Precipitation and Dissolution Phenomena in Al-Zn Alloys

The microstructure of Al-Zn alloys, with the Zn atom fraction, xZn 62%, has been studied in detail by XRD in dependence on the composition,temperature and previous thermal treatment. Precipitationphenomena in alloys, transferred to the supersaturated stateby rapid quenching from the solid solution temperature, Tss, in waterat RT, have been followed in dependence on the ageing time atRT or elevated temperature. The equilibrium state, reached by ageing,has been compared with that obtained by slow cooling from Tssto RT. Phase transitions, dissolution and precipitation processes inthe alloys, transferred to the equilibrium state, have been studiedin situ in dependence on temperature from RT to Tss. A temperaturehysteresis in reversal phase transitions has been observed oncooling from Tss to RT. Repeated heating and cooling cycles haverevealed different microstructure in alloys having undergone differentthermal treatments between RT and Tss. A change in thephase diagram of the Al-Zn system is necessary in the compositionregion xZn > 50%

Precipitation and Dissolution Phenomena in Al-Zn Alloys

The microstructure of Al-Zn alloys, with the Zn atom fraction, χZn ≤ 62%, has been studied in detail by XRD in dependence on the composition, temperature and previous thermal treatment. Precipitation phenomena in alloys, transferred to the supersaturated state by rapid quenching from the solid solution temperature, Tss, in water at RT, have been followed in dependence on the ageing time at RT or elevated temperature. The equilibrium state, reached by ageing, has been compared with that obtained by slow cooling from Tss to RT. Phase transitions, dissolution and precipitation processes in the alloys, transferred to the equilibrium state, have been studied in situ in dependence on temperature from RT to Tss. A temperature hysteresis in reversal phase transitions has been observed on cooling from Tss to RT. Repeated heating and cooling cycles have revealed different microstructure in alloys having undergone different thermal treatments between RT and Tss. A change in the phase diagram of the Al-Zn system is necessary in the composition region χZn > 50%

Precipitation and Dissolution Phenomena in Al-Zn Alloys

The microstructure of Al-Zn alloys, with the Zn atom fraction, χZn ≤ 62%, has been studied in detail by XRD in dependence on the composition, temperature and previous thermal treatment. Precipitation phenomena in alloys, transferred to the supersaturated state by rapid quenching from the solid solution temperature, Tss, in water at RT, have been followed in dependence on the ageing time at RT or elevated temperature. The equilibrium state, reached by ageing, has been compared with that obtained by slow cooling from Tss to RT. Phase transitions, dissolution and precipitation processes in the alloys, transferred to the equilibrium state, have been studied in situ in dependence on temperature from RT to Tss. A temperature hysteresis in reversal phase transitions has been observed on cooling from Tss to RT. Repeated heating and cooling cycles have revealed different microstructure in alloys having undergone different thermal treatments between RT and Tss. A change in the phase diagram of the Al-Zn system is necessary in the composition region χZn > 50%

In situ phase analysis of the thermal decomposition products of zirconium salts

X-ray powder diffraction at high temperature was used to determine the phase composition of the thermal decomposition products of two zirconium salts, Zr(SO4)(2). 4H(2)O and ZrO(NO3)(2). 2 H2O, and of a mixture of zirconium nitrates having Zr(OH)(2)(NO3)(2). 4.7 H2O and ZrO(NO3)(2). 2H(2)O as dominant components. Heating of the samples up to 1200 degrees C was performed inside a high-temperature chamber, attached to a diffractometer, at an air pressure of approximate to 2 x 10(-3) Pa. Regardless of the structural differences in the starting salts, thermal decomposition products crystallized to t-ZrO2 which remained stable up to 1200 degrees C. This result indicated that the structural nature of the starting materials was not the most important factor of metastable t-ZrO2 formation. The thermodynamically stable m-ZrO2 appeared after the cooling of the samples to room temperature. If the cooling was performed at low air pressure, the m-ZrO2 content was small. Introduction of air, even at RT, caused a considerable increase of m-ZrO2, which became the dominant phase in all cases. The important role of oxygen in the t-ZrO2 --> m-ZrO2 transition indicates that the lack of oxygen in the zirconia lattice favours the formation of metastable t-ZrO2

