Donald Francis Susan

A solidification diagram for Ni-Cr-Mo-Gd alloys estimated by quantitative microstructural characterization and thermal analysis

A γ-Gd solidification diagram is proposed as an aid to understanding solidification behavior of Ni-Cr-Mo-Gd alloys. In this system, the Ni-Cr-Mo solid solution γ primary austenite phase is treated as the “solvent” and Gd is treated as the solute. The proposed diagram, which has features characteristic of a binary “eutectic” system, was constructed by combining differential thermal analysis and quantitative microstructural analysis data. As a result of the partially divorced solidification microstructure in the ingots studied, determination of the fraction eutectic, and hence the eutectic composition, requires the use of advanced image analysis techniques. The diagram displays a number of features that are very similar to the Ni-Gd binary system and can be used to assess the influence of the Gd concentration on solidification behavior.

Surface alloy depletion and martensite formation during glass to metal joining of austenitic stainless steels

Abstract Preoxidised and glass to metal (GtM) sealed austenitic stainless steels displayed a ferritic (bcc) layer near the metal/oxide interface, as determined by electron backscatter diffraction and X-ray diffraction. Through electron probe microanalysis, it was determined that this layer was depleted of alloying elements due to the oxidation and sealing processes. Characterisation of the layer morphology suggested that it formed through the martensite transformation mechanism. Thermochemical modelling with ThermoCalc also supported a martensitic transformation as opposed to diffusional ferrite formation. The composition gradient through the layer was correlated to the Eichelman and Hull empirical relationship for martensite start (Ms) temperatures. Because of Cr, Mn and Si depletion during preoxidation and glass sealing, Ms temperatures near ambient are possible in this surface region. The martensite layer was non-uniform, however, with laths extending deeper into the alloy due to stabilised growth in the material above its Ms temperature. This behaviour was characterised by image analysis techniques and discussed in terms of martensite stability and microstructural effects. Possible negative aspects of bcc phase formation on GtM seal properties are discussed, and analyses of alternative alloys 21-6-9 (tradename Nitronic 40; Armco Holding Corp., West Chester, OH, USA) and 22-13-5 (Nitronic 50) showed reduction or elimination of martensite after GtM joining.

Application of Electron Backscatter Diffraction for Crystallographic Characterization of Tin Whiskers

Understanding the growth of whiskers or high aspect ratio features on substrates can be aided when the crystallography of the feature is known. This study has evaluated three methods that utilize electron backscatter diffraction (EBSD) for the determination of the crystallographic growth direction of an individual whisker. EBSD has traditionally been a technique applied to planar, polished samples, and thus the use of EBSD for out-of-surface features is somewhat more difficult and requires additional steps. One of the methods requires the whiskers to be removed from the substrate resulting in the loss of valuable physical growth relationships between the whisker and the substrate. The other two techniques do not suffer this disadvantage and provide the physical growth information as well as the crystallographic growth directions. The final choice of method depends on the information required. The accuracy and the advantages and disadvantages of each method are discussed.

Thermal Fatigue and Failure Analysis of Cu-Plated Through Hole Solder Joints

A thermal cycle fatigue study and post-mortem failure analyses were conducted on through hole solder joints. Two types of joints were evaluated: connector-to-board and internal circuit board through holes (vias). The through hole vias in the former assemblies contain a Cu-based alloy pin, Fig. 1a, while those in the latter are either empty or solder-filled, Fig. 2a. In both assembly types, the via walls (barrels) are made from electroplated Cu. During thermal cycling, cracks develop in the Cu via walls, Figs. 1b and 2b. The thermal cycle employed was typical for assessment of high-reliability military and aerospace microelectronics, with temperature limits of -55°C and +125 ̊C, 15 min hold times, and ramp rates of 10°C/min. The effects of thermal cycling and partial solder filling on the propensity for Cu via cracking were investigated experimentally. The Cu fatigue cracking is caused by differential thermal expansion between the circuit board materials and the copper. Cross-sectional metallography was used to analyze the solder joints in the as-received condition and after 100, 200, 300, 500, and 1000 thermal cycles. The observations of via cracking were quantitatively summarized – the crack counting procedure is given in Ref. [1]. The phenomenon of thermalmechanical fatigue of the Cu-plated barrels was also simulated by finite element analysis (FEA).

Comparing Field Emission Electron Microprobe to Traditional EPMA for Analysis of Metallurgical Specimens

The use of a thermal field emission electron source and other design changes have significantly increased the resolution of electron probe microanalysis (EPMA), especially in the 5-8kV range of accelerating voltage.[1,2] The analysis of particles/phases as small as 200 nm in diameter has been documented for geological and meteoritical specimens and a similar analytical volume was shown in a Sn-Ag solder alloy.[1,2] The following work highlights other examples of field emission EPMA for the analysis of fine-scale metallurgical microstructures. Features too small for traditional EPMA, such as fine-scale lamellar transformations and microsegregation in solidification microstructures, can now be analyzed successfully with FE-EPMA. The high resolution is obtained while simultaneously covering a long linescan length or a relatively large area for mapping.

