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On the interaction between Mg atoms and vacancies in the Al-Zn 10%-Mg 0.1% alloy
272
ACTA
METALLURGICA,
VOL.
12,
1964
fusion agents” in the binary and ternary alloys, respectively; (3) Finally a physical interpretation of the nature of quenched-in defects is given, namely vacancies in the binary alloy and stable, easy-moving, Mg-vacancy couples in the ternary alloy; in addition it is deduced that below lOO”C, Mg-vacancy couples are not easily eliminated from the matrix as single vacancies are. It is obvious that the first point is fundamental for the full interpretation: if it is not valid, the full model cannot be correct. In our work we have not been able to prove experimentally the validity of point (l), but we have simply assumed it; we have however based our opinion on the fact that the maximum resistivity observed at several aging temperatures is practically the same for the binary and ternary alloys (this fact would be very extraordinary if zones were of a diflerent nature in the two alloys); in addition the fact that the changes in resistivity observed when changing aging temperature are identical for the.ternary (see Fig. 10, Ref. 1) and for the binary alloy (see Fig. 9, Ref. 2) suggests the same conclusion. If we have well understood the considerations of G. BARTSCFI Imtitut fiir Metdlkunde Dr. Bartsch, he claims that the nature of zones in c&rTechnischen Uniuersitit Al-Zn alloys is strongly changed by the addition of a Berlin very small amount of Mg. In our opinion however Literature the experimental evidence reported by Dr. Bartsch is 1. C. PANSERI u. T. FEDERI~EI, A& Met. 11, 575 (1963). not sufficient to reject the validity of the above point 2. C. PANSEBI, F. GATTOu. T. FEDEEIQEI, Acta Met. 6, 198 (1958). (l), since it is easy to show that his results can be 3. G. BAXTSCH, Alwninium: Demdchst. easily explained in terms of our model. Received July 10, 1963. At this purpose let us first consider the results of Bartsch’s Fig. 2; one has to note that the “reversion” of 3’ at 150°C allows the sample to reach the equilibrium concentration of vacancies at this temperaOn the interaction between Mg atoms and ture; now the equilibrium concentration of vacancies vacancies in the Al-Zn lo%-Mg 0.1% alloy c, in the binary alloy is very low and by assuming Our method of investigating the interaction energy the value Ef = 0.70 eV for the formation energy, it is between a foreign atom and vacancies in Al,(l) making easily estimated in c, = 4.5 x 1O-e. In the ternary use at this purpose of the exceptional aging characteralloy the concentration of defects depends of course istics of the Al-Zn 10% alloy,@) is based on three on the Mg content and on the binding energy; successive steps: assuming a Mg content of cb = 0.1 oh and E, = 0.54 eV (1) First it is assumed that the presence of the for the binding energy, one obtains for the concentrasmall addition of a third element (Mg in the actual tion of coupled vacancies (see formula (22) in our case) does not change the general behaviour of the paper)(l), c, = 1.5 x 10-4. The higher concentration aging process (as density, shape and structure of of defects explains why in ternary alloy zones can zones); grow enough also after reversion, and overcome the (2) Hence a phenomenological model is developed, critical size for maximum resistivity. which is able to account for observed kinetic effects To account for the results of Bartsoh’s Fig. 1, one caused by the presence of Mg atoms; this model has to remember that the “amount of diffusion”, (which is strictly valid only for the case of Mg and not namely the final size of zones (and hence final values necessarily for other foreign elements) implies the of resistivity and hardness) depends both on the conexistence of two different kinds of quenched-in “difcentration of the defects and the mean number of
Der Widerstandsverlauf wird bereits zu einem Zeitpunkt in dem in zusatzfreien Legierungen durch die Bildung und das Wachstum von Zn-Zonen ein Widerstandsmaximum erreicht wird, durch diesen zweiten Prozess beeinflusst. Aus diesem Grunde ist die Zeit bis zum Erreichen des Widerstandsmaximums nicht geeignet, Schliisse auf die Aushllrtungsvorglinge die zur Bildung von Zn-Zonen fiihren, zu machen, insbesondere lassen sich hieraus keine Angaben iiber die Anderungen der Aktivierungsenergien fur die Leerstellenbildung und Wanderung gewinnen. Aus den am hiesigen Institut durchgefiihrten Widerstandsmessungen wurde versucht, die Aktivierungsenergie fur die Bildung der Leerstellen mit Hilfe der Grijsse t,, fiir Ap = const < Ap,, (d.h. die Zeit bis zum Erreichen eines festen Widerstandswertes der kleiner ist, ala das p-Maximum) zu berechnen. Die erhaltenen Werte lagen sowohl fiir die zusatzfreien ala such fiir die Mg-haltigen Iegierungen innerhalb der unserer Messmethode zugrunde liegenden Messunsicherheit, E, = 0,71 f 0,09 eV. Eine mogliche Wechselwirkung zwischen Leerstelle und Mg-Atome sollte daher mit einer Bildungsenergie E, < 0,09 eV stattfinden.
