Effects of Sc(Zr) on the microstructure and mechanical properties of as-cast Al–Mg alloys

Both the addition of 0.6% Sc and simultaneous addition of 0.2% Sc and 0.1% Zr exerted a remarkable effect on grain refinement of as-cast Al–Mg alloys, changing typical dendritic microstructure into fine equiaxed grains. Such effect was found to be related to the formation of primary particles, which acted as heterogeneous nucleation sites for α-Al matrix during solidification. Primary particles formed in Al–Mg–Sc–Zr alloy could be identified as the eutectic structure consisting of multilayer of ‘Al₃(Sc,Zr)?+?α-Al?+?Al₃(Sc,Zr)’, with a ‘cellular-dendritic’ mode of growth. In addition, an attractive comprehensive property of as-cast Al–5Mg alloy due to the addition of 0.2% Sc and 0.1% Zr was obtained.     

Investigation of primary Al3(Sc,Zr) particles in Al–Sc–Zr alloys

Abstract

This paper studied the primary Al₃(Sc,Zr) particles formed during solidification in three Al–Sc–Zr alloys with various Zr contents. It has been shown that the primary Al₃(Sc,Zr) particles formed during solidification since the Sc and Zr concentrations are above the solid solubility limit in Al matrix. Scanning electron microscopy line scanning results indicated that the distributions of Sc, Zr and Al atoms in the primary Al₃(Sc,Zr) particles differ a lot from those in the secondary Al₃(Sc,Zr) particles formed during annealing. In the primary Al₃(Sc,Zr) particles, both Sc and Zr contents are found from rim to the centre of the particles, with an increasing trend from the rim to the centre, while the Al content drops sharply on the rim of the particle and slightly from the rim to the centre.     

Microstructural investigations on as-cast and annealed Al–Sc and Al–Sc–Zr alloys

Al–Sc and Al–Sc–Zr alloys containing 0.05, 0.1 and 0.5 wt.% Sc and 0.15 wt.% Zr were investigated using optical microscopy, electron microscopy and X-ray diffraction. The phase composition of the alloys and the morphology of precipitates that developed during solidification in the sand casting process and subsequent thermal treatment of the samples were studied. XRD analysis shows that the weight percentage of the Al₃Sc/Al₃(Sc, Zr) precipitates was significantly below 1% in all alloys except for the virgin Al0.5Sc0.15Zr alloy. In this alloy the precipitates were observed as primary dendritic particles. In the binary Al–Sc alloys, ageing at 470 °C for 24 h produced precipitates associated with dislocation networks, whereas the precipitates in the annealed Al–Sc–Zr alloys were free of interfacial dislocations except at the lowest content of Sc. Development of large incoherent precipitates during precipitation heat treatment reduced hardness of all the alloys studied. Growth of the Al₃Sc/Al₃(Sc, Zr) precipitates after heat treatment was less at low Sc content and in the presence of Zr. Increase in hardness was observed after heat treatment at 300 °C in all alloys. There is a small difference in hardness between binary and ternary alloys slow cooled after sand casting.     

Ca-modified Al–Mg–Sc alloy with high strength at elevated temperatures due to a hierarchical microstructure

Al-Mg alloys are normally prone to lose part of their yield and tensile strength at high temperatures due to insufficient thermal stability of the microstructure. Here, we present a Ca-modified Al–Mg–Sc alloy demonstrating high strength at elevated temperatures. The microstructure contains Al₄Ca phases distributed as a network along the grain boundary and Al₃(Sc,Zr) nano-particles dispersed within the grains. The microstructure evolution and age-hardening analysis indicate that the combination of an Al₄Ca network and Sc-rich nano-particles leads to excellent thermal stability even upon aging at 300 °C. The tensile strength of the alloy for temperatures up to 250 °C is significantly improved by an aging treatment and is comparable with the commercial heat-resistant aluminum alloys, i.e., A356 and A319. At a high temperature of 300 °C, the tensile strength is superior to the above-mentioned commercial alloys, even more so when expressed as the specific strength due to the low density of Ca-modified Al–Mg–Sc alloy. The excellent high-temperature strength results from a synergistic effect of solid solution strengthening, grain boundary strengthening and nanoparticle order strengthening.

