Substitutional-Interstitial High Entropy Alloys

Abstract

Since the last two decades, a new alloying concept that involves the addition of multi principal elements in high concentrations has been introduced. These alloys, termed “high entropy” alloys, are under vast exploration due to the scientific and technological exploration possibilities in these new alloy systems. These alloys have shifted the alloy design strategy from a single element-based alloy system to a multi-element-based alloy system. Thus, offering a vast compositional space for the development of new alloy systems. Classically, high entropy alloys are defined as alloy systems with at least five major elements, each element content between 5 to 35 atomic percentage. Another definition of these alloys is based on configurational entropy, where alloys with configurational entropy, Sconf, larger than 1.61 R are categorised as “high entropy” alloys. Most of the investigations in these alloys show that Fe, Ni, Co and Cr are used with maximum frequency in these alloys. While the addition of interstitial elements such as Carbon, Oxygen, Nitrogen, Boron and Hydrogen into the interstitial sublattice is less explored. These interstitial elements can bring about additional configurational entropy to the alloy system. Also, the addition of these interstitial elements into these alloys brings about some additional interstitial strengthening effects, including some Transformation Induced plasticity (TRIP) and Twinning induced plasticity (TWIP) effects upon plastic deformation and compositional metastability effects such as spinodal decomposition and precipitation. As thorough investigations on the role of interstitials in HEAs are lacking, therefore it is of great interest to study High and Medium Entropy alloys based on “Substitutional- Interstitial” alloy design strategy. For solid solutions with both substitutional and interstitial elements present, a regular solution model with two sublattices are considered: substitutional and octahedral-interstitial. The configurational entropy “𝑆𝑐𝑜𝑛𝑓” of such solution is given by the following equation: � �𝑐𝑜𝑛𝑓= -R [{𝑎∑ 𝑌𝑀ln(𝑌𝑀) 𝑀 }+ {𝑐∑𝑌𝐼 𝐼 ln(𝑌𝐼)+(1−𝑌𝐼)ln(1−𝑌𝐼)}] where R is the Universal gas constant, 𝑌𝑀 is the fraction of substitutional sites occupied by M and � �𝐼 is the fraction of octahedral interstitial sites occupied by interstitial element, I. The relation between atomic fraction of elements and their occupied site fraction is given below � �𝑀= 𝑋𝑀 (1−𝑋𝐼) � �𝐼= 𝑎𝑋𝐼 𝑐(1−𝑋𝐼) ‘a’ and ‘c’ are the number of sites for the substitutional and octahedral-interstitial sublattices, respectively (for FCC, a = c = 1 and for BCC, a = 1;c = 3 ). XM and XI are the atomic fraction of substitutional and interstitial atoms, respectively. Due to the higher number of interstitial sites per substitutional site in BCC, the configurational entropy increase is higher for BCC alloyed with N as compared to FCC alloyed with N. Due to this additional entropy contribution from interstitial sublattice, one can alter the substitutional element composition to tune the properties without reducing the total configurational entropy of the alloy. This way a low or medium entropy alloy can be converted to high entropy alloy by introducing the interstitial N. Interstitials are at the heart of steels alloy design and heat treatment to bring in wide variety of properties. In HEA’s due to the differences in size and chemical activity of different elements the interstitial sites experience changes in size, shape and chemical surroundings making it as an interesting sublattice to understand the way interstitial solubility and ordering reactions emerge. Against the above background, this PhD thesis aims to understand the N alloying into medium entropy alloys by applying various nitriding treatments. The contents of this Ph. D thesis are organized as detailed below: First chapter of this thesis consists of an introduction to high entropy alloys with the presentation of literature information on the addition of interstitials into HEAs, Interstitial- Substitutional sublattice design approach, low-temperature nitriding of stainless steels and thermodynamics of salt bath and gaseous nitriding. At the end of this chapter, the objectives of this PhD thesis are detailed. Second chapter presents the research outcomes from the low temperature salt bath nitriding experiments on 316 L stainless steel powders. The N content taken up by the 316 L powders after different nitriding treatments is calculated based on the enhanced lattice parameter of austenite and the known dependence between lattice parameter and N content for austenitic phase of stainless steels. Thereafter, the increase in the configurational entropy as a result of dissolved nitrogen is calculated using Hillert-Staffanson two sublattice model. Colossal levels of N solubility (up to 30 at. % N) was realized which drastically enhanced the configurational entropy of the system. Hardness of powders is increased from about 200 HV before nitriding to above 1500 HV after nitriding. The obtained results were discussed based on the thermodynamic calculations and a discussion on utilizing these nitrided powders to make high N stainless steels via sintering or additive manufacturing utilizing nitrided steel powders.