Rajiv S. Mishra

• PhD
• Professor at University of North Texas

Understanding columnar-to-equiaxed transition (CET) is a critical aspect of microstructural evolution in addi-tively manufactured (AM) alloys, since proportions of columnar and equiaxed morphologies impact strength-ductility synergy. Traditional methodologies for CET prediction, primarily based on power-law relation between growth-rate and laser tr...

The high ductile brittle transition temperature of tungsten (W) poses challenges like cracking and porosity during its laser-powder bed additive manufacturing (L-PBFAM). Computational alloy design evaluated niobium (Nb) and carbon (C) to activate eutectic solidification, addressing the cracking issues during the terminal stage of solidification. Fu...

Metal-bonded magnetic composites (MBMCs) present a promising alternative to dense sintered magnets, particularly for intricate components. Compared to polymer-based bonded magnets, MBMCs have wider applicability in harsh environments. In this paper, we demonstrate a solid-state shear-based manufacturing technique to introduce localized magnetizatio...

Aluminum matrix composites hold promise for creating lightweight structural components with multifunctional properties. Nonetheless, achieving desired magnetic or electrical characteristics without compromising their fundamental mechanical properties presents a significant challenge. Here, we explore mechanical stir-based extrusion as a solid-state...

Strength and ductility vary inversely due to a rapid decrease in dislocation storage capacity with the pronounced increase in work hardening rate at the expense of ductility in most conventional and recently designed advanced ductile materials. This limitation can be overcome by creating heterogenous or hierarchical microstructures containing not o...

SolidStir® Extrusion, a newly-innovated manufacturing process, offers tooling design adaptability as a significant advantage over conventional extrusion processes. Aluminum 6061-T6 was extruded using a modular multi-hole three-piece die. Microstructural characterization of as-extruded 3 mm diameter wire revealed refined equiaxed grains of 9.5 ± 4.0...

The effect of two different backing plate (BP) conditions on the microstructure and resulting mechanical properties of friction stir welded Al-Cu-Li alloy (2195-O) was investigated. This study shows that Copper-BP with lower welding temperature and higher cooling rate, forms finer grains in the nugget, resulting in higher yield strength. The steel-...

Electrification is a path towards decarbonization. Identification of novel nanocarbon materials such as graphene or carbon nanotubes (CNTs) offers great potential as a reinforcement material in metal matrix composites (MMCs) due to their superior strength and electrical properties. However, challenges exist in achieving uniform dispersion of graphe...

In this work, a combination of in-situ high synchrotron X-ray diffraction and electron backscattered diffraction were used to systematically investigate the activation and evolution of the deformation mechanisms in an as-cast Fe42Mn28Co10Cr15Si5 metastable high entropy alloy deformed until fracture at room temperature. This work unveils the critica...

Aluminum alloys find wide applications in power and transmission cables, and electrical conductors due to their advantageous properties of high electrical conductivity, and lower cost per unit. However, the lower tensile strength compared to copper poses a challenge for the mechanical stability of aluminum-based cables. In this study, we addressed...

Phase-specific damage tolerance was investigated for the AlCoCrFeNi2.1 high entropy alloy with a lamellar microstructure of L12 and B2 phases. A microcantilever bending technique was utilized with notches milled in each of the two phases as well as at the phase boundary. The L12 phase exhibited superior bending strength, strain hardening, and plast...

The mechanical properties of an alloy depend on its microstructure. The strength-ductility trade-off is a paradigm that existed for a long time. Advanced alloys, such as high entropy alloys (HEAs), utilize a dual-phase strengthening mechanism, which originates from the microstructural phenomena consisting of twinning and phase transformation, to si...

An innovative approach to build a high-performance, thermally stable Al-8Ce-10Mg (wt.%) alloy via friction-stir based solid-state additive manufacturing, called additive friction stir deposition, has been demonstrated in this study. The deposited material displayed 22% higher yield strength and 181% improvement in ductility as compared to the base...

Deformation-based solid-state additive manufacturing techniques are known for high build rates, absence of process-induced defects that are detrimental to mechanical performance, refined equiaxed microstructure, large-scale cus-tomization, and manufacturing at ambient conditions. We introduce a novel severe plastic deformation-based SolidStir Ò add...