     

Laser powder bed fusion of high-strength Sc/Zr-modified Al-Mg alloy:phase selection,microstructural/mechanical heterogeneity,and tensile deformation behavior

Laser powder bed fusion (L-PBF) of Sc/Zr-modified Al-based alloys has recently become a promising method for developing a new generation of high-performance Al alloys.To clarify the modification roles of Sc/Zr elements,an Al-4.66Mg-0.48Mn-0.72Sc-0.33Zr (wt.％) alloy was processed using L-PBF.The ef-fect of the local solidification condition of the molten pool on the precipitation behavior of primary Al3(Sc,Zr) was analyzed based on time-dependent nucleation theory.it was found that primary Al3(Sc,Zr)inevitably precipitated at the fusion boundary,while its precipitation could be effectively suppressed in the inner region of the molten pool.This subsequently induced the formation of a heterogeneous α-Al matrix.After direct aging,the heredity of solidification microstructure introduced heterogeneous secondary Al3(Sc,Zr) precipitates within α-Al matrix.Owing to the inverse relationship between grain boundary strengthening and precipitation strengthening,the direct-aged sample with dual heterogeneous structures exhibited reduced mechanical heterogeneity,resulting in lowered hetero-deformation-induced hardening.The low strain-hardening capability in the direct-aged sample promoted necking instability while inducing a large Lüders elongation,which effectively improved the tensile ductility.     

Segregation behaviors of Sc and unique primary Al3Sc in Al-Sc alloys prepared by molten salt electrolysis

This work mainly deals with the segregating behaviors of Sc and the growth of unique primary Al₃Sc in Al-Sc alloys prepared by molten salt electrolysis. The alloys contain 0.23–1.38 wt%Sc where Sc segregation is observed. It is found that a high current density and long electrolysis time are in favor of high Sc content, and so do the high temperature and the addition level of Sc₂O₃. Sc content at the edge of Al based alloy (average Sc content: 0.75 wt%) can be as high as 1.09 wt%, while it is merely 0.24 wt% at the central area. The cooling rates have a strong impact on the morphology and particle size of primary Al₃Sc, but a weak influence on Sc segregation. The cusped cubic and dendritic primary Al₃Sc can precipitate in the prepared Al-Sc alloys. In a slightly hypereutectic Al-0.67 wt%Sc alloy, a large and cusped dendrite grows from the edge into the center. The primary and secondary dendritic arms can be as long as 600 and 250 μm, respectively. The Sc segregating behaviors in Al-Sc alloys is due to the mechanism controlled by the limited diffusion rate of Sc in liquid Al. This can involve the establishment of a near spherical discharge interface between liquid Al and the electrolyte. The Sc rich layer near Al-molten salt interface may provide the potential primary nuclei and sufficient Sc atoms for the growth of large dendritic primary Al₃Sc.     

Mechanical properties and microstructures of rapidly solidified Al89.5Ni8Zr2.5 and Al88.5Ni8Ti3.5 alloys

Al89.5Ni8Zr2.5 and Al88.5Ni8Ti3.5 alloys extruded from their rapidly solidified powders have tensile strength more than 800 MPa and Young's modulus about 100 GPa. The extruded Al89.5Ni8Zr2.5 alloy is composed of -Al, Al3Ni and a metastable tetragonal Al3Zr, and the extruded Al88.5Ni8Ti3.5 alloy consists of -Al, Al3Ni and equilibrium Al3Ti. Through investigation on microstructure change of rapidly solidified Al89.5Ni8Zr2.5 and Al88.5Ni8Ti3.5 alloys with temperature, it is found that a new tetragonal Al3Zr phase, together with -Al and Al3Ni precipitates from the supersaturated -Al phase in the rapidly solidified Al89.5Ni8Zr2.5 alloy at around 603 K and an equilibrium Al3Ti, together with -Al and Al3Ni forms from the supersaturated -Al phase in the rapidly solidified Al88.5Ni8Ti3.5 alloy at about 523 K. The lattice parameters of the new metastable tetragonal Al3Zr phase were calculated to be a=0.3896 nm and c=0.9006 nm. Both the metastable tetragonal Al3Zr and equilibrium Al3Ti phases keep a nano grain size, less than 50 nm even at 773 K. The existence of the nano scale Al3Zr, Al3Ti phases and fine grains of -Al, Al3Ni phases is the reason that Al89.5Ni8Zr2.5 and Al88.5Ni8Ti3.5 alloys have the high strength. © 1998 Chapman & Hall     

