe 1.85 Ga Sudbury Igneous Complex (SIC) and its thermal aureole are unique on Earth with regard to unraveling the effects of a large impact melt sheet on adjacent target rocks. Notably, the formation of Footwall Breccia, lining the basal SIC, remains controversial and has been attributed to impact, cratering, and postcratering processes. Based on detailed field mapping and microstructural analysis of thermal aureole rocks, we identified three distinct zones characterized by static recrystallization, incipient melting, and crystallization textures. The temperature gradient in the thermal aureole increases toward the SIC and culminates in a zone of partial melting, which correlates spatially with the Footwall Breccia. We therefore conclude that assimilation of target rock into initially superheated impact melt and simultaneous deformation after cratering strongly contributed to breccia formation. Estimated melt fractions of the Footwall Breccia amount to 80 vol% and attest to an extreme loss in mechanical strength and, thus, high mobility of the Breccia during assimilation. Transport of highly mobile Footwall Breccia material into the overlying Sublayer Norite of the SIC and vice versa can be attributed to Raleigh–Taylor instability of both units, long‐term crater modification caused by viscous relaxation of crust underlying the Sudbury impact structure, or both.