the system al  cu has been investigated intensively during the last decades , mainly due to the importance of al - based alloys , for example in aviation and transport industry . in view of this , the main focus of most studies in the system is the very al - rich part . although a lot of work was done in the cu - rich part as well , there are still some uncertainties and inconsistencies in the phase diagram present . his extensive paper gives an equilibrium phase diagram as well as manifold information on metastable phase equilibria which are not part of the current investigation . according to , the equilibrium phase diagram contains 5 intermetallic compounds stable at ambient temperature and 7 additional compounds stable at elevated temperature ( see fig .  1 ) . the phase diagram given by murray does not represent the current level of knowledge about the system . a more recent phase diagram combining the assessment of murray   with new data from liu et  al . thermodynamic calculations in the system were performed by several authors concentrating on transition- and ordering phenomena , , , , as well as on atomic mobility . an overview on the al  cu phases described by different authors , , , , , , , , , , , ,   is given in table  1 . the structure of the -phase with the composition al2cu was originally revealed by friauf   and found to be tetragonal . according to murray additional investigations in the region between 31 and 37.5  at.% cu by goedecke and sommer   indicate a composition of 32.4  at.% cu for  at its formation temperature of 592  c . the eutectic line of the reaction l  = ( al )  +   ends at 32.05 at.% cu . structural investigations of the compound -alcu have already been performed by preston   who found an orthorhombic structure in a sample quenched from 602  c . investigated slowly cooled samples of the same composition and proposed an allotropic transformation 1 2 on basis of structural differences compared to the work of preston . were able to solve the structure of the low temperature phase which was found to be monoclinic . preston suggested the structure to be orthorhombic ( op16 or oc16 ) , lukas and lebrun   mentioned in their assessment of the al  cu  si system an orthorhombic cell with lattice parameters a  =  4.087   , b  =  12.00   , c  =  8.635   and 32 atoms per unit cell . although the supposed type of transition reaction was not mentioned explicitly , the assessed phase diagram by murray   and riani et  al .   obviously suggest a transition of higher order between 2 and 1 . according to the assessment of murray , the introduction of the high temperature phase 1 and 2 goes back to 1920 . however , the structure of 2 was solved for the first time in 1972 by el - boragy et  al . the structure of 2-al2+xcu3 is of the nias - type with partial occupation of the additional interstitial position . the compound with the proximate composition al3cu4 ( 1/2  region ) was also described by preston   and bradley   and was found to show a high and a low temperature modification . the work of murray suggests a transition temperature between 530 and 570  c but mentions other reported thermal effects between 373 and 450  c as well . ,   investigated as - cast and annealed samples with the composition al3cu4 . in the as - cast samples the authors find a mixture of an orthorhombic face - centered and an orthorhombic body - centered structure as well as a small amount of -al4cu9 . after annealing at 500  c for 10  h the of structure became the major phase thus the authors suggested a transition al4cu9  +   oi electron probe micro analysis ( epma ) measurements indicated compositions of al43.2cu56.8 , al41.3cu58.7 and al39.6cu60.4 for  of  ,  oi  and -al4cu9 , respectively . the crystal structures of 1 ( fmm2 , structure type al3cu4 ) and 2 ( imm2 , structure type al3cu4- ) were finally solved by gulay and harbrecht using powder xrd , . the composition of the samples for structure analysis of 1 ( al42.5cu57.5 ) and 2 ( al43.2cu56.8 ) contradicts the findings of dong et  al . ,   who allocated the face - centered symmetry to the phase with lower cu - content . the assessment of murray shows a low temperature phase 2 and a high temperature phase 1 with a slightly higher cu - content ; the transition temperature is supposed to be between 530 and 570  c . gulay and harbrecht , however , found the cu - richer phase 1 ( only phase in a sample with the composition al42.5cu57.5 ) to be stable at 400  c . the cu - poorer phase 2 ( only phase in a sample with the composition al43.2cu56.8 ) was found to be stable at elevated temperatures ( 530  c ) and did not resist thermal treatment at 400  c . the authors claim that entropy provides an essential contribution to the stabilization of the 2 phase . the range from 60 to 70 at % bradley   claimed that three different phases are present in this region : a cubic (  ) , a monoclinic and a rhombohedral compound . westman   found the latter to crystallize in space group r3 m and confirmed the existence of a third phase of unknown structure between the cubic and the rhombohedral compound . seshadri and downie   claimed that there are only five intermetallic phases stable in the temperature range between 25 and 500  c , namely (1 ) ,  ( described by a cubic structure ) , 2 , 2 and . the separation of 1 and  is supported by an abrupt change in expansion coefficient from 1 to . in the assessed phase diagram the third phase of unknown structure mentioned above is not included since there is no consensus about its existence . murray pleads diffusion couple experiments of funamizu et  al .   