Niobium carbide (NbC) based cermets are emerging as strong contenders to replace conventional tungsten carbide (WC) based cermets for applications involving abrasion and wear resistance due to their superior mechanical, physical, and chemical properties. NbC cermets with various binders have been explored in literature and nickel has emerged as a p...

Nitrogen containing austenitic stainless steels (N-ASS) are widely utilized to fabricate various structural components in tokamak type fusion reactors owing to their suitable mechanical and functional properties. These components are exposed to a range of temperatures (4-500 K) and interact closely with the magnetic fields that are used to control...

Conventional macromechanical tests provide limited insights into complex hierarchical deformation behavior of a transformative high entropy alloy (HEA). In this work, a high throughput microstructure-micromechanical correlative study is presented by combining high-resolution nanoindentation, site-specific microscopy, and Gaussian mixture model (GMM...

Solid Stir Extrusion (SSE) is an innovative friction-stir technology based process developed to obtain an extrudate of the preferred shape. As a solid-state manufacturing process, SSE embodies several benefits such as lower power consumption, continuity over a greater time frame, wide material range applicability, and flexibility to large-scale cus...

In this paper, Inconel 718 (IN718) superalloy was processed by laser powder-bed fusion additive manufacturing (L-PBFAM), followed by heat treatment. High-resolution nanoindentation was used to investigate the complex deformation mechanisms that occurred at various length scales in both conditions. The nanoindentation elastoplastic maps show a stron...

Direct laser deposition of a mixture of 90 wt.% nickel (Ni) and 10 wt.% niobium-carbide (NbC) using the laser engineered net shaping process resulted in the formation of an in-situ Ni-NbC composite with a complex hierarchical multi-phase microstructure. Detailed microstructural characterization at multiple length scales revealed that the in-situ co...

Properties and radiation responses of a metastable high entropy alloy (HEA) exhibiting the transformation induced plasticity (TRIP) effect were studied. Innovative engineering used to manufacture this HEA has shown superior mechanical and corrosion properties in 3.5% NaCl than most advanced stainless steels. The microstructural evolution and corres...

Additive friction stir deposition (AFSD) is a novel solid-state additive manufacturing process which offers unique capability of producing 3-D parts with no solidification related defects and having homogeneous, equiaxed microstructure. However, to fulfill the full potential of the process for structural applications, AFSD needs intense process opt...

Solid state nature of additive friction stir deposition (AFSD) additive manufacturing process is very advantageous in terms of defect formation and microstructural refinement in the material. Current study presents the process optimization, microstructural evolution and kinetics of recrystallization for AFSD deposited low stacking fault energy mate...

Smart alloying and microstructural engineering mitigate challenges associated with laser-powder bed fusion additive manufacturing (L-PBFAM). A novel Al–Ni–Ti–Zr alloy utilized grain refinement by heterogeneous nucleation and eutectic solidification to achieve superior performance-printability synergy. Conventional mechanical testing cannot delineat...

The AlCoCrFeNi2.1 eutectic high entropy alloy is of great interest due to its unique mechanical properties combining both high strength and plasticity. Here, gas tungsten arc welding was performed for the first time on an as-cast AlCoCrFeNi2.1 alloy. The microstructural evolution of the welded joints was assessed by combining electron microscopy wi...

A non-equiatomic CoCrFeNiMo high-entropy alloy (HEA) was used as a binder for the TiB2-HEA composites. The TiB2–6 vol.% HEA and TiB2-10 vol.% HEA composites were successfully fabricated by mechanical alloying and spark plasma sintering. Phase analysis revealed the presence of FCC and TiB2 phases in the composites. A detailed microstructural study u...

This study explored the possibility of using friction stir welding (FSW) to join jacket web sections of two nitrogen-containing stainless steels for housing internally cooled superconducting cables which are utilized to generate magnetic fields in tokamak type fusion reactor systems. The two candidate materials chosen for the jacket are SS 316 LN a...

Fusion welding of age-hardenable high strength aluminum (Al) alloys can be challenging due to the propensity of these alloys to develop solidification-related issues such as porosity and microcracking. A solid-state welding technique, such as friction stir welding (FSW), offers an excellent alternative to fusion welding by minimizing solidification...

Excellent work hardening in transformation-induced plasticity (TRIP)-enabled metastable high entropy alloys (HEAs) owe to persistent austenite (γ) to martensite (ε) phase transformation; non-basal slip activity and deformation twinning in the transformed martensitic phase are additional deformation mechanisms that contribute to work hardening in se...

