Martensitic αʺ-Fe16N2-Type Phase of Non-Stoichiometric Composition: Current Status of Research and Microscopic Statistical-Thermodynamic Model

Abstract

The literature (experimental and theoretical) data on the tetragonality of martensite with interstitial–substitutional alloying elements and vacancies are reviewed and analysed. Special attention is paid to the studying the martensitic αʺ-Fe16N2-type phase with unique and promising magnetic properties as an alternative to the rare-earth intermetallics or permendur on the world market of the production of permanent magnets. The period since its discovery to the current status of research is covered. A statistical-thermodynamic model of ‘hybrid’ interstitial–substitutional solid solution based on a b.c.t. crystal lattice, where the alloying non-metal constituents (impurity atoms) can occupy both interstices and vacant sites of the host b.c.c.(t.)-lattice, is elaborated. The discrete (atomic-crystalline) lattice structure, the anisotropy of elasticity, and the ‘blocking’ and strain-induced (including ‘size’) effects in the interatomic interactions are taken into account. The model is adapted for the non-stoichiometric phase of Fe–N martensite maximally ordered by analogy with αʺ-Fe16N2, where nitrogen atoms are in the interstices and at the sites of b.c.t. iron above the Curie point. It is stressed an importance of adequate data on the available (in the literature) temperature- and concentration-dependent microscopic energy parameters of the interactions of atoms and vacancies. The features of varying (viz. non-monotonic decreasing with increasing temperature) the relative concentration of N atoms in the octahedral interstices of b.c.t. Fe, and therefore, the degree of its tetragonality (correlating with this concentration) are elucidated. Within the wide range of varying the total content of introduced N atoms, the ratio of the equilibrium concentration of residual site vacancies to the concentration of thermally activated vacancies in a pure b.c.c. Fe is demonstrated at a fixed temperature.

High-Temperature Entropy Effects in Tetragonality of the Ordering Interstitial–Substitutional Solution Based on Body-Centred Tetragonal Metal

“…У разі зміни зовнішніх термодинамічних умов (температури, тиску, магнетного поля) з'являється можливість реґулювати будову та тим самим властивості речовини даного тіла, а це означає, що одержується спосіб керування функціональністю матеріялу на його основі. Одну з таких можливостей розглянуто у даній роботі на прикладі одного з магнетних матеріялів сучасности [1][2][3][4]. Останні є складовими при використанні у магнетних пристроях, і певне вдосконалення таких матеріялів може мати значний вплив на енергетичні ресурси та сферу застосування.…”

“…Згідно з [14,15], припустімо, що атоми X по октаедричних міжвузловинах розподілені впорядковано за надструктурним типом фази α″-Fe16N2 [16,17] (про нинішній стан дос-ліджень цієї фази див. в огляді [3]). Тоді маємо [15]:…”

“…можуть бути легко розраховані в рамках наближення короткосяжної [11,24] міжатомової взаємодії. Наприклад, тоді й лише тоді 3…”

Comparative studies of the three-body abrasion wear resistance of hardfacing Fe-Cr-C-B-Ti alloy

“…The development of surfacing technologies is carried out by taking into account theoretical approaches to the selection of the alloy component composition [22,23,24], technological heredity [25], and optimization of detail manufacturing technologies [26] to ensure the functioning of products during the life cycle [27,28].…”

Re-examining the giant magnetization density in α′′-Fe16N2 with the SCAN+U method

“…Permanent magnets for functional applications are currently based largely on the costly rare earths such as Sm, Nd, and Gd and there is great need to find new cost-effective alternatives. [1][2][3] In this connection, magnetic nitrides, such as the iron nitrides, appear to be promising, 4 and among these a 00 -Fe 16 N 2 , 5,6 discovered in 1951, 7 stands out because it supports a giant magnetization density. [8][9][10] Kim and Takayashi 8 have reported a magnetization of 2.58 T and Sugita et al reported an even higher magnetization of 2.8-3.0 T. 11 These large values of the magnetization are surprising because they exceed the maximum magnetization expected from the Slater-Pauling curve, 12 which attributes the magnetic saturation of an itinerant ferromagnet solely to the number of empty states in the 3d shell and this number is maximized for the FeCo alloys at a magnetization density of about 2.45 T. Giant magnetization in a 00 -Fe 16 N 2 thus implies the emergence of localized magnetic moments in the material.…”