Laser-based directed energy deposition of novel Sc/Zr-modified Al-Mg alloys: columnar-to-equiaxed transition and aging hardening behavior

The control of grain morphology is important in laser additive manufacturing(LAM), as grain morphology further affects the hot cracking resistance, anisotropy, and strength–ductility synergy of materials. To develop a solidification-control solution and achieve columnar-to-equiaxed transition(CET) in Al-based alloys during LAM, Sc-and-Zr-modified Al-Mg alloys were processed via directed energy deposition(DED).CET was achieved by introducing high potent primary Al_3(Sc,Zr) nucleation sites ahead of the solidification interface. Furthermore, the relationship between the solidification control parameters and precipitation behavior of primary Al_3(Sc,Zr) nucleation sites was established using the time-dependent nucleation theory. Then, the CET was studied according to the Hunt criterion. The results indicated that coarse columnar grain structure was still obtained at the inner region of the molten pool at low Sc/Zr contents owing to the effective suppression of the precipitation of the primary Al_3(Sc,Zr) nucleation sites via rapid solidification during DED. In addition, the relatively low melt temperature at the fusion boundary unavoidably promoted the precipitation of primary Al_3(Sc,Zr) nucleation sites, which resulted in a fine equiaxed grains band at the edge of the molten pool. As the Sc/Zr content increased, the solidification cooling rate was not sufficient to suppress the precipitation of the primary Al_3(Sc,Zr) nucleation sites, and a fully equiaxed grain structure was obtained. Furthermore, the effect of the layer-by-layer manufacturing process on the subsequent precipitation strengthening of secondary Al_3(Sc,Zr) precipitates was discussed.Both the remelting and subsequent aging during thermal cycling should be considered to achieve greater precipitation strengthening.     

Precipitation in cold-rolled Al–Sc–Zr and Al–Mn–Sc–Zr alloys prepared by powder metallurgy

The effects of cold-rolling on thermal, mechanical and electrical properties, microstructure and recrystallization behaviour of the AlScZr and AlMnScZr alloys prepared by powder metallurgy were studied. The powder was produced by atomising in argon with 1% oxygen and then consolidated by hot extrusion at 350 °C. The electrical resistometry and microhardness together with differential scanning calorimetry measurements were compared with microstructure development observed by transmission and scanning electron microscopy, X-ray diffraction and electron backscatter diffraction. Fine (sub)grain structure developed and fine coherent Al₃Sc and/or Al₃(Sc,Zr) particles precipitated during extrusion at 350 °C in the alloys studied. Additional precipitation of the Al₃Sc and/or Al₃(Sc,Zr) particles and/or their coarsening was slightly facilitated by the previous cold rolling. The presence of Sc,Zr-containing particles has a significant antirecrystallization effect that prevents recrystallization at temperatures minimally up to 420 °C. The precipitation of the Al₆Mn- and/or Al₆(Mn,Fe) particles of a size ~ 1.0 μm at subgrain boundaries has also an essential antirecrystallization effect and totally suppresses recrystallization during 32 h long annealing at 550 °C. The texture development of the alloys seems to be affected by high solid solution strengthening by Mn. The precipitation of the Mn-containing alloy is highly enhanced by a cold rolling. The apparent activation energy of the Al₃Sc particles formation and/or coarsening and that of the Al₆Mn and/or Al₆(Mn,Fe) particle precipitation in the powder and in the compacted alloys were determined. The cold deformation has no effect on the apparent activation energy values of the Al₃Sc-phase and the Al₆Mn-phase precipitation.     

Influence of zirconium on grain refining efficiency of Al–Ti–C master alloys

The influence of Zirconium on the grain refinement performance of Al–Ti–C master alloys and the effect mechanism has been studied in this paper. The experimental results show that Zr not only results in poisoning the Al–Ti–B master alloy, but also poisons the Al–Ti–C master alloys. The poisoning effect is more obvious at higher melting temperature. When 0.12%Zr is added into the melt, the grain refinement performance of Al–5Ti–0.4C refiner with 0.2% addition level absolutely disappears at 800 °C. The experimental results also show that it is difficult to refine the commercial purity Al containing 0.15%Zr by Al–5Ti–0.4C master alloy. Further experiments show that the Zr element can interact with both TiAl₃ and TiC phases. If both of them are present, Zr preferentially reacts with TiAl₃ phase.     