LETTERS
TO
jumps for their elimination. The second is a very important factor; for example, if a foreign atoms is able to trap vacancies forming motionless couples (as Fe seems able to do) the amount of diffusion is surely reduced; if, on the contrary, couples can easily move (as we have supposed for Mg-vacancy couples), the amount of diEusion will depend on the fate of couples; now in our model, to explain the long time of reaction observed in the ternary slloy (see Fig. 9 of our paper)(‘), we have supposed, as stressed above (point (3)), that in contrast with the case of single vacancies, Mg-vacancy couples succeed in remaining for long times in the matrix. Therefore, our model is by no means in contrast with the results of Fig. 1. Coming Anally to the slight influence of quenching temperature for extended aging times (Dr. Bartsch’s Fig. 3), we think it unnecessary to discuss it again here since we had observed the same phenomenon and commented on it extensively in our paper(l) (see Fig. 9 and llrst column et page 583, discussion of point e). In any case it is to stress that the phenomenon is again to be connected to the long life of Mg-vacancy couples. In conclusion we believe that there is no concrete reason for the moment to reject our model which can be yet considered as the most simple way to interpret the extraordinary kinetio effects produced by the addition of Mg atoms. Of course a careful inspection of the influence of small amounts of magnesium on the nature of zones in ternary alloys would be very welcome; in our opinion however, the only way to reach a definitive conclusion on the matter is to employ the X-ray small angle scattering method. c. PANSERI IStdti Sperimen& T. FEDEFZIQHI Aietaui Leggeri c. P. 129-“ovuru (Italy) References and T. fiDERIWiI, hda itfet.11, 676 (1063). 2. C. PAXSEXI and T. F&DERIQEI, Acta Met. 8, 217 (1060). 1. c. PANSEIU
l
Received Auguet 6, 1963.
Tempering of martensite in the Cu-Zn-Ga system* The formation of martensite as a result of quenching /l Cu-Zn-Ga from high temperatures or cooling b Cu-Zn below room temperature is s, well documented phenomenon. A similar transformation is typical of many pure metals and binary intermediate phases
THE
EDITOR
273
having a b.c.c. crystal structure, to which group /l Cu-Zn-Ga and @ Cu-Zn may be assigned. The transformation temperatures among these systems are dependent on the identity and composition of the metal or alloy and are generally quite sensitive to small composition changes. The b.c.c. phase, however, invariably is the one which becomes unstable at low temperatures and transforms during cooling-never the reverse. The crystal structure of /l Cu-Zn martensite could not be determined by the original investigators,(l) and this problem has remained essentially unsolved until a recent paper by Kunze(2) suggested a monoclinic lattice. Kunze used single crystal techniques on specimens he assumed were twinned martensite crystals; however, his attempts to interpret the X-ray diffraction date end deduce the crystal structure of the martensite were not reproducible in this laboratory.@) Our interest in determining the crystal structure of Cu-Zn-Ga martensite has led to the observation of an interesting anomalous behavior occurring during tempering. Many of the marten&es formed on cooling revert to the parent phase on heating(*); the most common exception being Fe-C martensite, which produces a carbide through a nucleation and growth decomposition. In general, however, martensitic transformations are thermally reversible, although appreciable hysteresis may be involved. The following evidence is presented that the marten&e produced in /? Cu-Zn-Ga alloy d oes not undergo reversal to its parent ,!I phase upon heating, nor does it transform directly to the expected equilibrium phases, but rather transforms to an intermediate phase of unknown crystal structure. The Cu-Zn-Ga alloys used in the experiments were prepared by melting together predetermined amounts of the oomponent metals in Vycor capsules under a The capsules were shaken helium atmosphere. vigorously several times to ensure complete mixing of the molten metals. Weight of the cast pellets agreed with the sums of the weights of the constituent metals within two parts per 1096; subsequent spectrographic analysis revealed no detectable contamination from the Vycor. The cast pellets were annealed for one week at 660°C after casting to reduce the effects of segregation. Filings to pass 250 mesh were made at room temperature from the cast pellets, heated briefly to 735%, a temperature within the single b phase region, in evacuated (or He filled) Vycor capsules, and quenched in iced brine. The filings transformed upon quenching from 735%; the martensitic nature of this transformation has been discussed in detail by Massalski.(5) The diffractometer
ACTA
METALLURGICA,
VOL.
