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In this perspective, recent advances and current research challenges concerning the mechanical properties of stabilized nanocrystalline face-centered cubic (FCC) metals are discussed. First, a brief review of key experiments and modeling efforts over the last two decades is provided, with a focus on elucidating the mechanisms associated with plastic yield, hardening, and microstructure stabilization in nanocrystalline metals. This prior work has provided an understanding of the transition between dislocation-based and grain boundary-mediated mechanisms in plasticity and has identified several strategies to mitigate temperature or stress driven grain growth. Yet, the consequence of various stabilization methods on mechanical properties is not well understood. Future research challenges are presented in order to address this scientific gap, most critically the need to include grain boundary chemistry or grain boundary phases resulting from stabilization methods in new mechanistic theories for mechanical properties of nanocrystalline FCC metals. [ABSTRACT FROM AUTHOR]

Thin film stress is frequently controlled through adjustments applied to the processing parameters used during film deposition. In this work, we explore how the use of solutes with different intrinsic growth properties influences the residual growth stress development for a common solvent Cu film. The findings demonstrated that the addition of a high atomic mobility solute, Ag, or a low atomic mobility solute, V, results in both alloy films undergoing grain refinement that scaled with increases in the solute content. This grain refinement was associated with solute segregation and was more pronounced in the Cu(Ag) system. The grain size reduction was also associated with an increase in the tensile stresses observed in both alloy sets. These findings indicate that solutes can be used to control the grain size under the same deposition conditions, as well as alter the stress evolution of a growing thin film. [ABSTRACT FROM AUTHOR]

This paper explores the effect of Ti's segregation and corresponding effect on the intrinsic thin film growth stress and microstructural evolution in a series of W1-x(Ti)x alloys where x is varied from 0 to 20 at. %. We report that the addition of the Ti solute reduces the compressive W growth stress, with further reductions achieved through in-situ annealing during growth. Upon examination of the microstructure, Ti did not appear to have a dramatic effect in altering the film's grain size and distribution, but it did increase the fraction of low angle grain boundaries. We confirmed that the A15 to bcc W phase transformation, which occurs in the early stages of W growth, diminished with increasing Ti content. This has been explained with respect to Ti's preference for gettering residual oxygen, a known stabilizer for the A15 phase. Collectively, this work demonstrates the impact of solute segregation in the control of residual stresses, specific grain boundary formations, and phase transformation control in growing thin films. [ABSTRACT FROM AUTHOR]

A section of the time-temperature-transformation (TTT) curve has been measured in the millisecond regime to describe the A1 to L10 transformation of 10 nm FePt thin films. Short time annealing was accomplished using a pulsed neodymium-doped yttrium aluminum garnet laser operating at a wavelength of 1064 nm. The temperature-time profile of the films was measured using an optical pyrometer and a platinum thin film resistor and it was numerically modeled. Effective thermal pulse widths were determined from the time dependence of the atomic diffusion coefficient calculated from the measured temperature profile. The measured TTT diagram involving average order parameter is consistent with theoretical predictions of TTT diagrams involving ordered volume fraction and shows that partial ordering can be obtained with a single effective thermal pulse as short as 1.1 ms. [ABSTRACT FROM AUTHOR]

A series of ≈12 nm thick FePt thin films deposited onto glass substrates have been annealed with multiple 1064 nm wavelength laser pulses. The fluence was varied using pulse widths of 10.0, 5.0, and 2.5 ms. The peak temperature for each individual pulse was kept near 700 °C. The A1 to L10 phase transformation was confirmed by x-ray diffraction. A single pulse was not sufficient to obtain a fully ordered state. A maximum order parameter of 0.89 and coercivity of 10.6 kOe was obtained after 5×10 ms pulses. This particular annealed film showed the greatest amount of grain growth with a mean size of 55.1 nm. This grain size is 20% smaller than that of a furnace annealed sample which was annealed for 60 s and yielded an approximately equivalent order parameter. Similar order parameters, grain sizes, and coercivity values were observed for films that had equivalent total annealing times regardless of pulse widths. [ABSTRACT FROM AUTHOR]

To eliminate compositional and size variabilities between individual binary nanoparticles, it is essential to control the mechanistic steps involved in nanoparticle synthesis. A common method for synthesizing FePt nanoparticles involves the simultaneous decomposition and reduction in iron and platinum precursors, respectively. This simultaneous nucleation and growth method yields wide composition and size distributions. This paper describes and experimentally validates a methodology needed to tighten composition and size distributions for this process. By engineering the surfactant chemistry with tertiary phosphines to tightly bind the iron atoms in the iron precursor, uniform platinum rich seeds form during the initial stages of the synthesis. A thermodynamically preferred heterogeneous nucleation of iron atoms into these uniform platinum seeds in the subsequent stages produces a final dispersion with uniform particle-to-particle compositions. The paper addresses the understanding for optimizing the nucleation and growth sequences for compositional control in FePt nanoparticles. [ABSTRACT FROM AUTHOR]