which do not show any other phase between 1 and . more importantly , very slow cooling experiments performed by van sande et  al . these two experiments support the non - existence of a third equilibrium phase in the indicated region . the existence of the high temperature phase 0 was demonstrated by thermal analysis but the transition 0 to 1 could not be confirmed metallographically . examined the cu - rich part of the phase diagram by diffusion coupling , differential scanning calorimetry and high temperature xrd . the authors state that 0 crystallizes in the cu5zn8-type and they do not find a two phase field between 0 and 1 , thus proposing a higher order transition between the two phases , in contradiction to murray s assessment . according to murray , quoting dawson , the high temperature phase 0 is formed peritectically from  and liquid at 1037  c . dawson determined the composition and stability range of 0 by metallography and dilatometry but the findings have never been reconfirmed and the structure of 0 remains unknown . nevertheless , 0 was included in the equilibrium phase diagram . in 1998 diffusion couple experiments performed by liu et  al . showed a two phase region between  and 0 and no single phase 0 was found . additionally , the authors found only one peak in dsc measurements at 1019  c which they interpreted to be the solidus of the  phase rather than the reaction temperature of the eutectoid transformation 0 investigated the region between 85 and 89 at.% cu at temperatures from 450 to 850  c , finding a square - like shaped phase which has , however , not been confirmed by other authors . the -phase and the two - phase region between  and ( cu ) was frequently investigated and the assessment of murray   gives a broad overview about the results of this research . it shows that the eutectoid temperature was found between 560 and 575  c which can be explained by the sluggishness of the reaction . reaction temperatures between 515 and 540  c can be considered due to metastable eutectoid and peritectoid reactions . , later studies confirmed the peritectoid decomposition temperature to be 363  c at 77.25 at.% cu . according to murray s assessment , 2 has an ordered fcc structure with a long - period superlattice based on cu3au and al3ti ( strukturbericht designations : l12 and d022 , respectively ) . a more detailed description about investigations of the low temperature phase 2 , including thermal analysis experiments in this region is given by adorno et  al . . the samples were prepared from aluminum slug ( 99.999% ) , and copper wire ( 99.95% ) , both supplied by alfa aesar , karlsruhe , germany . the cu wire was reduced in a h2-flow at 300  c for 3  h. the calculated amounts of al and cu were weighted to an accuracy of 0.05  mg ; the sample weight usually was 1000  mg . sample homogenization was done in an arc furnace mam-1 by edmund buehler with a water - cooled copper plate and zirconium as the getter material . for homogenization of the sample , the resulting bead was turned and re - melted two times . the occurring mass loss during this procedure was found to be below 1% and therefore considered not to affect the sample composition significantly . the resulting bead was wrapped in molybdenum foil ( 99.97% , plansee se , reutte , austria ) and annealed at 500  c under vacuum in a quartz glass tube for 24 days . representative sections of all annealed samples were investigated by means of optical microscopy using a zeiss axiotech 100 microscope . the quantitative chemical analyses were performed on a zeiss supra 55 vp in combination with energy dispersive spectroscopy ( eds ) using the pure elements for calibration . measurements of the phase composition were performed with a minimum of three different spots and the results were averaged . x - ray powder diffraction analyses were performed using a bruker d8 advance diffractometer operating in reflection mode ( cu k1 radiation , lynxeye silicon strip detector ) . for selected samples high temperature x - ray powder diffraction analysis was applied using an anton paar xrk900 reactor chamber with an automated alignment stage . the temperature resolved measurements were performed under evacuated conditions . for evaluation of the resulting diffractograms , both at ambient as well as at elevated