12,
1964
fusion agents” in the binary and ternary alloys, respectively; (3) Finally a physical interpretation of the nature of quenched-in defects is given, namely vacancies in the binary alloy and stable, easy-moving, Mg-vacancy couples in the ternary alloy; in addition it is deduced that below lOO”C, Mg-vacancy couples are not easily eliminated from the matrix as single vacancies are. It is obvious that the first point is fundamental for the full interpretation: if it is not valid, the full model cannot be correct. In our work we have not been able to prove experimentally the validity of point (l), but we have simply assumed it; we have however based our opinion on the fact that the maximum resistivity observed at several aging temperatures is practically the same for the binary and ternary alloys (this fact would be very extraordinary if zones were of a diflerent nature in the two alloys); in addition the fact that the changes in resistivity observed when changing aging temperature are identical for the.ternary (see Fig. 10, Ref. 1) and for the binary alloy (see Fig. 9, Ref. 2) suggests the same conclusion. If we have well understood the considerations of G. BARTSCFI Imtitut fiir Metdlkunde Dr. Bartsch, he claims that the nature of zones in c&rTechnischen Uniuersitit Al-Zn alloys is strongly changed by the addition of a Berlin very small amount of Mg. In our opinion however Literature the experimental evidence reported by Dr. Bartsch is 1. C. PANSERI u. T. FEDERI~EI, A& Met. 11, 575 (1963). not sufficient to reject the validity of the above point 2. C. PANSEBI, F. GATTOu. T. FEDEEIQEI, Acta Met. 6, 198 (1958). (l), since it is easy to show that his results can be 3. G. BAXTSCH, Alwninium: Demdchst. easily explained in terms of our model. Received July 10, 1963. At this purpose let us first consider the results of Bartsch’s Fig. 2; one has to note that the “reversion” of 3’ at 150°C allows the sample to reach the equilibrium concentration of vacancies at this temperaOn the interaction between Mg atoms and ture; now the equilibrium concentration of vacancies vacancies in the Al-Zn lo%-Mg 0.1% alloy c, in the binary alloy is very low and by assuming Our method of investigating the interaction energy the value Ef = 0.70 eV for the formation energy, it is between a foreign atom and vacancies in Al,(l) making easily estimated in c, = 4.5 x 1O-e. In the ternary use at this purpose of the exceptional aging characteralloy the concentration of defects depends of course istics of the Al-Zn 10% alloy,@) is based on three on the Mg content and on the binding energy; successive steps: assuming a Mg content of cb = 0.1 oh and E, = 0.54 eV (1) First it is assumed that the presence of the for the binding energy, one obtains for the concentrasmall addition of a third element (Mg in the actual tion of coupled vacancies (see formula (22) in our case) does not change the general behaviour of the paper)(l), c, = 1.5 x 10-4. The higher concentration aging process (as density, shape and structure of of defects explains why in ternary alloy zones can zones); grow enough also after reversion, and overcome the (2) Hence a phenomenological model is developed, critical size for maximum resistivity. which is able to account for observed kinetic effects To account for the results of Bartsoh’s Fig. 1, one caused by the presence of Mg atoms; this model has to remember that the “amount of diffusion”, (which is strictly valid only for the case of Mg and not namely the final size of zones (and hence final values necessarily for other foreign elements) implies the of resistivity and hardness) depends both on the conexistence of two different kinds of quenched-in “difcentration of the defects and the mean number of
Der Widerstandsverlauf wird bereits zu einem Zeitpunkt in dem in zusatzfreien Legierungen durch die Bildung und das Wachstum von Zn-Zonen ein Widerstandsmaximum erreicht wird, durch diesen zweiten Prozess beeinflusst. Aus diesem Grunde ist die Zeit bis zum Erreichen des Widerstandsmaximums nicht geeignet, Schliisse auf die Aushllrtungsvorglinge die zur Bildung von Zn-Zonen fiihren, zu machen, insbesondere lassen sich hieraus keine Angaben iiber die Anderungen der Aktivierungsenergien fur die Leerstellenbildung und Wanderung gewinnen. Aus den am hiesigen Institut durchgefiihrten Widerstandsmessungen wurde versucht, die Aktivierungsenergie fur die Bildung der Leerstellen mit Hilfe der Grijsse t,, fiir Ap = const < Ap,, (d.h. die Zeit bis zum Erreichen eines festen Widerstandswertes der kleiner ist, ala das p-Maximum) zu berechnen. Die erhaltenen Werte lagen sowohl fiir die zusatzfreien ala such fiir die Mg-haltigen Iegierungen innerhalb der unserer Messmethode zugrunde liegenden Messunsicherheit, E, = 0,71 f 0,09 eV. Eine mogliche Wechselwirkung zwischen Leerstelle und Mg-Atome sollte daher mit einer Bildungsenergie E, < 0,09 eV stattfinden.