The mechanical properties of transformation induced plasticity high entropy alloys (TRIP HEAs) are affected by tailoring the metastability via alloying and processing. The current work focuses on as-cast and friction stir processed alloy where the initial microstructure is altered by Cu addition (a γ-fcc phase stabilizer) to the ε-hcp dominated HEA...

In metallic materials, the nature of passivation film plays an important role in governing corrosion behavior. The present work focuses on a detailed analysis of the passivation film characteristics of a transformation induced plasticity Fe39Mn20Co20Cr15Si5Al1 (at%) high entropy alloy (HEA) in 3.5 wt% NaCl solution at room temperature. The characte...

Additive manufacturing (AM) has completely altered the traditional component manufacturing and qualification paradigm. It provides unitisation and topological optimisation opportunities simultaneously. Broadly, the additive manufacturing processes are classified as fusion-based or solid-state. The solid-state additive manufacturing processes are re...

Monitoring tool degradation during manufacturing can ensure product accuracy and reliability. However, due to variations in degradation conditions and complexity in signal analysis, effective and broadly applicable monitoring is still challenging to achieve. Herein, a novel monitoring method using ultrasound signals augmented with a numerically tra...

Multi-principal element alloys represent a new paradigm in structural alloy design with superior mechanical properties and promising ballistic performance. Here, the mechanical response of Al 0.3 CoCrFeNi alloy, with unique bimodal microstructure, was evaluated at quasistatic, dynamic, and ballistic strain rates. The microstructure after quasistati...

C103 is a refractory-based niobium alloy for high temperature applications in aerospace. This paper aims to evaluate the compatibility of C103 alloy as a choice of material for laser-based additive manufacturing (AM) technologies. Crack free AM specimens with relative densities of greater than 99% were successfully fabricated via laser powder bed f...

Fatigue failure is ubiquitous in structural components. In additively manufactured (AM) components, the processing induced defects limit the fatigue performance. Further, the stochastic nature of defects in laser-powder bed fusion (L-PBF) make it difficult to predict the fatigue life in these components. In this work, we explore exceptional work ha...

This paper describes the main results from an experimental investigation into tailoring the phase content and grain structure for high strength of a microstructurally flexible high entropy alloy (HEA), Fe42Mn28Co10Cr15Si5 (in at%), using rolling, friction stir processing (FSP), and compression. Optical microscopy, neutron diffraction, and electron...

Strain hardening in metallic materials delays catastrophic failure at stresses beyond the yield strength by the formation of obstacles to dislocation motion during plastic deformation. Conventional measurement of the instantaneous strain hardening rate originates from load–displacement data acquired during uniaxial mechanical testing, rather than t...

Laser-powder bed fusion (L-PBF) additive manufacturing offers unprecedented microstructural fine-tuning capabilities. Naturally, benefitting from such capability requires alloys that are amenable to microstructural heterogeneity and hierarchy (MHH) and that exhibit a low hot-cracking susceptibility (HCS). However, columnar growth, which is characte...

This paper introduces a new microstructural template for high entropy alloys (HEAs), where the face centered cubic (FCC) complex concentrated solid solution is reinforced with a high density of strong, yet deformable, nanorods of an ordered multi-component intermetallic L12 compound. Thermodynamic modeling has been employed to design this HEA with...

Additive friction stir deposition (AFSD) is a novel thermo-mechanical solid state additive manufacturing process. AFSD enables manufacturing of near net shape, fully dense builds with refined equiaxed grain structure resulting in excellent mechanical properties. AFSD has the potential to produce ingots and components/builds from recycled metals (ch...

Using the calculation of phase diagrams approach and Scheil solidification modeling, the Al-2.5Mg-1.0Ni-0.4Sc-0.1Zr alloy was designed, intentionally with an extraordinarily high cracking susceptibility, making it prime for solidification cracking during laser powder bed fusion. This study demonstrates the ability to mitigate even the most extreme...

A systematic study of butt friction stir welding of a recently developed Cu-containing metastable high entropy alloy (HEA) was conducted. Different zones of the weld were evaluated using mechanical testing, microstructural characterization, and differential scanning calorimetry (DSC). The nugget exhibited high tensile strength as a result of the re...