temperature , the software topas   was used . the dta measurements were performed on a setaram setsys evolution 2400 ( setaram instrumentation , caluire , france ) and a netzsch dta 404 pc ( netzsch , selb , germany ) . the measurement devices used pt / pt-10%rh thermocouples ( type s ) which were calibrated using the melting points of pure sn , au and ni . the samples with a weight of approximately 20  mg were placed in open alumina crucibles and measured under an argon flow of 50  ml  min for the netzsch and 20  ml  min for the setaram device . applying a heating / cooling rate of 5  kmin , the possible mass loss during the dta investigations was checked routinely and no relevant mass changes were observed . combining results from both dta and sem measurements , it was possible to obtain the complete description of the al  cu system which is plotted in fig . 2 . it is , for the most part , in good agreement to the phase diagram of liu et  al .   and riani et  al . the occurring invariant reactions together with the composition of the reacting phases and the reaction temperature are given in table  2 , selected sem images taken in back - scattered electron ( bse ) mode are shown in fig . the al - rich part of the phase diagram has not been investigated extensively in the current study . concerning the reaction temperatures and the phase composition , the solubility of cu in al was found to be 2.2(1 ) at.% at 500  c . the lattice parameters of the  phase  ( ti12 , al2cu - type ) vary from a  =  6.0718(1 )  and c  =  4.8802(1 )  at 32(1 ) at.% cu to a  =  6.0613(1 )  and c  =  4.8724(1 )  at 33.6(2 ) at.% cu . the phase boundaries of the binary phases in the al - rich part of the phase diagram have been determined by means of sem measurements and are indicated as black dots in fig the phases 1-alcu and 2-alcu are supposed to be stable around 50 at% cu with a solubility range of 12 at.% cu . the results in the current investigations , however , reveal a shift of the composition toward the cu - rich side . the solubility limits of the phase 2-alcu at 500  c were confirmed by sem measurements . the cu - poor and the cu - rich composition limit were found at 51.9(5 ) at.% cu and at 54.8(5 ) at.% , respectively . xrd analysis of the samples with the nominal composition al49cu51 ( showing  and 2 in equilibrium ) , al47.5cu52.5 and al46.5cu53.5 ( showing single phase 2 ) and al45cu55 ( showing 2 plus traces of 1 ) indicate a cu - rich solubility limit between 54 and 55 at.% cu which supports the sem measurements . the lattice parameters for 2 ( mc20 , alcu - type ) range from a  =  12.0925(1 )  , b  =  4.1001(1 ) , c  =  6.9085(1 )  , =  55.03(1) on the cu - poor side to a  =  12.2012(1 )  , b  =  4.0997(2 )  , c  =  7.0047(3 )  and   =  54.787(1) for the cu - rich side of this compound . the phase diagrams of riani and murry indicate a transition temperature from the high temperature 1-phase to the low temperature 2-phase at 563  c at the cu - poor side and 560  c at the cu - rich side , . according to our measurements the transition temperatures are 574(3 ) c for the cu - poor side and 580(1 ) c for the cu - rich side . consequently , we propose also different solid state reactions in this area : the eutectoid decomposition 1  =  2  +   at 574(3 ) c and the peritectoid reaction 1 the peritectic decomposition temperature of the phase 1 was found to be 625(2 ) c which is in agreement with the phase diagrams mentioned above . the region of al4cu9 with the supposed high temperature modification 1 and the low temperature phase 2 is also complex . gulay and harbrecht find the cu - rich phase 1 stable at 530 and 400  c , but the cu - poor phase 2 stable at 530  c and not stable at 400  c , . they discovered that after a heat treatment at 400  c , 2 is decomposed into 1 and small amounts of 2 . additionally , gulay and harbrecht state that at 450  c the phase 2 segregates in a mixture of 1 and 2 . therefore , the authors conclude that the temperature of the eutectoid decomposition of 2 is between 400 and 450  c . the present work shows a sample with the nominal composition of al45cu55 exhibiting 2 as major phase with very small traces of a second phase . comparison of systematic extinctions of 1 and 2 suggests that the second phase is 1 . due to the very low amount of 1 in the respective sample , sem analysis of this sample shows only one suitable measurement point at 56.3 at.% cu which can be assigned to the cu - poor solubility limit of 1 . xrd analysis of a sample with the nominal composition al42.5cu57.5 shows 1 