LETTERS
TO
jumps for their elimination. The second is a very important factor; for example, if a foreign atoms is able to trap vacancies forming motionless couples (as Fe seems able to do) the amount of diffusion is surely reduced; if, on the contrary, couples can easily move (as we have supposed for Mg-vacancy couples), the amount of diEusion will depend on the fate of couples; now in our model, to explain the long time of reaction observed in the ternary slloy (see Fig. 9 of our paper)(‘), we have supposed, as stressed above (point (3)), that in contrast with the case of single vacancies, Mg-vacancy couples succeed in remaining for long times in the matrix. Therefore, our model is by no means in contrast with the results of Fig. 1. Coming Anally to the slight influence of quenching temperature for extended aging times (Dr. Bartsch’s Fig. 3), we think it unnecessary to discuss it again here since we had observed the same phenomenon and commented on it extensively in our paper(l) (see Fig. 9 and llrst column et page 583, discussion of point e). In any case it is to stress that the phenomenon is again to be connected to the long life of Mg-vacancy couples. In conclusion we believe that there is no concrete reason for the moment to reject our model which can be yet considered as the most simple way to interpret the extraordinary kinetio effects produced by the addition of Mg atoms. Of course a careful inspection of the influence of small amounts of magnesium on the nature of zones in ternary alloys would be very welcome; in our opinion however, the only way to reach a definitive conclusion on the matter is to employ the X-ray small angle scattering method. c. PANSERI IStdti Sperimen& T. FEDEFZIQHI Aietaui Leggeri c. P. 129-“ovuru (Italy) References and T. fiDERIWiI, hda itfet.11, 676 (1063). 2. C. PAXSEXI and T. F&DERIQEI, Acta Met. 8, 217 (1060). 1. c. PANSEIU
l
Received Auguet 6, 1963.
Tempering of martensite in the Cu-Zn-Ga system* The formation of martensite as a result of quenching /l Cu-Zn-Ga from high temperatures or cooling b Cu-Zn below room temperature is s, well documented phenomenon. A similar transformation is typical of many pure metals and binary intermediate phases
THE
EDITOR
273
having a b.c.c. crystal structure, to which group /l Cu-Zn-Ga and @ Cu-Zn may be assigned. The transformation temperatures among these systems are dependent on the identity and composition of the metal or alloy and are generally quite sensitive to small composition changes. The b.c.c. phase, however, invariably is the one which becomes unstable at low temperatures and transforms during cooling-never the reverse. The crystal structure of /l Cu-Zn martensite could not be determined by the original investigators,(l) and this problem has remained essentially unsolved until a recent paper by Kunze(2) suggested a monoclinic lattice. Kunze used single crystal techniques on specimens he assumed were twinned martensite crystals; however, his attempts to interpret the X-ray diffraction date end deduce the crystal structure of the martensite were not reproducible in this laboratory.@) Our interest in determining the crystal structure of Cu-Zn-Ga martensite has led to the observation of an interesting anomalous behavior occurring during tempering. Many of the marten&es formed on cooling revert to the parent phase on heating(*); the most common exception being Fe-C martensite, which produces a carbide through a nucleation and growth decomposition. In general, however, martensitic transformations are thermally reversible, although appreciable hysteresis may be involved. The following evidence is presented that the marten&e produced in /? Cu-Zn-Ga alloy d oes not undergo reversal to its parent ,!I phase upon heating, nor does it transform directly to the expected equilibrium phases, but rather transforms to an intermediate phase of unknown crystal structure. The Cu-Zn-Ga alloys used in the experiments were prepared by melting together predetermined amounts of the oomponent metals in Vycor capsules under a The capsules were shaken helium atmosphere. vigorously several times to ensure complete mixing of the molten metals. Weight of the cast pellets agreed with the sums of the weights of the constituent metals within two parts per 1096; subsequent spectrographic analysis revealed no detectable contamination from the Vycor. The cast pellets were annealed for one week at 660°C after casting to reduce the effects of segregation. Filings to pass 250 mesh were made at room temperature from the cast pellets, heated briefly to 735%, a temperature within the single b phase region, in evacuated (or He filled) Vycor capsules, and quenched in iced brine. The filings transformed upon quenching from 735%; the martensitic nature of this transformation has been discussed in detail by Massalski.(5) The diffractometer