In situ phase analysis of the thermal decomposition products of zirconium salts

X-ray powder diffraction at high temperature was used to determine the phase composition of the thermal decomposition products of two zirconium salts, Zr(SO4)2 ⋅ 4 H2O and ZrO(N03)2 ⋅ 2 H2O, and of a mixture of zirconium nitrates having Zr(OH)2(NO3)2 ⋅ 4.7 H2O and ZrO(NO3)2 ⋅ 2 H2O as dominant components. Heating of the samples up to 1200 °C was performed inside a high-temperature chamber, attached to a diffractometer, at an air pressure of ≈ 2 × 10-3 Pa. Regardless of the structural differences in the start-ing salts, thermal decomposition products crystallized to t-ZrO2, which remained stable up to 1200 °C. This result indicated that the structural nature of the starting materials was not the most important factor of metastable t-ZrO2 formation. The thermodynamically stable m-ZrO2 appeared after the cooling of the samples to room temperature. If the cooling was performed at low air pressure, the m-ZrO2 content was small. Introduction of air, even at RT, caused a considerable increase of m-ZrO2, which became the dominant phase in ali cases. The important role of oxygen in the t-ZrO2 → m-ZrO2 transition indicates that the lack of oxygen in the zirconia lattice favours the formation of metastable t-ZrO2

Ovisnost mikrostrukture slitina Zn-Al o temperaturi

The temperature dependence of microstructure in Zn-Al alloys, transferred to the equilibrium state by quenching from the solid solution temperature, TSS, to room temperature (RT) and prolonged ageing at RT, was studied in situ by Xray powder diffraction. Instead of the phase transitions expected according to the phase diagram, namely α(M/β) + β → α’ + α(M/α’) → αSS for the alloy with 54 at% Zn and α(M/β) + β → α’+β → αSS for the alloy with 62 at% Zn, the following sequence was observed for both alloys: α(M/β) + β → α’ + β + α(M/α’,β) → α’+β → αSS. In the cooling run, a temperature hysteresis in reversal phase transitions was observed. During a repeated heating, the microstructure depended on the previous thermal treatment of the alloys.Primjenom rendgenske difrakcije istraživali smo temperaturnu ovisnost mikrostrukture slitina Zn-Al, koje su bile prevedene u ravnotežno stanje brzim kaljenjem s temperature čvrste otopine, TSS, na sobnu temperaturu, te dugotrajnim starenjem pri sobnoj temperaturi. Umjesto faznih pretvorbi koje su se očekivale prema faznom dijagramu, naime α(M/β) + β → α’ + α(M/α’) → αSS za slitinu s 54 at% Zn, te α(M/β) + β → α’+β → αSS za slitinu s 62 at% Zn, opazili smo sljedeći niz faznih pretvorbi za obje slitine: α(M/β) + β → α’ + β + α(M/α’, β) → α’+β → αSS. Tijekom hlađenja slitina uočili smo temperaturnu histerezu za obrnute fazne pretvorbe. Tijekom ponovnog grijanja slitina od sobne temperature do TSS utvrdili smo da mikrostruktura slitina ovisi o prethodnoj termičkoj obradi

Konstanta kristalne rešetke faze α u ravnoteži s raznim precipitatima u slitinama Al-Zn