  and some unidentified peaks ( see table  3 ) . these xrd results narrow the solubility limits of 1 ( of88 , al3cu4-type ) between approx . 56 and 57.5 at % cu . in the present investigation , dta analysis of the sample with the nominal composition of al45cu55 does not show any effects related to the transition of 1 to 2 , caused by the fact that the amount of 1 is very small in the sample . dta analysis of the sample with the nominal composition al42.5cu57.5 shows an invariant effect at 561(2 ) c which is considered to be related to the reaction 2 these results do confirm the previous authors ,   concerning the stability of the low temperature phase 1 which is considered to be stable between ambient temperature and 561(2 ) c . since the xrd results of the sample al45cu55 show small traces of 1 we suggest a transition temperature 2  =  1  +  2 above 500  c . the solubility limit at the cu - rich side of 1/2 was not accessible due to low contrast and fine microstructure . however ,  was found as the only phase present in al40cu60 , while al42.5cu57.5 showed 1 with traces of . therefore , the situation of the two phase field can be specified quite accurately . it was not possible to determine the phase boundaries between the two phases  and 1 by sem measurements due to the lack of contrast and possibly very fine microstructure . in xrd it was possible to distinguish the single phase region  ( r3 m , a  =  12.285(1 )  , c  =  15.1486(1 )  ) from the single phase region 1 ( p-43 m , a  =  8.7068(3 )  ) by peak splitting and selective peak broadening even though the patterns look very similar . samples with the nominal composition al40cu60 to al37cu63 show  as the only present phase . the sample al36cu64 shows a main pattern corresponding to  plus some small extra peaks that could not be explained by  or 1 . they may be caused by super structure reflections , corresponding to the monoclinic structure proposed by bradley   and westman , which was omitted in the assessment of murray . +  1  =   in the sample al38cu62 and al37cu63 can be explained by a shift of the 1 phase field toward the cu - poorer region at elevated temperatures and , therefore , a smaller amount of  taking part in the reaction . this leads to a smaller endothermic effect and since the respective endothermic peaks in the samples al40cu60 and al39cu61 are already small the resulting effect in the samples al38cu62 and al37cu63 might be insufficient to observe . the region between 1 and ( cu ) has been the subject of an intensive research in the past and the present work does not provide any contradictory information . the solubility of al in ( cu ) as well as the upper solubility limit of 1 was confirmed by sem measurements . dta analysis of samples in the respective area show very small thermal effects close to the solubility limit of ( cu ) at 567  c and thermal effects related to the formation of  at higher temperature , which are indicated as diamond shaped points in fig .  2 . in general , solubility ranges and thermal stability of the high temperature compounds 1 and 2 could be confirmed in the present work by dta observations . slight changes concerning reaction temperatures and solubility ranges are shown in table  2 . since there is no consensus in literature concerning the transition of the high temperature phase 0 to the low temperature form 1 this area is of special interest . analysis of thermal effects of samples in the respective field show very weak effects varying continuously with the composition . we did not observe any pointer for an invariant decomposition of 1 in any of the investigated samples . therefore we conclude that the transition 1  =  0 is of higher order , in agreement with the previous obtained results by liu et  al . . structural analysis of a sample with the nominal composition al32cu68 confirm the structure given for 0 by liu ( i-43 m , cu5zn8-type ) , and reveals a lattice parameter of a  =  8.8692(1 )  at 900  c . according to murray , the high temperature phase 0 was included in the equilibrium phase diagram although its existence could not be confirmed . did not find any evidence of its existence and consequently was not incorporated in the assessed phase diagram of riani et  al . . results of the present work confirm that there is no evidence of the existence of 0 and all observed dta effects in this composition area can be explained by the formation of the phases  and 0 . preston   suggested the space groups cmmm or pban and orthorhombic lattice parameters were given by lukas and lebrun . in the current study , we used high temperature powder xrd data to establish a structural model for 1 . measurements were carried out in a temperature range from 500 to 750  c at intervals of 25  c using a sample with the nominal composition al50cu50 . up to 550  c show the low - temperature 2-phase in equilibrium with traces of the -phase . at 575  c a third pattern , 1 , appears which is the only phase present at 600 and 625  c . above 650 c only 2 is present and significant peak broadening can be observed , indicating the partial melting of the powder . after cooling back the sample to 500  c one again observes 2 and  , the diffraction lines , however , are significantly broadened . some unidentified peaks of very low intensity may be attributed to oxide formation at the sample surface during the long stay