The structural stabilities of Al3(Sc1−xMx) by first-principles calculations

The positive effects of Al₃Sc phase on Al–Sc alloys can be promoted by adding to the third elements. The structure properties of Al₃Sc when it dissolves the third elements are investigated by first-principles calculations. Special quasirandom structures are developed for the quasi-binary L1₂ structures. The calculations indicate that the elements of Ti, Zr, Y and Ta tend to substitute for Sc in Al₃Sc, while Ni and Si prefer to substitute for Al. The present lattice parameters generally follow the Vegard’s law. The high solubility of Ti and Y in Al₃Sc is revealed in the calculated quasi-binary phase diagram.     

Effect of heat-treatment on phase stability and grain coarsening in a powder-processed Al–Ni–Co–Zr–Y alloy

Processing of Al alloys via metastable amorphous intermediates can give much higher volume fractions of dispersed strengthening phases than in conventional precipitation- or dispersion-hardened systems. Here, we report a study on an Al–Ni–Co–Zr–Y alloy processed by gas atomization and consolidated/devitrified by warm extrusion. X-ray diffraction and electron microscopy are used to reveal the effects of heat-treatments at 300–500 °C for up to 96 h on the phase stability and coarsening behavior of the alloy. In all samples, the microstructure contains 22 % by volume of Al₁₉(Ni,Co)₅Y₃ plates surrounded by grains of FCC Al. Samples heat-treated at 350 °C and above also contain fine Al₃Y and Al₃Zr particles as minority phases. The softening of the alloy is limited for heat-treatment temperatures of up to 400 °C, and the Al₁₉(Ni,Co)₅Y₃ plates coarsen slowly. At higher temperatures, abnormal coarsening is observed with the development of a secondary population of much larger Al₁₉(Ni,Co)₅Y₃ plates. An analysis of the coarsening kinetics gives a constant coarsening exponent of 3, but a distinct transition in the activation energies. These values suggest that the normal coarsening at lower temperatures occurs by short-circuit diffusion, whereas the abnormal coarsening at higher temperatures involves lattice diffusion. The Al grain size is dictated by the Al₁₉(Ni,Co)₅Y₃ inter-plate separation, and grain growth is limited by the extent of plate coarsening. Such systems could form the basis of new high-strength high-temperature Al alloys for structural applications.     

Superior mechanical properties of a selective-laser-melted AlZnMgCuScZr alloy enabled by a tunable hierarchical microstructure and dual-nanoprecipitation

Achieving high mechanical strength and ductility in age-hardenable Al7000 series (Al–Zn–Mg) alloys fabricated by selective laser melting (SLM) remains challenging. Here, we show that crack-free AlZnMgCuScZr alloys with an unprecedented strength–ductility synergy can be fabricated via SLM and heat treatment. The as-built samples had an architectured microstructure consisting of a multimodal grain structure and a hierarchical phase morphology. It consisted of primary Al₃(Sc_x,Zr_1−x) particles which act as inoculants for ultrafine grains, preventing crack formation. The metastable Mg-, Zn-, and Cu-rich icosahedral quasicrystals (I-phase) ubiquitously dispersed inside the grains and aligned as a filigree skeleton along the grain boundaries. The heat treated SLM-produced AlZnMgCuScZr alloy exhibited tunable mechanical behaviors through trade-off among the hierarchical features, including the dual-nanoprecipitation, viz, η′ phase, and secondary (Al,Zn)₃(Sc₉Zr), and grain coarsening. Less coarsening of grains and (Al,Zn)₃(Sc₉Zr) particles, due to a reduced solution treatment temperature and time, could overwhelm the more complete dissolution of I-phase (triggering more η′ phase), resulting in higher yield strength. Optimal combination of the hierarchical features yields the highest yield strength (∼647 MPa) among all reported SLM-produced Al alloys to date with appreciable ductility (∼11.6%). The successful fabrication of high-strength Al7000 series alloys with an adjustable hierarchical microstructure paves the way for designing and fine-tuning SLM-produced aluminum engineering components exposed to high mechanical loads.     

