Uticaj odgrevanja na strukturne transformacije i magnetna svojstva legure Fe72Cu1V4Si15B8
Influence of annealing on structural transformations and magnetic properties of the Fe72Cu1V4Si15B8 alloy
Докторанд
Surla, RadoslavМентор
Mitrović, NebojšaЧланови комисије
Vasić, MilicaIvanovski, Valentin
Метаподаци
Приказ свих података о дисертацијиСажетак
APSTRAKT:
U ovoj disertaciji su prikazani rezultati ispitivanja legure Fe72Cu1V4Si15B8, koja
pripada familiji FINEMET-tipa legura. Legura je izrađena ultrabrzim hlađenjem
rastopa legure na rotirajućem disku. Dobijena legura ima već formirane nanokristalne
faze (α-Fe(Si) i Fe23B6 faze), koje se nalaze u amorfnoj matrici, što je ustanovljeno
XRD analizom i Mesbauerovom spektroskopijom. Pored toga, Mesbauerovom
spektroskopijom je ustanovljeno postojanje znatno više kristalnih faza koje zbog male
zastupljenosti nisu detektovane XRD analizom. Legura je termički tretirana, što je
indukovalo strukturne promene koje su doprinele promeni magnetnih svojstava. DTA
analizom je legura zagrevana do 1073 K, različitim brzinama (β= 5K/min, 10K/min i 20
K/min), kojom su ustanovljena dva kristalizaciona pika: (i) od 750 K do 780 K i (ii) od
875 K do 900 K (za β= 5 K/min). Oba pika su nesimetrična, što ukazuje na složenost
procesa kristalizacije. Ovi pikovi odgovaraju jednom termomagnetnom pi...ku u
intervalu od 745 K do 875 K (za H=7960 A/m i β= 4 K/min). Termomagnetnim merenjima
je ustanovljena Kirijeva temperatura amorfne faze legure koja iznosi oko 615 K i
Kirijeva temperatura iskristalisane legure koja iznosi oko 875 K.
Nakon toga su uzorci oblika trake odgrevani različitim temperaturama (Tan= 573 K –
973 K). Ustanovljeno je da je pri Tan= 773 K došlo do najvećeg povećanja magnetizacije
zasićenja, Ms uz minimalno povećanje koercitivnog polja, Hc. Povećanje Ms se objašnjava
uvećanjem zastupljenosti α-Fe(Si) faze, sa porastom Tan, usled transformacije amorfne
u kristalnu fazu. Prosečna veličina novoformiranih kristalita α-Fe(Si) faze ista je
kao i veličini kristalita u neodgrevanoj leguri do Tan=773 K. Pri Tan>773 K dolazi do
naglog povećavanja prosečne veličine kristalita ove faze, što uzrokuje slabiju
pokretljivost magnetnih domena, zbog čega se smanjuje Ms. Isti trend promena Ms uočen
je i za longitudinalnu i normalnu (perpendikularnu) orijentaciju uzorka u magnetnom
polju u odnosu na temperaturu odgrevanja.
Konstantan porast Hc sa porastom Tan, povezano je sa boridnim fazama gvožđa, čiji se
udeo sa odgrevanjem takođe povećava. Naime, odgrevanjem se metastabilna faza Fe23B6
transformiše u stabilnu Fe2B fazu, a proces se završava na Tan= 723 K. Nemagnetni
atomi bora (B) koji okružuju α-Fe smanjuju izmensku interakciju atoma Fe, što je razlog
degradacije magnetno mekih svojstva.
Odgrevane trake su analizirane i Mesbauerovom spektroskopijom, gde su rezultati
pokazali da ni jedan od podspektara u potpunosti ne odgovara identifikovanim fazama
Fe23B6 i Fe2B. Ovo se objašnjava rastvorljivošću elemenata Si i V u kristalnoj rešetki
Fe, kao i bliskog atomskog faktora rasejanja atoma Fe i V, pa se pretpostavlja da u
kristalnim rešetkama ovih faza ima u manjoj meri ugrađenih atoma V i Si. Pored toga
na osnovu podspektara koji korespondiraju Fe(Si) fazi, pretpostavlja se da u pojedinim
delovima ove faze ima različitog rasporeda prvih suseda atoma Fe, kao i ugrađenog V,
što ukazuje na postojanje i drugih oblika kristalnih rešetki ove faze. Ove
pretpostavke su potvrđene skenirajućim elektronskim mikroskopom gde su EDS
analizom detektovana određena zrna bogata vanadijumom. Takođe, ovom metodom
pronađena su i mesta bogata bakrom, što sugeriše na postojanje klastera atoma bakra
koji se prvi izdvajaju prilikom kristalizacije FINEMET sistema i isti pospešuju
nukleaciju α-Fe(Si) faze...