in the non ambient device . since the amount of possible oxide is very small , further investigation was not performed . the pattern measured at 600  c could be successfully indexed using the orthorhombic unit cell suggested by lukas and lebrun . cell refinement in space group cmmm yielded the lattice parameters a  =  4.1450(1 ) , b  =  12.3004(4 ) and c  =  8.720(1 )  . according to the phase diagram discussion at 600  c the measured sample with the nominal composition al50cu50 is in equilibrium with a small amount of liquid phase . this affects the background of the xrd measurement , which was compensated by modeling an additional broad peak at 43.9(1 ) 2. further details of measurement and structure refinement of the 1 phase including the calculated errors of the parameters are listed in table  3 . the refined pattern of 1 is shown in fig .  6 . the structural model for 1 was established by a twofold approach . given the similarities of the lattice parameters between the monoclinic phase 2 and the orthorhombic 1 , aortho  bmono , bortho  amono and vortho    1.5vmono , we tried to develop the structural model by rearranging the atomic positions of the low temperature phase in the orthorhombic high temperature cell using the space group c222 . the atomic coordinates of this structural model were finally transformed to cmmm and standardized by applying the program structure tidy , . during the consecutive rietveld refinement , unusually large differences at the individual isotropic displacement factors were observed , indicating a decrease of electron density at some atomic positions . the occupations of al2 , al3 and cu2 were found to be significantly reduced while all other sites were found to be fully occupied within 3 esd s and were therefore fixed during the final refinements . the final structural model shows reasonable displacement factors and the refined overall composition al14.1cu14.8 ( equivalent to 51.2 at.% cu ) is in excellent agreement with the al - rich phase boundary of 1 ( 51.5(5 ) at.% cu ) . more details on the crystal structure investigation can be obtained from the fachinformationszentrum karlsruhe , 76344 eggenstein - leopoldshafen , germany , ( fax :  + 497247 808666 ; e - mail : crysdata@fiz.karlsruhe.de ) on quoting the depository number csd 423053 . the coordination polyhedra for the 7 independent positions of 1 are shown in fig . 7 . apart from the high - symmetry positions cu3 and cu4 , the coordination figures are quite irregular with coordination numbers between cn  =  10 and cn  =  14 . a comparison of the atomic arrangements in the low temperature phase 2 and the high temperature phase 1 is shown in fig . the figure shows the layer in ( 001 ) of orthorhombic 1 in comparison to the layer in ( 010 ) of the monoclinic 2 , i.e. both structures are projected along their short axis . all atoms shown are situated within the mirror plane at z  =  0 and y  =  0 , respectively . the corresponding second layer of each structure ( situated at z  =  1/2 and y  =  1/2 , respectively ) shows the same atomic arrangement shifted by  in   for 1 and in   for 2 according to space group symmetry . 8 shows that both structures have a common structural motif ; i.e. a diamond shaped unit consisting of 5 cu- and 4 al - atoms . these motifs are arranged in a rectangular pattern and interconnected along their corners in case of the high temperature structure . in the monoclinic structure while only one atom , al3 , is not part of the diamond - shaped motif of 2 , three of the seven sites in 1 ( al1 , cu2 and cu3 ) are not part of this motif . although the two structures are obviously related it should be pointed out that it is not possible to transform one structure into the other in a simple way and a second order transition between 1 and 2 can be definitely ruled out . this is consistent with our phase diagram investigation which clearly indicates an invariant reaction related to the transformation from 2 to 1 . as an example , the dta curve for the sample with the nominal composition al50cu50 ( used also for the high temperature xrd ) is shown in fig . this effect is followed by the peritectic decomposition of  ( 590  c ) and 1 ( 625  c ) and finally the liquidus effect at 804  c . combining results from both dta and sem measurements , it was possible to obtain the complete description of the al  cu system which is plotted in fig . 2 . it is , for the most part , in good agreement to the phase diagram of liu et  al .   and riani et  al . the occurring invariant reactions together with the composition of the reacting phases and the reaction temperature are given in table  2 , selected sem images taken in back - scattered electron ( bse ) mode are shown in fig . the al - rich part of the phase diagram has not been investigated extensively in the current study . concerning the reaction temperatures and the phase composition , the solubility of cu in al was found to be 2.2(1 ) at.