Microstructure and mechanical properties of Al–Si cast alloy with additions of Zr–V–Ti

The microstructure and tensile properties at temperatures up to 300 °C of an experimental Al–7Si–1Cu–0.5Mg (wt.%) cast alloy with additions of Ti, V and Zr were assessed and compared with those of the commercial A380 grade. The microstructure of both alloys consisted of Al dendrites surrounded by Al–Si eutectic containing, within its structure, the ternary Al–Al₂Cu–Si phase. Whereas the Al₁₅(FeCrMn)₃Si₂ phases were present in the A380 alloy, Ti/Zr/V together with Al and Si phases, Al(ZrTiV)Si, were identified in the experimental alloy. As a result of chemistry modification the experimental alloy achieved from 20% to 40% higher strength and from 1.5 to 5 times higher ductility than the A380 reference grade. The role of chemistry in improving the alloy thermal stability is discussed.     

Laser-induced combustion synthesis of Zr–Ti–Al–Ni amorphous-based alloys

Bulk metallic glasses, like many crystalline intermetallics, have large negative enthalpy of mixing among the major constituent elements, and hence are potential candidates of self-propagating high-temperature reaction systems. Based on this characteristic, the laser-induced combustion synthesis (LCS) technique has been applied to fabricate amorphous-containing alloys. In the present paper, we report the LCS of the Zr–Ti–Al–Ni alloys. A series of Zr–Ti–Al–Ni alloys is designed and synthesized by LCS. The LCS products mainly consist of intermetallic phases, but in Zr₅₅Ti_10.8Al_17.1Ni_17.1 and Zr₅₀Ti_21.6Al_14.2Ni_14.2 amorphous phases are found. The hardness and tribology characteristics are closely related to the phase contents. The amorphous phases, ductile and soft, lower the hardness and increase the friction coefficient of the LCS samples.     

Effect of silver addition on formation of secondary phases in squeeze cast Al–Cu–Mg alloys

Abstract

The effect of silver addition on the formation of secondary phases in squeeze cast Al–4.0Cu–1.5Mg and Al–4.0Cu–1.5Mg–0.7Ag (all wt-%) alloys has been investigated using optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffractometry, and transmission electron microscopy. The as cast microstructure of both alloys consists of primary dendritic α-Al and various types of secondary solidification phase, e.g. Al₂Cu, Al₂CuMg, Al(Cu,Ag)Mg, and icosahedral (I) and decagonal (D) quasicrystalline phases. However, the solidification path in the interdendritic region during squeeze casting is different for each alloy, i.e. L→ternary α-Al–Al₂Cu–Al₂CuMg eutectic in Al–4.0Cu–1.5Mg and L→L′+Al₂Cu→α-Al–Al₂Cu–Al(Cu_0.75Ag_0.25)Mg eutectic in Al–4.0Cu–1.5Mg–0.7Ag. This indicates that silver acts as an alloying element stabilising the formation of Al(Cu,Ag)Mg Laves phase. The remaining copper and iron rich liquid in the interdendritic region at the final stage of solidification solidifies into a mixed structure of α-Al, Al₂Cu, and AlCuFe I (or D) phases. The composition of the I and D phases, measured by energy dispersive X-ray spectroscopy, is in the range Al–(27～28)Cu–(9～10)Fe and Al–(26～27)Cu–(7～9)Fe (all at.-%) respectively.     

Yield strength of twin-belt cast Al–Mg–Sc–Zr alloy after annealing

AA5xxx aluminium alloys are used in the automotive and packaging industries owing to their high strength and ductility. The addition of Sc and Zr to these alloys has shown promise for improving high temperature stability and therefore broadening the range of applications. This high temperature stability is due to the formation of fine Al₃(Sc, Zr) precipitates during aging. In this work, two twin-belt cast Al–3%Mg alloys, one with 0·4% Sc and the other without Sc, were annealed at 300 and 400°C. Hardness, tensile yield stress and electrical resistivity measurements were used to examine the evolution of microstructure and strength of the alloys. These results were then utilised to develop a yield stress–precipitation model to describe simultaneous precipitation hardening and recovery.     