High entropy alloys (HEA) are an emerging class of novel metallic alloys with the ability to develop unique microstructural modification by using thermomechanical processing techniques. In this study, friction sir processing (FSP), widely regarded as an effective thermomechanical processing technique, was applied to a rolled HEA of nominal composit...

Mechanical response of transformation-induced plasticity (TRIP)-enabled metastable high entropy alloys that display austenite (γ) to martensitic (ε) phase transformation under uniaxial tension is characterized by a constant work hardening segment preceded and succeeded by stages of gradual drop in work hardening. A four-parameter empirical model ba...

Dissimilar material joints or multilayered metals have become inevitable in the manufacturing industry due to the increasing demand for multifunctional materials with variable mechanical, thermal, or electrical characteristics in a single assembly. Lattice mismatch of materials at the interface of dissimilar materials leads to inferior mechanical c...

Recent metastable alloy designs have demonstrated simultaneous attainment of high ultimate tensile strength (UTS) and ductility in high entropy alloys but with low yield strength. Here we present new strategy for improving the work hardenability and yield strength (YS) together in Fe38.5Mn20Co20Cr15Si5Cu1.5 high entropy alloy (Cu-HEA). Drastic incr...

Mechanical behavior of the binder phase in a ceramic-metal (cermet) composite is critical for its overall properties, especially toughness and shock resistance. High entropy alloys (HEA) are promising binder phase materials owing to their excellent mechanical properties and impact on obtaining fine-grain cermet microstructure. In the current work,...

Required microstructural attributes of an alloy vary with structural applications. The microstructural fine-tuning capability of laser-powder bed fusion (L-PBF) additive manufacturing (AM) enables application specific manufacture of the components. Such manufacture with L-PBF AM requires alloys that exhibit wide processing window and are amenable t...

The unprecedented increase in component design space has led to significant focus on fusion-based additive manufacturing (AM) technologies. The new design possibilities integrate features and functionalities that are not supported by conventional manufacturing, and simultaneously achieve unitization of components. However, fusion-based AM suffers f...

The paradigm shift of alloying approach that led to high entropy alloys (HEAs) is now well established. Although the initial years were dominated by equiatomic approach, recent years have seen expansion in non-equiatomic compositional space that can be termed as complex concentrated alloys (CCAs). These HEAs/CCAs provide opportunities for tunable p...

Compressive response of a novel Fe38·5Mn20Co20Cr15Si5Cu1.5 high entropy alloy with transformation induced plasticity made by laser powder bed fusion was studied at quasi-static, medium and high strain rates. Mechanical response and variation in work hardening rate with strain rate were correlated with γ (f.c.c.) → ε (h.c.p.) martensitic transformat...

Tuning deformation mechanisms is imperative to overcome the well-known strength-ductility paradigm. Twinning-induced plasticity (TWIP), transformation-induced plasticity (TRIP) and precipitate hardening have been investigated separately and have been altered to achieve exceptional strength or ductility in several alloy systems. In this study, we us...

Transformation induced plasticity (TRIP) leads to enhancements in ductility in low stacking fault energy (SFE) alloys, however to achieve an unconventional increase in strength simultaneously, there must be barriers to dislocation motion. While stacking faults (SFs) contribute to strengthening by impeding dislocation motion, the contribution of SF...

This short review paper examines the uniqueness of metastable high entropy alloys (HEAs) that transform during plastic deformation. The linkages among alloy design induced metastablilty, processing induced “high enthalpy states” and evolving deformation micromechanisms are intriguing. One key aspect is the adaptive nature of deformation mechanisms...

The compositional possibilities in high entropy alloys (HEAs) is very vast and effective strategies are needed to establish potential alloy chemistries. In this study, friction stir gradient alloying (FSGA), a recently-introduced high-throughput (HT) technique incorporating compositional and microstructural gradients , was used to explore the possi...

An in-depth understanding of microstructure and resultant properties is paramount in the design of a novel alloy system, especially for additive manufacturing (AM). The present investigation aims to characterize a prototypical AM Al alloy with great potential for structural applications. An Al-1.5Cu-0.8Sc-0.4Zr alloy designed using integrated compu...

Laser powder bed fusion additive manufacturing (LPBF‐AM) of a low‐alloy, high‐performance AF‐9628 steel results in exceptionally high strength and good ductility. The reasons for such mechanical properties are investigated through detailed microscopy performed at several length scales. Thus, the characterization of melt pool, porosity, grain morpho...