% at 500  c . the lattice parameters of the  phase  ( ti12 , al2cu - type ) vary from a  =  6.0718(1 )  and c  =  4.8802(1 )  at 32(1 ) at.% cu to a  =  6.0613(1 )  and c  =  4.8724(1 )  at 33.6(2 ) at.% cu . the phase boundaries of the binary phases in the al - rich part of the phase diagram have been determined by means of sem measurements and are indicated as black dots in fig the phases 1-alcu and 2-alcu are supposed to be stable around 50 at% cu with a solubility range of 12 at.% cu . the results in the current investigations , however , reveal a shift of the composition toward the cu - rich side . the solubility limits of the phase 2-alcu at 500  c were confirmed by sem measurements . the cu - poor and the cu - rich composition limit were found at 51.9(5 ) at.% cu and at 54.8(5 ) at.% , respectively . xrd analysis of the samples with the nominal composition al49cu51 ( showing  and 2 in equilibrium ) , al47.5cu52.5 and al46.5cu53.5 ( showing single phase 2 ) and al45cu55 ( showing 2 plus traces of 1 ) indicate a cu - rich solubility limit between 54 and 55 at.% cu which supports the sem measurements . the lattice parameters for 2 ( mc20 , alcu - type ) range from a  =  12.0925(1 )  , b  =  4.1001(1 ) , c  =  6.9085(1 )  , =  55.03(1) on the cu - poor side to a  =  12.2012(1 )  , b  =  4.0997(2 )  , c  =  7.0047(3 )  and   =  54.787(1) for the cu - rich side of this compound . the phase diagrams of riani and murry indicate a transition temperature from the high temperature 1-phase to the low temperature 2-phase at 563  c at the cu - poor side and 560  c at the cu - rich side , . according to our measurements the transition temperatures are 574(3 ) c for the cu - poor side and 580(1 ) c for the cu - rich side . consequently , we propose also different solid state reactions in this area : the eutectoid decomposition 1  =  2  +   at 574(3 ) c and the peritectoid reaction 1 the peritectic decomposition temperature of the phase 1 was found to be 625(2 ) c which is in agreement with the phase diagrams mentioned above . the region of al4cu9 with the supposed high temperature modification 1 and the low temperature phase 2 is also complex . gulay and harbrecht find the cu - rich phase 1 stable at 530 and 400  c , but the cu - poor phase 2 stable at 530  c and not stable at 400  c , . they discovered that after a heat treatment at 400  c , 2 is decomposed into 1 and small amounts of 2 . additionally , gulay and harbrecht state that at 450  c the phase 2 segregates in a mixture of 1 and 2 . therefore , the authors conclude that the temperature of the eutectoid decomposition of 2 is between 400 and 450  c . the present work shows a sample with the nominal composition of al45cu55 exhibiting 2 as major phase with very small traces of a second phase . comparison of systematic extinctions of 1 and 2 suggests that the second phase is 1 . due to the very low amount of 1 in the respective sample , sem analysis of this sample shows only one suitable measurement point at 56.3 at.% cu which can be assigned to the cu - poor solubility limit of 1 . xrd analysis of a sample with the nominal composition al42.5cu57.5 shows 1 
  and some unidentified peaks ( see table  3 ) . these xrd results narrow the solubility limits of 1 ( of88 , al3cu4-type ) between approx . 56 and 57.5 at % cu . in the present investigation , dta analysis of the sample with the nominal composition of al45cu55 does not show any effects related to the transition of 1 to 2 , caused by the fact that the amount of 1 is very small in the sample . dta analysis of the sample with the nominal composition al42.5cu57.5 shows an invariant effect at 561(2 ) c which is considered to be related to the reaction 2 these results do confirm the previous authors ,   concerning the stability of the low temperature phase 1 which is considered to be stable between ambient temperature and 561(2 ) c . since the xrd results of the sample al45cu55 show small traces of 1 we suggest a transition temperature 2  =  1  +  2 above 500  c . the solubility limit at the cu - rich side of 1/2 was not accessible due to low contrast and fine microstructure . however ,  was found as the only phase present in al40cu60 , while al42.5cu57.5 showed 1 with traces of . therefore , the situation of the two phase field can be specified quite accurately . it was not possible to determine the phase boundaries between the two phases  and 1 by sem measurements due to the lack of contrast and possibly very fine microstructure . in xrd it was possible to distinguish the single phase region  ( r3 m , a  =  12.285(1 )  , c  =  15.1486(1 )  ) from the single phase region 1 ( p-43 m , a  =  8.7068(3 )  ) by peak splitting and selective peak broadening even though the patterns look very similar . samples with the nominal composition al40cu60 to al37cu63 show  as the only present phase . the sample al36cu64 shows a main pattern corresponding to  plus some small extra peaks that could not be explained by  or 1 . they may