Sc and Sc–Zr effects on microstructure and mechanical properties of Be–Al alloy

Herein, we investigated the effects of Sc and Sc–Zr on the microstructure and mechanical properties of Be–Al alloy, showing that Sc alloying resulted in Be grain refinement and reduced the secondary dendritic arm spacing (SDAS) of these grains by 1/3, whereas Sc–Zr alloying further decreased the SDAS to 7.5?µm and afforded equiaxed/cellular-like morphology with further refined Be grains. The above alloying resulted in the formation of intermetallic compounds (Be₁₃Sc, Be₁₃Zr, and Al₃(Sc_1–xZr_x)), increasing the macrohardness of the Be–Al alloy, with the microhardness and elastic modulus of the Be phase increasing to a larger extent than those of Al. Importantly, Sc–Zr alloying resulted in better microstructure modification and mechanical reinforcement than Sc alloying.     

Microstructure and wear characteristics of spray formed and hot extruded Al–Si alloys

Abstract

The microstructural and wear properties of spray formed Al–6.5Si, Al–18Si and Al–18Si–5Fe–1.5Cu (wt-%) alloys have been investigated. The microstructure of the Al–6.5Si alloy exhibits the equiaxed grain morphology of the primary α-Al phase with eutectic Si at the grain boundaries. The size of the primary Si particulates in the Al–18Si alloy varied from 3 to 8 μm embedded in the eutectic matrix. Complex intermetallic phases such as β-Al₅ SiFe and δAl₄ Si₂ Fe are observed to co-exist with primary Si in the spray formed Al–18Si–5Fe–1.5Cu alloy system. The periphery of the preforms invariably showed pre-solidified particles with a large amount of interstitial pores. An extrusion ratio of 6 : 1 for these alloys led to drastic porosity reduction and extensive breaking of second phase particles. These microstructural features showed distinct variation in the wear behaviour and the coefficient of friction of the alloys. The Al–18Si–5Fe–1.5Cu alloy shows better wear resistance compared with the other two alloys, particularly at higher loads. The coefficient of friction shows a dependence upon the applied load. However, this becomes steady at higher loads. The wear behaviour of these alloys is discussed in light of the morphology of debris particles as well as that of the worn surfaces.     

Effect of transition metals on energy absorption during strain-controlled fatigue of an aluminum alloy

Tensile and low cyclic fatigue tests were used to assess the influence of micro-additions of Ti/V/Zr on the performance of Al–7Si–1Cu–0.5Mg (wt.%) alloys in the as-cast and T6 heat-treated conditions and their improvement was compared to the base alloy. The microstructure of the as-cast Al–7Si–1Cu–0.5Mg (wt.%) base and modified alloys consisted of α-Al, eutectic Si, and Cu, the Mg- and Fe-based phases Al_2.1Cu, Al_8.5Si_2.4Cu, Al_7.2Si_8.3Cu₂Mg_6.9 and Al₁₄Si_7.1FeMg_3.3. In addition, the micro-sized Ti/V/Zr-rich phases Al_6.8Si_1.4Ti, Al_21.4Si_4.1Ti_3.5VZr_3.9, Al_6.7Si_1.2TiZr_1.8, Al_2.8Si_3.8V_1.6Zr and Al_5.1Si_35.4Ti_1.6Zr_5.7Fe were identified in the modified alloys. It was also noticed that increasing the content of Ti–V–Zr changed the morphology of Ti/V/Zr-rich phase. The tensile test results showed that the T6 heat-treated alloy modified with the addition of a higher content of Ti–V–Zr achieved the highest tensile strength of 343 MPa over the base alloy and alloys modified with additions of Ti, Ti–Zr and lower contents of Ti–V–Zr. The plastic strain energy density coefficient of the alloy modified with the addition of a higher content of Ti–V–Zr in the T6 temper condition was higher than the other studied alloys and reached 162 MJ m⁻³. The fatigue life of the same alloy was considerably longer than that of the other studied alloys, including the base alloy. The fractography revealed that all the studied alloys showed similar fracture behavior. The tensile cracks propagated through the eutectic Si and primary phases, exhibiting intergranular fracture along with some cleavage-like features of the plate-shaped Zr–Ti–V-rich intermetallics with the presence of fatigue striations on the latter, indicating their ductile nature. It is believed that the morphological changes of intermetallic precipitates containing Zr, Ti and V enhance the fatigue life of the alloy modified with additions of larger amounts of Ti–V–Zr in the T6 condition.