Accelerated aging and thermal-mechanical fatigue modelling of Cu-plated through holes with partial solder filling

The reliability of connector-to-board solder joints was investigated by accelerated aging experiments and finite element analysis (FEA), with a strain-based criterion for fatigue failure of Cu vias. The accelerated aging temperature cycle was ?55?C to +125?C. The pin/through hole solder joints were examined by metallography after test intervals up to 1000 thermal cycles. Partial solder filling of the joints was observed and attributed to Au contamination of the solder and thin solder gaps between the connector pins and Cu-plated vias. All fatigue cracks observed in the vias after thermal cycling were associated with partial solder filling of the joints. The effects of partial solder fill on failure location were confirmed by FEA and acceptable agreement was found between the predicted cycles to failure and the experimentally observed onset of via cracking. The model and experimental results were used to predict component lifetimes for milder thermal-cycle conditions typically found in service.

Measuring Carbon in Steel Using Calibration Curves on the Microprobe; Failed Cap Screw Study

A fractured cap screw was presented for failure analysis to determine the cause of the failure. The fractured screw is shown in Fig. 1. Fig. 2 is a SEM secondary electron image of the fracture surface that shows the low ductility nature of the fracture. The broken screw and a matching “sister” screw from the same flange have measured hardness values that significantly exceed the maximum allowed hardness. The hardness of the out of specification screws was on average 52.6 HRC. The specified hardness is 3845 HRC. Hardness measurement is a straight forward technique to estimate strength, but the question remained as to whether the anomalous hardness came from the steel chemistry, a heat treatment problem, or some other source.

3D RoboMET Characterization

The goal of this project is to generate 3D microstructural data by destructive and non-destructive means and provide accompanying characterization and quantitative analysis of such data. This work is a continuing part of a larger effort to relate material performance variability to microstructural variability. That larger effort is called “Predicting Performance Margins” or PPM. In conjunction with that overarching initiative, the RoboMET.3D™ is a specific asset of Center 1800 and is an automated serialsectioning system for destructive analysis of microstructure, which is called upon to provide direct customer support to 1800 and non-1800 customers. To that end, data collection, 3d reconstruction and analysis of typical and atypical microstructures have been pursued for the purposes of qualitative and quantitative characterization with a goal toward linking microstructural defects and/or microstructural features with mechanical response. Material systems examined in FY15 include precipitation hardened 17-4 steel, laser-welds of 304L stainless steel, thermal spray coatings of 304L and geological samples of sandstone.

Investigation of Steel Lug Nut Failures with Brittle Fracture Characteristics

Tractor trailer dual wheel inner cap nuts failed periodically over several years. The cap nuts were specified as 10B21 steel, a low carbon steel with boron addition. For corrosion protection, the cap nuts were zinc plated and chromate coated. In this study, the microscopic characteristics of the cap nut fractures were investigated along with companion tensile testing, hardness measurement, and novel insitu fracture with Auger spectroscopy. The failure mechanism was brittle intergranular fracture due to hydrogen and/or temper embrittlement. Contributing causes for delayed cracking due to hydrogen could include the following: 1) inadequate H bakeout after plating, 2) higher strength steel (more susceptible to H embrittlement) with lower fracture toughness, 3) hydrogen from corrosion reactions due to inadequate Zn and chromate coverage, 4) higher Cr in the failed samples could make the material more susceptible to temper embrittlement, and 5) periodic re-torqueing of lug nuts also may contribute to cracking, essentially mimicking a delayed hydrogen cracking test.[1] Supporting evidence of “thumbnail” cracks, with more corrosion than other areas of the fractures, suggested pre-existing cracks induced during maintenance.

Confirming the Composition of Shape Memory Alloys by Microstructural Characterization

It is well known that the properties of shape memory alloys, such as binary Ni-Ti (nitinol), are very sensitive to composition.[1-3] Especially on the Ni-rich side of stoichiometry, the martensite/austenite phase transformation temperature decreases abruptly when Ni is increased only a few tenths of a weight percent.[4] In ternary high temperature shape memory alloys (HTSMAs) containing Pt, Pd, or Hf, this compositional sensitivity can cause even greater shifts in transformation temperature, potentially hundreds of degrees C.[2,3] Figure 1 shows the Austenite-start temperatures of several heats of Ni-Ti-Pt alloys, measured by differential scanning calorimetry (DSC), as a function of aim Pt concentration. Some samples exhibit comparatively low As temperatures. The target composition for all specimens was slightly Ti-rich (50.5 at. % Ti). However, slight changes in the melt composition due to Ti oxidation, inhomogeneity, or other effects can sometimes push the alloys to the (Ni,Pt)-rich side of stoichiometry. While DSC provides a good measure of phase transformation temperature, it is only indirect evidence of the compositional effect. In addition, the small discrepancies in matrix composition that lead to property changes are often masked within the experimental error of bulk chemical analysis methods. To definitively confirm the (Ni,Pt,Pd)-rich or (Ti,Hf)rich character of a specific lot of material, microstructural characterization is necessary.