A series of Al-rich Al-Zn alloys were studied by X-ray diffraction at room and elevated temperatures. Using the present results and those cited in literature, the following conclusion can be drawn. The lattice constant, a[α(M/P)], of the matrix, i.e. the phase α(M) in contact with various sorts of precipitates, P, depends on the initial Zn content, x, only in case of fully coherent P, i.e. GP zones. In other cases, where P’s are semicoherent or incoherent α′ R, α′ or β(Zn) phases, a[α(M/P)] is independent of x, but increases with diminishing degree of coherency. The difference in the Zn content between P and α(M) determines the observed changes of the lattice constant.Niz slitina Al-Zn u području bogatom Al istraživano je rentgenskom difrakcijom pri sobnoj i povišenoj temperaturi. Prema rezultatima sadašnjeg rada i literaturnih podataka može se izvesti sljedeći zaključak. Konstanta rešetke, a[α(M/P)], matrice, tj. faze α(M) u ravnoteži s raznim vrstama precipitata, P, ovisi o početnom udjelu Zn, x, samo u slučaju potpuno koherentnih P, tj. GP zona. U ostalim slučajevima, gdje su P polukoherentne ili nekoherentne faze α ′ R, α ′ ili β(Zn), a[α(M/P)] ne ovisi o x, ali raste sa smanjivanjem stupnja koherencije. Razlika u x između P i α(M) određuje opažene promjene konstante rešetke

Powder-Pattem-Fitting Methods in Structure Determination

The Rietveld method is a well known powder-pattem-fitting method which consists in adjusting the complete theoretical diffraction pattern, calculated on the basis of the model for the sample crystal structure, to the experimental powder diffraction pattern. In the fitting procedure, the structure model is being refined. The Rietveld method also enables determination of some other structural properties of the material, like crystallite size and strains, and the quantitative phase analysis of a multicomponent mixture. Complementary fitting methods to the Rietveld method are the individual profile fitting method and the whole-powder-pattern decomposition method, which do not require structural models. The individual profile fitting method enables decomposition of the overlapping diffraction lines in a small range of the powder pattern, while the whole-powder-pattern decomposition method simultaneously decomposes the whole powder pattern into individual lines and refines the unit-cell parameters of the sample. Although the powder-pattern-fitting methods are not methods for direct structure determination, they are very powerful tools in the course of structure determination when the sample is not available in a single crystal form. Several examples of the application of the described methods in structure determination are presented

Konstanta kristalne rešetke faze α u ravnoteži s raznim precipitatima u slitinama Al-Zn

A series of Al-rich Al-Zn alloys were studied by X-ray diffraction at room and elevated temperatures. Using the present results and those cited in literature, the following conclusion can be drawn. The lattice constant, a[α(M/P)], of the matrix, i.e. the phase α(M) in contact with various sorts of precipitates, P, depends on the initial Zn content, x, only in case of fully coherent P, i.e. GP zones. In other cases, where P’s are semicoherent or incoherent α′ R, α′ or β(Zn) phases, a[α(M/P)] is independent of x, but increases with diminishing degree of coherency. The difference in the Zn content between P and α(M) determines the observed changes of the lattice constant.Niz slitina Al-Zn u području bogatom Al istraživano je rentgenskom difrakcijom pri sobnoj i povišenoj temperaturi. Prema rezultatima sadašnjeg rada i literaturnih podataka može se izvesti sljedeći zaključak. Konstanta rešetke, a[α(M/P)], matrice, tj. faze α(M) u ravnoteži s raznim vrstama precipitata, P, ovisi o početnom udjelu Zn, x, samo u slučaju potpuno koherentnih P, tj. GP zona. U ostalim slučajevima, gdje su P polukoherentne ili nekoherentne faze α ′ R, α ′ ili β(Zn), a[α(M/P)] ne ovisi o x, ali raste sa smanjivanjem stupnja koherencije. Razlika u x između P i α(M) određuje opažene promjene konstante rešetke

Temperature dependence of microstructure in Zn-Al alloys

The temperature dependence of microstructure in Zn-Al alloys, transferred to the equilibrium state by quenching from the solid solution temperature, T_SS, to room temperature (RT) and prolonged ageing at RT, was studied in situ by X-ray powder diffraction. Instead of the phase transitions expected according to the phase diagram, namely α (M/β) + β → α' + α(M/α') → α_SS for the alloy with 54 at% Zn and α(M/β) + β → α'+β → α_SS for the alloy with 62 at% Zn, the following sequence was observed for both alloys: α(M/β) + β → α' + β+ α(M/α', β) → α'+β → α_SS. In the cooling run, a temperature hysteresis in reversal phase transitions was observed. During a repeated heating, the microstructure depended on the previous thermal treatment of the alloys