Engineering applications of high strength alloys are often restricted due to their poor tensile elongation or ductility. Alloys with high yield strength typically exhibit limited strain-hardenability (the difference between tensile and yield-strengths), leading to reduced tensile ductility. Deformation twinning, resulting in high strain hardenabili...

A probabilistic model to estimate the fatigue life of an additively manufactured material with polished surface was developed based on the statistical size distribution of grains and various manufacturing defects such as pores and unfused powder particles embedded in the microstructure. The probabilistic model includes the prospect of each microstr...

Development of multicomponent alloys, popularly known as high entropy alloys (HEAs) provides abundant compositional space for designing a variety of HEAs, including equiatomic, dual phase and microstructurally flexible (MF-HEAs). Among them, the design of MF-HEAs further extends the HEA domain in terms of understanding adaptive phase evolution with...

Effect of strain rate on the tensile behavior of two friction stir-processed dual-phase high entropy alloys was investigated. The dual-phase high entropy alloys have a high fraction of metastable f.c.c. phase. Engineering the f.c.c. phase metastability through alloy chemistry leads to different levels of transformation and impacts the retained f.c....

Use of laser powder bed fusion (LPBF) additive manufacturing (AM) in structural applications requires development of a) damage-tolerant alloys (alloys that exhibit high strength and ductility despite the presence of pores and/or microcracks), and b) corrosion resistant alloys. High entropy alloys (HEAs) offer abundant alloy design space that can be...

In this manuscript the ballistic impact response of a high entropy alloy is demonstrated along with detailed description of the corresponding deformation and fracture mechanisms. The high entropy alloy studied could be potentially transformative for next generation armor applications. Further details can be found in article, number 2000124, by Sund...

Complex concentrated alloys (CCAs) extend the compositional paradigm shift of high entropy alloys (HEAs) to new microstructural opportunities. CCAs provide opportunities for tunable performance by manipulating deformation mechanisms. Fe–Mn–Co–Cr–Si alloys exhibit potential for a combination of phase transformation and twinning. These alloys give gr...

A metastable transformative Fe39Mn20Co20Cr15Si5Al1 high entropy alloy (Al-HEA) was friction stir butt welded with Al-7050 alloy. Extensive mechanical mixing was evident in the weld nugget wherein sheared HEA particles were dispersed in Al-alloy matrix. Detail investigation of the weld interface showed suppression of intermetallic formation thereby...

High‐entropy alloys, consisting of multiple principal elements, represent a new paradigm in structural alloy design with excellent mechanical properties and potentially promising ballistic performance. Herein, the ballistic response of a single‐phase Al0.1CoCrFeNi high‐entropy alloy is evaluated with spherical E52100 steel (RC60) projectiles at vel...

Additive manufacturing (AM) technology for metallic alloys such as laser powder bed fusion (LPBF) brings tremendous opportunities for development of novel alloys specifically designed for AM that would desensitize the inherent process variability and requires a refined understanding of processing–structure–property relationship that would contribut...

Lamellar eutectic structure in Al0.7CoCrFeNi high-entropy alloy (HEA) is emerging as apromising candidate for structural applications because of its high strength-ductility combination.The alloy consists of a fine-scale lamellar fcc + B2 microstructure with high flow stresses > 1300 MPaunder quasi-static tensile deformation and >10% ductility. The...

Lamellar eutectic structure of Al0.7CoCrFeNi high-entropy alloy (HEA) is emerging as a promising candidate for structural applications because of its high strength-ductility combination. The alloy consists of a fine-scale lamellar fcc+B2 microstructure with high flow stresses >1500 MPa under quasi-static conditions. The response to shear loading wa...

Fatigue is a major cause of failure and requires dedicated investigation to validate the integrity of potential structural materials. As high and medium entropy alloys (HEAs and MEAs) are garnering significant attention from scientific communities, it is important to test the structural integrity of these novel alloys in cyclic loading environment....

This work builds on our previous investigation of the room temperature fatigue cracking mechanisms of an A356 Al alloy [1]. Here, we analyze the elevated temperature fatigue cracking mechanisms in cast and friction stir processed (FSP) A356, and contrast them with the room temperature behavior. Two sets of FSP parameters were used to modify the mic...