be caused by super structure reflections , corresponding to the monoclinic structure proposed by bradley   and westman , which was omitted in the assessment of murray . +  1  =   in the sample al38cu62 and al37cu63 can be explained by a shift of the 1 phase field toward the cu - poorer region at elevated temperatures and , therefore , a smaller amount of  taking part in the reaction . this leads to a smaller endothermic effect and since the respective endothermic peaks in the samples al40cu60 and al39cu61 are already small the resulting effect in the samples al38cu62 and al37cu63 might be insufficient to observe . the region between 1 and ( cu ) has been the subject of an intensive research in the past and the present work does not provide any contradictory information . the solubility of al in ( cu ) as well as the upper solubility limit of 1 was confirmed by sem measurements . dta analysis of samples in the respective area show very small thermal effects close to the solubility limit of ( cu ) at 567  c and thermal effects related to the formation of  at higher temperature , which are indicated as diamond shaped points in fig .  2 . in general , solubility ranges and thermal stability of the high temperature compounds 1 and 2 could be confirmed in the present work by dta observations . slight changes concerning reaction temperatures and solubility ranges are shown in table  2 . since there is no consensus in literature concerning the transition of the high temperature phase 0 to the low temperature form 1 this area is of special interest . analysis of thermal effects of samples in the respective field show very weak effects varying continuously with the composition . we did not observe any pointer for an invariant decomposition of 1 in any of the investigated samples . therefore we conclude that the transition 1  =  0 is of higher order , in agreement with the previous obtained results by liu et  al . . structural analysis of a sample with the nominal composition al32cu68 confirm the structure given for 0 by liu ( i-43 m , cu5zn8-type ) , and reveals a lattice parameter of a  =  8.8692(1 )  at 900  c . according to murray , the high temperature phase 0 was included in the equilibrium phase diagram although its existence could not be confirmed . did not find any evidence of its existence and consequently was not incorporated in the assessed phase diagram of riani et  al . . results of the present work confirm that there is no evidence of the existence of 0 and all observed dta effects in this composition area can be explained by the formation of the phases  and 0 . preston   suggested the space groups cmmm or pban and orthorhombic lattice parameters were given by lukas and lebrun . in the current study , we used high temperature powder xrd data to establish a structural model for 1 . measurements were carried out in a temperature range from 500 to 750  c at intervals of 25  c using a sample with the nominal composition al50cu50 . up to 550  c show the low - temperature 2-phase in equilibrium with traces of the -phase . at 575  c a third pattern , 1 , appears which is the only phase present at 600 and 625  c . above 650 c only 2 is present and significant peak broadening can be observed , indicating the partial melting of the powder . after cooling back the sample to 500  c one again observes 2 and  , the diffraction lines , however , are significantly broadened . some unidentified peaks of very low intensity may be attributed to oxide formation at the sample surface during the long stay in the non ambient device . since the amount of possible oxide is very small , further investigation was not performed . the pattern measured at 600  c cell refinement in space group cmmm yielded the lattice parameters a  =  4.1450(1 ) , b  =  12.3004(4 ) and c  =  8.720(1 )  . according to the phase diagram discussion at 600  c the measured sample with the nominal composition al50cu50 is in equilibrium with a small amount of liquid phase . this affects the background of the xrd measurement , which was compensated by modeling an additional broad peak at 43.9(1 ) 2. further details of measurement and structure refinement of the 1 phase including the calculated errors of the parameters are listed in table  3 . the refined pattern of 1 is shown in fig was established by a twofold approach . given the similarities of the lattice parameters between the monoclinic phase 2 and the orthorhombic 1 , aortho  bmono , bortho  amono and vortho    1.5vmono , we tried to develop the structural model by rearranging the atomic positions of the low temperature phase in the orthorhombic high temperature cell using the space group c222 . the atomic coordinates of this structural model were finally transformed to cmmm and standardized by applying the program structure tidy , . during the consecutive rietveld refinement , unusually large differences at the individual isotropic displacement factors were observed , indicating a decrease of electron density at some atomic positions . the occupations of al2 , al3 and cu2 were found to be significantly reduced while all other sites were found to be fully occupied within 3 esd s and were therefore fixed during the final refinements . the final structural model shows reasonable displacement factors and the refined overall composition al14.1cu14.8 ( equivalent to 51.2 at.