ABSTRACT:
This dissertation presents the results of examination of the alloy Fe72Cu1V4Si15B8, which
belongs to the FINEMET-type family of alloys. The alloy was prepared with a rapid quenching
of the melt on a rotating disc. The as-prepared alloy already contains nanocrystals of α-Fe(Si)
and Fe23B6 phases, in an amorphous matrix, that is observed by XRD analysis and by
Mössbauer spectroscopy. In addition, Mössbauer spectroscopy revealed the existence of
significantly more crystal phases that, due to their low presence, were not detected by XRD
analysis. The alloy was thermally treated, which induced structural changes that contributed to
the changes in magnetic properties. First, the alloy was heated to 1073 K, at different rates (β =
5K/min, 10K/min and 20 K/min), and two crystallization peaks were found by DTA analysis:
(i) from 750 K to 780 K, and (ii) from 875 K to 900 K (for β = 5 K/min). Both peaks are
asymmetric, which indicates the complexity of the crystallization pr...ocess. These peaks
correspond to a single thermomagnetic peak in the range from 745 K to 875 K
(for H = 7960 A/ m and β = 4 K/ min). Thermomagnetic measurements revealed the Curie
temperature of the amorphous phase of the alloy at 615 K, and the Curie temperature of the
crystallized alloy at 875 K.
After that, the ribbon-shaped samples were annealed at different temperatures (Tan = 573 K -
973 K). It was found that at Tan = 773 K there was the huge increase in saturation magnetization
(Ms) with a minimal increase in the coercive field (Hc). The increase in Ms is explained by the
increase of the abundance of α-Fe (Si) phase, with the increase of Tan, due to the transformation
of amorphous into crystalline phase. The average size of the evolved crystallites of α-Fe (Si)
phase is the same as the size of crystallites in the as-prepared alloy up to Tan = 773 K. At Tan >
773 K there is a sharp increase in the average size of crystallites of this phase, which causes
slow mobility of magnetic domains due to reduced width of the domain walls, that is why the
Ms decreases. The same trend of changes in Ms was observed for both longitudinal and
perpendicular orientation of the sample in the magnetic field with increase of the annealing
temperature.
The constant increase of Hc with the increase of Tan is connected with the boride phases of iron,
whose weight fraction also increases with the annealing. By annealing, the metastable phase
Fe23B6 is transformed into a stable Fe2B phase, and the process ends at Tan = 723 K. Nonmagnetic boron atoms (B) surrounding α-Fe reduce the exchange interaction of Fe atoms, and
consequently, the degradation of magnetically soft properties occurs.
The annealed ribbons were also analysed by Mössbauer spectroscopy, where the results showed
that none of the sub-spectra fully corresponded to the identified Fe23B6 and Fe2B phases. This
is explained by the solubility of the elements Si and V in the crystal lattice of Fe, as well as by
the similar atomic scattering factor of Fe and V atoms. It is assumed that some of V and Si
atoms are embedded in the crystalline lattices of these phases. In addition, based on the subspectra corresponding to the Fe (Si) phase, it is assumed that in some parts of this phase there
is a different arrangement of the first neighbors of the Fe atom, as well as a built-in V, which
indicates the existence of other crystalline forms of this phase. These assumptions were
confirmed by scanning electron microscopy where certain vanadium-rich grains were detected
by EDS analysis. Also, with this method, copper-rich sites were found, which suggests the
existence of clusters of copper atoms that are formed at the onset of crystallization of the
FINEMET system and they accelerate the nucleation of the α-Fe (Si) phase...