A series of FePt nanoparticles was synthesized by the thermal decomposition of iron pentacarbonyl and reduction in platinum acetylacetonate in phenyl ether solvent. A range of precursor molar ratios of 2, 1.5, and 1 between iron pentacarbonyl and platinum acetylacetonate was studied. After 30 min of reflux, the synthesis method produced a wide distribution in composition and size for the nanoparticles. Given 200 min of reflux, it was observed that the particle-to-particle composition and size narrowed, and the atomic ratio of Fe to Pt, for the majority of nanoparticles, approached the initial precursor molar ratios except for the molar ratio of 1. It is speculated that the compositional variability may be a result of the slow kinetics of iron pentacarbonyl’s decomposition in the reaction. [ABSTRACT FROM AUTHOR]

Self-assembled FePt nanoparticle arrays are candidate structures for ultrahigh density magnetic storage media. One of the factors limiting their application to this technology is particle-to-particle compositional variation. This variation will affect the A1 to L10 transformation as well as the magnetic properties of the nanoparticles. In the present study, an analysis is provided for the formation mechanism of these nanoparticles when synthesized by the superhydride reduction method. Additionally, a comparison is provided of the composition distributions of nanoparticles synthesized by the thermal decomposition of Fe(CO)5 and the reduction of FeCl2 by superhydride. The latter process produced a much narrower composition distribution. A thermodynamic analysis of the mechanism is described in terms of free energy perturbation Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

In this work, all group-IV band-to-band lasers based on SiGeSn/GeSn/SiGeSn multi-quantum-well structures were demonstrated. Lasing performance was investigated via two 4-well samples. The thinner GeSn well sample exhibits a maximum lasing temperature of 20 K and a threshold of 55 kW/cm2 at 10 K, while the thicker well sample features a higher maximum operating temperature of 90 K and lower lasing thresholds of 25 and 62 kW/cm2 at 10 and 77 K, respectively. The distinct results were tentatively interpreted mainly by the difference of gain volume. This result provides guidance for the future GeSn quantum well laser optimization for higher performance. [ABSTRACT FROM AUTHOR]

In this study, the authors address how surface roughness alters secondary electron emission. By using specific grades of metallographic polishing pads, controlled levels of roughness and surface features were imparted. As expected, the smoothest surface (root mean square roughness 0.110 ± 0.022 µm) produced the highest secondary electron yield; however, a moderate rough surface (0.990 ± 0.019 µm) produced a slightly lower yield as compared to a rougher surface (7.10 ± 1.23 µm) at lower primary electron energies. This inversion, that a macroscopic rougher surface yields a higher emission, has been explained by differences between large and small scale variations in the surface roughness and the frequency that these features appeared on the surface. The surface roughness was quantified using optical profilometry and a fast Fourier transform of the surface topology. [ABSTRACT FROM AUTHOR]

The issue of stress in thin films and functional coatings is a persistent problem in materials science and technology that has congregated many efforts, both from experimental and fundamental points of view, to get a better understanding on how to deal with, how to tailor, and how to manage stress in many areas of applications. With the miniaturization of device components, the quest for increasingly complex film architectures and multiphase systems and the continuous demands for enhanced performance, there is a need toward the reliable assessment of stress on a submicron scale from spatially resolved techniques. Also, the stress evolution during film and coating synthesis using physical vapor deposition (PVD), chemical vapor deposition, plasma enhanced chemical vapor deposition (PECVD), and related processes is the result of many interrelated factors and competing stress sources so that the task to provide a unified picture and a comprehensive model from the vast amount of stress data remains very challenging. This article summarizes the recent advances, challenges, and prospects of both fundamental and applied aspects of stress in thin films and engineering coatings and systems, based on recent achievements presented during the 2016 Stress Workshop entitled “Stress Evolution in Thin Films and Coatings: from Fundamental Understanding to Control.” Evaluation methods, implying wafer curvature, x-ray diffraction, or focused ion beam removal techniques, are reviewed. Selected examples of stress evolution in elemental and alloyed systems, graded layers, and multilayer-stacks as well as amorphous films deposited using a variety of PVD and PECVD techniques are highlighted. Based on mechanisms uncovered by in situ and real-time diagnostics, a kinetic model is outlined that is capable of reproducing the dependence of intrinsic (growth) stress on the grain size, growth rate, and deposited energy. The problems and solutions related to stress in the context of optical coatings, inorganic coatings on plastic substrates, and tribological coatings for aerospace applications are critically examined. This review also suggests strategies to mitigate excessive stress levels from novel coating synthesis perspectives to microstructural design approaches, including the ability to empower crack-based fabrication processes, pathways leading to stress relaxation and compensation, as well as management of the film and coating growth conditions with respect to energetic ion bombardment. Future opportunities and challenges for stress engineering and stress modeling are considered and outlined. [ABSTRACT FROM AUTHOR]