% cu ) is in excellent agreement with the al - rich phase boundary of 1 ( 51.5(5 ) at.% cu ) . more details on the crystal structure investigation can be obtained from the fachinformationszentrum karlsruhe , 76344 eggenstein - leopoldshafen , germany , ( fax :  + 497247 808666 ; e - mail : crysdata@fiz.karlsruhe.de ) on quoting the depository number csd 423053 . the coordination polyhedra for the 7 independent positions of 1 are shown in fig . 7 . apart from the high - symmetry positions cu3 and cu4 , the coordination figures are quite irregular with coordination numbers between cn  =  10 and cn  =  14 . a comparison of the atomic arrangements in the low temperature phase 2 and the high temperature phase 1 is shown in fig . the figure shows the layer in ( 001 ) of orthorhombic 1 in comparison to the layer in ( 010 ) of the monoclinic 2 , i.e. both structures are projected along their short axis . all atoms shown are situated within the mirror plane at z  =  0 and y  =  0 , respectively . the corresponding second layer of each structure ( situated at z  =  1/2 and y  =  1/2 , respectively ) shows the same atomic arrangement shifted by  in   for 1 and in   for 2 according to space group symmetry . 8 shows that both structures have a common structural motif ; i.e. a diamond shaped unit consisting of 5 cu- and 4 al - atoms . these motifs are arranged in a rectangular pattern and interconnected along their corners in case of the high temperature structure . in the monoclinic structure while only one atom , al3 , is not part of the diamond - shaped motif of 2 , three of the seven sites in 1 ( al1 , cu2 and cu3 ) are not part of this motif . although the two structures are obviously related it should be pointed out that it is not possible to transform one structure into the other in a simple way and a second order transition between 1 and 2 can be definitely ruled out . this is consistent with our phase diagram investigation which clearly indicates an invariant reaction related to the transformation from 2 to 1 . as an example , the dta curve for the sample with the nominal composition al50cu50 ( used also for the high temperature xrd ) is shown in fig . this effect is followed by the peritectic decomposition of  ( 590  c ) and 1 ( 625  c ) and finally the liquidus effect at 804  c . the current work revealed significant improvements on the established phase diagram in the system al cu by solving several inconsistencies in literature . the al - rich part of the phase diagram could be confirmed . the phases 1 and 2 show a significant shift to the cu - rich side of the phase diagram and exhibit , contrary to previously published phase diagrams , , , a first order transition reaction . the 1/2 region was re - investigated and the recent findings of gulay and harbrecht ,   were widely confirmed . the transition between 0 and 1 does not show a two phase field thus indicating a higher order transition and confirming the results of liu et  al . . the phase is orthorhombic ( space group cmmm ) and the lattice parameters are a  =  4.1450(1 )  , b  =  12.3004(4 )  and c  =  8.720(1 ) .
the phase equilibria and reaction temperatures in the system al  cu were re - investigated by a combination of optical microscopy , powder x - ray diffraction ( xrd ) at ambient and elevated temperature , differential thermal analysis ( dta ) and scanning electron microscopy ( sem ) . 
 a full description of the phase diagram is given . 
 the phase equilibria and invariant reactions in the cu - poor part of the phase diagram could be confirmed . 
 the cu - rich part shows some differences in phase equilibria and invariant reactions compared to the known phase diagram . 
 a two phase field was found between the high temperature phase 1 and the low temperature phase 2 thus indicating a first order transition . in the 1/2 region of the phase diagram 
 recent findings on the thermal stability could be widely confirmed . 
 contrary to previous results , the two phase field between  and 1 is very narrow . 
 the results of the current work indicate the absence of the high temperature 0 phase as well as the absence of a two phase field between 1 and 0 suggesting a higher order transition between 1 and 0 . 
 the structure of 0 ( i-43 m , cu5zn8-type ) was confirmed by means of high - temperature xrd . 
 powder xrd was also used to determine the structure of the high temperature phase 1-al1cu . 
 the phase is orthorhombic ( space group cmmm ) and the lattice parameters are a  =  4.1450(1 )  , b  =  12.3004(4 )  and c  =  8.720(1 )  ; atomic coordinates are given .